Gyro stability Test Criteria and Methods - Introduction

[gyrogreg]

This thread is to discuss and seek peer review on flight test methods to determine the quantitative static and dynamic stability of a gyro. This thread presents, for peer review and education, the currently proposed stability standards in the ASTM gyroplane standard, and the testing methods currently envisioned.

There are lots of forum discussions about theories of gyroplane stability. Some of these get very intricate. I have been one who suggests that these theories, with all the complicated interactions in a gyro, may not be fully defined or proven. I also espouse that there are no “cookbook” solutions to gyro stability. I suggest that such theories and “cookbook” may be valuable concepts for initial designs, but that the final determination if the final configuration is actually stable, must be determined by flight testing.

Let me also mention that “flight testing” does not mean going out to see how if “feels” or handles wind, or how easy it is to fly. Such “subjective” determinations, especially when provided by pilots experienced in flying that machine, are not true indications of how that machine might fly in the hands of other, less experienced pilots, in other realms of the flight envelope (speed, power, loading, wind turbulence, etc.) Often, it is the experienced pilot who actually stabilizes the gyro – it is very difficult for an experienced pilot to avoid commanding stabilizing control inputs because they may be so unapparent to even that pilot. Subjective pronouncements of “stable” can be very misleading and often, even unconsciously, biased.

The only true way to rate a gyro’s stability performance is to perform objective flight tests that basically isolate pilot flight inputs from the equation. This is what professional test pilots do, and what aircraft standards specify as criteria. Such criteria should be readily comparable to similar testing data from other machines so that objective comparisons and decisions can be made.

The object of the stability criteria and the testing methods is to assure, as well as practicable, that a particular gyro would not exhibit flight and stability characteristics that would lead to accidents. We all know that there are certain pitch stability accident types (mostly PIO and buntover) that have been the historic risk of flying gyros. The intent of these stability criteria is to address the stability issues that may lead to such accidents, and to help identify what those issues might be in any particular gyroplane. The intent of these criteria, and associated testing methods are to help gyro pilots evaluate and understand the potential stability-related risks in any gyro. These criteria may not be so confining or strict as some older aircraft stability standards, but we do want them to effectively address issues that might prevent likelihood of these traditional accidents.

These stability criteria, attempt to establish objective “flight performance” criteria. The criteria should be based on “results” of flight testing. The criteria and test methods should isolate, as much as practical, any pilot intervention or bias on the results. It is often tempting to base criteria on a “prescriptive solution”, rather than objective and analytical testing results. These criteria are intended to avoid, as much as possible, “prescribing” what a design should look like. These criteria should not care what a design looks like, they should only care about how it works – “Flight results testing”.

If possible, these criteria and test methods should be phrased in terminology readily understandable to a normal pilot with normal training – not confusing terminology or intricate theoretical descriptions.

These test methods are intended to be relatively simple and inexpensive to accomplish – require no exotic sensor or recording instrumentation. This is to both allow most gyro pilots to actually conduct some testing themselves, and to minimize costs of manufacturers who wish to verify compliance with the standard.

Below are individual posts addressing the four stability areas, or criteria, that the ASTM gyroplane standards subcommittee has proposed. These are divided into basically the following four areas:

Static Airspeed Stability
Static G-Load (or maneuvering) Stability
Static Power Stability
Dynamic Stability

The following posts initially address these four areas. For this initial thread, I hope we attract constructive peer review and comment. We are also hoping to refine some testing methods for each of these stability criteria. More information is included in these first several posts below. If extensive discussions on any particular effort evolve, to avoid confusion from too many subjects, I will break that area off into a new thread for more specific discussion.

[gyrogreg]

Standards Criteria:

Static Longitudinal Airspeed Stability:

The longitudinal control must be such that, with constant engine power, an aft force and movement of the cyclic control is necessary to achieve an airspeed less than any available trim airspeed;

The longitudinal control must be such that, with constant engine power, a forward force and movement of the control is necessary to achieve an airspeed greater than any available trim airspeed; and,

The control force gradient must not reverse during any progressive application of control movement at airspeeds greater than VMIN up to VNE.

Conditions--Static longitudinal airspeed stability must be met at the following power and airspeed conditions: Trimmed at:
(a) Steady altitude at MPRS,
(b) Full power at the lesser of VH or of VNE,
(c) Engine idle at MPRS, and
(d) Engine idle at 80 % VNE.

Explanation: This is the classic “test pilot” criteria for static airspeed stability – the slope of the control vs. airspeed curve must be positive. This means that a steady higher airspeed must not require an aft stick or pressure, and steady lower airspeeds must not require a forward stick or pressure. This criteria results in the tendency of the aircraft to self-restore itself to “trimmed” airspeed upon a disturbance to airspeed. A tendency for airspeed to diverge (to higher or lower airspeeds), without active pilot intervention, could result in severe aircraft divergent higher or lower airspeed. Aircraft with statically unstable airspeed static characteristics, requires constant pilot attention to maintain “trimmed” airspeed upon a disturbance to airspeed.

Suggested testing method:

Perform tests at different trimmed airspeeds, and loadings, in calm air

Increase airspeed above trimmed airspeed – note that forward stick pressure and position is required to maintain the increased airspeed.

Decrease airspeed below trimmed airspeed – note that an aft stick pressure and position is required to maintain the decreased airspeed.

Repeat test for different trimmed airspeeds from Minimum Power Required Airspeed (MPRS) up to Vne.

Repeat test for various allowed aircraft loading – gross wt. and CG limits

Note 1: Stick position can be determined or measured by a retractable tape measure secured to a solid point on the instrument panel and extended to the cyclic stick. Any other reasonable method to determine stick position deviation can also be used.

Note 2: Stick pressure or stick position is not required to be identified quantitatively other than the direction and that some force and stick movement is required.

[gyrogreg]

Standards Criteria:

Static Longitudinal Maneuvering (G-Load) Stability:

The pitch control forces during turns or load factor maneuvers greater than 1.0g must be such that an increase in load factor is associated with an increase in aft pilot control force, and a decrease in load factor is associated with a decrease in aft pilot control force.

Conditions--Static longitudinal maneuvering stability must be met at the following power and airspeed conditions: Trimmed at:
(a) Steady altitude at MPRS,
(b) Full power at the lesser of VH or of VNE,
(c) Engine idle at MPRS, and
(d) Engine idle at 80 % VNE.

Explanation: This is the classic “test pilot” criteria for maneuvering (G-Load) stability. This criteria results in the tendency of the aircraft to self-restore itself to 1g rotor loading upon a disturbance to rotor G-loading. A tendency for G-Loading to diverge (to higher or lower G-Loadings), without active pilot intervention, could result in severe rapid aircraft divergent pitch up or pitch down. Aircraft with statically unstable maneuvering static characteristics, requires constant pilot attention to maintain 1g loading upon a disturbance. Such G-Load instability upon rapidly decreasing and divergent G-Loading is thought to be a critical element in gyro buntover incidents – especially in the hands of an inexperienced pilot.

Suggested testing method:

Perform tests at different trimmed airspeeds, and loadings, in calm air
Steady the airspeed in straight and level flight.

Maintaining the same airspeed and power setting, bank the aircraft into an approximate 30 degree spiraling bank (to increase rotor G-Load) – steady the airspeed at the original trimmed airspeed.

Note that aft stick forces are required to maintain the original trimmed airspeed in the spiraling descent.

Repeat test for different trimmed airspeeds from Minimum Power Required Airspeed (MPRS) up to Vne.

Repeat test for various allowed aircraft loading – gross wt. and CG limits

Note 1: Stick pressure is not required to be identified quantitatively other than the direction and that some aft force is required.

[gyrogreg]

Standards Criteria:

Static Longitudinal Power Stability:

At the cyclic stick position established in level flight at MPRS at MPRS power, a change in power from MPRS power to full power shall not result in a change in a steady state trimmed airspeed of more than 10 % from MPRS.

At the cyclic stick position established in level flight at MPRS at MPRS power, a change in power from MPRS power to idle power shall not result in a change in a steady state trimmed airspeed of more than 10 % from MPRS.

At the cyclic stick position established in level flight at MPRS at MPRS power, a change in power from MPRS power to engine off shall not result in a change in a steady state trimmed airspeed of more than 20 % from MPRS.

Explanation: This criteria establishes that the “Sum of Static Moments”, or balancing of the HS to other static moments such as prop thrustline offset and drag line offset, reasonably prevents rapid aircraft pitching upon sudden power changes. This criteria also avoids severe commanded stick inputs upon power changes, such as during power reduction for flare or power application to “save” a botched landing. This criteria addresses the concerns for “unbalanced” high or low propeller thrustlines that might result in a buntover upon sudden rotor thrust or G-load decrease. This criteria results in a tendency for the aircraft AOA attitude and airspeed to remain relatively constant upon power changes – a characteristic the FAA suggests to avoid pilot over-reaction upon power changes. This criteria also initially assures that the CG/RTV relationship will not severely change under different power applications. Because this CG/RTV relationship is a critical factor in the first two static stability criteria above, this test Power Stability testing is recommended to be conducted, and any deficiencies corrected, prior to conducting the other stability tests.

Suggested testing method:

Perform tests at different loadings in calm air

Steady the airspeed in straight and level flight at MPRS.

Maintaining the same fixed stick position, increase power to full power.
Note that airspeed does not increase or decrease more than 10% from the original MPRS.

Repeat test for a power reduction from MPRS to idle power, noting that the airspeed does not deviate more than 10% from the original MPRS.

Repeat test for a power reduction from MPRS to idle power and then shutting the engine down. Note that, with power off, the airspeed does not deviate more than 20% from the original MPRS.

Repeat test for various allowed aircraft loading – gross wt. and CG limits

NOTE: It is suggested that an acceptable means to maintain the “fixed stick” condition for this test, would be to force the stick against a bar or spacer between a solid point on the instrument panel or pilot’s seat.

[gyrogreg]

First, I must note, that this DYNAMIC testing should be done only by professional test pilots who are experienced in the specific gyro type. Dynamic testing on a truly unstable gyro can cause rapid dynamic divergence.

CAUTION: This testing requires experienced testing methodology and technique. This DYNAMIC testing should not be conducted by an inexperienced pilot. This DYNAMIC testing should not, in any case, be attempted prior to meeting the STATIC stability test criteria above!

Standards Criteria:

Dynamic Longitudinal Stability

The gyroplane under smooth air conditions must exhibit no dangerous behavior at any speed between MPRS and VNE with:
(1) Primary cyclic controls fixed and
(2) With primary cyclic controls free.

No longitudinal oscillations with periods less than 5 s shall be exhibited with:
(1) Primary cyclic controls fixed and
(2) With primary cyclic controls free.

Any excitable longitudinal oscillations with periods longer than 5 s do not diverge with:
(3) Primary cyclic controls fixed and
(4) With primary cyclic controls free.

Conditions--Dynamic stability must be met at the following power and airspeed conditions: Trimmed at:
(a) Steady altitude at MPRS,
(b) Full power at the lesser of VH or of VNE,
(c) Engine idle at MPRS, and
(d) Engine idle at 80 % VNE.


Explanation: This criterion establishes two dynamic requirements:

1. That there be no rapid natural oscillation tendencies with periods less than 5-seconds. This is because more rapid oscillations or response rates to disturbances may be too quick for the pilot to properly apply commanded corrective control inputs. Rapid oscillation or response tendencies can lead to pilot reactive over-control or Pilot Induced Oscillations. This minimum of 5-second period criteria essentially means that any tendency to oscillate so quickly should be at least “critically damped” (no over-shoot) so as to not require or excite pilot corrective response.

The 5-second period limit is not a magic number concluded by extensive testing or historical evidence. This limit was presumed reasonable in that it is slow enough that normal pilot skills are assumed to be adequate to respond correctly to such rates. Some airplane criteria go much further than this, requiring the Phugoid oscillation period to be as much as 10-40 seconds. This gyro criterion does not distinguish between Phugoid oscillations or other short-period oscillations. This 5-second criterion allows that high maneuvering rates are a desirable characteristic of gyros and that a 5-second period seems reasonable for a normal pilot to correct.

2. That ANY natural oscillatory tendency be “damped” – this means induced oscillations would not inherently continue or diverge to higher amplitudes of oscillation. This assumes that the pilot should be able to apply proper corrective control responses to “pilot-dampen” or correct any oscillations or disturbances slower than those defined by the 5-second period requirement above. Such oscillations might be initiated by external uncommanded inputs (wind gusts), or by pilot over-reactive commanded control inputs. Many airplane criteria go further than this requirement and often require damping rates to 10% within 3 cycles. This gyro criteria presumes that normal pilot skills can stop or correct these longer period oscillations – as long as they are at least inherently damped somewhat.

Suggested testing method:

Suggest that indicators of oscillations can be airspeed indicator and/or rotor RPM indicator (G-load indicator). Testing of gyros suggests that these two parameters dynamically vary in proportion to aircraft AOA, altitude, climb rate,g-load, etc. Only one indicator would be required to establish damping and/or oscillation tendencies.

The magnetic pulse-wheel type rotor RPM indicators are handy for recording the RRPM deviations on a standard audio tape recorder. The recorded “hum” frequency variations are readily perceived and timed and can be measured electronically afterwards if necessary.

All testing must be done with for both “fixed-stick” and “free-stick” modes. Fixed-stick can be accomplished with the use of a spacer or bar to hold the stick against after the input excitation.

Positive damping (and measurement of damping rates) for longer period oscillations can be determined by using a singlet control input (step displacement of the cyclic stick from one position to another). This induces the longer-period Phugoid oscillations to be observed to be damped. This should not be attempted by any pilot that is not thoroughly accomplished and experienced in THAT gyro! This testing should not be attempted unless the above static criteria are fully met! A gyro with poor dynamic stability might be induced into a transient reaction that a less skilled pilot may not be able to stop!

Investigation for SHORT-period oscillations requires careful “doublet” control inputs - slowly increasing the “doublet” rate, and watching carefully for onset of oscillations. (A “doublet” is one complete cycle of control input.) This is the really critical testing safety concern that requires an experienced test pilot to accomplish properly and safely. Short period natural oscillating tendencies may only be induced when the exciting “doublet” input is close to the same frequency as a natural short-period oscillation frequency of the gyro (within 20%?). The test pilot starts with longer period “double” inputs, slowly increasing the frequency of the inputs while watching for oscillation tendencies. This is called a frequency “sweep” test. This is similar to how test pilots slowly approach “flutter” tendencies on airplane testing. The objective is to slowly approach the critical frequency, without actually matching that frequency where severe aircraft response might be possible. A full and careful “sweep” of the higher frequencies is needed to assure that there are no natural oscillation tendencies in that short-period range.

I have proposed that this DYNAMIC testing is probably not essential for gyro owners who want to verify their gyro is stable safe! I believe, that the HS volume and power required to meet the above static stability criteria will be more than adequate to provide adequate dynamic damping to meet these dynamic criteria. Much more testing data is required to make that assurance completely though. However, manufacturers wishing to comply with the ASTM gyroplane LSA standard WILL need to do the dynamic testing in order to determine compliance with the LSA requirements and to sell LSA gyroplanes in the U.S.

[Dean_Dolph]

Greg, first of all I want to make it clear to everyone that I don't have the credentials to be addressing any issue in this thread. But I do have a comment and question or two and maybe three!

1. What the heck is MRPS in the post dealing with Static Longitudinal Airspeed Stability? The acronyms you have used are another example of why a list of acronyms needs to be created to support the gyro glossary. Not all of us are knowledgeable enough to follow the presentation without knowing what they stand for. (Ha! Just found the definition further down but my comment still stands.)

2. I would think that the flight conditions would need to be recorded. You mention conducting tests in calm air but what does that mean? Zero, 5 mph, less than 10 mph or…….

3. What about specified trimmed air speeds? Is there some reason not too?

4. Why not specify how much above trimmed airspeed the airspeed needs to be increased? Also why not specify specific speed, loading Cg limits and etc. for all the tests?

5. Why not specify a required method of measurements? I'm specifically referring to the stick movement measurement but there may be other measurements taken that I won't comment on.

6. What about including in a final stability report the measured Cg, pilot weight (part of MOI?) and all the other design factors that influence stability?

What I'm driving at is that left on their own, the testers may properly conduct the tests as you have outlined and determined that the machine passes the standards. But without specific test conditions there will be no way that a comparison can be done between makes, models or types by anyone interested. This includes the manf. when changes are made or a new machine is developed. Sure, they can subjectively say some thing was improved but without presenting test results that were conducted to a specific standard they are leaving themselves open to question. I recognize that without data gathering equipment that a lot of the testing will be subjective anyhow but I would like to see personal opinion/judgement reduced to a minumum.

I can appreciate keeping the testing as simple as possible for the individual owner but as a consumer, I want to see standardized data from manufacturers of the light gyro. There might not be any value for the consumer in being able to compare data but I suspect there is. I would think that a data table would indicate not only that a gyro passed/failed the stability test but also the degree of stability/instability in all three axis.

[MGman]

Greg - I think this is probably the most important post I have seen on this forum since I have been reading it ( a couple of months now). It goes to the heart of gyro flying, in my opinion - To attempt to define and document the criteria for making gyros safer to operate and overcome the bad publicity the sport seems to have attained.

I am not a pilot (yet) so I don't have enough experience to comment on most of the aerodynamic concepts and criteria you have presented. However, I am a mechanical engineer and do understand the engineering concepts involved, and they definitely make sense to me. Part of my background has been involved in specification writing and applications, and also in testing procedures writing and documentation, and interpretation of the test results.

I hope that others on this forum that do have extensive experience will jump in and offer their comments and thoughts. This may be the only chance to affect the outcome of this very significant proposal before it is enacted.

The only significant comment I think I can offer is to suggest that the criteria provide for specific documented "success" criteria for each testing category. The specific success criteria for passing (or failing) each specific test must be clear and recognizable as the specific machine passing or failing the criteria. It should not be left to the individual builders to provide their interpretation of the test results. I guess my comments are really very much the same as Dean's, in that a data table could be produced to allow prospective purchasers of any specific machine to be able to easily see and understand the performance of a given machine in order to decide if it has accepatable operating characteristics for their conmfort level.

Please keep up the good work - This is a great piece of work.

Dave Bohler

[Dean_Dolph]

Greg, Dave clarified what I should have said! Spec writing? Test layout? Result interpretation? Yeah, been there, done that and it was always understood that if clear specifics were not built into the test procedure then the it was likely that Murphy would follow whoever was responsible for the testing and run the test exactly opposite of the way you intended. Not Good!

There is always danger in leaving anything open for interpretation in a test procedure. It is always black and white with absolutely no gray areas. This is one area where I believe being prescriptive is not only warranted but is an absolute necessity.

Now it is time to get out of the way while the knowledgeable people get some serious work done here.

