Can anyone please explain why aircraft pitches up when the speed increased? (please consider an aircraft, Wing AC, CG and Tail AC lies on a line.)
The ideal explanation I'm looking for should have something to do with the static margin.
Here I am talking about a powered or unpowered aircraft, trimmed for steady flight, reacting to an EXTERNAL velocity perturbation which increases the airspeed. (Like a sudden, substantial, sustained increase or decrease in the speed of the wind, e.g. due to wind shear.)
I have seen that when faced by this kind of a perturbation, the aircraft tries to slow down/keep the airspeed unchanged by increasing the pitch angle. My question is how it happens
However, I can imagine what might have happened to let you observe a pitch-up. This needs several conditions: A propeller-diven airplane with the propeller in the front A sufficiently large static margin so the empennage produces a down-force. Speeding up by opening the throttle.
Let me clarify these are not necessarily the cases for what I'm asking, 1. If we throw a glider hard enough, which is trimmed for a certain airspeed it will pitch up and climb. 2. At least for now I think it doesn't matter whether the tail produces down-force or upward force. Say the velocity is increased by factor of 2 and the forces on both wing and tail increased by a factor of 4. Equilibrium is still maintained as moment around CG has not changed by this. (and this is what stands against my observation)
The horizontal stabilizer always provides a downward force to balance the forces of lift and weight with the center of gravity. This also provides stability because if the aircraft pitches down and starts to speed up, the increased airflow over the tail will result in more downward force and cause the nose to rise and the aircraft to slow.
I think there is no need for the tail to make downward lift always. The tail can be uplifting as well. Anyway, if we agree that to be the case for now, when the aircraft gains airspeed, the flow speed over the wings increases as well as tails. Isn't it? What I can't understand is, what makes the aircraft pitch up when both the wing force and tail force increased by the same factor, due to increase of airspeed.
Any further input is much appreciated.