(I was trying to understand the longitudinal stability of aircraft and how it is related to elevator)

If I pull the stick back by, say 2 inches, will it have the same affect as trimming the horizontal stabilizer to make the aircraft fly at that angle of attack?

I mean, does setting the elevator at constant 2 degree will make the aircraft swing its nose up as long as the stick is held or does it trim itself to the new angle of attack and ride steadily in its new position?

  • $\begingroup$ Are you asking if pulling the stick back makes the nose continuously rise until returned to the neutral position? $\endgroup$ – Ron Beyer May 9 '16 at 18:34
  • $\begingroup$ Yes. Or does the longitudinal stability comes into play and levels the aircraft at, may be, reduced speed? $\endgroup$ – user2927392 May 9 '16 at 18:51
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    $\begingroup$ Somebody like Peter can answer this pretty succinctly but yes, the aircraft will slow down and fly level at a higher angle of attack, lower speed, and higher drag. If you want to initiate a sustained climb, you'd have to increase the power as well. $\endgroup$ – Ron Beyer May 9 '16 at 19:10
  • $\begingroup$ I expect the trim tabs to be adding some drag (no free lunch), but it would probably be small or negligible. Holding the stick back manually should be very slightly more efficient. If you are hydraulically holding the stick back, then any drag savings goes into hydraulic pump load. $\endgroup$ – doug65536 May 12 '16 at 0:59

Yes, both are equal for small angle changes. Exceptions do apply, especially in transsonic flow.

Both changing the stabilizer incidence and the elevator deflection will change the lift distribution between wing and tail surfaces and will trim the aircraft for a different angle of attack. Changing the incidence gives the empennage a new angle of attack while changing the deflection will change the camber (and with it the zero-lift angle of attack) and the angle of attack of the empennage. For small deflections the effect is the same; however, the change in stabilizer incidence is smaller than the elevator deflection change for the same effect in proportion to the square root of the relative elevator chord.

Say you change the stabilizer nose-down rsp. the elevator trailing edge up. The tail will produce less lift / more downforce and the aircraft pitches up. Now the angle of attack of both the wing and the tail will increase which increases lift on both until the distribution of lift between both is as before - after all, we did not change the position of the center of gravity, and the angle of attack will change until the center of pressure is exactly below or above the center of gravity again. There the aircraft finds a new equilibrium speed and will fly on in its new attitude.

Nevertheless, depending on the initial trim point, the aircraft will now climb or sink because the thrust does not match the new drag at the new trim point. The aircraft will settle at a new vertical speed, too.

Before you think that one of them would be enough: Both are needed because each is a specialist for tasks that the other would handle poorly:

  • The elevator moves quickly and needs small actuation forces, so it is built for manoeuvring or gust response.
  • The stabilizer moves slowly but can effect much higher lift changes. It is meant for retrimming during or after changes in configuration. Deflection of the lift devices on the wing will shift the center of pressure backwards, so in order to balance all lift forces, the tail now needs to produce massive downforce, something only the adjustable stabilizer is capable of.

This answer covers these aspects in more detail.

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  • $\begingroup$ Seems like the elevator deflection would have a slightly higher drag than the stabilizer trim because of the camber. Or would it be the same since both ways would produce the same amount of lift/downforce? $\endgroup$ – TomMcW May 9 '16 at 20:25
  • $\begingroup$ @TomMcW: Not at small deflections - camber helps to create lift without shifting the stagnation point too far away from the center of the airfoil nose. Actually, an elevator is more effective than an all-flying tail and allows to use a smaller tail surface for the same control power. $\endgroup$ – Peter Kämpf May 9 '16 at 20:55
  • $\begingroup$ @PeterKämpf, so why do all (subsonic) jets use trimmable stabilizer? Due to the larger trim range required for the larger speed difference and large moment caused by flaps? $\endgroup$ – Jan Hudec May 9 '16 at 21:41
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    $\begingroup$ @PeterKämpf, So I understand that with elevator up, aircraft does not just keep climbing. But rather, it finds a new equilibrium and stays level at that? (which is pretty much same as trimming horizontal stabilizer to higher angle of attack) $\endgroup$ – user2927392 May 10 '16 at 11:51
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    $\begingroup$ @JanHudec: The trimmable stabilizer is needed for the strong pitching moment changes when the high lift devices are deflected. They add a lot of lift aft of the neutral point of the clean configuration, which causes a strong nose-down moment. The trim range of the stabilizer is much larger in the negative range than in the positive range for exactly that reason. A second reason is a higher permissible c.g. range, but the main requirement is the flap moment. $\endgroup$ – Peter Kämpf Jul 28 '16 at 18:41

Yes. See Chuck Yeager re man’s discovery that trim tabs don’t work once you hit supersonic. What you describe saved his life and why Mach fighters today use stabilators to trim instead of trim tabs.

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