[gyrogreg]

Dean and Dave,
1) Thank you Dave for pointing out that MPRS should probably be added to the “Glossary of Gyro Terms”. I haven’t forgotten your request to including acronyms in the “Glossary”, and I will do that when I get a bit more time. I do think, however, that the “Glossary” has been helpful to some people, despite its deficiencies.

2-6) Specifying numbers in the criteria: Your experience with writing procedures is really coming through. This is exactly the kind of feedback I am hoping to get from this forum thread! There would be no problem with quantifying and documenting the conditions and data from this testing. I think I draw a distinction between the standard’s criteria (minimum requirements) and a Test Method to verify compliance to those criteria. I think you are suggesting the Test Method should include more data and precision. The “Test Methods” is not developed, and may not have to be, for the LSA rules. But, if we develop formal “Test Methods” for the gyroplane LSA standard, we may indeed SUGGEST more precise specifics. The standards criteria specifies the minimum requirement, and a manufacturer might, and probably would choose to document these specifics, and more, for their confidence and files – but the LSA rules only require that a manufacturer state compliance with the standard, the intention is to leave the method of verifying compliance (test methods) up to the manufacturer if possible. I do agree that a manufacturer providing a more complete data set – above and beyond the standards requirement – could be an advantage for a sale. But, the manufacturer has that choice for competitive reasons – whatever they feel indicates their compliance to the standard the best. The standard does not make it a requirement for a more complete data set, but that may be an advantage for a manufacturer and some assistance for the more technically astute customer – manufacturer’s choice – not an FAA requirement.

The FAA also intends that we make verification of these criteria as simple and inexpensive as possible, while still assuring safety. And, to encourage, or at least not to discourage, the average gyro flier to understand and assess their own machine, I am inclined to keep things as simple as possible – trying to think outside the usual FAR box in language and procedures a layman can understand and accept!

I’m not sure we know how to more specifically quantify the conditions and criteria more precisely at this point anyway. I have conducted these tests numbers of times and have found no real technical need for more precision. For instance, if the stick is further forward and the pilot is having to also push the stick forward, positive static airspeed stability (positive slope of the curve) is indicated – current thinking is that positive slope is all that is required to make the gyro stable-safe from a static airspeed criteria. Also, for instance, for the “calm wind” criteria, if the wind is too turbulent, it is simply difficult for the test pilot to make the assessment – they will have to try again in calmer air – I have readily assessed most of the static criteria in winds with up to 10 mph gust factors – but, in some cases, it may be easier to make the assessment in calmer winds! As far as specifying exact conditions, my testing has found that three test points within the allowable or critical range (airspeed and power and load) – especially if the point near the worst extreme condition is tested - is more than adequate to eliminate the possibility of different results at some finite intermediate test condition. In other words, my testing hasn’t found it necessary to get over-precise here, and nobody has yet provided safety arguments for otherwise.

For better understanding and encouragement to use these criteria by less technically professional people, I am inclined to keep the guidance to the general public as friendly and simple as possible. But, we will likely be providing some guidance to manufacturers as well. And that guidance might be that manufacturers might be much more comfortable if they developed and recorded and filed a very complete testing data set – as part of their design development, and for their own, and their customer’s confidence of compliance with the standard. We will also be encouraging persons shopping for a gyro to ask for convincing data that shows a machine complies – a very complete data set might be more likely to make that sale – even if it doesn’t really make any better assurances. Although, in my testing, I do not find it necessary to require more specific data points and measurements than the standards requirements currently state, a manufacturer might more surely make a sale if they can provide a full data set with more data points. I think competition might also raise the standards of the test reporting as well – but I’m not sure that is really necessary for safety and should be required by the standard.

Our ASTM LSA standard is only intended to verify minimum criteria adequate to assure the gyro is stable safe. This is not intended for any more comparison between models other than that. IMHO, the degree of stable-safe beyond the simple requirements of these criteria would not really improve the safety of the gyro – but might be more a matter of “harmony” of control “feel” or personal preferences. For instance, a tuned “harmony” between airspeed stability and g-load stability might present a different “feel” between one machine or another – but not really influence the risk of an instability accident. Such variations – i.e.: stronger airspeed or g-load stability, might be more of a marketing choice, rather than a safety issue requiring more precise specification. By not specifying or quantifying parameters beyond what is needed for safety, we allow more choices for manufactures and customers.

But, at this point, we are not totally sure (at least I’m not) just what degree of stability really minimizes the risk of stability related accidents, without compromising desirable attributes of a gyro. That is one reason for my creating this thread – I am seeking experienced judgments or comments on whether the criteria are good enough to ensure stable-safe! I will certainly keep your “Testing Methods” suggestions in mind for if/when we provide such guidance to manufacturers. And, if you would have some good safety arguments for more tightly specifying conditions or results, we need to consider those. But, to this point, these are the degree of requirements that the LSA consensus says does assure stable-safe gyros. I’m looking for good arguments to support this or to change the requirements - many of these criteria are the best opinions of experienced people but may not have a lot of analytical or accident statistical support. The FAA has encouraged us to “start somewhere”. If we find from experience or technical argument that some criteria need to be tightened up (or loosened), that is what the ASTM review and revision process is for. If this thread uncovers improvements in these criteria, our ASTM gyroplane subcommittee will consider such suggested changes – the reason I have started this thread.

Thanks, Greg Gremminger

[Mike Jackson]

Standards Criteria:

Static Longitudinal Maneuvering (G-Load) Stability:

Explanation: This is the classic “test pilot” criteria for maneuvering (G-Load) stability. This criteria results in the tendency of the aircraft to self-restore itself to 1g rotor loading upon a disturbance to rotor G-loading. A tendency for G-Loading to diverge (to higher or lower G-Loadings), without active pilot intervention, could result in severe rapid aircraft divergent pitch up or pitch down. Aircraft with statically unstable maneuvering static characteristics, requires constant pilot attention to maintain 1g loading upon a disturbance. Such G-Load instability upon rapidly decreasing and divergent G-Loading is thought to be a critical element in gyro buntover incidents – especially in the hands of an inexperienced pilot.

.

In fixed wing testing , one usually looks for a positively increasing slope with no objectionable "nonlinearities" up to about 3 Gs. The slowly varying G method at constant airspeed as you described is one of the better methods to use. If the curve "flattens" , the stick force per G lightens, at higher G we might begin to find objectionable maneuver flight characteristics which can aggravate or possibly initiate a PIO situation.




Mike

[Mike Jackson]

1. That there be no rapid natural oscillation tendencies with periods less than 5-seconds. This is because more rapid oscillations or response rates to disturbances may be too quick for the pilot to properly apply commanded corrective control inputs. Rapid oscillation or response tendencies can lead to pilot reactive over-control or Pilot Induced Oscillations. This minimum of 5-second period criteria essentially means that any tendency to oscillate so quickly should be at least “critically damped” (no over-shoot) so as to not require or excite pilot corrective response.

The 5-second period limit is not a magic number concluded by extensive testing or historical evidence. This limit was presumed reasonable in that it is slow enough that normal pilot skills are assumed to be adequate to respond correctly to such rates. Some airplane criteria go much further than this, requiring the Phugoid oscillation period to be as much as 10-40 seconds. This gyro criterion does not distinguish between Phugoid oscillations or other short-period oscillations. This 5-second criterion allows that high maneuvering rates are a desirable characteristic of gyros and that a 5-second period seems reasonable for a normal pilot to correct.

2. That ANY natural oscillatory tendency be “damped” – this means induced oscillations would not inherently continue or diverge to higher amplitudes of oscillation. This assumes that the pilot should be able to apply proper corrective control responses to “pilot-dampen” or correct any oscillations or disturbances slower than those defined by the 5-second period requirement above. Such oscillations might be initiated by external uncommanded inputs (wind gusts), or by pilot over-reactive commanded control inputs. Many airplane criteria go further than this requirement and often require damping rates to 10% within 3 cycles. This gyro criteria presumes that normal pilot skills can stop or correct these longer period oscillations – as long as they are at least inherently damped somewhat.



Greg, I have found that most FW systems have a short period between 2 - 5 seconds. Most systems will NOT be critically damped and will have some degree of overshoot. We want to make sure these overshoots are damped and have a short "time constant" - time to 63% of your stable initial value of whatever you are using to measure your short period. This could be fuselage ref line, rotor rpm, disc AOA - but not airspeed by assumption. We assume the doublet you described is sufficiently quick to leave airspeed constant. This time constant (Greek letter TAU) will be on the order of a second or less for higher perf FW aircraft and probably close to that for a rotor system. I need to look at Houston's data. Point is, it will be quick and none of us will adequately be able to respond quickly enough if we wanted to. That's where sufficient HS size and placement comes in. Let the design take care of it.

The Phugoid is one of those things that is more annoying than anything else - especially to autopilot designers. Most are damped, maybe neutral with a long period 20+ seconds and I believe fall into the WGAS category where gyro flying is concerned. To excite, slowly pull back on the nose (FRL) a finite amount and watch the AS go down, yawn a little then watch the AS build and the nose come up. You can do it stick free or fixed.

Here's a little beauty that will allow you to get a feel for both maneuvering flight (stick force/G), and the short period mode (gust stability). NASA pilots and engineers refers to this as the "Concave Downward" requirement emphasizing the short-time response. Might be helpful where the FAA LSA requirement descriptions are concerned. It requires that the time history of normal acceleration (G ) in response to a cyclic control step be concave downward within 2 seconds. It is intended by this feature, to provide a que of the coming acceleration (G response) , and to guarantee a finite (damped) response to the control deflection. The requirement provides for reasonable maneuvering characteristics in forward flight - possibly the most compelling single handling quality.

How do we do it in practice? Pick an airspeed in the heart of your gyro's envelope and trim hands off at a constant power setting. Quickly pull to a specified G level. Don't get too carried away on the pull - try about 2 to start. When you've reached your target G, quit pulling. You can hold the stick fixed or release it (stick free). What do you feel? The G may increase for up to 2 seconds but you should then feel the G load decay. This is your indication of a finite damped response.

You may have doubts about G continuing to increase after you've stopped pulling but there seems to be lag in any pitch response. One of the better films I've seen of this is Ernie Boyette's Dominator video where he is pumping his stick fore and aft flying straight and level. Watch the stick position and the slight lag in fuselage (G) response as he does this.

You can now change airspeeds and CG locations if you wish but it is a quick test technique to get a feel for dynamic stability and maneuver flight. You might also say, all we've really done is excite the short period with a single pulse. This is the case, but because the airspeed will decay here, you might see the phugoid also excited .

Mike

[MGman]

Greg - Thanks for your reply - You are extremely patient with people that don't have direct flying experiience, and I appreciate it. I feel a little reticent doing any posting because of my lack of experience, but what the heck - I guess everyone who is interested has something to offer :-).

I need to give Dean the credit for the acronym comments BTW. My comments were toward seeing what your thinking was in providing more hard numbers for the pass/fail criteria. From your response you have obviously given these same ideas a lot of though and I can see where you are coming from. You can't make them hard and fast for a lot of flying situations where it is mostly a pilot feel sort of thing.

It looks like you are starting to get the experienced flyers making comments now, so I'll sort of drop back and stay out of the way (unless I read someting I don't agree with (or don't understand), at which point you'll hear from me again :-).

Thanks again for your efforts - they are helping me learn - Dave Bohler

[raghu]

Thanks Greg for putting out the proposed tests for peer review. In this post I will give my comments on the dynamic stability test and on a later one I will post comments on the static tests.

I have two major comments 1) the manner in which oscillations are measures; 2) the criterion that no oscillation must have a time period less than five seconds I see Mike J. has brought this up as well;


No longitudinal oscillations with periods less than 5 s shall be exhibited with:
(1) Primary cyclic controls fixed and
(2) With primary cyclic controls free.
.....
.....

Suggest that indicators of oscillations can be airspeed indicator and/or rotor RPM indicator (G-load indicator). Testing of gyros suggests that these two parameters dynamically vary in proportion to aircraft AOA, altitude, climb rate,g-load, etc. Only one indicator would be required to establish damping and/or oscillation tendencies.


1. THE NATURE of DYNAMIC MODES
-----------------------------------------------------
In order to measure the oscillations one must first be aware of the different dynamic modes and what parameter ( airspeed, rotorspeed, pitch varry). It turns out in a gyro that passes the G load static stability test ( this is really AOA static stability...more on that later) there are essentially three oscillation modes
a. short period mode: varies in time period between ~2-10 seconds and is alwys damped though almost never critically. The damping for example in the magni VPM m16 is ~ 1 second to half in amplitude. The oscillation is primarily one of pitch attitude but it may involve (particularly at high speeds) some rotor speed variation. In my opinion the way to measure it would be by looking at pitch variations, given that at low speeds you may see no rotor speed changes in this mode) . A video camera should do the job. As Greg has said the way to capture this mode is by a stick doublet.

b.Long period or Phugoid mode: In the order of 10+ seconds and at best weekly damped and can be mildly unstable as well. The oscillation involves airspeed and also rotor speed. The rotor speed slightly lags airspeed due to rotor inertia. For the magni vpm 16 the time period is 12-15 seconds and time to damp to half amplitude is between 30 and 40 seconds.

c. rotor mode. This by the nature of autorotation is critically damped ( deadbeat response) and can be ignored ( see *)

2. MEASURING PERIOD and DAMPING
Given this my view is that the long period should be measured by air speed, though rotor speed may also suffice. But for the short period measuring pitch variation may be the best option.

3. FIVE SECOND CRITERION
If the criterion is just for the long period mode then its fine. However the short period mode will be in the 5 second or below range. For example the magni VPM 16 at 30 mph has a short period mode of 4 seconds! Similarly I would guess the littlewings would be low as well particularly the elevator model that relies on pitch changes to make cyclic input. Both these aircraft from all accounts are very pilotable and must pass the stability test.

Things are a little different with gyros that have a lack of g load (what is really AOA stability, more on this later) stability, the short period merges with the rotor mode and a new second phugoid mode of a smaller time period ( ~6 seconds) develops and things get very hairy....... though the mode may be marginally stable and could induce PIO. For this reason, this mode must be taken out by insisting on G load ( AOA ) static stability.

Hope this helps and I will post other comments as and when I get a chance to write.

[gyrogreg]

Mike and Raghu, thank you very much for your thoughts. It is very helpful to have experienced and qualified technical input. I think these are very important and helpful issues to explore. I’m not sure I am technically savvy enough to understand all that you have presented, but let me see if I can summarize – and then I have a few specific questions to help clarify this:

Both Mike and Raghu suggest that there WILL be natural oscillation tendencies below 5 second periods – AND, these WILL NOT be “critically damped” – they will have some overshoot. BUT, Raghu suggests that these short-period oscillations will be quickly damped (1 second or more?) IF the aircraft has G-Load static stability (Raghu says this is really called “AOA stability”).

QUESTION 1: Are we sure of these short-period assumptions for Gyroplanes, or are they unproven transferences from FW fundamentals – can we trust these assumptions?

QUESTION 2: Our ASTM standards criteria says “No longitudinal oscillations with periods less than 5 seconds”. In my explanation, I had assumed this would be met if those oscillations were at least “critically damped”. Since they are not “critically damped”, but may be quickly damped (1 second +), how would you recommend we define “no longitudinal oscillations”? Should some point of “quickly damped” be considered acceptable? - Or is “no oscillations” an improper simplification (remember, keep it simple!). What could we use as a measurable dividing point between “oscillations” or "no oscillations"?

QUESTION 3: If a gyro meets the Static G-Load Stability requirement (AOA stability?), would this technically suffice for adequate damping of short-period oscillations – therefore making the “no oscillations shorter than 5 seconds” criteria unnecessary?

Raghu, is suggesting that the short-period oscillations would not be observable via RRPM. I would agree that a video camera recording nose attitude would certainly be a simple means to observe and record those oscillations – if this is a necessary requirement (Question 3 above).

QUESTION 4: Mike, you seem to agree that the requirement that longer-period be “damped” is adequate – not a safety issue! - rather than requiring a specific damping rate. Or, are you suggesting that even undamped longer-period oscillations are not really required?

QUESTION 5: Raghu, you are pointing out that the VPM16 (as tested by Dr. Houston) has a short period of 4 seconds, but is damped to ½ amplitude in 1 second. The short period of less than 4 seconds would arguably not meet the standard’s 5 second minimum. You suggest similar results are likely for the LW. But, since you suggest that both these aircraft are “very pilotable”, are you suggesting that the criteria of a 5 second minimum oscillation period is too stringent – for safety? Would you define a 4 second period with a 1 second damping rate as being an “oscillation” per the criteria in the ASTM standard?

QUESTION 6: I have tried hard to initiate a “short-period” oscillation in the Magni M-16 (“sweep testing” with “doublet” excitation!). I would expect this to repeat what Dr. Houston reports. What am I missing? Is the damping rate so rapid that they cannot be readily detected? Was Dr. Houston’s determination from actual flight testing, or from the computer model? Are there better ways to verify there are “no oscillations” - short period? In light of QUESTION 3 above, is this even necessary?

QUESTION 7: What is the difference, or is there any difference, between “Maneuvering (G-Load) Static Stability” and “AOA Static Stability”? Are these just terminology differences, or are there real differences that influence real safety determinations we should be considering?

I ask the above questions for clarification and advancing our understanding of what the stability requirements to assure a stable-safe gyro should be. I invite anyone with thoughts on all this to please post.

Thanks, Greg Gremminger

[raghu]

Greg, Here are the answers. feel free to ask for any clarifications.

QUESTION 1: Are we sure of these short-period assumptions for Gyroplanes, or are they unproven transferences from FW fundamentals – can we trust these assumptions?

Ans: I was giving data from actual flight test of a gyro. So, yes the short period does exist. Now, do all gyros have such a mode? Well the pure short period mode will always be there as long as you have AOA stability. This mode will often be resonably damped ( I will explain why in a later post if you want an explanation) but there could be exceptions.

On the other hand if you do not have AOA stability ( say neutrally stable) then the pure short mode will ironically become critically damped ( yes this is true) but as you increase the instability it will rapidly merge with the rotor mode and form a new mode that is very much like the phugoid in terms of damping ( weekly damped) but is of a lesser time period ( though not as low as the pure short period mode). Based on this high frequency and low damping, I argue that this may well be the mode that is suseptible to PIO and shoul be taken out of the equation by requiring sufficient AOA stability.

Nb. in theory the transition from short period mode to critically damped mode will occur at zero AOA stability, but in practice this transition may occur at some point just above or below AOA stability as other factors may come into the equation, though AOA is the dominent factor and so the transition is very nearly at neutral AOA stability.


QUESTION 2: Our ASTM standards criteria says “No longitudinal oscillations with periods less than 5 seconds”. In my explanation, I had assumed this would be met if those oscillations were at least “critically damped”. Since they are not “critically damped”, but may be quickly damped (1 second +), how would you recommend we define “no longitudinal oscillations”? Should some point of “quickly damped” be considered acceptable? - Or is “no oscillations” an improper simplification (remember, keep it simple!). What could we use as a measurable dividing point between “oscillations” or "no oscillations"?

