# How does change in airspeed affect pitching moment?

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?

• Note that, depending on location of the cog the horizontal stab is providing lift in a downward directing, thus an increase in speed would create a pitch up rather than down Commented Apr 27, 2017 at 18:23

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).

• Do u mean that to trim the a/c at any new airspeed, only changing the pitch angle by elevator is required and no throttle change is done. Throttle change is done only to change flight path angle? Commented Apr 28, 2017 at 4:49
• @Swati. well. if you change trim, the aircraft will assume the new speed, but at that speed drag is different. If you don't adjust throttle, your potential energy will change by the difference resulting in climb or descent (there is a particular speed where drag is minimal, so we can't say slowing down always results in climb). Commented Apr 29, 2017 at 21:03