stick fixed or stick float?

[Redbaron]

just wondering what is the best way to measure stability? with the stick locked or just floating the stick relying on the gimble head/trim spring?

YouTube - bee

expert opinions welcomed, thanks!

[gyromike]

Stick-fixed.

[Chuck Roberg]

Fixed stick.

[Vance]

A small part of what Mr. Gremminger wrote “For instance, a gyro which passes the above STATIC tests is not necessarily assured to have moments of inertia that might not “resonate” or “feed” each other at certain disturbance rates so as to make the gyro very difficult to fly - PIO. The DYNAMIC stability of a particular gyro configuration also does not have easy “cookbook” answers. Assurance of DYNAMIC stability also requires flight testing. Unfortunately, although we know how to perform such dynamic stability flight tests, we are not sure what the “safe” criteria for DYNAMIC stability for a gyro should be – to be “safe”, does it need to be the same criteria as used for airplanes? Also, performance of such testing in a gyro which may have hidden dynamic stability issues may not be totally safe and must be approached cautiously. The airplane industry and military use professional Test Pilots for this job.”

I feel that some people are ignoring what I feel is an important caveat.

In the motorcycle world that I am familiar with people who imagine that handling is a specific “thing” often try to quantify that “thing” so they can announce that a particular motorcycle has good handling.

A company that I am very familiar with and whose engineering staff I am in touch with managed to produce several particularly dangerous motorcycle models based on this kind of testing.

As I write this they are manufacturing a motorcycle with quarter inch thick tubing in the frame as a testament to their lack of understanding of motorcycle handling dynamics.

In my opinion air speed stability is a very small part of a dynamic flight system.

It appears from the above quote that Mr. Gremminger agrees.

Fixed stick testing is interesting.

In my opinion it leaves a lot of questions unanswered.

It is probably a good start.

In my opinion, challenges that arise from a lack of airspeed stability are easy for a pilot to manage.

In my opinion how a flight system responds to a perturbation is much more important than airspeed stability.

I feel that frequency, dampening and overshoot after a perturbation have more to do with gyroplane stability than airspeed stability or any other kind of static stability.

In my opinion an aircraft in flight is a dynamic system that includes the pilot and should be treated as such.

Most motorcycles won’t fall over when they are on the side stand even if the engine is running.

Thank you, Vance

[Redbaron]

thanks chuck and vance! full power from zero airspeed equals zero pitching! I guess I got lucky and my machine is true clt! has anyone ever thought of putting on an adjustable seat that you could raise up and down for different pilot weights to adjust the vertical cg?

[birdy]

has anyone ever thought of putting on an adjustable seat
Solong as the peddles and stick follow, it could be dun.

[Doug Riley]

Vance: Regarding your post #4 -- yes, certainly both static and dynamic stabilities matter. Stabilities with respect to different parameters -- airspeed, angle of attack, G-load and power setting -- matter. Stabilities around different axes matter, although pitch is the most troublesome axis.

When we started on this industry-wide gyro-safety adventure, we first addressed the grossest and most deadly form of instability: PPO. Lots of gyros designed in the 80's went down, fatally, as a result of PPO. PPO is a matter of gross static instability with respect to rotor disk AOA.

Dynamic instability can't exist without static stability. Dynamic instability is, in fact, caused by the very same restoring forces that produce static stability. Given the deadliness of PPO, however, our first priority was (and should have been, and still should be) static stability. The critical need, specifically, is for static pitch stability with respect to rotor angle of attack and power setting. CLT and H-stabs are tools to achieve these stabilities.

Dynamic instability can rear its head if the gyro has static stability. Many of the Bensen crashes of yesteryear probably were cases of dynamic instability, since so many of them involved oscillations. If so, then those Bensens had some level of static stability,* but poor damping.

Fortunately, if a gyro is equipped with a reasonable H-stab and one of today's heavier, lower-RRPM rotors, it likely will have far better damping than the old Bensens. We know from the Bensen era that dynamic instability can be a killer. Still, partly as an unintended benefit of using H-stabs to augment static stability, we have moved away from designs that typically have dynamic-stability problems.

We should still test for it, though, just in case.
_______________________
*and you're right, this static stability was provided by the pilot-aircraft system, not solely by the aircraft. That's why the resulting dynamic instability could be labelled "pilot-induced." The PIO label gave the designer a free pass that he didn't deserve, though, IMHO.

[Vance]

Originally Posted by Doug Riley

Vance: Regarding your post #4 -- yes, certainly both static and dynamic stabilities matter. Stabilities with respect to different parameters -- airspeed, angle of attack, G-load and power setting -- matter. Stabilities around different axes matter, although pitch is the most troublesome axis.

When we started on this industry-wide gyro-safety adventure, we first addressed the grossest and most deadly form of instability: PPO. Lots of gyros designed in the 80's went down, fatally, as a result of PPO. PPO is a matter of gross static instability with respect to rotor disk AOA.

Dynamic instability can't exist without static stability. Dynamic instability is, in fact, caused by the very same restoring forces that produce static stability. Given the deadliness of PPO, however, our first priority was (and should have been, and still should be) static stability. The critical need, specifically, is for static pitch stability with respect to rotor angle of attack and power setting. CLT and H-stabs are tools to achieve these stabilities.

Dynamic instability can rear its head if the gyro has static stability. Many of the Bensen crashes of yesteryear probably were cases of dynamic instability, since so many of them involved oscillations. If so, then those Bensens had some level of static stability,* but poor damping.