Ans: I think we are going into handling qualities issues here and you may need more input from test pilots, but I would say make no limitation to the maximum frequency as long as the damping is in the range of damp to half in 1 second. These oscillations will not be noticable to the pilot possibly just as you do not spot the magni short period mode



QUESTION 3: If a gyro meets the Static G-Load Stability requirement (AOA stability?), would this technically suffice for adequate damping of short-period oscillations – therefore making the “no oscillations shorter than 5 seconds” criteria unnecessary?

Ans: It is likely in practice given current configurations you will be OK ( well damped SP mode), but you cannot guarentee it ( at least until we have more data). For example, it is concevable that you have a scenario where a very short lever arm is used with a sufficient HS to provide static AOA stability. In this case damping may be low and things could get squirly ( though there will almost always be positive damping). One would thus need to test.


Raghu, is suggesting that the short-period oscillations would not be observable via RRPM. I would agree that a video camera recording nose attitude would certainly be a simple means to observe and record those oscillations – if this is a necessary requirement

Ans: The sp mode may have some RRPM digressions ( particularly at high speed) but that is not the primary component and so pitch variations is a good attribute to measure the sp mode.


(Question 3 above).

QUESTION 4: Mike, you seem to agree that the requirement that longer-period be “damped” is adequate – not a safety issue! - rather than requiring a specific damping rate. Or, are you suggesting that even undamped longer-period oscillations are not really required?

QUESTION 5: Raghu, you are pointing out that the VPM16 (as tested by Dr. Houston) has a short period of 4 seconds, but is damped to ½ amplitude in 1 second. The short period of less than 4 seconds would arguably not meet the standard’s 5 second minimum. You suggest similar results are likely for the LW. But, since you suggest that both these aircraft are “very pilotable”, are you suggesting that the criteria of a 5 second minimum oscillation period is too stringent – for safety? Would you define a 4 second period with a 1 second damping rate as being an “oscillation” per the criteria in the ASTM standard?

Ans: see my answer to question 2.

QUESTION 6: I have tried hard to initiate a “short-period” oscillation in the Magni M-16 (“sweep testing” with “doublet” excitation!). I would expect this to repeat what Dr. Houston reports. What am I missing? Is the damping rate so rapid that they cannot be readily detected? Was Dr. Houston’s determination from actual flight testing, or from the computer model? Are there better ways to verify there are “no oscillations” - short period? In light of QUESTION 3 above, is this even necessary?

Ans: The figure I quoted where from actual flight testing. It is tricky to catch and will require you pumping the controls at the right frequency, but you should spot it, though you cannot measure it as easily as the phugoid. you may need to video and verify after the flight. Perhaps even that may not be accurate enough as freezing the stick during such a short duration is hard and you are going to get inadvertend cyclic noise. In general as long as you see enough damping this mode not a problem

QUESTION 7: What is the difference, or is there any difference, between “Maneuvering (G-Load) Static Stability” and “AOA Static Stability”? Are these just terminology differences, or are there real differences that influence real safety determinations we should be considering?

Ans: This is something that needs some clarification. I hope to devote a separate post ( when I get a chance) to articulating how engineers think about dynamic stability and what tools they use to better get an insight. The current approach in many posts is a mish mash and very unstructured approach with direct and indirect effects all combined, often resulting in circular arguments. In the meantime more to the point...

1. Static AOA stability: Gives the relationship between AOA and pitching moment. If an increase in AOA results in a nose down moment then the gyro is AOA stable. The turn test actually measures this.

2. static Rotor G load stability: This is a relationship between RRPM and pitching moment. If an increase in RRPM produces a down pitching moment then the gyro is rotor G load stable. A test for this may need more thinking. Out of the top of my head photos at trimmed flight could help determine CG versus RTV relationship. Or perhaps a more radical one, dropping a sack of potatoes in flight and noteing the effec on pitch!

Usually AOA stable gyros are also rotor G load stable because AOA satbility is often derived by placeing RTV behind the CG. However this is far from guarenteed. Consider a gyro that has RTV in front of a CG ( at some air speed) and also has a large HS with zero incidence to the relative wind. Now this gyro may very well be AOA stable if the HS is large enough as the slope of the HS is much steaper than the rotor. However, the gyro would be G-load unstable.

why bother with this distinction? Well it turns out that if you are G load unstable the long period mode losses a bit of damping ( by interacting with the rotor speed). This may still be perfectly fine, but the trend is adverse. Thus the RTV BEHIND THE CG is a very good criterion as it kills the proverbial two birds with one stone- both AOA and G load are stable.

[Udi]

I think the proposed tests and stability criteria are an excellent work. Greg, we are all in debt to you and the rest of the team.

I am trying to think whether these tests and stability criteria will detect other potentially hazardous modes of operation. One such mode is a zero-G event, in which the rotor thrust vector is not available to help stabilize the aircraft. Although this event may be rare, it can happen none the less due to weather conditions or pilot action (like pushing the stick on top of a zoom). There have been PPO accidents in which zero-G event - without a prior PIO - was seen as the root cause of the accident. I think these accidents were referred to as "spontaneous PPO".

Are there any current gyroplane designs, which will pass the above tests, but are still prime candidates for a spontaneous PPO? I can't prove it, but my gut feeling is that some high thrust line gyros, with an effective stab, may pass the above tests but will not survive a zero-G event. I think that a large and effective stab can help the gyro pass the proposed tests, even when the sum of all moments is such that the RTV is always forward of the CG. We talked about the stab contribution to static and dynamic stability in another thread.

This question is unique to gyroplanes. Unlike fixed wing airplanes, in a gyroplane, all control is lost in a zero-G event. At that point "the sum of all moments" will determine whether the gyro will pitch, roll, or both. How can we make sure that the sum of all moments in a LSA gyroplane will always try to restore a positive G-load?

And how can we make sure the gyro will not roll to an inverted position due to engine torque? This potential problem is also unique to gyroplanes.

I have a suggestion for another set of tests. These are not flight tests; they can be performed in the shop or, preferably, in a wind tunnel. Since we want to test the behavior of the gyro in a zero-G event, lets simulate such an event.

Mount the gyro on a test stand equipped with strain gauges. Start the engine and measure the torques in the pitch and roll axes. If the engine thrust line is passing through the CG, or if a stabilizer cancels the engine pitch moment, there will be no torque in the pitch axis. The standard may require that the sum of all moment about the CG, excluding the RTV, must be nose up, within certain radial acceleration limits.

There could be a similar standard for torque in the roll axis.

Any thoughts?

Udi :cool:

[gyrogreg]

Mike Jackson sent me an email with some points in it I would like to present and document on this forum for all to consider. These are responses to my set of questions above:

Don't be afraid of SP testing. It can be done with a single step pulse. You can use a doublet, but it should be quick enough to excite the SP response which will not be far off the machines natural freq which can be found with freq sweeps.

I'm currently just throwing down thoughts and book stuff but I will answer all your forum Qs as succinctly as I can under each question topic.

Back to SP - Q 1, I believe SP and Phugoid phenom are well thought out in theory and displayed in actual flt test. I've got a couple of text quotes along these lines.

Q2 - This ASTM std is way off base and should be stricken. I'll try to discuss why a combination of pitch or viscous damping AND damped natural freq are BOTH important and why critical damping is not necessarily desireable. I'll explain how we can easily find the damping ratio and damped freq in flt testing. We can also talk about relative values (real #s) for these parameters.
No oscillations is not a realistic option. We can also talk about phugoid parameters and flight test.

Q3 mind melds into Q2. I think Fourcade has a nice discussion of disk AOA stability. For SP testing we can measure fuselage reference lines (FRL), disk angle, pitch rate etc. We'll probably use FRL. How close the RLV is to the CG is a strong player. It is analagous to FW "static margin" discussions and strongly affects damped nat freq of the SP. What is cool is the gyro's "static margin" moves!

Q4 All flight vehicles I'm familiar with will have a long period mode (phugoid mode). Most texts agree it is more annoying than anything. Because of the long periods involved 20 - 45 sec, this mode is easily controllable even if it is slightly divergent. Most to all pilots wouldn't recognize it for what it is. Houston's Phugoid is ~ 20 sec according to his charts. BTW, his SP mode period is ~ 5 sec.

Q5 See Q4. Don't confuse these times. The 4 sec period is just that - a period (top of one crest to the top of the next crest). The amplitude will decay to the original pre-disturbed value of what you are measuring. The amplitude of overshoot will decay along the previously discussed period (or frequency). We use a standard as the time it takes the amplitude to decay to 1/2 the original disturbed amplitude. Thru some shake and bake, this time = t1/2 = .69/damping ratio x natural freq -------these can be measured from flt test.

Q6 What are you seeing Greg? Do you think you've excited the mode? Are you doing it stick fixed or free? You can try it from a positive singlet. Stick quickly aft to about 1/2 G initially then quickly back to the trimmed position. Start the clock and observation here.

Q7 - This is terminology for the same things in my view.

[GyroRon]

Udi I agree with you.

Hey how you guys like this? the shortest post in this whole thread!

[gyrogreg]

ALL: As you can see, we are getting some really good thoughts on all this. I have to admit, I may not be as “rounded” on all of these issues as some of you. Since we have these really deep thinkers working on this forum now, I think I’d should play the role of facilitator (or maybe Devil’s Advocate) here to see if we can reach a consensus among each of these experts.

Each of you are now expressing views on specific issues or QUESTIONS. I'll try to point out the issues on which there seems to be consensus or not. Could I ask each of you to spot issues that you are in difference to, and present arguments that might start resolving a consensus among the “experts”?

I will try to be the one also to point out differences or ask “dumb” questions to stir further consensus development, or to simplify the "consensus" opinion for mass consumption.

Udi, I’d like to stir a little more discussion on a couple of your points in your post above. Some “dumb” questions:

QUESTION 8: Udi, you said: “Unlike fixed wing airplanes, in a gyroplane, all control is lost in a zero-G event.” I recognize that under zero-G conditions, the airframe is no longer “hanging” under the rotor – so it might more readily roll or pitch during a zero-G event. But, is “all control” lost? At least immediately upon loss of G-Load, the rotor is still spinning and cyclic action still works - even at negative G-load! Doesn’t this at least mean that a commanded cyclic input can still restore positive G-load, and/or stop a roll? And, if the Sum of Static Moments” (in pitch) is proper, the airframe and spindle would naturally pitch in the direction (nose-up if the CG is forward of the RTV) so as to inherently restore positive G-load upon a sudden loss of G-Load? This Static G-Load stability action tries to restore 1g loading upon ANY disturbance from that steady state 1g condition!?? It seems to me that Positive G-Load Stability is perhaps the "Holy Grail" of gyroplane stability safety - even dynamic stability issues! And the stronger this factor, the safer! Does this not mean that such postively G-load stable gyros are much less risky or even immune from "spontaneous PPO"? Am I wrong?

QUESTION 9: Udi, you went on to say: “At that point "the sum of all moments" will determine whether the gyro will pitch, roll, or both. How can we make sure that the sum of all moments in a LSA gyroplane will always try to restore a positive G-load?” Unless I am thinking wrong, if the (“effective”) RTV is aft of the CG, loss or reduction of Rotor Thrust will cause the nose to pitch up and therefore spindle cyclic action would inherently restore positive G-Load – trying to return to 1g load (Positive G-Load Stability). If the gyro naturally restores positive G-load immediately, is the momentary torque/roll instability an issue?

QUESTION 10: Upon loss of positive rotor thrust, any (torque induced) uncommanded roll, IMHO would be continued and accelerated by the spindle cyclic applied to the rotor – Power Roll-Over?! I don’t know that we have evidence that this has been a major safety issue – somehow pilots don’t seem to have trouble with the inherent roll instabilities. The “Lateral and Directional Stability” criteria we have proposed in the ASTM Gyroplane standard is correspondingly not nearly as specific as the pitch stability criteria. I don’t propose we diverge this thread into roll stability – but I think at some time I might start a different thread to gather our expert’s advice on the issue of roll/yaw also. But, for now, unless someone has good arguments of how this might couple with our pitch issues, I suggest we don’t further complicate this thread with the roll issue.,

Thanks, Greg Gremminger

[Udi]

Greg - I will try to answer your questions, one at a time, to the best of my ability:

QUESTION 8: Udi, you said: “Unlike fixed wing airplanes, in a gyroplane, all control is lost in a zero-G event.” I recognize that under zero-G conditions, the airframe is no longer “hanging” under the rotor – so it might more readily roll or pitch during a zero-G event. But, is “all control” lost? At least immediately upon loss of G-Load, the rotor is still spinning and cyclic action still works - even at negative G-load! Doesn’t this at least mean that a commanded cyclic input can still restore positive G-load, and/or stop a roll? And, if the Sum of Static Moments” (in pitch) is proper, the airframe and spindle would naturally pitch in the direction (nose-up if the CG is forward of the RTV) so as to inherently restore positive G-load upon a sudden loss of G-Load? This Static G-Load stability action tries to restore 1g loading upon ANY disturbance from that steady state 1g condition!?? It seems to me that Positive G-Load Stability is perhaps the "Holy Grail" of gyroplane stability safety - even dynamic stability issues! And the stronger this factor, the safer! Does this not mean that such postively G-load stable gyros are much less risky or even immune from "spontaneous PPO"? Am I wrong?

You are right - my statement was not completely accurate. The rotors may still be flying and controllable within whatever cyclic range you have left. What I meant to say was that, at zero-G, your controls have no direct effect on airframe attitude, because even when the rotors are flying to their new position this change is meaningless to the airframe. Unless positive rotor G-load is regained, the airframe will pitch and roll according to other moments that may be out of your control.

The airframe would naturally pitch in a nose up direction ONLY if the sum of the aerodynamic and engine moments is nose up. I think that one of your assumptions, Greg, is that, if a gyroplane is G-load stable, than the RTV must be passing aft of the CG, thus the sum of all the other moments must be nose up. This may be a bad assumption. Any gyro, including the Magni, has some regions within its flight envelope in which the RTV is passing forward of the CG. This, all by itself, does not make the gyro G-load unstable. The gyro will still exhibit positive G-load stability thanks to the difference in AOA and lift slopes of the rotor and the stab.

One example of a region in which any stabbed gyro is most likely flying with the RTV forward of the GC is during slow flight (powered or not). Think about the stab AOA at normal cruise speed. Close to zero degrees right? Now, think about the stab AOA during vertical descent. Positive (lifting) 90 degrees right? During vertical descent the stab is of course stalled, so it doesn't produce much lift but consider a given airspeed in between cruising speed and vertical descent, in which the stab is flying at, say positive 20 degrees. If you hang some yarn from your stab, you may be able to observe this airspeed.

During this airspeed, your stab is producing a lifting force. To counteract this lifting force, your RTV must be shifted forward of the CG! You know it is, because the stick is pulled back considerably aft of its cruising location during slow flight, right? Still, your gyro will exhibit stable G-load stability. How? Any G-load maneuvering will have a more significant lift effect on your stab than on your rotor (in relative terms). The stab will be affected more than the rotor because: a. the stab AOA is smaller than the rotor AOA, so a 1-degree change in the stab AOA makes a relatively larger change on the stab lift than on the rotor (1 out of 20 degrees, 5%, is larger than 1 out of 30 degrees, 3.3%) and, b. the stab lift slope is steeper than the rotor lift slope because the stab is operating at a significantly lower loading factor.

Bottom line is that the RTV does not have to be aft of the CG for the gyroplane, as a whole, to exhibit positive G-load stability.

Udi :cool:

[raghu]

Udi, I think I have not succeded in making myself clear. I will try and keep it to the point as I think the point I am makeing is quite subtle and valuable.

1) I understand your explanation of gyro G load stability and how it works. Lets for the moment keep to your terminology.

2) I totally agree that RTV behind CG is not important to arrive at this stability, namely for the gyro as a whole to be G load stable

3) The essence of my point is encapsulated in this question:

If the gyro as a whole exhibits G-load stability can I conclude that the gyro is dynamically stable?

Answer: YES ( in general)for the **short term mode** and NO for the LONG PERIOD mode.

In the short term mode G-load static stability is the dominent factor, and with sufficient G-load stability you can guarentee a dynamically stable short term mode.

However, for the long term (phugoid stability) there are other dereivative that matters. Dr. S. Houston found that rotor specific static stability derivatives do affect the damping of the phugoid mode. One derivative in particular, is the relationship between rotor RPM and the pitching moment of the entire gyro viz. **rotor G-load stability** does matter.

Rotor G- load stability: stable if an increase in RRPM causes a nose down moment and unstable if an increase in RPM causes a Nose up reaction.

It turns out ( thanks to S.houstons analysis ) that if the rotor G-stability derivative is unstable it reduces the damping from the phugoid mode. Now this does not mean the phugoid is automatically unstable. It just mean that one has to be watchfull as the trend is adverse.

My suggestion was that by having the RTV behind the CG you guarantee rotor G-load stability and go a long way towards gyro G-load stability.

4) Without dwelling into the math ( if you want more details I would be happy to give them) , the rotor speed variations because of its frequency may couple with the phugoid mode, and this coupling is adverse to the phugoid damping. Ensuring a stable rotor G stability can take this coupling out of the picture.

If the rotor had no inertia then the gyro would behaave just as a FW and all we need is G-load stability, as there will be no lag in the rotor response and we can ignore rotor specific derivatives.

5) the terminology I have been using I though minimized confusion- AOA stability for gyro G-load stability and rotor G load stability for the rotor specific derivative. But it appears it did not work.

6) I am happy with either terminology but the point I am trying to make is that we MUST MAKE a distinction between the gyro as a whole and the rotor, because it turns out it matters. Above all it is gratifying to know sophisticated analysis validates that simple old maxim- keep the RTV behind the CG.

[Udi]

Raghu – In my previous post I was trying to explain how a gyroplane in which the RTV is forward of the CG may still meet the proposed ASTM tests for static and dynamic stability. I was not saying this is a good way to design a gyroplane. My concern is that such a gyro may not survive a zero/low-G event. Any gyro with a high engine thrust line and/or low drag line, in which the stab is not designed to cancel out these nose-down moments, is subjected to this risk. I was merely pointing out to Greg that the proposed gyroplane stability criteria may fail to address one of the more dangerous, and maybe overlooked, attributes of gyroplanes.

My suggestion was that by having the RTV behind the CG you guarantee rotor G-load stability and go a long way towards gyro G-load stability

Maybe I don't understand your terminology, but what does the RTV vs. CG has to do with rotor G-load stability? A typical gyroplane rotor, to the best of my knowledge, is G-load unstable. The offset gimbal head/spring mechanism was invented to counter the natural G-load instability of the rotor. I guess we are not talking about the same thing when we say “rotor G-load stability”...

the terminology I have been using I though minimized confusion- AOA stability for gyro G-load stability and rotor G load stability for the rotor specific derivative. But it appears it did not work

Totally unintentional. Since you are the expert, I will be more than happy to use your terminology. Please understand that I have no formal education in aeronautical engineering, and I am learning this stuff as I go!

