Gyroplane longstab

[Genghis the Engineer]

Just interested if anybody else has a view on this.

The new draft of BCAR Section T (the UK safety requirements for Gyroplanes up to 650kg) crossed my desk the other day. Most of the changes are pretty uncontentious - one might even say sensible, but one worried me a bit.

There is a re-write of the apparent longitudinal static stability requirements. Basically what it now says is that the speed/control slope has got to be continuously positive or zero - and that the most critical factor is stick displacement, which is considered much more important (and more measurable) that stick force.

I fully accept that I'm not a gyroplane expert, but like to feel that I've a nodding acquaintance with handling qualities in general. This requirement just doesn't make sense to me - firstly it would accept zero force / zero displacement per airspeed change in pitch - all my FW experience tells me that's unnecessarily putting pilot workload right up. Secondly, all my flying experience - in anything - tells me that pilots sense control force first, and position a low second (just look at the F16 for example). So why the reliance upon control displacement in preference to force?

Or is my lack of practical experience of gyroplanes causing me to miss something here?

G

[Shawn Coyle]

Genghis:
Great question. Made me think for a minute or two...
Lots of helicopters have neutral longitudinal static stability in autorotation, and yet are reasonably easy to fly, and will even maintain airspeed once established. So this isn't a new issue as far as handling goes.
I understand the FARs are going to change to allow neutral statics in more cases than they do now, but not exactly sure what is going to change.
The reason the position is allowed over force is probably because the displacements are probably pretty large, and would be noticeable.
I agree that our hands are relatively poor displacement sensors for small displacements, but these are probably large displacements. Forces may be quite small if the rotor head is designed properly (to give minimal feedback forces)

[Genghis the Engineer]

Is that a safe assumption - that the displacements will be large enough to give adequate feedback? The actual wording is:-

(definition of conditions) the slope of the curves:-

(i) control force versus speed, and
(ii) control position versus speed, must be zero or stable.

(The interpretative material then says that displacement must be demonstrated quantitatively, but force may be qualitative).


Interesting also to note that JAR-27 (I assume that FAR is the same) doesn't actually express any preference for force or displacement except in the hover, where displacement seems to be the player.

G

[Fergus Kavanagh]

Most gyro pitch stability concerns centre around
prevention of PIO or PPO(power push-over).
and a report on the AAIB site on G-CBAG might
put it in perspective.

Hope this helps

Fergus Kavanagh.

['India-Mike]

Genghis

Shawn has largely hit the spot I think. I recommended many years ago (1997 I think) that Section T should consider position as well as force.

My principal reason was that I wanted to get speed stability established (Mu to those of us that know what it is). With good Mu the aircraft is partly en route to having good phugoid characteristics ( the mode that is the one that causes PIO problems in gyros, worse than in helicopters because it couples into rotorspeed as well).

Stick shake on these machines can be horrendous (Glasgow University's Montgomerie is a pretty good example of this, but the VPM M16 that was tested in 1996 by me was awful in this regard as well). Couple this with a generally poor trim system (if one exists at all), pilots that aren't tp's, operating without the benefit of an FTE, light stick forces (nothing like fixed-wing aircraft), then on balance I thought that stick position should have been given some consideration as well.

How CAA implemented this recommendation was up to them. I've been under contract to them on gyroplane research since 1993, but I haven't been sent a copy on the lastest Section T that others (including yourself) got for comment......ho hum!!!

Anyway, now that I've got a Chipmunk I'm hardly ever around to do gyro research anyway! That offer's still open Genghis!

Regards

[NickLappos]

Gen,
It has been my experience for many years that neither position nor force gradient are particularly important for speed stability, since most helos are so dynamically unstable that the pilot is constantly correcting cyclic to keep the aircraft level. The most critical term for speed stability is the Theta fuselage vs speed slope, that is the nose down gradient with speed. The only helos that give me trim troubles are those where the nose is flat or negative with speed.

Often this attitude/speed gradient follows the stick gradient, but usually because the attitude is the dominant factor, and the stick follows along.

The horizontal tail is the bad actor, where rotor dowwash playing on and off it will make the nose pop up with a slight speed increase, bewildering the pilot and the static stability.

I have been a champion of flat stick gradients, as long as the pilot has the cue environment to trim with ease. The new draft 29.175 FAR/JAR words were pounded out in a subcommittee meeting with some of us reaching consensus that these other cues (nose gradient) might suffice.

[Genghis the Engineer]

I-M, nice to know that CAA's requirements department is on the ball as ever, much like when they sent the only copy of a revision to Section S to SBAC, who unsurprisingly were rather uncertain as to what to do with it. Anyhow, I'll drop a copy in the post to you when I'm next in the same place as it and a photocopier, probably about Tuesday. It's past the official consultation date, but I think given it's largely your research it's apparently based upon, you've every right to shout at them at any time.

Nick's point is rather interesting, because I wonder where we treat the Gyro in this respect. Accepting that aerodynamically it's a helicopter in autorotation (or at-least, a close relative), there are people who would maintain that it's closer in terms of HQ to a fixed wing (I'm not necessary saying that I'm one of those, but I know who they are). In a FW aircraft a continuous gradient of Fs.v.CAS is considered a non-negotiable absolute, whilst displacement.v.CAS is only in the "nice to have" category. But, it begs the question - how stable is a gyro, typically, in flight compared (for example) to either a helicopter in autorotation, or to a light aircraft in forward flight. I suppose what I'm wondering is what level of speed or flightpath stability can, or should, reasonably be demanded?

G

Still inconvinced that stick position is a better indicator of anything than force where speed stability is concerned.

[Shawn Coyle]

You might want to check with the folks at Groen Bros, who are the only ones I know of who are still working on gyrocopters in the pure sense. They have a lot of experience, but don't necessarily speak engineering flight test.

[Genghis the Engineer]

For those with a continuing interest in this subject, may I strongly recommend taking the time to read this fatal accident report (http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_030918.hcsp) which has just come out. A remarkably thorough and informative piece of work, even by the high standards of AAIB.

The comments regarding stick geometry, versus LSS and LDS, and how they affect PIO tendency I thought were particularly useful and bear deep thought on all aircraft classes.


G

[Fergus Kavanagh]

It is quite distressing that this type of accident still occurs.

Most current designs either feature Centre-line thrust or have
generous horizontal stabilisers fitted.

Either of these options,(or both) seem to be very effective
in avoidance of PIO/PPO.

I fitted such a stab to my machine recently and was surprised
at how much pitch stability improved.