Hi waela320,
The title of your thread seems to have caused some confusion! I am guessing that you may not have expected the discussion to get so technical, and am not convinced that it has yet answered the questions you had in mind. (Forgive me if I'm wrong, as I don't know your background.)
Quote:
"Lowering the trailing edge flaps moves the CP aft of the CG and causes a pitch down moment but is this applicable to all aircraft types or only for T tail airplanes ?"
Staying with big jet transports that use tailplanes for horizontal stabilisation, in my experience on 6 types any difference caused by a T-tail is not noticeable to the pilot during flap extension. Taking the example of the B707/KC-135, which has a fuselage-mounted THS (trimable horizontal stabiliser), the HS has to be trimmed to a greater and greater negative angle of incidence (that is, nose-up trim) as the flaps are run out in stages on the approach, in order to allow the elevators to return to neutral. The same applied to the VC10, which is a similar-sized aeroplane with a T-tail, and all the others in my experience.
With conventional trailing-edge flaps, particularly those of the fowler type that all these jets have, the total negative lift required at final approach speed with full flap is presumably greater than at minimum-clean speed to stabilise the pitch attitude. But the lower IAS will itself demand a greater angle to produce the same negative lift, so those two additive factors explain why such a large change in the THS angle is needed.
A good, but sad illustration of the above was probably involved in a fatal accident to a B707-320C at Lusaka in 1975. When the crew selected full flap at about 4 miles from touchdown, the increase in load on the fatigued THS caused a failure on one side (I forget which) and the aircraft bunted so violently that it actually passed the vertical in pitch before hitting the ground.
(By the way, the CP is always behind the CG in a conventional aeroplane with a tailplane HS.)
Quote:
"What happens to the pitch attitude for an A320 for example when lowering the flaps ? "
The A320 is a bad example because the FBW control laws keep the pitch-attitude roughly constant as the flaps (and slats) run out. The B737 would have been better, but I haven't flown it. However, the first thing to remember is that, if you are trying to maintain a steady FPA (for example, maintaining the ILS glide-slope) the greater flap angle will initially demand a lower pitch-attitude until the aircraft slows down to the IAS appropriate for the new flap setting. If the pilot does not push the yoke or stick slightly forward to lower the nose (with down-elevator) while the flaps are running out, the a/c will seem at first to "balloon", and stop descending. (That applies equally on the A320.) Then, at a constant thrust, the IAS will decay rapidly and the nose will drop, partly because the CP has moved aft. So at that stage up-elevator will be needed until extra nose-up trim is selected on the THS.
There may come a point in the stages of flap extension, however, where the CP may not move as much for each degree extra. Perhaps the aerodynamicist(s) posting on this thread will comment?