I am happy with either terminology but the point I am trying to make is that we MUST MAKE a distinction between the gyro as a whole and the rotor, because it turns out it matters. Above all it is gratifying to know sophisticated analysis validates that simple old maxim- keep the RTV behind the CG.

I totally agree. I have much more to lean with regard to rotor dynamics, among other things, and I really value the expertise brought to this forum by you, Chuck, Doug, and others.

Udi :cool:

[mceagle]

Keep going please fellas - more of this sinks in each time I read it

[raghu]

Totally unintentional. Since you are the expert, I will be more than happy to use your terminology. Please understand that I have no formal education in aeronautical engineering, and I am learning this stuff as I go!


Udi :cool:

I feel compelled to point out that neither am I an expert not do I have aeronautical engineering degrees. I too have learnt a lot from Chuck B who in addition to haveing a good theoretic understanding has valuable on field experience that is invaluble in reconsiling theory with practice.

My humble objective is to make some of the more recent technical findings, particularly those by Stewart Houston, more accessible. It would be a pity to waste over a million dollars worth of very pertinent research. There seems to be a lot of misunderstanding and distrust of theory in our community. While we cannot (yet) accurately predict the behaviour of an on- paper design, there is a lot that theory can tell us about the trends and interrelationship of various gyro design parameters that will guide us to make sensible paper designs and help test, evaluate and tune the final design.

[raghu]

Raghu –

Maybe I don't understand your terminology, but what does the RTV vs. CG has to do with rotor G-load stability? A typical gyroplane rotor, to the best of my knowledge, is G-load unstable. The offset gimbal head/spring mechanism was invented to counter the natural G-load instability of the rotor. I guess we are not talking about the same thing when we say “rotor G-load stability”...

Udi :cool:

A common misconception is that a rotor is unstable wrt AOA. Jean fourcade, even S. Houston in one place fall prey to this. While this is true for a helicopter rotor that runs at constant speed, things are not as cut and dry for an autorotating one.

A perfectly inertialess autorotating rotor is in fact stable wrt AOA and neutrally stable wrt velocity stability. A real world autorotating one is some where in between the perfectly inertialess and the helicopter one.

Imagine an inertialess rotor subjected to an increase in AOA. It will immediately speed up (no inertia) until the relative wind again approaches the rotor at the same AOA. However now mu ( the ratio between the forward speed and rotor speed) has decreased as the rotor has sped up while there has been no change in the forward speed. A reduction in mu means flapping will reduce.

Thus an increase in AOA results in

1. RRPM increase resulting in more rotor thrust

2. Reduction in mu resulting in less cyclic flapping and so RTV moves aft.

Both these effects combined give rise to static rotor AOA stability.

[Aussie_Paul]

You said,My humble objective is to make some of the more recent technical findings, particularly those by Stewart Houston, more accessible. It would be a pity to waste over a million dollars worth of very pertinent research. There seems to be a lot of misunderstanding and distrust of theory in our community. While we cannot (yet) accurately predict the behaviour of an on- paper design, there is a lot that theory can tell us about the trends and interrelationship of various gyro design parameters that will guide us to make sensible paper designs and help test, evaluate and tune the final design.

Excellent post Raghu, excellent.

Aussie Paul.

[C. Beaty]

I feel compelled to point out that Ragu is a computer geek, a faculty member of Carnage Melon University in Pittsburgh, is married to an aeronautical engineer and is whiz bang at math and physics.

Now I have a question, Raghu. If you need to know something about fluid mechanics, do you first ask your wife or just go to the library and look it up?

[Dean_Dolph]

Chuck, appreciate the background info on Raghu.

It is always helps to know peoples credentials even though that is no guarantee that they are passing on valid info. Broke in, or broke down, to many chemical engineers over the years to the point that there were times that I questioned the job our academic institutions were doing. I suspect that it is more a case of application of learning, which is the individuals burden, than a problem with the institutions. Some people have the knack and some don't.

Consequently I have a nasty habit of not always accepting what is presented on the first pass. But those with credentials do get more respect than those of us that got our schooling in an evironment that used chalk and slate. And physics wasn't a science but something you took to cure a digestive problem!

After all that text, what I'm getting at is that I wish the members of this forum would provide more info in their public profile so we would have some idea what their back ground is.

[C. Beaty]

You picked an example that skews the mean, Dean.

Chemistry is not the exact science that chemists would have you believe. But then neither is aerodynamics.

[Udi]

...After all that text, what I'm getting at is that I wish the members of this forum would provide more info in their public profile so we would have some idea what their back ground is.

Then why don't you lead by example, Dean? If you don't practice what you preach, you lessen the effectiveness of your message. ;)

I agree, by the way with your assessment of Chemical Engineers in this country. Feel free to check my public profile for my credentials regarding this statement.

Regards,

Udi :cool:

[Dean_Dolph]

Chuck, you are absolutely correct about the chemistry. That's why there is R&D! And that is why there is flight testing. You have to prove the paper theories. It isn't surprising when the theories don't always match the testing.

Udi, you are right, of course, that is if I wanted to lead! :D Actually I should add something to my profile since I wouldn't have to keep adding disclaimers about my credentials to most of my posts. I didn't mean to disparage the chemical engineering professionals because by and large most of the ones I worked with produced meaningful work but there were a few that were works of their own.

[darrellwittke]

I am sorry I have nothing to contribute but curiosity. What is the consensus regarding a horizontal stabilizer damping to 1/2 amplitude in appx. 1 sec and no requirement of a short period to damped time? (To my untrained mind, the heart of the matter.)

Thank you to everyone, I really enjoy your posts!

[gyrogreg]

I would much prefer that we continue to refine this discussion toward consensus. I had a separate phone conversation with Rahgu. Rahgu is a mathematics PHD who has taken up an interest in applying those skills to interpret the studies by Dr. Houston at the University of Glasgow in Britain. I find his mathematical insights convincing and helpful and I am grateful he is interested in donating those skills toward gyroplane safety. I would like to try to summarize what we have come up with so far in this thread. I eventually hope to present the discussions in this thread to the ASTM gyroplane standards subcommittee for possible refinements to the stability criteria in the ASTM standard.

Please correct me where I am not summarizing the discussion accurately:

STATIC STABILITY:
There are differences in label terminology as suggested by Rahgu. However, Rahgu has told me that he has thought a lot about the three static stability criteria, and finds reasons that all three criteria are appropriate and should be used. (Rahgu, I would appreciate your details on this assessment sometime.) Perhaps we might consider more technically correct names for these static stability criteria, but the static criteria themselves are proper.

Rahgu initially expressed that it might be difficult to make a determination of G-Load Static stability (“his term AOA stability”). But, he agreed that the “turn test”, where a simple determination if aft stick force and position are required to maintain original trimmed airspeed while in a banking turn at the same power and airspeed. This test method is advised to eliminate the need to record g-load forces correlated with stick forces and position. I think Rahgu and I have agreed that this “turn test” is adequate and readily determinate.

Rahgu has suggested that a determination of "Rotor G-Load stability" might also be required - but that a test for this is very difficult - Rahgu, could you please elaborate and suggest how such a test would be of additional value?

DYNAMIC STABILITY:
Rahgu is suggesting that our original criteria that there are no oscillations with periods quicker than 5 seconds is not an accurate criteria. Rahgu maintains that there should and will be short period oscillations in the range under 5 seconds. These short-period oscillations should be heavily damped – in the order of 1 second to ½ amplitude! Rahgu maintains that this slight overshoot short-period response, rather than a critically damped response, is desirable for flying qualities and quick response of the aircraft. Furthermore, Rahgu maintains that this heavily damped short-period response is assured if the static G-Load stability criteria (AOA Stability? - "turn test") is achieved. Also, if this heavily damped short-period response is present, it would be difficult to detect it by flight testing because it is so quickly damped.

This all suggests that we might not have to actually look for short-period dynamic responses if the G-Load static criterion is met! However, Rahgu says there is the possibility of the long-period Phugoid responses merging with the short-period responses to create a lesser damped oscillatory response at an intermediate frequency – in the range of 7-10 second periods. According to Rahgu, this is possible if the HS damping is not strong enough – long enough moment arm. This 7-10 second response period could be the root of some residual PIO risk in even a gyro that is actually G-Load stable and has the desired heavily damped short-period response.

So, perhaps it is not so simple, to avoid PIO tendencies, to simply verify the static G-Load criteria is met!!?? I suggest that we should not even test for dynamic stability until all static criteria are met – and this should assure that there are no un-damped oscillatory responses in the short-period range under 5 seconds!!?? But, even if all the static criteria are met, because of this possible “merging” of long-period with short-period response, we do still need to test for any such “merged” 7-10 second oscillatory responses.

Rahgu and others seem to be in consensus that the long-period response criteria proposed – just “damped” is adequate - long-period oscillations are readily “damped” by the pilot and present no dangerous quick PIO tendencies except that they may help create a “merged” oscillatory response in the quicker 7-10 second period range!

In summary, my next two questions:

QUESTION 11: Do we all agree that the proposed static criteria appear to be appropriate and encompassing (even if not consistent in title terminology) according to what we have learned from the mathematical insights into the Dr. Houston study and data? What would be the additional value of additiona l criteria or testing?

QUESTION 12: Because there is a possibility of some “merged residual” oscillatory tendency in the range of 7-10 seconds (right numbers?), what would be the appropriate criteria and flight test to assure these don’t exist? Should we define the criteria as “no oscillations with periods between 7 and 10 seconds”? (Since the G-Load static criteria is met, is it at all necessary to verify that there are highly damped short-period oscillations, or that those short period oscillations are indeed damped?) Or, would the criteria be as simple as saying that all oscillations with periods less than 10 seconds should be damped within X seconds? Would looking for the “merged residual” intermediate oscillations be all that is necessary to do and would looking for those be the same as for short-period oscillations – i.e.: “sweep” doublet excitations? How easy is it to find such short and intermediate period responses? Could any specific dynamic testing assure there are no short or intermediate period responses and therefore no PIO tendencies?

I hope that at least Rahgu, Udi, Doug, Chuck and Mike will be able to respond to this post. I am looking for consensus and simplicity please. Your perspectives please!

I would appreciate any specific suggestions as to the exact wording of criteria and test methods.

Thanks, Greg Gremminger

[C. Beaty]

Stick position Vs. “G” load in a steady turn is the important parameter, Greg. Stick force Vs. “G” load is mostly a measure of rotorhead offset once rotor speed has stabilized.

In my opinion, the only meaningful dynamic stability testing must be performed stick fixed, more easily said than done. Almost anything that will leave the ground will exhibit some degree of stick free dynamic stability with appropriate rotorhead offset and trim spring rate.

An individual in Swainsboro GA, Pete Johnson, experimented extensively with various combinations of offsets and trim spring rates back in the 1970s and generated some informative data that was published in a Sunstate Rotor Club newsletter. Wish I had a copy.

Roger Wood in Cincinnati also did some stick free Vs. stick fixed stability testing on his stock Bensen. His conclusion, as I recall, was that it was dynamically stable stick free and unstable stick fixed. His findings were published in the Cincinnati Club newsletter.

The problem with stick fixed testing is most people, myself included, can’t absolutely hold the stick fixed. Reflexes take over and most people fudge without being aware of doing so.

Also, the elasticity of the cyclic control system can seriously skew the results.

What’s called for are trimable spring detent locks installed at the rotorhead. Then the stick can be bumped out of detent and released to excite such oscillatory responses as may be present.

[raghu]

In my opinion, the only meaningful dynamic stability testing must be performed stick fixed, more easily said than done. Almost anything that will leave the ground will exhibit some degree of stick free dynamic stability with appropriate rotorhead offset and trim spring rate.


You are absolutely right Chuck that stick fixed criterion will be the more critical one particularly given that the offset gimbals is widely used and performs the stick free stabilizing function quite adequately in otherwise marginal designs.

However, given that the proposed standards are meant to be generic and are not prescriptive in any way ( for example a design does not have to incorporate an offset gimbals) it is important that designs meet both criterion. After all both stick fixed and stick free are idealisations and a real pilot neither flies with the stick fixed or free.

I think you would agree that an otherwise stick fixed stable gyro without an offset gimbals ( or any other stick free stabilization mechanism) will be a handful and very unpleasant to fly due to the counterintuitive stick feel.

[raghu]

. Rahgu is a mathematics PHD who has taken up an interest in applying those skills to interpret the studies by Dr. Houston at the University of Glasgow in Britain.

This bit is probably irrelavent as I am not a qualified aeronautical engineer and anything else perhaps make little difference, but for what it is worth it turns out I am not a mathematician either- my Phd is from the department of engineering and I spealized in the area of simulation and modelling. Contrary to popular belief mathematics is not just the domain of mathematicians but is a valuable tool in any modern engineers aresenal.

[C. Beaty]

"I think you would agree that an otherwise stick fixed stable gyro without an offset gimbals ( or any other stick free stabilization mechanism) will be a handful and very unpleasant to fly due to the counterintuitive stick feel."

I'm not so sure about that, Raghu. I've flown several gyros with feathering cyclic control that had no rotor thrust feedback.

One, with propeller thrust line well above the CG and with no horizontal stab was nearly impossible to keep right side up with feathering cyclic control but was tolerable with an offset gimbel rotorhead.

Two other of my gyros with propeller thrust nearly centered on the CG were comfortable to fly without rotor thrust feedback.

I've also flown Karol DeGraw's "DeBird" that had a zero feedback cyclic control system and found it to be quite pleasant. I only flew it down the runway because the rudder pedals, set up for Karol, forced my knees up against my chin. But it did have a fairly stiff spring centering mechanism. I understand that both of the DeGraw machines, once trimmed will fly indefinitely with the stick locked.

But I do agree that a component of rotor thrust fed back into the cyclic control system in a stable direction confers important benefits such as guiding the pilot to avoid disturbances as well as being a stabilizing mechanism.

Incidentally, the first Bensen rotorheads, the "spindle" head had rotor thrust feedback in an unstable direction.

[darrellwittke]

Say Chuck, why detents installed on the head? Why not the stick? Simplicity?

Thanks in advance, I seen you were browsing.

[C. Beaty]

Locating the detents at the rotorhead, Darrell, eliminates the effect of control springiness. With a whole lot of offset and the stick locked, there could be enough springiness in the pushrods, etc to muddy the results of fixed stick testing.

[Mike Jackson]

I'll try to relate some of the terminology and school progression the Edward's boys use in their discussion of longitudinal stability. I'll admit I'm having a difficult time with some of the nomenclature in our current discussions.

1. Aircraft Statics
2. Longitudinal Static Stability
3. Longitudinal Dynamic Stability
a. Phugoid (P)
b. Short Period (Sp)
4. Maneuvering Flight
5. Flight Test Techniques (FTT)


From a consensus standpoint (LSA) I'd like to ID the major stability derivatives for P and SP as they apply to gyroplanes and determine what we think are acceptable damping and frequency/periods for the type of flying we do. For ManFlt, determine what stick force per G range (Fs/g) we want to live with. How to we design the gyro to "comply" or be w/in the envelopes we desire? What are some simple FTTs we can do to see where our machines lie in these envelopes.

1. Aircraft statics - simply all forces and moments about a standard point (usually cg) sum to zero in trimmed non accelerating flt (longitudinal axis). That's it.

2. Longitudinal Static stability - the tendency of the aircraft (AC) to maintain trimmed AOA. Emphasis on initial tendency. All the rest of the stuff comes after the initial tendency. If an AC has + long. stability and it is disturbed from it's initial AOA, moments are produced tending to return it. There are 2 simple ways to test for LongStat stability. We can see this by noticing the stick force to maintain a "g" or a velocity other than trim. If we have to increase back stick force to increase g (increase AOA) or push forward decreasing g/AOA - the AC is statically stable. Another way to tell is to maintain level flight while accelerating above trim AS - stick fwd and AOA decreases. Conversely, by slowing AS, AOA increases and aft stick forces increase. Design factors: HS area, HS moment arm, Position of cg wrt RLV - in FW parlance this is the static margin. Make this margin approach zero and the stick can become very light and possibly PIO prone. For the cowboys of OZ this is good. Wouldn't want to get overshot by a steer.

3. Long. Dynamic Stability - refers to the motion of many systems produced by a disturbing force. Note: POSITIVE STATIC STABILITY IS REQUIRED for a system to be oscillatory. If you care, longitudinal dynamics are defined by a 4th order equation (whew). Who could figure it out? Someone much smarter than I factored it out with the help of a guy named Laplace and we got 2 second order equations. Behold the Phugoid mode and the short period mode side by side. We then made some assumptions of strong players vs weak players, threw out the weak ones to clean up the math and came up with
something useable.

Both eqns look like this characteristic eqn. S^2 + 2dFn S + Fn^2 = 0 (eqn1)

I know I've butchered the math symbols - can't help it here:

d - damping ratio (should be a zeta symbol)
Fn - undamped natural freq (should be an omega symbol)
2dFn - damping term
Fn^2 - frequency term

Some of you will say WGAS! Well stick around. These terms can be estimated and flight tested.

a. Phugoid - long period mode where we assume AOA is constant. Variables to be measured could be velocity, altitude, deck/pitch angle, rotor rpm. The easiest is deck angle where we will count and time peak amplitudes to get d & Fn. Turns out thru some magic the phugoid eqn becomes:

S^2 + 2g/V Cd/Cl S + 2 (g/V)^2 = 0 (eqn 2)

From eqn 1 & 2 : the freq of the Phugoid is dependent primarily on speed:

Fnp = (2)^0.5 g/V (eqn 3)

The damping ratio dp = Cd/Cl*1/(2)^0.5 It's interesting to note the better the lift to drag (if there is such a thing in gyros) the smaller the damping. Makes common sense.

If you take Dr. Houston's Magni test on pg 395 and look at the flt test chg in AS, the peak to peak time (period) is about 18 sec. Using the rrpm, T = 19 sec. Iff you plug and chug in eqn 3, the Phugoid estimation
T = 2 pi/Fnp = 14.3 sec - respectable estimate?

The FTT is easy. Pull the nose up 10 - 15 deg and count the peak while timing. Try it stick fixed and free.

I'll try more tomorrow - it's late.

Just a note on the getting wrapped about the axle on overshoots - don't. It's a combination of damping and overshoots that give you acceptable flying qualities.

Most of my inputs are FW in origin. I have problems with the additional deg of freedom with the rotor.both stick fixed and free. I still think we're on the right track.

Cheers,

Mike

[raghu]

Spring centered sticks provide pseudo stick forces. Although the stick forces are not from the rotor directly the direction of the forces are in the right direction. From a pilots perspetive, for the most part, it does not matter whether the forces are created artificially by the spring or actually from the rotor. Fly by wire aircraft use spring centred stick forces to provide feedback to the pilot quite effectively.

