Date: 10 Sep 97 19:38:44 From: michael piersdorff <email@example.com> Organization: Industry Canada References: 1 Followups: 1
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The elevator tends, at low speed, to ride in the burble of air behind the main wing, hence the rougher ride. The very earliest t-tails used to suffer from a condition known as "deep stall" where the main wing was stalled due to high angle of attack, and the t-tail was in the turbulent air behind the main wing. Because the air was turbulent, the elevator could not provide the control power necessary to kick the nose down for stall recovery. Because the wing was stalled, the ailerons would not work, either, to allow a roll recovery. The aircraft would descend, stable in the stall, right into the ground. Needless to say, it would spoil the day for everyone on board. Later changes to the certification standards required systems to prevent the condition (stall warning, stick pushers and the like), sufficient engine power available in the stalled condition to push the aircraft out of a stall (increased forward speed will reduce effective angle of attack), and that the inner wing stall beofre the outer wing, leaving the aileron portion unstalled later into the deep-stall condition. Given the problemms, why ever use t-tails? It does tend to solve the problem of jet exhaust beating on the tail surfaces, which can be a killer on structures. And in most normal flight conditions, the high mounted tail is flying in cleaner (less disturbed) air, increasing its effectiveness. More effectiveness results in less surface needed equals lower weight and drag.