Pitch & Speed Trim

[paulo]

I decided to demo to a friend the principal of trimmed to pitch and speed. For the demo, I set up a stall, 'parachuted' for a bit to show some stability (in the particular type), then demo'd full hands off behaviour. All went as expected - it found nose down, gained airspeed then lift, and did the expected climb. (We are both about the same weight, so not much roll input, side-by-side seating)

Are there any types where, apart from when forcing the stall, this can go pear shaped? I can't think of anything, apart from spinning (which I practice anyway)... but haven't really heard of anyone demo'ing this behaviour?

I'm not a test jock, but thought this would be the best place to ask.

[Genghis the Engineer]

What you are describing is the characteristics that, in flight testing, we'd expect to see in any properly sorted aeroplane.

Most of us have probably seen an aircraft at some point which did not show that characteristic. The reason is likely to be one of:-

- A low speed reversal of pitch stability, causing the aircraft to continue to decelerate (sometimes called a "deep stall").

- Similar to the above, a low speed neutral pitch stability, causing the aircraft to stay at the stalled attitude. This is only likely to be nasty close to the ground or in turbulent air where the aircraft may hit something or spin before the pilot realises what is happening and recovers.

Both of these characteristics should be regarded as unacceptable and I'd not expect you to see either in anything but an unsorted prototype, or perhaps one or two homebuilts (which occasionally have the potential to fall into the same category).



Alternatively however there is another characteristic which could give you a problem, and might well be considered acceptable in a certified light aircraft. If the aircraft has very high apparent pitch stability (which would be indicated by very high control forces to stall the aircraft), combined with a moderately powerful elevator / poorish pitch damping there is a reasonable risk that releasing the control at the point of stall could put you into some combination of moderate to high transient negative g, and/or exceed a pitch attitude limit - which in something with a very narrow speed range could potentially cause a subsequent overspeed. This is pretty close to what in some branches of aviation is known as a whip stall (and usually prohibited).

So, for this reason, I'd not try this manoeuvre in some (perfectly safe) types. I've not flown one for a few years, but from memory you might regret it in a Slingsby Firefly trimmed fast, and am quite certain that you would regret it in many microlights and hang-gliders. I think that with the high pitch forces but moderately crisp pitch control of a PA28 it might rattle the teeth a little, particularly if trimmed fairly fast.


So, whilst I think that you'd probably get away with this demonstration in most certified light aircraft, I'd not try it too abruptly or with a fast trim speed - doing it by gently backing off on the nose-up control force with the aircraft trimmed to about 1.3Vs should give the demonstration you want without endangering anything.

G

[paulo]

I did spend some time pondering the more obvious stuff (e.g. trend towards vne) on the pitch down, but you've given me a few more things to consider. Currently I always fly trim neutral (rel to: level upright), unless climbing out, so that will have been my entry trim.

Whip stall wasn't an issue on this type - a momentary and firm nose down, but the start point was Vs obviously (55kt) and pitched up nicely at around 100kt IIRC. I felt like carrying on to prove the reducing oscillations, but we were running down on height so it felt like a good point to conclude.

Interesting to hear how things might behave. Or misbehave!

Cheers sir.