Chuck, Stick free neutral stability may be OK to fly as your experience with a non stick centered gyro shows but I am more concerned that stickfree instability and I think it would make for a very uncomfartable piloting experience. How was the original bensen with spindle control to fly?

[C. Beaty]

I’ve never flown a spindle head gyro, Raghu.

In case you’re not familiar with its geometry, the spindle head used a spherical roller bearing as the main rotorhead bearing with the rotor teetered at the top of the spindle and cyclic control applied to the bottom. In forward flight with cyclic flapping, the rotor thrust vector passed forward of the pitch control axis.

In a story related to me by Dave Prater, he and Bill Parsons teamed up and built gyros together; Dave using the spindle head and Bill using the gimbel head.

Dave said that when flying together, he was all over the sky while Bill was rock steady. He couldn’t understand why until he tried a gimbel head himself. I’ve heard similar stories from others that have flown spindle heads.

With appropriate offset and trim spring rate, a riding lawnmower could be made to display some degree of stick free stability, the reason for my de-emphasizing the importance of stick free stability testing. Trim spring rate has a more important influence on stick free stability than most people realize.

I agree that while it’s important to have some sort of stable stick force because the pilot is always more or less in the loop depending upon how the stick is grasped, only stick fixed stability testing demonstrates intrinsic stability of the machine.

Let’s have no more of the huckster’s pitch; “It’s so stable it flies hands off!” The most important indication of stability is the question; How stable it is when a low time pilot has a choke hold on the stick?

[gyrogreg]

Chuck and all,

Just FYI, the ASTM standard for Dynamic stability does require that the dynamic criteria be met with BOTH fixed-stick and free-stick modes.

The less than 5 second period criteria is mainly the DYNAMIC criteria that Raghu takes issue with and that I would like to address specifically at this time with suggested and supported alternative DYNAMIC criteria and “FTT”.

I much appreciate the detailed posts, but I’m kind-of hoping for a simple answer to my static QUESTION 11: Do we all agree that the proposed STATIC criteria appear to be appropriate and encompassing?

Thanks for the valuable posts - Greg

[C. Beaty]

A qualified yes to question 11, Greg.

Some rotorcraft controlled by swashplate means have a mechanism for disengaging the trim springs; if that is the case with the McC J-2, it would not meet the stick force requirement with the springs disengaged, or; if the springs are too soft to fully meet the stick force requirement, an out of trim forward bias could be cranked in. Ron Herron knows the answer to this one.

[raghu]

The three static stability tests.
------------------------------------

1) Test one: G-load test (turn test)

What does this prove: Stick fixed AOA (or g-load) stability.

Why do we need this:This is the most important factor in dynamic stability. It ensures that the gyro is always pointing into the relative wind. This is what gives the speed holding tendency to a stable aircraft. This point is often not appreceiated in the case of gyro due to the existence of velocity stability.


2) Test two: Speed test

What does this prove: Stick free aoa (or g-load) stability.

Why do we need this: Helps with pilot feel and also ensures that in case the pilot is temporarily incapacitated and the stick is free, the gyro will remain stable. The effect of the offset gimball is the dominating factor here.

3) Test three: Power test
What does this prove: It does not prove any standard stability criterion.

Why do we need it: Primarily for safety. In case the engine suddenly fails or full power is added rapidly the gyro must not make any abrrubt pitch changes. In my view this is the least critical aspect. Also, it is debateble what the tolerence should be. The exact speed difference is not as important as the instant pitch reaction.

** Do we need any other static stability tests?

Answer: A test to check weather the RTV is very close to or behind the CG at all speeds will be useful. This static stability is termed rotor G-load stability

Why?

Because this stability dereivative influences the damping of the long period mode. If it is unstable then the long period looses a bit of its damping. At an extreme this may mean that the long period mode is actually unstable.

Also, probably more importantly, rotor G-load stability acts aginst a PPO tendency.

How do we perform this test?

An obvious way is to photograph the gyro in flight and work out the RTV/ CG geometry. This method could work but I have no idea how accurate it would be.

Another method ( very impractical) is to carry a sack full of potatoes at the CG and drop it during flight. If the instant reaction is nose down then we have rotor G-load stability, if it is
nose up we have negaative G-load stability. The key is the instant reaction this may be hard to monitor. In both cases the gyro will nose up in the more medium to long term.

I guess you could ignore this test and just test for dynamic stick fixed stability and verify that the long period mode is sufficiently damped.

[Udi]

Question #11

Conditions--Static longitudinal airspeed stability must be met at the following power and airspeed conditions: Trimmed at:
(a) Steady altitude at MPRS,
(b) Full power at the lesser of VH or of VNE,
(c) Engine idle at MPRS, and
(d) Engine idle at 80 % VNE.

I suggest to add the following power speed combinations-
- Full power at 0.9 VMIN
- Full power at VX (Best climb airspeed)
- Engine idle at 0.9 VMIN

I would also add the above combinations to the G-load stability testing.

I would also expand the envelope for the static power stability tets. All the proposed tests Occure at MPRS. I would expand the range as follows:

- At the cyclic stick position established in level flight at VMIN at full power, a change in power from full power to engine off shall not result in a change in a steady state trimmed airspeed of more than 20 % from VMIN.

- At the cyclic stick position established in descent at VMIN at idle power, a change in power from idle power to full power shall not result in a change in a steady state trimmed airspeed of more than 20 % from VMIN.

-At the cyclic stick position established in flight at 0.9 VNE at full power, a change in power from full power to engine off shall not result in a change in a steady state trimmed airspeed of more than 20 % from 0.9 VNE.

Udi-

[Udi]

...Also, probably more importantly, rotor G-load stability acts aginst a PPO tendency...

Raghu - did you mean “against PIO tendency”? Aligning the engine thrust line with the CG is the only way I know of to reduce PPO tendency.

Also, for the benefit of having a clear terminology - when you say "rotor G-load stability", you are talking about the rotor specific derivative of the aircraft G-load (or AOA) stability, correct? I believe that although you can separate the two on paper, short of removing the HS from the machine, I don’t think you can test in flight rotor G-load stability. Every test, including the sack of potatos test, would include the airframe/stab effect on total G-load/AOA stability.

Udi-

[Mike Jackson]

Back at it. To summarize the Phugoid Mode - low freq (large period), lightly damped, alpha (AOA) assummed constant. Fn ~ 1/V , dp ~ Cd/Cl
Typical period ` 20+ sec, Houston's Magni ~ 18 - 19 sec (flt test). There is not much to "design" here. Easily controlled/trimmed in cruise fltt. No impact on Maneuver Flt.

SHORT PERIOD

This is the bread and butter of longitudinal stability. The same 2nd order eqns apply but the variables having strong influences on Fn and dsp (short pd damping ratio) are different. The static stability design factors have a large influence on short period characteristics.

Variables include AOA, deck angle, vertical g, and rotor rpm (rrpm). For simple testing, deck angle is the easiest to observe. Here's where rotor / airframe coupling discussions can enter. I will assume (?) for small pertubations about a trimmed condition, the rotor will follow the airframe with maybe some lag. For the math and flt test we define/set AS and Alt constant.

What are the variables?

Fnsp ~ Cma, 1/(Iyy)^0.5 , AS

dsp ~ Cmq, 1/(Cma)^0.5 , 1/(Iyy)^0.5

where

Fnsp - short pd. undamped natural freq.
Cma - restoring moment chg due to AOA chg, sign is neg by convention for statically stable AC. This is a strong term for both freq and damping and per static stability criteria affected by HS size and location as well as CG vs RLV
Iyy - here's your mom. of inertia about longit. (y) axis by convention.
Cmq - pitch rate damping - guys are getting creative naming this one. Moment change due to pitch rate or AOA rate chg (assuming rotor follows fuselage).

Changes in SP characteristics:

AS ^ - Fnsp v( v = down or lower)
Iyy ^ - Fnsp & dsp v Increassing mass about Y axis decreases damping &
freq
Cma ^ - Fnsp & dsp ^
Cmq ^ - Fnsp & dsp ^
Static Margin (cg vs RLV) v - Cma v and Fnsp v

Just like the Phugoid, thee characteristic eqn consists of a damping term
2 dps Fnsp and the frequency term Fnsp^2.

dsp - damping ratio can be described by the number of overshoots of deck angle, vertical g, AOA, or rrpm. For dsp between 0.1 and 0.7, dsp can be approximated by dsp ~ 0.1 (7 - #peaks). These are peak overshoots until totally damped.

Here's some common terminology wrt damping ratios:

Overdamped - d > 1
Critically damped - d = 1
Underdamped - 0<d<1 this is where we'll usually operate ( 0.1 - 0.7)
Undamped - d = 0
Negatively damped d < 0


2 dsp Fnsp - damping term which can be described as inversely proportional to the time to damp. We can measure this in flt test. Sometimes we refer
to what is called a time constant (Tau) = 1/dFn where one time constant = 63% of the final original value. Sometimes we use time to half amplitude -
T1/2 = .69/dFn.

I had an opportunity to fly a highly augmented Lear jet belonging to the Calspan Corp out of NY. We could "vary" many stability derivatives to see how flight characteristics changed. This was an outstanding teaching aid. Quoting from their manual:

It is important to remember iin stability and control work that neither frequency nor damping ratio, nor time to damp alone, will be sufficient to predict handling qualities. It is the combination of Fn and d and consequently 2*Fn* (time to damp) that is meaningful

It's difficult, if not impossible for specific cookbook numbers to apply to the LSA standards but we CAN talk about ranges of damping ratios and natural frequencies which are based on a large data base of pilot surveys while flying this augmented airplane. These same "goodness" ratings were bounded and put on a logarithmic chart with Fn vs d. The best tested boundary had d varying from 0.4 to 1.2 and Fn varying from 0.5 to 1.2 cycles per sec. More qualitative descriptions of these damping and freq ranges later.

I will try to get this graph thrown up on this forum. I contend the same favorable goodness, or pilot in the loop, ranges apply to sport gyros.
I'll later try to discuss simple FTTs for the short period including what some may be seeing as periods less than the phugoid but longer than a "typical" short period response.

I will also try to tie it all up in a Man. Flt discussion. Like Greg G. said all we may find is we need an adequate tail, reasonable cg wrt rlv and thrust line. I'm just trying to help put some #s (or reasonable ranges of numbers)on values we're not too used to defining. When all is said and done, it will be flt test and the inputs of many gyro pilots to come to a consensus. It will be as much qualitative as quantitative.

[Dean_Dolph]

Mike, in the end, I'm waiting for you guys to boil it all down to where the back yard builder can apply the standards. Chances are, there are going to be a lot more backyard built machines than manufactured ones for sometime. In my mind, this whole exercise is about safety.

What the ASTM sub-committee is doing is establishing standards for the manufacturer and even there I think we are going to have to be careful that they are as easy to understand and implement as possible. Backyard builders have been known to become manufacturers, you know! This means minimum calcs and/or canned software to handle them.

And I think everyone agrees that flight testing has to be done in order to prove the paper design and the final construction.

[raghu]

Also, for the benefit of having a clear terminology - when you say "rotor G-load stability", you are talking about the rotor specific derivative of the aircraft G-load (or AOA) stability, correct? I believe that although you can separate the two on paper, short of removing the HS from the machine, I don’t think you can test in flight rotor G-load stability. Every test, including the sack of potatos test, would include the airframe/stab effect on total G-load/AOA stability.

Udi-

Not quite Udi! I have defined this earlier but here it is again:

Rotor G load stability: Keeping all other parameters fixed, if an increase in rotor RPM (RRPM) causes a nose down reaction of the gyro then this derivative is stable. Conversely, if the gyro pitches up due to an increase in RRPM the derivative is unstable. Note, what we are interested ( as in all static stability derivatives) is the instantaneous reaction.

Based on this it is obvious to see that the only way to make this derivative stable it to place RTV aft of CG. Consequently, the photo test will work. Also, the sack of potatoes will work because when you drop the sack there is an instantaneous increase in G -load- it does not matter whether the g-load increase is due to a drop in weight or an increase in RRPM. As you are looking for an instantaneous reaction the speed of the gyro does not change so none of the aerodynamic forces change and so HS or no HS is irrelevant. The catch though as I said in my earlier post is whether you can effectively spot this instantaneous reaction and not confuse it with the more medium trend.

The other test (particularly g-load of the entire gyro or AOA) are structured cleverly that they indirectly measure static stability qualities and thus avoid measuring the instantaneous reaction. The photo test I guess is indirect as well, but the sack of potatoes is a direct test and so its practicability may be an issue. You could do the same test by un banking from a turn, but again you should look for the instantaneus reaction and not other medium/longer term reactions. For example in both the unstable and stable case the medium term reaction is that the gyro will climb and so it will pitch up.

[C. Beaty]

I’ve performed the potato drop test, Raghu and was unable to spot the instantaneous reaction.

Well, not exactly. I didn’t actually drop a sack of potatoes but the rotor rpm can be cranked up by holding a steep turn until rotor speed stabilizes and then abruptly rolling out and observing response.

All I’ve ever observed was the medium period response; the gyro simply climbs until excess rotor energy is consumed.

We’ve also had a skydiver jump from a gyro.

The young lady skydiver was an employee of mine at the time and was after unusual entries for her logbook.

The pilot to be, Lloyd Poston, was concerned as to what the reaction of the gyro might after she jumped. The spot on the gyro from which she jumped was very nearly on the CG, so I assured him (I didn’t know with certainty but OTH I wouldn’t be flying the thing) that all he need expect was a gentle climb until excess rotor rpm was dissipated.

As it turned out, that’s all that happened. Lloyd didn’t observe any significant pitch change.

The gyro was a Lycoming powered machine with fuselage pod made from a surplus drop tank. We bolted a plank to the side of it to serve as a running board of sorts and a handrail crosswise to the mast to prevent her from falling into the prop. She was instructed to climb out, sit on the plank and simply roll off from a seated position.

They rehearsed the exit routine on the ground and the actual jump went off without a hitch.

[Mike Jackson]

Mike, in the end, I'm waiting for you guys to boil it all down to where the back yard builder can apply the standards. Chances are, there are going to be a lot more backyard built machines than manufactured ones for sometime. In my mind, this whole exercise is about safety.

What the ASTM sub-committee is doing is establishing standards for the manufacturer and even there I think we are going to have to be careful that they are as easy to understand and implement as possible. Backyard builders have been known to become manufacturers, you know! This means minimum calcs and/or canned software to handle them.

And I think everyone agrees that flight testing has to be done in order to prove the paper design and the final construction.

Hi Dean,

You're exactly correct in my view.

I guess what we're trying to find are design guidelines and associated "ranges of numbers" which is one way to describe a safe design. All of us want to keep it simple, but I think that a manufacturer may be interested in a range of damping ratio/frequency combinations which might emulate a Dominator. Or he may be more interested in a cruising XC machine, possibly like the Sparrow Hawk. Why are the damping and natural frequencies of these 2 machines different (if at all)? How do I modify the physical airframe/rotor to give me higher damping, higher Fs/g, lower frequency, etc. This is where we go if we want to dig a little below the surface.

The backyard builder, if he's smart, will jump on the shoulders of giants and could physically build a good design simply by inspection of successful designs, using the right materials. All the great design "gouge" over the years (possibly written in blood) might be collated and used on a machine that "looks about right". We've built a lot of great aircraft over the years before we analyzed them to death.

[gyrogreg]

Good posts – keep them coming!

Chuck, we should not have to worry about swash plate gyros – the ASTM LSA standard at least is limited to fixed pitch rotors (or ground adjustable pitch rotors) – no collective controls! – Complexity eliminates in-flight pitch adjustments (for now).

Raghu, thank you for your comments on the three static criteria and tests. I think you are saying, that in lieu of the Rotor G-Load criteria (sack of potatoes test), the “turn” test criteria (what we have so far called static G-Load or Maneuvering stability) should suffice to cover this – even if it also covers static AOA stability as well. In my simple mind, I just feel that if we can show that the airframe nose reacts the correct direction to a change in G-load, we have covered this really important static stability issue.

Raghu, you also commented that the “Static Power Stability” criteria and test is not really a stability issue. But then, Udi is suggesting making this criteria more stringent. In my thinking, this IS actually a stability issue – that airspeed or AOA is stable wrt power changes – for the safety reasons you point out But also in my simple thinking, this is the issue that gets right at the heart of a possible PPO from the “popular” un-balanced high prop thrustline crowd. This criteria tells us that if the Sum of Static Moments is suddenly changed by the loss of RTV thrust, the machine will not be “pushed over” into a PPO. From a study of the static moments on a gyro, if an increase of power causes the nose to lower (increased trimmed airspeed), a loss of RTV thrust on that machine will also cause the nose to lower – possibly initiating the popularly classic PPO buntover. This criterion originally started out addressing only the nose lower with increased power issue, specifically to avoid the likelihood of a PPO if the rotor loses its thrust (or in some people’s more simple visualization – the rotor drag goes to zero). The criterion was expanded then, when the FAA Rotorcraft Directorate expressed to us that they expected gyroplanes to be “Power Stable” in both nose-down and nose-up directions! With no objections, I think this power stability criterion is appropriate, but I do admit, the tolerances are somewhat a guess that we would hope to refine with more data. My thought is that at 10% or a 20% trimmed airspeed change would not result in enough of an instantaneous attitude (cyclic) change that might stall a blade enough to vilently flap – precess stall. For now, the major gyro manufacturers have had no objection to the tolerances suggested at this time.

As for Udi’s suggestion to test this more vigorously at more speeds and power ranges, I can put that to the ASTM subcommittee for consideration. But, to me, this is not realy necessary to eliminate our initial concern for a PPO potential.

Mike and Dean, We really do intend that this criteria and the testing methods be simple and inexpensive enough to be accomplished by the individual gyro owner as well as designers/manufacturers. This is essential in order to achieve acceptance and understanding by gyro pilots in what is required for gyros to be stable/safe. But, also, we do not want to add undue costs to a manufacturer to verify compliance with the standard. It is my goal that we might be able to set the criteria and testing methods so that we can readily communicate those to the regular gyro pilot – and have them understand them, respect them and require them in what they buy or fly! The design details in what Mike and others are providing will be valuable for designers and manufacturers and more technically adept builders to help them meet, or adjust to meet, the criteria. And, some of the details will help us in determining – with time and data just what criteria and flight testing elements are necessary for a stable/safe gyro. For instance, we would want to watch the data to determine if just “damped” long-period dynamic responses are adequate for safety. Time and data may tell us we are overly-conservative in some areas, where our intuition had possibly set too liberal criteria in other areas. Our intention is to be biased on the safe side to start with right now!

When we have wrung this out as far as anyone cares to, I will present this material to the ASTM Gyroplane subcommittee for consideration of refinements to the existing standard. Your posts are great material for consideration.

