Re: T-tail

Date:         10 Sep 97 19:38:44 
From:         michael piersdorff <piersdorff.michael@ic.gc.ca>
Organization: Industry Canada
References:   1
Followups:    1
Next article
View raw article
  or MIME structure

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.