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In aircraft with conventional elevator control, if I increase the speed, the horizontal tail sees higher velocity and hence higher lift. Does it mean that increasing airspeed creates nose down pitching moment (i.e. decrease in AoA) and vice versa ?
If So, it means if i want to increase the speed keeping altitude constant, the aircraft automatically gets trimmed at new trim AoA or i manually have to change the AoA using elevator?
How exactly do you plan to increase speed? You can add thrust. The aircraft is trimmed for a specific airspeed (normal flight assumed) so the added thrust will introduce a climb.
If you want to increase speed in level flight, you trim down (the engine power force vector rotating down, shrinking the vertical component and increasing the horizontal, so converting more engine power to forward movement and losing a little lift) AND add power to compensate for the lift loss.
It follows that there is an optimum: when there is no more vertical component, you cannot increase forward component of the engine force vector. But by that time the aircraft is already converted into a rock (no vertical force = only gravity = fall).
If you increase speed, you do not only increase the aerodynamic forces on the tail (be they lift or downforce, doesn’t matter), but on the wing as well. The weight of the aircraft will stay the same, however (if we neglect the fuel consumed during acceleration).
This increase in wing lift needs to be compensated by a lower pitch attitude of the aircraft, so the tail surface will see a reduction in angle of attack which more than compensates for the increased dynamic pressure. As a consequence, a naturally stable aircraft will experience a nose-up pitch moment change when being accelerated.
In order to trim the new speed, the elevator deflection needs to move to a more positive setting (= trailing edge down).
A simple answer:
Normally the wing has an upward force (lift) and the tail has a (usually) downward force (negative lift if you will). So as the aircraft increases airspeed, the lift of the wing, and the (usually) downward lift of the tail will increase. There are other factors which include the engine / prop orientation, center of mass, and so on.
The issue which makes a clear, always true, answer elusive is that the lift profile of the wing and of the tail are not the same. And of course there are complications from trim tabs and the like.
So when a designer considers pitch changes with airspeed changes, they usually try to minimize the effect to provide longitudinal stability. (Exceptions can be fighter, aerobatic and fly-by-wire configurations.) Another normal design goal is to have a lower angle of attack on the tail, than the wing. They can only be identical at more than one point, unless there is an unconventional or active flight control.
