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What is required to happen to control surfaces and trim tabs to remain trimmed in straight and level flight, as speed increases? (for a GA propelled aircraft)
My manual tells me that the elevator should be further deflected downwards, and its trim tab further upwards.
I understand the reason for the trim to deflect in the opposite direction of the elevator (to help maintain it in that position), but don't get it for the elevator.
The angle of attack is lower for the faster flying aircraft. More speed means more dynamic pressure, and in order to create the same lift (weight did not change, after all), less angle of attack is needed to create the needed amount of lift.
Now it is important to know that the angle of attack (more precisely, the difference between the actual and the zero-lift angle of attack) on the wing is higher than on the tail if the aircraft is statically stable. If the aircraft accelerates, the relative amount of lift loss on the wing is less than on the tail, because both experience the same reduction in angle of attack. To keep the balance between both the same, this extra lift loss on the tail needs to be compensated by some downward elevator deflection. The center of gravity did not change, and the center of lift must be shifted to the same longitudinal station in order to maintain level flight.
Fun fact: When transitioning from sub- to supersonic speed, the elevator must be trimmed trailing-edge-up, because now the center of pressure moves back on both wing and tail, so their lever arms around the center of gravity change.
As speed increases, the horizontal stab generates more lift... downwards. This would then force the tail down and the nose up, the wing would be at a higher AoA, and create more drag, and the aircraft would return to its trimmed-out speed.
If you want to fly faster, you have to hold the nose down against the wanting-to-go-up tenfency, and thus you need down-elevator. And to hold this you then need forward-trim.
