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If the aircraft is not trimmed correctly and either the pilot or autopilot is having to use constant pressure on the elevator does this increase drag?

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  • $\begingroup$ This should have the same effect, minus feedback loop issues if the pilot doesn't input exactly the right amount of pressure $\endgroup$
    – Antzi
    Mar 25 '16 at 19:42
  • $\begingroup$ I've been told that an aircraft will find its trim speed/attitude if left untrimmed, which leads me to believe that you may be increasing drag by trimming it (aka forcing it to hold an attitude), but somebody a little smarter than me will have to confirm or refute that claim... $\endgroup$
    – Ron Beyer
    Mar 25 '16 at 19:51
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    $\begingroup$ @RonBeyer You're forcing it to hold an attitude trimmed or not. My non-pilot intuition tells me it depends on the trim system. If the whole stabilizer moves it prob helps to trim but if it uses trim tabs then it's really just using the elevator anyway. It just helps you hold it in position $\endgroup$
    – TomMcW
    Mar 25 '16 at 19:58
  • $\begingroup$ @TomMcW Not really, think of it this way, if you are going too slow, the nose will drop and you'll pick up speed, as you pick up speed lift becomes greater and the nose will rise. Same if you are going too fast, your nose will come up, speed will bleed off, lift decreases and the nose drops. Aside from the oscillations after some period of time the aircraft will find its trim speed (even if the trim is neutral). $\endgroup$
    – Ron Beyer
    Mar 25 '16 at 20:09
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    $\begingroup$ @RonBeyer Here's my thinking: You're in an ac with a trimmable stabilizer trimmed for climb. When you level off you push forward the yoke forcing the elevator down. You set the a.p. to maintain speed and altitude without re-trimming. The tailplane exerts the appropriate downforce to maintain the wings at the correct AoA for the set speed. In climb config the stab is trimmed slightly downward and now the elevator is also pointing downward to maintain level flight. Seems like it would produce more drag like that than if you re-trimmed so that the elev is neutral and aligned with the stab. $\endgroup$
    – TomMcW
    Mar 25 '16 at 20:52
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Normally not; details depend on the control system and by how much the aircraft is out of trim. The force produced by the empennage will be the same in the trimmed state and when out of trim, only the control forces are different.

Let's examine the different possibilities:

  • Adjustable spring (as used on some light aircraft and gliders): The elevator deflection will be identical to the trim position, so there is no difference at all.
  • Trim or servo tab on the elevator: The tab deflection will be different, but the elevator position will be practically identical. For tab angles of less than 10° relative to the elevator there should not be any difference in drag.
  • Adjustable stabilizer: Now the elevator deflection will be different, and depending on the trimmed state the deflection angle might cause higher airfoil drag. If the elevator deflection relative to the stabilizer is more than 10°, a slight drag increase can be expected.

If the power consumption of the autopilot is higher in the out of trim case, this might cause higher generator loads and more engine power is needed to keep it powered. But this effect should be very small.

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