DYNAMIC criteria: I think we still need to find the DYNAMIC criteria that would avoid PIO. I think Mike is agreeing that the criteria can be expressed in the shortest oscillation period allowed, and the damping rate for that oscillation. I think everyone is in good agreement that the long-period oscillations just need to be “damped. But, where is the breakpoint between the short-period criteria and long-period? And, how do we test for the criteria?

I like Rahgu's arguments that there might be some "residual intermediate" natural oscillations in the 7-10 second period range that might be THE concern. Is 10 second period the breakpoint, below which any oscillations should be damped to x percent within x cycles?

Thanks, and keep up the valuable input - Greg

[Udi]

As for Udi’s suggestion to test this more vigorously at more speeds and power ranges, I can put that to the ASTM subcommittee for consideration. But, to me, this is not realy necessary to eliminate our initial concern for a PPO potential.

My intention was not to make the tests more stringent. I thought that the proposed tests do not cover the entire flight envelope. In my opinion, these tests should cover the entire range of normal operating conditions. Why leave entire regions of the envelope untested? I, as a customer, would like to know that my FSA gyroplane was tested before I fly it. This is no longer experimental aircraft!

One example in particular is engine out at airspeed VNE. Some gyros are at the end of the cyclic range at this airspeed and may not respond favorably in the event of engine out.

I say, cover the entire normal flight envelope. If you want to provide wider tolerances for airspeed at the edges, fine. But at least test it.

Thanks

Udi-

[gyrogreg]

Udi,
I don’t really disagree with the wider range for Power stability testing. Your proposed speeds and power changes actually target some really critical issues – the slow speed at idle power then increasing to full power, seems to me to specifically target possible PPO tendencies at reduced g load at the push over the top of a zoom. The high speed with full power reduced to idle also aggressively targets the potential for bunt upon sudden power loss on a gyro that artificially holds the nose high under this condition.

Development of this criteria in the standards group was a bit dicey, as the whole issue of “unbalanced” prop thrustline was a bit touchy for some, and poorly understood by others. I’m still not sure how the group would feel about this – might be taken by some as a direct attack on CLT designs. Of course, a CLT should not have any problems with this issue, but some very low prop thrustlines are considered CLT by many. Your suggested speed and power change ranges would certainly target gyros with highly unbalanced prop thrustlines – high or low. And, that is probably a good idea – I just have not been comfortable pushing that issue so strongly because I have run into a lot of resistance when this issue might suggest some “perfect” designs aren’t so perfect. We’ll see how the suggestion goes, but I really feel the problem machines will be identified with the more moderate criteria.

Thanks, Greg

[eruttan]

Would Udi's test not target unstable machines, not thrust line offsets?

I am not confident I am keeping up with the discussion, but I belive (think, guess?) udi's suggestion would be passable by a large thrust offset machine with enough coping mechinisms (tail)

[Udi]

That's right, eruttan. Some of the additional tests I have suggested are targeted at the edges of the envelope where the gyroplane must be well balanced for it to pass the test. Any gyro that is passing these tests may be considered pitch and power stable, even if it has a thrust line offset along with a correcting mechanism.

I know of at least two gyroplanes that would pass these tests right now, possibly three.

Udi-

[Mike Jackson]

My intention was not to make the tests more stringent. I thought that the proposed tests do not cover the entire flight envelope. In my opinion, these tests should cover the entire range of normal operating conditions. Why leave entire regions of the envelope untested? I, as a customer, would like to know that my FSA gyroplane was tested before I fly it. This is no longer experimental aircraft!

One example in particular is engine out at airspeed VNE. Some gyros are at the end of the cyclic range at this airspeed and may not respond favorably in the event of engine out.

I say, cover the entire normal flight envelope. If you want to provide wider tolerances for airspeed at the edges, fine. But at least test it.

Thanks

Udi-

Hi Udi,

We have tossed the term "flt envelope" around quite a bit. I don't know how we are defining "entire flt envelope". Most experimental gyros don't come with well defined published flt envelopes. I have not seen much discussion of what really should/might define a VNE limit. Just because we can reach a high calibrated airspeed, for example, doesn't mean we want to be there, maybe due to cyclic boundaries or pitch sensitivity.

IOW, we may want to define our envelope/limit to a point reasonably short of falling off the edge of the earth. Sound flt testing is conservative and works slowly from the heart of any proposed envelope. As we move closer to the boundaries, the testers say, "that's fast enough because...., here's the published limit - don't exceed it because..........."

Mike

[eruttan]

Mike;
I humbly disagree.

If I sell a plane that can do 150MPH, but has undesirable traits that show at the speed. Not to mention break Sprot Pilot perhaps. I might put a Vne of 100 on it. It test it in this envelope and it is good. It is safe in this envelope. If a pilot goes beyong my tested and published envelope, thats pilot error. It is on the pilots head.

Conversly, If the standard gets into , "well this aircraft can go 150, so you have to test it there", we are beyong the scope of assureing stability and safety. We are forcing people to test where no one would want ANYONE to fly.

If a designer/builder/manufacturer says Vne = X, thats what it is.
Getting into the reasons for it is irrelivant and beyong the scope of the ASTM standard.

As long as flight testing is done for the entire published envelope, then the craft is tested.

I cant see anything other that the Published flight envelope working. But I will listen to the counter point.

[Aussie_Paul]

I agree. I believe that we should test stability to the published flight envelope. If some U BEAUT designer can have his machine meet the standards with 20' of rope out the back with a brick tied to it, then it passes!!!!

Aussie Paul

[Mike Jackson]

Mike;
I humbly disagree.

If I sell a plane that can do 150MPH, but has undesirable traits that show at the speed. Not to mention break Sprot Pilot perhaps. I might put a Vne of 100 on it. It test it in this envelope and it is good. It is safe in this envelope. If a pilot goes beyong my tested and published envelope, thats pilot error. It is on the pilots head.

Conversly, If the standard gets into , "well this aircraft can go 150, so you have to test it there", we are beyong the scope of assureing stability and safety. We are forcing people to test where no one would want ANYONE to fly.

If a designer/builder/manufacturer says Vne = X, thats what it is.
Getting into the reasons for it is irrelivant and beyong the scope of the ASTM standard.

As long as flight testing is done for the entire published envelope, then the craft is tested.

I cant see anything other that the Published flight envelope working. But I will listen to the counter point.

Hi Eruttan,

Thanks for your reply :D . I'm not sure we disagree about much here.

I totally agree with your first para. If an LSA mfgr wishes to "limit" his flt envelope for good reasons, it should be so! I feel the standards folks, whoever they might be, should NEVER dictate an aircraft be flown to their perceived limit just because they think it can get there. Tail waggin' the dog. That's nuts. I hope it would never come to that. Common sense to me says it won't - but what does common sense have to do with governing bodies? That's why I think Greg Gremminger's work with ASTM is so important. Jump in here Greg!

I disagree with your statement,

"If a designer/builder/ mfgr says VNE = X, that's what it is. Getting into the reasons for it is irrelevant and beyond the scope of the ASTM standard.

I may be wrong, but I thought the details of the flt envelopes, and hence safety, for all the LSA mfgrs were an important aspect of the standard. VNE, for example might be limited by handling characteristics which several of us are trying to work out in the Forum and ASTM discussions.

IOW, an LSA mfgr might establish a limit, for whatever reason, but it should be verified under defined standards (our standards submitted to ASTM) by flt test before it's put on the market.

Cheers :) ,

Mike

[Udi]

...I feel the standards folks, whoever they might be, should NEVER dictate an aircraft be flown to their perceived limit just because they think it can get there...

Mike,

The additions I have proposed do not dictate any limits. They dictate standards for airspeed and power stability. eruttan has correctly stated that it is in the manufacturer's discretion to limit their machines flight envelope, so that it can meet these standards. An LSA gyroplane will have a published Vmin and Vne. Any airspeed in between must be proven safe and stable.

Nobody will dictate the flt envelope for a manufacturer, but an LSA gyroplane will have to demonstrate that it is meeting the standards for the flt envelope that the manufacturer is publishing. This is no different from the certification of any other aircraft.

Udi-

[Mike Jackson]

Agreed. We need to sit around beers and have this discourse instead of the computer!

UDI, I used to fly my Tiger Moth to a QG Aviation in Fort Collins to a guy named Ray Middleton. Super mech. Have you ever run into him? He used to have alot of great machines in his hangars.

Cheers,

Mike

[Hognose]

Some comments --

- Udi, I am not sure I understand your suggestion to test certain modes at 80 or 90% of Vne. IMHO if the aircraft has only demonstrated safe short period dynamic stability to, say, 80kt when the Vne is set by the designer at 100kt, then the machine should have its Vne placarded at 80kt -- particularly in something like Light Sport Aircraft which is not SUPPOSED to be an experimental aircraft.

- To put it another way, we don't want the general run of pilots who purchase LSA compliant aircraft to become, in effect, test pilots by taking the machine into an undocumented flight regime.

One factor present in at least one RAF 2000 accident with which I am acquainted is a pilot who was determined to operate his gyroplane at very high speed, perhaps not faster than the designers had ever tested but definitely higher than where most people customarily operate gyros. High speed -- high power settings -- rough air -- he PPOd.

cheers

-=K=-

[Udi]

Kevin,

When I suggested 0.9 Vne I thought that would be close enough to the upper airspeed limit, and we can expect the gyroplane not to have nasty surprises at the upper end. I was also considering the higher risk for the test pilot at Vne.

But I agree with you. The customers should not be the test pilots. Test pilots should test every conceivable manouver, as well as engine failure, withing the published flt envelope.

Mike,

I don't know Ray Middleton, but there are a few people at 3V5 who've got hangars full of interesting airplanes. Let me know if you happen to land at FC.

Udi-

[gyrogreg]

My thought on flt envelope is that the envelope may be defined by its ability to meet the standard's criteria - in otherwords, if it meets the criteria up to 90 mph, but it fails the criteria above 90 mph, then that might be (with some safety margin applied) the flt envelope. This would Vne would need to be defined at the extremes of loading and power application as well.

By this thought, it might be possible for some dangerous machines to actually meet the LSA standard if their Vne is limited to the flt range in which it does pass. This is a little scary since most of these less stable gyros would likey still be capable of much higher airspeeds - beyond the published Vne! But, I doubt there would be much market for a production gyro that would have to declare their Vne to be very low in order to qualify as LSA. At least, to my thinking, the pilot of such machine would be made aware of that limitation and the possible consequences of exceeding that limit.

One slight confusion to this might be to actually perform a flight test at Vne - when that test method calls for a further increase of airspeed. That is why, some stability criteria specify 90% Vne - rather than Vne - so Vne is not exceeded - especially if exceeding that Vne might mean entering a possbily unstable flight range.

[eruttan]

What about maneuvering speed. I am aware that some certificate aircraft at Vmax or Vne, if you wiggle the stick, off come the wings.

We should not confuse Vne with Vmaneuvering (for lack of the correct term, somebody correct this, please)

A maker may have a Vne, but not expect maneuvering (or radical maneuvering) at this speed.
So in summary, we need to me very specific over what envelope we expect stability. Or where it has been tested. Or where the manufacture has tested it.

[gyrogreg]

The ASTM team has specifically avoided requiring a maneuvering speed or even allowing a manuevering speed to be defined for any requirement - the LSA standard requirements must be met for all speeds up to Vne - or put another way, when the standards criteria are exceeded, that sets the Vne. So essentially Vne is also the maneuvering speed, so maneuvering speed is irrelevent.

We specifically did this because that is one advantage of a gyroplane - it is highly maneuverable, and the rotor spills its lift at relatively low g loads so the g-load requirement in the standard will be difficult to even achieve - the standard requires a rotor and airframe maneuvering positive g load of 3.0 for all maneuvers up to Vne! And, a good gyroplane design inherently doesn't really need to have such limitations for structural reasons - an advantage of gyroplanes over other aircraft types. If the gyro can't meet all of the requirements above a certain speed - that is both the Vne and maneuvering speed! And, putting a manuevering speed on an aircraft that is, by nature and endorsement, highly maneuverable, would likely not be respected anyway! This is one advantage of gyroplanes that we did not want to diminish by suggesting or allowing a maneuvering speed.

Thanks, Greg Gremminger

[Udi]

I’d like to add to Greg’s reply. There is no need for a maneuvering speed (Va) in gyroplanes. Rotor lift is proportional rotor RPM, and rotor RPM is limited by drag. So the rotor cannot generate a G-force that would exceed the design limitations.

A different limiting airspeed may be appropriate for gyroplanes though. One of the risks of flying very fast in gyroplanes is the low AOA of the rotor. When flying fast with a low AOA, a strong downdraft may result in a negative AOA. Thus, there may have to be a speed limit specified for operations in rough air.

If Ron Herron is reading this, it would be interesting to know whether the autogiros had any speed limitations for flying in turbulent air.

Udi-

[RHerron]

Udi,

None that I am aware of. I will look through the Specifications and Type-Certificates to see if I find anything. As far as I know, only VNE was listed.
Captain Miller operated regularly in tremendous winds, both in the Pitcairn and Kellett machines.

[Udi]

Thanks, Ron. This is very interesting.

Udi-

[gyrogreg]

Udi, thanks for the additional comment.

As far as a rough air speed limit - this is what maneuvering speed is intended to cover in an airplane as well. But for a gyroplane, IMHO, I don't see any reason that a stable gyroplane's speed needs to be limited because of a low rotor disk AOA. A statically stable gyro - especially one that is G-load stable, will react to the g-load disturbance to restore the rotor AOA upon a disturbance - even a strong disturbance. And, according to some discussions in this thread, the G-load stable gyro will inherently have very quick (sp) reactions to immediately correct for the disturbance. Also as identified in this thread, at the higher airspeeds, the natural reactions of the gyro are even quicker. Also, at high speed with a stable gyro, the stick gets fairly stiff discouraging rapid stick movement. But, even forced rapid stick movements at high speed in a stable gyro are followed immediately by rapid airframe pitch and restoration of 1g - essentially preventing even an intentioanl buntover. Done this numbers of times in the Magni also!

As my evidence that this is so, I have flown at high speed (High Command, Dominator and Magni) - 100 mph plus - in very gusty winds (thunderstorm gust fronts) and experienced short negative gs. For a stable gyro the G disturbance is so quick, and positive Gs are restored so quickly, that there is very little effect on RRPM.

These are my opinions - Greg Gremminger

[eruttan]

So we should note that we have dicussed this and for the reasons sited we will require all stability tests to be preformed through the entire envelope up to the published Vne.

Then should not UDI's "more vigorous" tests be done at 100% Vne?

[gyrogreg]

Eric,

I will certainly present the issue of testing at full Vne to the ASTM gyroplane subcommittee again when we start to consider further changes to the gyroplane standard per these discussions. Especially, I would want to consider Udi’s suggestions for a more vigorous evaluation of Power Stability airspeed range.

For your helpful review, I am making the full text of the proposed gyroplane standard available at this link:

http://www.magnigyro.com/ASTM%20Standards/Gyro%20D&P%20V3.1.pdf

(As this standard is not yet approved, please do not distribute this further than your own personal reference for these conversations.)

Please review the entire 4.1 section – Controllability and Maneuverability. Note, that these requirements do mostly include ALL airspeeds up to Vne. Note that there are some "catch all" criteria beyond the strict stability performance criteria. Except for the Power Stability criteria that Udi comments on, Vne or some safe margin from Vne is included. (In some cases, where testing for the requirement requires an increase in the initial airspeed, 80% Vne is specified for the criteria to maintain safety margin for the testing within Vne.

Everyone is certainly welcome to join the ASTM Gyroplane standards subcommittee when we initiate the first review of the approved standard.

- Greg

[gyrogreg]

Mike will be unavailable for participation in this forum for a while. Mike sent me an interesting email, and asked me to pass it on to the forum if I thought it would be helpful. I consider anything that Mike presents as useful – Mike’s career in the Air Force was as a Test Pilot, and he has a lot of both technical insight and technical resources to draw from. I feel we are fortunate that Mike is also very interested in promoting gyroplane safety – as are all of you participating in this forum.

Before I post Mike’s comments and my response, you may find this link helpful in understanding Mike’s comments. This is also a very good explanation some of the dynamic issues we are addressing here. Check out this link:

http://adg.stanford.edu/aa241/stability//dynamicstability.html

- Greg

[gyrogreg]

This might be a bit long, but here is an email comment from Mike that I would like to pass on to this working group:

I thought some of the military (Mil Spec) requirements would apply to gyro handling - specifically sections of Mil Spec 8785 - C. Your presentation to the Standards folks might have more teeth referencing a mil spec. It is used industry wide - not just Uncle Sam's toys.

I am presenting (hopefully) an email picture of a graph of Undamped natural frequency (Omega sp) vs n/alpha. In the forum discussion I called this Fnsp. If it didn't come thru look up the following:

http://adg.stanford.edu/aa241/stability//dynamicstability.html

n/alpha is a measure of how much G can be extracted from an aircraft thru a change in angle of attack (AOA) either by gusts or quick step inputs into the controls to excite the SP mode.

The natural freq can be measured by flt test but I'm having difficulty how to measure or simply find n/alpha. I know there is a simple way since it's all tied to the system lift coeff (Cl). I may call the Test School.

If we stay within the Level 1 boundaries . All the combinations of damping and freqs fall into place and we should have a controllable auto gyro. We can refine it a little further by looking at the collation of hundreds of pilot comments about flying at various damping ratios and varying the freq and vice/versa holding a freq constant and varying the damping ratio. We did this in the Calspan variable stability Lear jet - amazing teaching tool.

Here's the results verbatim:

1. Effect of Changing DAMPING of Short Period Motion

With medium frequency ~ 4 r/sec (r=radians); 8 lb/g (Lear)

Damping Ratio / Remarks:

Medium .4 / Small tendency to overshoot but quickly damps out by itself. Useable in calm air.

Low .2 / Overshoot eaasily noticed, interferes with quick maneuvers

Zero .0 / Not useable, although flyable. With (control) friction, becomes almost unflyable

High .7 / Looks dead beat (no overshoots). Steady for tracking, easy to maneuver, but not particularly quick

2. Effect of Changing FREQUENCY of short period motion

With high damping ratio ~ .7; 8 lb/g

Frequency or quickness of response / Remarks

Medium 4 r/sec / Typical of medium sized airplane. Good.

Fast 6 r/sec / Quicker to get moving, quicker to settle down. More like a fighter or small plane. Good for maneuvering, easy to get desired G

Very Fast 8 r/sec / Approaching upper limit of Mil Spec for n/alpha = 20. Strong tendency to overshoot and bobble especially with lighter stick forces.

Slow 2 r/sec / More like transport. Stable, good for IFR, not good as fighter

Note: CG moving aft towards neutral point (RLV @ CG) has the effect of reducing AOA stability resulting in a weaker restoring moment in AOA and a lower frequency. CG forward increases the SP frequency. The 2 * dsp * Fnsp remains constant when CG moved so as Fn increasees , dsp decreases and vice versa.

