Gyro stability Test Criteria and Methods - Introduction


Platinum Member
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...



Senior Member
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

Senior Member
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 Jackson

Senior Member
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.




Senior Member
Stick force per G

Stick force per G

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.
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Senior Member


gyrogreg said:
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.
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Senior Member
Getting closer to consensus?

Getting closer to consensus?


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


Platinum Member


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.



Senior Member
Rotor / Airframe coupling

Rotor / Airframe coupling

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


Senior Member
Mr. Gremminger,

Mr. Gremminger,

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


Active Member

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?


A reforming stirrer!!!!!
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.
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.


AR-1 gyro manufacturer
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

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.

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.
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Senior Member
approved ASTM standard

approved ASTM standard

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


Senior Member
Flight Stability Criteria - Standard

Flight Stability Criteria - Standard

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) 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. 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. 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. 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. 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
(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: 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. 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. 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: 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. 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
(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: 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
(2) 30° banking turn at full engine power, and
(3) Descending 30° turn at MPRS at engine idle.

4.5.7 Dynamic Longitudinal Stability: 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. 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.
(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.
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