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I'm having an argument regarding elevator trim. According to a book my friend showed me, trim is for constant airspeed. So, let's say we are flying level at 100 kts with 2300 RPM and it's trimmed. My buddy claims that if we reduce power (1800 rpm for example), and do not touch elevator trim and yoke, pitch will be set for 100 kts by airplane itself and we can descend at 100 kts with 1800 rpm.

However, what I think is to maintain 100 kts with reduced power, the nose of airplane should be lowered and forward trim should be readjusted by a pilot.

Can somebody give me an answer to end this argument?

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Your friend is right. Elevator trim isn't setting a pitch, it is compensating for control forces on the elevator, which vary with speed. If you are trimmed for level flight at 100 kias and reduce power the plane will initially slow. As it slows the trim will cause a nose-down moment causing the plane to pitch down. Given no input, the plane will equilibrate at whatever nose-down pitch is needed to regain the forces it was trimmed for, which means speeding back up to 100 kias.

You are correct that to maintain 100 kias with reduced power the nose should be lowered, but the trim will take care of that for you.

You might have a misconception about how trim works, based on your argument. A typical trim setup in light airplanes is essentially another control surface on the elevator:

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The elevator is the large control surface and that smaller control surface is the elevator trim. When you adjust the pitch trim you are adjusting this trim tab which provides a force on the elevator that counters aerodynamic forces. This is why the airplane responds in pitch to maintain constant aerodynamic forces (e.g. constant indicated airspeed) for a given trim setting.


You can test this in the airplane easily. Take your plane up to the altitude you practice maneuvers at, set 75% power (or some other number) and trim for straight and level. Add power and watch the plane climb, decrease power and watch it descent, both without touching the trim. Go back to straight and level and trimmed and accelerate without climbing -- note you are having to retrim as your speed increases. Decelerate without descending -- note you are retrimming as your speed decreases.

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    $\begingroup$ Good explanation. Two additional notes: 1: because the restoring force affects roll acceleration, the plane will oscillate around the trimmed speed and pitch in a phugoid oscillation, which might be more or less damped depending on particular aircraft type and 2: because the engine thrust is not directly on aerodynamic axis in most planes, the change in power affects the trimmed speed somewhat, how much again depends on particular type. $\endgroup$ – Jan Hudec Aug 3 '15 at 9:11

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