Effect of STICK FORCE PER G

With medium frequency ~ 4 r/sec; high damping ratio ~ .7

Stick Force / g / Remarks

Medium 8 lb/g / Suitable for this airplane (Lear). Adequate g protection

Light 2 lb/g / Pilot maneuvers more quickly. Appears like higher freq of Short Period. Open loop response unchanged but closed loop response quicker. Pleasant to maneuver. Remember from demo of effect on frequency,
Forces too light make pilots bobble with high freq, overshoot with low freq.

High 25 lb/g / Acts like transport. Pilot maneuvers slower. Looks like lower freq of SP because heavy forces preveent pilot from applying enough control input to get a quick response. Heavier forces prevent inadvertent quick maneuvers.


There it is in a nutshel. I didn't make this up. Consensus of hundreds of guys. What is pleasing to these folks should likely please the gyro community. With the above nums we might come to a consensus envelope for damping and freq response. Possibly damping between .4 - .7; frequency 2 - 6 r/sec Stick forces 2 - 8 lb/g. Keep in mind most gyros will have some spring forces to overcome above trim. This will add or be incorporated in the overall Fs/g but will not affect the airframe dynamics. We can discuss friction and control elasticity later.

- Mike Jackson

[gyrogreg]

I'm not sure I completely understand all of this. But, it looks like one of the important factors to be measuring might be the n/alpha factor (G-load per rotor AOA) - but, this does not seem simple to accurately measure! Maybe we will have to use it in our dynamic stability criteria, but I hope we could find an easy way to measure it. I suspect this measurement, as well as the measurement of the sp frequency, is a bit difficult and requires very professional flight testing. I'd like to find ways to keep this all in the realm of the average manufacturer and pilot and pocketbook and technical expertise.

But, I am still clinging to the hope that we might not need to be so refined in our dynamic stability criteria or testing. Our goal is safety - for dynamic stability issues, I think this means little or no propensity for PIO. The handling qualities, beyond what will avoid PIO, to me, are more a marketing advantage or disadvantage to be set by the manufacturer or designer according to their overall market and design goals.

The link you sent, and Raghu, suggest that the short period responses are likely to be acceptable if the static G-load criteria are met. Raghu does suggest there might be a mostly undamped "merged" dynamic response in the range of 7-10 second periods that might be a risk element to PIO. I'm hoping it might be enough for the standard to simply identify if there are any poorly damped oscillatory responses under 10 seconds in period - in order to avoid PIO tendencies. But, I am unclear if the measurement of n/alpha might be the definitive measurement for this 7-10 second period concern.

I actually think that simply meeting the static stability criteria might establish adequate dynamic responses - adequate to avoid PIO tendencies. I'm not sure if the Raghu’s suggested possibility of "merged" dynamic responses is a real factor in the real world. I am sort of inclined to focus on the static stability criteria, with just a simple dynamic evaluation (no poorly damped oscillatory tendencies under 10 second period?). Then, let's see if data and pilot reports (and accidents) might verify that approach is effective to eliminate PIO events. My encouragement for this is simple - over the past several years, where the emphasis and acceptance of HSs has started to grow, we have started to bring down the PIO/buntover accident rate dramatically already. This emphasis, so far has mostly been on HSs - essential to meet the static stability criteria!

Thanks, Greg

[Udi]

As far as a rough air speed limit - this is what maneuvering speed is intended to cover in an airplane as well.

Greg - Va, by definition, is the airspeed above which the load factor of the wings (FW) can exceed the aircraft loading design limits. Rough weather is only one scenario that could lead to a high load factor. Another one is making a steep turn, or performing aerobatics. Flying slower than Va guarantees that you don't exceed the aircraft loading limits because the wings will stall before the critical loading is reached. Va is based on the wings airspeed/loading/stall curve. There is no such curve for gyroplanes (I think...?), so I think we should not use the term “Maneuvering Speed”, or Va, when discussing high airspeed limits for gyroplanes. This term is meaningless for gyroplanes.

With regard to operation in rough weather conditions, both you and Ron Herron are saying that pitch-stable gyroplanes appear to be very safe in these conditions. That's great! In this case I am taking back my suggestion that we limit the airspeed for rough air operations.

So, what are the criteria for Vne? I am not clear on that.

Thanks Greg!

Udi-

[gyrogreg]

Forum Team, we have had a lot of very technical input on this forum. I still do not see consensus on the dynamic criteria – to minimize or avoid possible PIO risks, what are the REAL WORLD criteria that might be necessary to determine if there are PIO risks?

We have plenty of deeply technical chum to chew on. Bit, I would like to ask each of you now to try to put this all into practical sense – that can be tested, verified, and understood readily and inexpensively. I believe that is possible. And the success of this standard in improving gyroplane safety is not only that it be technically supportable, but that it be understood, accepted and applied by the gyroplane community.

It appears that the technical aspects are starting to converge to consensus. Could each of you, in consideration of all of this material, make some concrete suggestions as to criteria and testing methods that would be simple and understandable to the masses, and supportable by the our best technological understandings at this time.

My basic question is, beyond the static criteria, what DYNAMIC criteria and measurements might be necessary to minimize the risk of PIO. I am not sure we need to do much more than the static criteria already assures!

Thanks, Greg Gremminger.

[Hognose]

Greg, Udi, Eric, all:

I see you all have hashed out the Va issue, which is, as Udi points out, not applicable to rotorcraft. I am unaware of rotorcraft that have suffered inflight breakup due to overstress alone (unlike airplanes, where such accidents are relatively common, most usually stemming from VFR into IMC). There are probably some examples in the military files... but the truth is there are many more threats to a rotorcraft that are more, er, threatening.

IMHO -- and to address Greg's sensible question -- the major stability concerns are poor static stability, first, and in dynamic stability a divergent short mode. THe pilot can usually learn to handle the long mode (phugoid) w/o risk. Which thread did Raghu raise the possibility of an intermediate mode in? I want to read what he wrote (which I have probably read, and forgotten...) Most of the lit seems to mention just the short period, and the long or pugoid. (So does Mike's link).

I am not completely sure that criterion-based performance tests alone are the best approach to ASTM; although I have read and understand Greg's arguments for the same. I believe that some paper exercises, for instance, illustrating the sums of moments on the aircraft in various flight regimes, could also help. At the same time we don't want to turn this into the nightmare committee bureaucracy the PFA has in England. Their rigid standards have actually worked against safety by forbidding safety-enhancing modifications to unsafe, but standards-compliant, gyroplanes.

So to recap (for Greg):
1. The static criteria are important. Without static stability you have no hope of dynamic stability
2. Long period (phugoid) motion is not going to cause PIO (or people would be PIO'ing Mooneys every day).
3. Therefore, any dynamic stability criteria ought to address the short mode. Ideally it should be con- rather than divergent. However, testing this seems to be "easy to say, hard to do."

Oh, you wanted me to suggest particular criteria? Gulp. Pass for now.

cheers

-=K=-

[Udi]

I really enjoyed Mark's email/post describing the Calspan variable stability testing for Lear jet. I think mark's conclusion for damping criteria makes a lot of sense, and we may accept Marks numbers as a good starting point.

The problem might be with the testing method itself. If you have to have specialized test flight equipment to record the SP frequency and damping ratio, who, other than AAI, can afford it? Are FW LSA aircraft manufacturers required to demonstrate SP frequency and damping ratio? Aren’t we going a little overboard for an LSA aircraft?

If we want to have dynamic stability criteria that any manufacturer can measure with nothing more than a stopwatch, than we should stop looking at SP frequencies, damping ratios and stick feedback forces.

For the purpose of avoiding PIOs, we want to look for easily measurable flight characteristics that can tell whether a gyroplane is prone to PIO, or not. So here is a question for Greg, Chuck, Doug, Jim Mayfield, Ernie, and all the other gyroplane gurus:

If I let you fly a gyroplane that you know nothing about and you can't even inspect it before you fly it; how would you be able to tell if this gyroplane is prone to PIO or not. What would you measure, with a stopwatch, to decide if this gyro is dynamically stable or not? This is a paper exercise, so don't worry, you can't crash.

I have never flown an unstable gyroplane so I wouldn't know what to look for...

Udi-

[gyrogreg]

Udi, My answer to your question:

If it had a large HS, at a good distance aft of the prop, airfoil shaped, and a reasonable propeller thrustline (IMHO appearing to be slightly high if anything), I would familiarize myself with the control sensitivities by practice "balancing on the wheels". Then, I would perform the STATIC flight tests in this order:

Power Stability,
Airspeed Stability,
G-Load stability.

I would start these tests at MPRS and and at increasing increments of airspeed up to Vne.

If this gyro then passed these static tests, I would perform a singlet fixed stick and free stick dynamic test at MPRS and at 5 mph increments up to Vne. If the resulting oscillations are damped, I would feel confident in flying this machine throughout the speed, power and wind envelope.

I would not rule out a CLT or moderately low thrustline, or T-Tail gyro, or any gyro, as long as it has a large and airfoil HS (like the current Dominator) and apparently reasonable prop offset. But, if the static tests indicated any large pitch or airspeed deviations with power changes, I would not conduct any dynamic testing on this machine and I would limit my flying to airspeeds within which there is not so much deviation with power. IMHO poor power stability results indicates there is an unbalance of static moments that could result in quick, nose-down pitching upon certain conditions or disturbances to (turbulence or power).

If any static tests did not meet the criteria, I might limit flying that gyro to the airspeed and power envelope where it does pass. But, I would really not be comfortable flying a machine that failed in any area of the static tests!

One more thing, I would not provide training to anyone who was planning to fly a gyro that did not at least meet the static criteria. I would encourage that student to make the modifications to improve the static results - that isn't really hard to do - the worst possibly would be to change the prop thrustline and install a good HS far back on the keel.

Thanks, Greg Gremminger - anxious to hear other answers to this question!

[Mike Jackson]

Hi guys,

I have been pondering how to very simply encompass simple tests (2) to ensure a vehicle has 2 of 3 test criteria Greg has just mentioned.

Two tests include a Fs/g exercise and a "Concave Downward" (CD)requirement developed ny NASA pilots and engineers. I've mentioned the latter in a previous post. I will try to attach homegrown graph after the test whic illistrates the CD requirement.

Test 1
Fs/g is a strong indicator of static stability Cmalpha (Cma). The strongest design player is the static margin(SM) (for us - where and how far apart is the cg relative to the RLV). The larger the slope, the larger the SM, the higher the Fs/g is.

Flt test involves taking a 5 lb bag of sugar and get a feel for it. You are the force gauge. Simple hand held force gauges exist for the LSA mfgr. Pick a heart of envelope trim speed and roll into a "wind up turn". Adjust pitch as necessary to keep AS constant. Pull at a rate which will allow an estimate of increasing G and stick force. It's important to note whether the increase Fs/g is linear - it shouldn't change slope or "lighten" Do this to approx 3 G - or whatever you establish. A gyro with a small to zero SM will have lighter to very light/no stick force (other than artificial spring) and a flatter Fs/g slope. A pull up maneuver may be used but be careful not to end up in a low G nose high situation.

According to one of my dusty school books by Seckel, Stability and Control of Airplanes and Helicopters, this might be sufficiently enough testing. The claim is if Fs/g is a "reasonable" value, the dynamic stability will follow - QED. What is "reasonable" value? To light - gyro prone to light damping and increased natural frequency - PIO prone. To heavy Fs/g, heavy damp - low freq and not "fun" to maneuver.

Test 2

"Concave Downward" requirement.

This is a requirement emphasizing "short time" response characteristics. It requires that the time history of normal acceleration "G" in response to a cyclic control STEP to be concave downward w/in 2 sec.

It provides a cue of upcoming acceleration and to guarantee a finite response. The requirement provides for "reasonable" maneuvering characteristics. What are we seeing in the input? - A pitch rate, AOA rate, and a finite G input resulting in, hopefully in to AOA rate (rotor) and pitch rate (tail geometry) damping. We want to see/feel the G excursion above the step input decay within 2 sec for a "reasonably" stable machine. If it takes longer, AOA and pitch rate stability is less than favorable.

Flt test technique - From a trimmed level state input a quick finite (step) stick input. Keep the stick displacement small initially, then fix the stick in that step position. Feel/observe the G input, especially the response after stick input. We want to do it quick enough to consider the test at a constant AS. Count potato(e?)s and see if it feels like after a G rise, the G is decreasing within 2 sec.. The G will decay regardless, but it is the time were interested in. If you don't see it within a reasonable amount of time you will probably see a Phugoid mode.

These one or two tests are simple and can be done with NO instrumentation. You might want to calibrate seat of the pants G - 60 deg lvl turn = 2G etc, and go do some light wt curls to calibrate your stick force feel. This may simplify the ASTM procedures and criteria.

Get out the Magni Greg.

I'll try to attach a diagram in the next post - I'm very confident I might make this written input disappear if I try to input it here now.

I am to be out of touch for about 12 days to respond to comments.

Quoting from the text: The CD requirement is essentially a specification on the maneuver margin, since an aircraft with any reasonable acceleration trim gradient (Fs/g) could not fail by much to meet the criterion. Conversely, an unstable trim gradient certainly indicated failure to meet it. The trim gradient is far easier to determine in flt testing the dynamic response, and so perhaps it should be used as an indicator of the "downward concavity."

What do you guys think?

Mike

[Mike Jackson]

I'm trying to attach a "concave downward" graph. I use a high tech dry erase crayon on my kid's board.

Looks like file was too big to send. Try later.

Cheers,

Mike

[raghu]

Mike, stick force per G is an effective means of determining the degree of AOA stability (or static margin) in FW. Unfortunately, the offset gimbel control system and the velocity stability in gyro prevents us from using the same technique in gyros. A statically unstable (AOA) gyro such as an RAF will have a perfectly acceptable stick forces and stick force gradients thanks to the offset gimbel and velocity stability due to cyclic flapping.

Downward concave response criterion
---------------------------------------------

This is used in helicopters as it is not unusual for helicopters to be statically unstable wrt AOA. Consequently, the stick acts as a pitch rate controls rather than a pitch position control. So, a change in pitch is made by jabbing the control to start the motion and jabbing it back to stop the motion. Lag and overshoot make the control of pitch rate controls harder to use and more prone to PIO. The downward concave response criterion is used to make sure that the responce is acceptable in using a pitch rate control (as in a helicopter)

Unlike helicopters, gyros can quite easily ( and should be) be made statically stable wrt AOA. Thus, the stick acts, much as in FWs, as a pitch positional control rather than a pitch rate controls. Hence there is NO NEED for the downward concave response criterion.

[raghu]

I actually think that simply meeting the static stability criteria might establish adequate dynamic responses - adequate to avoid PIO tendencies. I'm not sure if the Raghu’s suggested possibility of "merged" dynamic responses is a real factor in the real world.

Thanks, Greg

Any control system with a human in the loop can and is prone to operator-induced oscillation (PIO in the case of aircraft). Anyone who has trained as a glider pilot will appreciates this. During the first handful of aero-tows, a glider pilot lasts at best a couple of seconds before the glider goes into a PIO ( either in pitch or laterally, or both). I have seen experienced military helicopter pilots humbled by this. But soon enough, after a handful of training flights, almost by magic the pilot gets the feel and wonders what the big deal was all about. And gliders are orders of magnitude safer than gyros and arguably the safest form of aviation.

My point is that a big part of PIO is P and so there is no escaping good training. Having said that we can try and design out some of the likelihood of this tendency. Also its not the PIO but the PPO that follows it that is the killer. Reducing the risk of PPO thus is vital.

The big boys like NASA use pilot in loop models and simulations to determine PIO propensity. These are very sophisticated techniques that are well out of range of the sport plane industry.

Experience has shown that it is the high frequency modes that are poorly damped that lead to PIO. Typically the time period is in the 2-7 second range. There is a lot of literature in handling qualities and if it is of interest I can throw around some ball park numbers, but I think there is value in keeping things simpler.

Here are my suggestion as a start

1) I believe, and the recent UK report on the RAF fatality concurs, that the oscillation mode due to the coupling between the rotor and body is the most likely culprit of PIO in almost all the PIO incidence. This mode is dynamically unstable but OCCURS ONLY WHEN THE GYRO IS STATICALLY PICTCH UNSTABLE (fails the g-load turn test) . So, guaranteeing that a gyro is statically pitch stable ( turn test) is a good first step.

2) In a sufficiently pitch stable gyro the short period mode should be well damped. However it is possible that this mode, particularly in low speed, may be excited into PIO. But given that there is no evidence of any of the pitch stable gyros ever having PIO problems, I think this is unlikely to be a problem in practice. I can again suggest some ball park numbers for damping and frequency to reduce the propensity of PIO but unless you use well instrumented gyros, you are not going to be able to accurately measure this mode. My suggestion is ignore having any short period criterion as a start.

3) the phugoid or long period mode may have reduced damping due to coupling with the rotor. Also this mode has a higher frequency than in FWs and so, unlike in FWs, cannot be entirely ignored. My recommendation is that this mode be measured as part of the dynamic stability test (easily done) and it be shown to be stable and also above say a time period of say above ~10 seconds.

These should be good first steps. Finally it may help to know what the objective of the stability standard is. Is it just a minimum standard that is reasonable but still maintains that these are experimental aircraft or is it more akin to the FAR 103 certification standard? If it is the former then the recommendations here could well suffice.

[gyrogreg]

Raghu,

Thank you, and everyone for your clarity and hard work on this. To answer your question, Raghu, this LSA standard is intended to be a reasonable MINIMUM standard – the best we can do with the data and resources we can muster at this time. The LSA standard does not intend that these aircraft be experimental, but due to the relative lack of data and technology understanding for gyroplanes, we cannot assure that these criteria are perfect at this point either. It is my intention and hope that the best we can do at this time will save lives. It is then my hope that, with further focused data in light of these guideline criteria, we can further refine any deficiencies or problems that come up. For instance, when manufacturers and others start trying to meet the criteria in the standard, I am sure they will have some issues or problems and request/suggest some changes. The ASTM standard is intended to be a “living” document that grows with our experience.

Raghu, as I understand your last post, you are suggesting that the existing Dynamic criteria change by the following:

1) Remove the “short-period” (less than 5 second period) criteria. This forum has convinced me that the existing requirement for “no oscillations below 5 second period” is not appropriate, as originally suggested by Raghu. It appears to me that both the Test Pilot perspective (Mike Jackson’s input) and high level mathematical analysis (Raghu and Houston) suggest that there should actually be short-period highly damped responses and that these are already fairly assured by the static stability criteria.
2) Retain the long-period criteria (simply damped), but specify this is for oscillatory response periods ABOVE 10 seconds.

