It is known how stabilizers affect the longitudinal stability of an airplane.

What about longitudinal stability in airplanes using stabilators (all-flying tails) or trimmable stabilizers?

Is it to be expected that certain combinations of angles of attack of the main wing and stabilators or trimmable stabilizers should lead to longitudinal instabilities?

  • $\begingroup$ Stabilizers, in this case, means the whole thing including the control surface, whatever that control surface is. $\endgroup$ – Jan Hudec Apr 27 '16 at 8:13

There is no fundamental difference between them in such context. If you don't go to the extremes like 90° deflection (and generally stay in the linear range), you can mentally replace an elevator with a stabilator at a different equivalent angle. All-moving surfaces are typically a design solution when additional efficiency is required for whatever reason.

There are finer effects, especially at supersonic, but they don't change the overall picture for 'normal' aircraft. You can always find border cases when, say, elevator will stall but an equivalent stabilator will not yet, but they will generally be beyond the operational envelope.

Stability is primarily determined by derivatives, i.e. the way moments change with changing angles (of attack in this case). Absolute angles (tail vs wing) are less important and are usually the consequence of the desired derivatives.

  • $\begingroup$ I actually thought about static stability. I can imagine some situations when an angle of attack combination of main wing and stabilator (the all-moving part used to control the airplane) could lead to a static longitudinal instability. Static longitudinal stability is also determined by the angle of attack difference of main wing and stabilizer (the non-moving part used for static longitudinal stability). I expect this has to be considered when using stabilators. I agree with you that a stabilator could be approximated by a stabilizer in order to estimate it. $\endgroup$ – a1337q May 23 '16 at 15:06
  • $\begingroup$ What I mean by this is the following: Not the whole deflection range of a stabilator is allowed in the whole angle of attack range of the main wing (and following from this also e.g. the whole airspeed range when in level flight). Otherwise this would lead to loss of static longitudinal instability. $\endgroup$ – a1337q May 23 '16 at 15:08

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