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Old 17th Jun 2011, 20:31
  #126 (permalink)  
henra
 
Join Date: May 2010
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Originally Posted by Lonewolf_50
What does this do to the aircraft's response ...

even with the THS not being stalled at that unusual angle of attack, it would take longer for both the elevator to influence THS, and for the lift acting on the THS to move the tail up (and thus the nose down) since the lift force isn't pushing as hard for a given flight control pitch command.

At some point at very low airspeed, depending upon CG, the airspeed ( airflow? ) may be so low that there isn't sufficient pitching moment (due to the combination of low speed influencing lift reduction, low air density, and sine of the angle vector subtraction) to move the nose at all.
I agree with the much slowed response at the low speeds and AoA. It is quite safe to assume that any recovery at these speeds and angles would have been relatively slow.
Where I'm not so sure if I understand it correctly is the thing with the Sine.
I will give it a try based on how I understood your approach:
As long as the flow does not separate it is not the speed along the chord but the speed along the Flight Path which is relevant for the lift. Therfore the lift component which is trying to tilt the aircraft Nose Down around an unknown center of rotation will be simply a function of the square of the speed along the trajectory.
The sine will apply with regard to the center of the Earth (i.e. the external coordinate system) but not for the coordinate system of the aircraft itself. Therefore it should be irrelevant for Nose Down with regard to the aircraft coordinate system. This assumes that the sum of external forces acts at one point on the aircraft and acts in the direction of the flight path.
This is a little oversimplification because the external forces consist of gravity, momentum, aerodynamic effects and thrust of the engines. It has to be noted that the latter two do not act exactly on the same point on the aircraft therefore making my assumption not 100% correct. Therefore the aircraft coordinate system is not completely independent.

Was a bit lengthy and probably even more difficult to understand what I mean but I hope I understood your description correctly if not you can ignore my comment.
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