Fortunately, if a gyro is equipped with a reasonable H-stab and one of today's heavier, lower-RRPM rotors, it likely will have far better damping than the old Bensens. We know from the Bensen era that dynamic instability can be a killer. Still, partly as an unintended benefit of using H-stabs to augment static stability, we have moved away from designs that typically have dynamic-stability problems.

We should still test for it, though, just in case.
_______________________
*and you're right, this static stability was provided by the pilot-aircraft system, not solely by the aircraft. That's why the resulting dynamic instability could be labelled "pilot-induced." The PIO label gave the designer a free pass that he didn't deserve, though, IMHO.

Thank you for clarifying that Doug.

Are you suggesting that if a gyroplane doesn’t maintain plus or minus 10 miles per hour with a fixed stick throughout its power range it is prone to a power push over?

Thank you, Vance

[Doug Riley]

Vance, for PPO prevention, I think that static pitch stability with respect to power setting and disk AOA are the keys. PPO isn't a result of runaway airspeed, it's a result of runaway low disk AOA. But...but...

Runaway airspeed tendecies should make us suspicious that something isn't right with the design. Rotor flapping-blowback normally makes the rotor trend toward zero airspeed, stick locked. If the aircraft does just the opposite in practice, the SOMETHING is overpowering the blowback effect. The "something" may be an incorrect rotorblade airfoil, a low center of frame drag without adequate HS, or HTL.

So, indirectly, airspeed instability could be evidence of PPO tendency. There are more direct ways to sniff out PPO tendencies, though. Just do a double hang test.

[Vance]

Thank you Doug,

I ask because a SparrowHawk that I flew in my opinion would not pass and an RAF with a Stabilator but no horizontal stabilizer would.

This is in conflict with my understanding of the tendency toward a power push over of these particular gyroplanes based on the location of the center of gravity and the lack of a horizontal stabilizer on the RAF.

Thank you, Vance

[Doug Riley]

Right, Vance. Airspeed stability/instability isn't a direct or reliable predictor of PPO tendency. PPO happens when rotor thrust is abruptly reduced, which may not happen in the course of an airspeed stability test.

Airspeed stability is a good thing for other reasons, though.

[Vance]

Originally Posted by Doug Riley

Right, Vance. Airspeed stability/instability isn't a direct or reliable predictor of PPO tendency. PPO happens when rotor thrust is abruptly reduced, which may not happen in the course of an airspeed stability test.

Airspeed stability is a good thing for other reasons, though.

Thank you Doug, that helps to rearrange some of my confusion.

What other reasons is fixed stick air speed stability a good thing for?

Thank you, Vance

[Redbaron]

thanks for sharing doug! I went for a flight this evening "a very short flight" in light winds. there was lots of tail wagging and momentary low g. Felt like flying my old ultralight. I landed immediatelly, call me a wuss but I hate this feeling.

imo with a low disk loading like on a bee you should have plenty of angle of attack stability "large tail waay back centered in the prop wash"

[Doug Riley]

Vance:

Fixed-stick airspeed stability is handy to reduce workload. It allows you to get your head out of the cockpit, do your VHF, GPS or other tasks, and not end up overspeeding or mushing while you are distracted. It's also more natural* to hold the stick firmly in turbulence and allow the airframe stability to hold airspeed for you.

If your gyro has only stick-free stability (the kind created by the offset gimbal head), you have to "float" the stick. That technique can be learned, but it's pretty counter-intuitive when you're tense -- as one is apt to be in turbulence.
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*natural, unless you first learned in an unstable gyro. We who did that actually have to re-learn some things when we switch to a stable machine.

[Vance]

Originally Posted by Doug Riley

Vance:

Fixed-stick airspeed stability is handy to reduce workload. It allows you to get your head out of the cockpit, do your VHF, GPS or other tasks, and not end up overspeeding or mushing while you are distracted. It's also more natural* to hold the stick firmly in turbulence and allow the airframe stability to hold airspeed for you.

If your gyro has only stick-free stability (the kind created by the offset gimbal head), you have to "float" the stick. That technique can be learned, but it's pretty counter-intuitive when you're tense -- as one is apt to be in turbulence.
___________________________
*natural, unless you first learned in an unstable gyro. We who did that actually have to re-learn some things when we switch to a stable machine.

Thank you Doug,

I pick an angle of attack for the fuselage for airspeed so I can have my head outside.

I find that my indicated airspeed generaly slows when I am performing complex radio or navigation tasks. I like the extra quiet.

When flying in turbulence my indicated airspeed seems to vary by about what the gusts are. Does this portend instability? I have seen the indicated airspeed drop or rise by more than 30kts. My GPS ground speed is much more steady.

With a fixed stick the Predator will maintain plus or minus ten knots through most of her power settings. On the Predator the stick shake wants to quarrel with the rotor control system with a fixed stick so it is not something I do much.

I did use fixed stick to measure the angle of bank created by a specific stick displacement. Without a stop on the stick it was difficult for me to be consistent.

I was taught to relax when flying in turbulence.

It seems natural to me; if I don’t know what to do to stop the turbulence related deviations from my intended path then it seems like I should get out of the way and let the aircraft find its way through the turbulence. I often fly in turbulent conditions in the hills around here so close to the ocean.

I have found my response is not fast enough to manage excursions caused by turbulence and they always seem short lived.

I am afraid I still don’t understand the importance people seem to place on airspeed stability as a measure of gyroplane stability. Gyroplane stability seems more complex than that to me.

Thank you for your help Doug; you always shine a light on my dark gyroplane ignorance.

Vance