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The trim tab is deflected downward, which deflects the elevator control surface upward, which then causes the airplane to pitch 'up'. (and vice versa)

But why does it work that way. Why doesn't the relative wind push the control surface back down again?

(edited once for clarity)

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  • $\begingroup$ I'm a little uncertain of this, but it could be because the control surface is "balanced". Part of it extends forward of the hinge. Also because the trim tab is at the back edge, where it has more leverage. $\endgroup$ – Mike Dunlavey May 15 '17 at 11:23
  • $\begingroup$ Would Aviation be a better home for this question? $\endgroup$ – Qmechanic May 16 '17 at 11:24
  • $\begingroup$ I believe I have the answer from a discussion with a CFI. The trim tab is located at the rear of the elevator and as such it has more leverage. A deflection of the trim tab up will create a force to drive the elevator down. These forces balance each other $\endgroup$ – Brian May 16 '17 at 17:09
  • $\begingroup$ Ignore the comment above..cant delete it.. time expired.. $\endgroup$ – Brian May 16 '17 at 17:16
  • $\begingroup$ Answer: The elevator will be pushed downward by an upwardly deflected trim tab. The small trim tab has more leverage and can therefore deflect the larger elevator control surface down a good bit before they up and down forces are balance. With the elevator down, the camber is increased and lift is therefore increased. Result is that the nose of the aircraft is pitch downward. THANK YOU everyone for your input. I really find this discussion useful $\endgroup$ – Brian May 16 '17 at 17:20
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Why doesn't the relative wind push the control surface back down again?

But it does, however, some additional lift at the trailing edge from the downward-deflected trim tab remains and prevents the full return to the earlier position. What the trim tab does to the elevator, the elevator in turn does to the whole aircraft: Changing its pitch attitude.

See it this way: Before, the elevator was flying at its force-free angle. Then the added trim tab deflection changed the local pressure distribution on the elevator (in the case of a downward deflection it adds lift), so the elevator assumes a new equilibrium position. This new position is found when the lift change on the elevator just compensates for the newly added lift by the tab. Of course it must go trailing-edge up now.

Trim tabs

Trim tabs (picture source)

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    $\begingroup$ @Brian: The comments are not meant for thank-you notes. Better upvote good answers and accept the one which helped most. $\endgroup$ – Peter Kämpf May 16 '17 at 18:25
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See this article on the hows and whys of elevator trim tabs. They don't perform as you suggest. They are used this way... The pilot uses the stick to move the elevator to adjust the pitch of the aircraft. This requires a constant force by the pilot on the stick to maintain that attitude. An elevator trim tab is adjusted to aerodynamically apply the holding force required so the pilot doesn't have to.

http://www.airspacemag.com/flight-today/how-things-work-trim-tabs-129376547/

Note something in this article... it states that the pilot pulls back on the stick to pitch the nose up to make the plane climb. This is incorrect. Pitching the nose up slows the airspeed of the plane, pitching it down speeds up the air speed. The throttle is used to make the plane clime or descend. More power makes the plane climb, less power makes it descend.

At least that's how I was taught on a single engine prop plane. http://www.fearlessflighttest.com/pitchandpower.html

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  • $\begingroup$ Surely you've got the roles of pitch and throttle mixed up. $\endgroup$ – DanielSank May 16 '17 at 7:57
  • $\begingroup$ Yes. Excess thrust determines your angle of climb, and excess power determines your rate of climb. I edited my question to be more specific and clear. The question is not what make an airplane climb. The question is focused on why the trim tab is able to overpower the elevator and make a meaningful pitch change in the aircraft without the relative wind pushing the elevator back down again. $\endgroup$ – Brian May 16 '17 at 17:04
  • $\begingroup$ i dont think i should be down voted by people who havent had flight instruction. im adding a reference for pitch controls airspeed, power controls climb/descend even though its not needed for the question because the reference fo trim tabs is misleading on this $\endgroup$ – scm May 17 '17 at 0:58

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