This would suggest that our existing standard is not too far from what we are starting to condense to in this forum discussion. I do intend to experiment with the flight test suggestions that Mike Jackson has made above (Fs/g and “concave downward”) to see if anything is consistent with Mike’s thoughts. I do understand Raghu’s suggestion that the offset gimble might mask any true Fs/g. I am interested to see if I can detect any “concave downward" phenomena. But, Mike’s suggestion that 2-3 gs might be required in the testing might be difficult to do – it is difficult to pull more than 2 gs in a gyro – but, I’ll try it in the Magni to see what happens!

I also feel that to conduct these dynamic tests, where there is an attempt to quantify G-load, is very subjective and would be difficult to quantitatively verify. Reported results from a highly subjective test would likely be disputable – we are looking for simple and inexpensive test methods that are minimally disputable.

I have asked another helicopter experienced Test Pilot – an FAA employee who is working with the ASTM teams – to review our forum discussion here and present his opinions. Ed Kolano had emailed me with some similar concerns about the dynamic criteria. If Ed might concur with our discussions here, that might be strong support for the work we have done here.

I would still like further discussion or concurrence from any of you who have thoughts on all this – are you following the arguments? - are you reaching any conclusions? Udi, Chuck, Ron H., and others, please comment. Any ASTM sub members who are monitoring, please comment!

- Greg Gremminger

[Udi]

I completely agree with Raghu's last post. I would like to expand the discussion though, on a very important point that Raghu has touched briefly. Raghu said:

the oscillation mode due to the coupling between the rotor and body is the most likely culprit of PIO in almost all the PIO incidence

I always thought it was THE LACK of coupling, or feedback, between rotor and body that was the root cause of all PIOs. A pilot needs feedback for control inputs. In a gyroplane, feedback is achieved in a few ways. First, when we move the cyclic, the rotor disc moves in plane, and the rotor thrust vector (RTV) changes its location about the CG, initiating a change in airframe attitude. This feedback is slow, probably in the range of 3-7 seconds. The reason it is slow is that when the RTV is moving a few inches about the CG, the moment (force x arm) is small, so the airframe accelerates slowly to a new steady state location. You can actually see this form of feedback very clearly if you watch a stab-less RAF-2000 flying; the airframe appear to be "swinging" under the rotor.

The second form of control feedback is G-force. G-force is quicker, almost immediate. But G-force is not a very helpful feedback as long as the G-forces remain small. G-force feedback may be very helpful to combat pilots, but not to gyro pilots (unless you are Birdy, I guess).

The third form of feedback is aerodynamic feedback. Aerodynamic feedback is achieved with the tail feathers, allowing the airframe to align itself with the relative wind. Depending on the size and location of the tail feathers, aerodynamic feedback may be almost instantaneous. When the pilot is making a control input, the rotor reacts almost instantly, changing the direction of flight. The tail feathers make the airframe pitch right away towards the new flight direction, and the pilot gets a quick feedback to his control input.

Now, does this anti-PIO feedback mechanism relate directly to stability?

In other words – can a gyroplane be made statically stable (i.e. pass Greg’s tests), without having a horizontal stabilizer? If so, having a properly sized stab should be a specification for LSA gyroplanes, regardless of stability criteria. The stab IS the anti PIO mechanism.

Udi-

[gyrogreg]

My thoughts on coupling between rotor and airframe: For a stabless gyro, you are right that this coupling can be the root of PIO. The airframe reactions may be slow with overshoot. So, for this gyro, the coupling is an important reason for over-control and PIO – it takes skill to anticipate and time the correct control response reaction.

But, as you point out, with good tail feathers, the airframe response is almost immediate, and if the airframe is properly statically stabilized (reacting to G-load, airspeed and AOA of relative wind), the coupling actually improves stability performance because it immediately provides a cyclic action to the rotor to counter the disturbance. In this case, it is very beneficial for the cyclic stick to have friction or be held tightly by the pilot – so the cyclic action of the airframe pitch reaction immediately causes a corrective pitch change of the rotor disk (AOA). Do this right, and there is no control reaction either required by or excited in the pilot.

Requiring a HS anyway, if the static criteria can be met by other means (such as a stabilator!??): I think that it is highly doubtful that any other passive means to stabilize a gyro could be contrived. The main requirement is that the airframe must track the path provided by the rotor. This then essentially means you have a fixed-wing aircraft. I see it very difficult for this to be accomplished by any other means! (How about a long massive pole such as on a bottle rocket! Maybe, but there are a lot of penalties with that!) That is one reason I am promoting all three static tests – Essentially assures FW response! I don’t think it can be done, short of a highly sophisticated auto pilot - and even an aouto pilot stabilizer would result in confusing pitch and G-Load responses that a pilot would ahve to get used to! But, I do hope some people try, this would be extremely interesting to analyze. And, we do not know for sure whether such a scheme would actually have any dynamic stability issues different from the HS scheme. We fought hard to eliminate the “prescriptive” requirement for a HS, and I would not want to revert that without very good arguments.

Thanks, Greg Gremminger

[darrellwittke]

Thought I would add a note just to make my viewpoint understood. Regarding your last sentence "We fought hard to eliminate the prescriptive requirement for a horizontal stabilizer, and I would not want to revert that without very good arguments."

As I understand your post, you do not want to have tightly written rules which preclude innovation and invention, of which I completely concur.

However, as you know, horizontal stabilizers are the prescription required for perhaps 98% of light gyroplanes. My viewpoint is that this should be made plain (as a suggestion or acceptable example) to any casual reader of the standards.

My reasoning is that we do not know where the next gyro manufacturer (ala RAF) may come from, and although talented perhaps in machining and manufacture, do not have or desire to have a deep technical insight to infer the "prescriptive" solution.

Thank you for your hard work, time, effort, money and understanding of this issue! Sincerely Darrell Wittke

[PW_Plack]

Greg,

I'm as passionate as anyone about avoiding being overly prescriptive, but if the LSA consensus gyroplane rules require a horizontal stab, IMHO innovation will not suffer. We're talking about machines for the masses, which must be somewhat proven in their designs. Better to get LSA gyros out and available ASAP, so pilots who've already lost years of enjoyment due to restrictive medical issues can get back in the air in something safe.

People who want to experiment will still be building experimentals. If one of them someday discovers a worthy substitute for an effective horizontal stab, the consensus rules can be amended, right?

[Aussie_Paul]

When the term "Horizontal stab" is used it should and must be "an effective Horizontal stab".

When I was president of ASRA, the Aus, Sports Rotorcraft Assn, who oversees the operations on gyroplanes here in Oz on behalf of CASA our eqqivilent of your FAA, in 1990/1, and with the help of McEagle Tim, we implemented mandatory h/stabs.

Later on when one of our most narrow minded gyro pilots became president he had the h/stab requirement removed!!!!!!!!!!

At least Tim and I were heading in the right direction, but at that time we did not really understand that putting a small flat plate on the end of a short rear keel met the requirements, BUT did not neccessarily work. That is why I never refer to the h/stab unless I use the "effective Horizontal stab" terminology.

I have investigated a number of PPO accidents where there was a h/stab fitted, but would have been as useless as "tits on a bull" (Birdy you will understand that!!!) on the 10" and up to 16" thrust offsets machines involved.

Just my few demented thoughts after arriving home from NZ.

Aussie Paul.

[Dean_Dolph]

Paul, I think that the thinking is that the stability tests will show that the 'effective horizontal stab' as Mr. Bruty describes it, will be a necessity. So, those that want the horizontal stab are probably going to get it but at the same time the standard leaves room for innovation. There isn't any legitimate reason to require an effective horizontal stabilizer when the standards define what stability requirements have to be met. If the machine can't meet those requirements it cannot be sold as a LSA gyro without consequences. Remember, a horizontal stab doesn't guarantee that a gyro will be stable and safe.

The stability standards are the key to a safe/stable gyro regardless of configuration. If someone is innovative enough to design a gyro that doesn't require a horizontal stabilizer then why penalize them?

If everyone understands that a LSA gyro means a gyro designed to required standards then that is all that should be required.

[fara]

Forum Team, we have had a lot of very technical input on this forum. I still do not see consensus on the dynamic criteria – to minimize or avoid possible PIO risks, what are the REAL WORLD criteria that might be necessary to determine if there are PIO risks?

We have plenty of deeply technical chum to chew on. Bit, I would like to ask each of you now to try to put this all into practical sense – that can be tested, verified, and understood readily and inexpensively. I believe that is possible. And the success of this standard in improving gyroplane safety is not only that it be technically supportable, but that it be understood, accepted and applied by the gyroplane community.

It appears that the technical aspects are starting to converge to consensus. Could each of you, in consideration of all of this material, make some concrete suggestions as to criteria and testing methods that would be simple and understandable to the masses, and supportable by the our best technological understandings at this time.

My basic question is, beyond the static criteria, what DYNAMIC criteria and measurements might be necessary to minimize the risk of PIO. I am not sure we need to do much more than the static criteria already assures!

Thanks, Greg Gremminger.


Hi Greg,
Part 27 may be a good start and it only requires Static Longitudinal stability tests and a demand for control-ability in all other modes by the pilot if VFR operations are intended
http://www.flightsimaviation.com/data/FARS/part_27.html
http://www.flightsimaviation.com/data/FARS/part_27-173.html
http://www.flightsimaviation.com/data/FARS/part_27-175.html

However, if the rotorcraft is to be IFR which almost every type certificated helicopter does offer that option I think, they have to then also show Dynamic Longitudinal stability through testing.
http://www.flightsimaviation.com/data/FARS/part_27-appB.html

In the case of LSA which can be flown by lower experience Sport Pilots, I would be of the opinion that it be treated in a similar manner as Part 27 type certificated aircraft are when they are allowed to fly IFR ... meaning more stability is required. In this case, it is due to the lower hour pilot who can start flying these S-LSA and corresponding E-LSA aircraft and not because of IFR option.
Perhaps not as demanding and specific as in Part 27 Appendix B for oscillation amplitude dampening requirements but still requiring a dampening oscillation with progression (weak positive dynamic stability - ok) with and without power.

That will include pretty much most gyroplanes except those few that have an extremely short moment arm and surface area and rely tremendously on the propeller blast (engine thrust) to become effective (not a great way to solve the issue).

As to someone asking if HS WILL be a requirement.
Absolutely not. ASTM standards are never prescriptive. HS is one way of solving the problem cheaply, easily and with relatively small penalty at speeds that LSA travel at during normal operations.
If some manufacturer wanted to they could develop a whole very expensive SAS and knock themselves out. ASTM standards only specify what is expected in behavior not how to achieve it. They do specify construction norms that should be followed etc.

[gyrogreg]

Hi Abid,

This is a rather old thread, one our ASTM subcommittee used to develop the original gyroplane standard. As you know, this gyroplane D&P standard was approved by both the ASTM LSA committee and the FAA back in 2005. Since then the subcommittee has made some adjustments - from things we learned since, and from developments with the UK Section T standard.

(FYI: Originally, the ASTM standard used the older Section T as a start, and made efforts to simplify criteria verification and limit some of the requirements the UK CAA put on Section T. Section T actually started out as a modification of their Section S - FW - standard. As a result they carried over numbers of FW requirements that were not appropriate, in our opinion for gyroplanes.)

We referenced both the existing FAA Part 23 (airplane) and Part 27 (helicopter) in development of the ASTM LSA Gyroplane standard as well. But, as you know, neither Part covers some of the critical needs of gyroplanes - especially flight stability. Due to the not-fixed rotor, some areas such as short-period natural oscillation rates and damping factors are critical for gyroplanes.

Yes, the subcommittee did not include prescriptive requirements for flight stability - such as HS, tail length or size, prop thrustline, Drag lines, painted color, etc. :) It's up to the designer and producer to verify that it performs with results required in the standard - not how it is configured or how those results are achieved.)

Since there were no legal requirements to use the gyroplane standard – FAA does not allow SLSA or ELSA gyroplanes - this standard has not been able to evolve with use and ID of issues as the other ASTM LSA standards have. As a result, I am not anxious to pursue a lot of theoretical and anticipatory changes until we can actually put it into use. I am monitoring the developments in the UK Section T, and there are probably a few areas that could use some refinement from what the British have discovered with their certifications under Section T.

The ASTM standard is copyrighted by the ASTM - have to join or buy the standard. But, the ASTM has allowed me to publish some of the standard in the efforts to educate the community and improve safety. If you are a member, you can download the whole standard. If not able, my next post here will be some of the critical criteria on gyroplane flight stability in the standard.

- Greg

[gyrogreg]

The following exerpts from Gyroplane ASTM Standard F2352-11 is printed here with the permission of the ASTM. The full standard is copyrighted and may be purchased from the ASTM or is available to ASTM members (ASTM (http://www.astm.org))

4.5.1.2 The gyroplane must be able to be flown without
undue piloting skill, alertness, or strength in any normal
maneuver for a period of time as long as that expected in
normal operation.

4.5.1.3 Each requirement of this section must be met for the
most adverse combinations of engine power and airspeed
within which the gyroplane will be operated. Unless otherwise
specified, all requirements of this section shall be met at engine
power settings ranging from idle power to maximum allowed
engine power. Unless otherwise specified, all requirements of
this section shall be met at airspeeds ranging from MPRS to
VNE.

4.5.2.1 A power change from trimmed MPRS level flight at
MPRS power must result in a steady state trimmed airspeed not
to differ by more than 25 % from the initial trimmed MPRS
airspeed for the following conditions:
(1) In level flight, MPRS power increased to full power.
(2) In level flight, MPRS power reduced to engine off.
(3) Conducted with a cyclic stick fixed in pitch at the initial
MPRS stick position.
(4) Conducted with a the cyclic stick free in pitch at the
initial MPRS pitch trim.
4.5.2.2 Without trim adjustment, the cyclic pitch control
range must be adequate to reduce airspeed from trimmed VNE
to VMIN airspeed without excessive forces on the cyclic control
system at the following conditions:
(1) From VNE to VMIN with engine power off.
(2) From VNE to VMIN with engine at full power.

4.5.2.3 A rapid power change from trimmed MPRS level
flight at MPRS power must result in an airframe pitch attitude
rate of change not to exceed 5° per second for the following
conditions.
(1) MPRS power rapidly increased to full power.
(2) MPRS power rapidly reduced to idle power.
(3) Conducted with a cyclic stick fixed in pitch at the initial
MPRS stick position.
(4) Conducted with a the cyclic stick free in pitch at the
initial MPRS pitch trim.

4.5.3 Static Longitudinal Airspeed Stability:
4.5.3.1 The longitudinal control must be such that: (1) with
constant engine power, an aft force and movement of the cyclic
control is necessary to achieve an airspeed less than any
available trim airspeed; and (2) with constant engine power, a
forward force and movement of the control is necessary to
achieve an airspeed greater than any available trim airspeed.
The control force slope must not reverse during any progressive
application of control movement at airspeeds greater than
VMIN up to VNE. Static longitudinal airspeed stability must be
met at the following power and trimmed airspeed conditions:
(1) Steady altitude at MPRS,
(2) Full power at VNE,
(3) Full power at VMIN,
(4) Engine idle at MPRS,
(5) Engine idle at 80 % VNE, and
(6) Engine idle at VMIN.

4.5.3.2 The longitudinal control must be such that, with
constant engine power and with airspeed temporarily increased
at least 20 % above trimmed airspeed, upon release of the
cyclic pitch control the airspeed shall not diverge and shall
return to within 10 % of the following initially trimmed
airspeed condition with the cyclic pitch control free. Initial and
return trimmed conditions:
(1) Steady altitude at MPRS,
(2) Full power at 80 % VNE,
(3) Engine idle at MPRS, and
(4) Engine idle at 80 % VNE.

4.5.3.3 The longitudinal control must be such that: (1) with
constant engine power and with airspeed temporarily increased
at least 20 % above trimmed airspeed, upon return to the
following fixed stick conditions the airspeed shall return to
within 10 % of the initial fixed stick steady state airspeed; and
(2) with constant engine power and with airspeed temporarily
decreased at least 20 % below trimmed airspeed, upon return to
the following fixed stick conditions the airspeed shall return to
within 10 % of the initial fixed stick steady state airspeed.
Initial and return fixed stick conditions:
(1) Steady altitude at MPRS,
(2) Full power at 80 % VNE,
(3) Engine idle at MPRS, and
(4) Engine idle at 80 % VNE.

4.5.4 Static Longitudinal Maneuvering (G-Load) Stability:
4.5.4.1 The pitch control forces during turns or load factor
maneuvers greater than 1.0 g must be such that an increase in
load factor is associated with an increase in aft pilot control
force, and a decrease in load factor is associated with a
decrease in aft pilot control force for the following initial
trimmed conditions:
(1) Steady altitude at MPRS,
(2) Full power at the lesser of VH or VNE,
(3) Engine idle at MPRS, and
(4) Engine idle at 80 % VNE.

4.5.4.2 The airspeed during turns or load factor maneuvers
greater than 1.0g at a fixed cyclic pitch position must be such
that an increase in load factor is associated with an increase in
airspeed, and a decrease in load factor is associated with a
decrease in airspeed for the following initial fixed stick
conditions:
(1) Steady altitude at MPRS,
(2) Full power at the lesser of VH or of VNE,
(3) Engine idle at MPRS, and
(4) Engine idle at 80 % VNE.

4.5.5 Static Spiral Divergence:
4.5.5.1 For banked turns up to 1.5 g or 30° of bank with the
stick fixed, there must be no tendency for the gyroplane to
increase the turn rate rapidly at all allowable power settings for
the following conditions:
(1) Level 30° banking turn at straight and level MPRS
airspeed,
(2) 30° banking turn at full engine power, and
(3) Descending 30° turn at MPRS at engine idle.

4.5.7 Dynamic Longitudinal Stability:
4.5.7.1 The gyroplane under moderately turbulent air conditions
must exhibit no dangerous or divergent behavior with
cyclic pitch control fixed or with cyclic pitch control free for
the following conditions:
(1) Steady altitude at MPRS,
(2) Full power at VNE,
(3) Engine idle at MPRS,
(4) Engine idle at 80 % VNE, and
(5) Engine idle at VMIN.

4.5.7.2 Longitudinal Oscillation Damping:
(1) Any excitable longitudinal oscillations with periods less
than 5 s must damp to one half amplitude in not more than one
cycle with cyclic pitch control fixed or with cyclic pitch control
free. There should be no tendency for undamped small amplitude
oscillations to persist for more than 2 cycles with cyclic
pitch control fixed or with cyclic pitch control free.
(2) Any excitable longitudinal oscillations with periods
between 5 and 10 s should damp to one half amplitude in not
more than two cycles. There should be no tendency for
detectable undamped small oscillations to persist for longer
than 20 s.
(3) Any excitable longitudinal oscillations with periods
between 10 and 20 s should be damped, and in no circumstances
should a longitudinal oscillation having a period longer
than 20 s achieve more than double amplitude in less than 20 s.
Conditions:
(a) Steady altitude at MPRS,
(b) Full power at VNE,
(c) Engine idle at MPRS,
(d) Engine idle at 80 % VNE, and
(e) Engine idle at VMIN.