Gyro stability Test Criteria and Methods - Introduction

[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 . 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]

Another wrench for the machine.

Another wrench for the machine.

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]

Speed Limits

Speed Limits

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]

Vne testing

Vne testing

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 Standards/Gyro D&P V3.1.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 Jackson comments

Mike Jackson comments

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]

Mike Jackson's comments

Mike Jackson's comments

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]

My comments to Mike – Greg

My comments to Mike – Greg

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]

Still looking for consensus on DYNAMIC criteria:

Still looking for consensus on DYNAMIC criteria:

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=-