Same reasons why they sweep the wing [many excellent explanations for this phenomena abound] remember the tail exerts a downward force but this has not much to do with your question--- basically, it delays the onset of compresibility effects on the tail that hinder its effectiveness.
the Cp shift that aides Mach tuck may be just one term the other is loss of downward balance force by fiddling with these two parameters.
designers try to impart as much positve natural longitudinal stability before turning to artificial stabilty augmentation like pitch trim conpensators etc
PA
Last edited by Pugilistic Animus; 14th October 2009 at 22:25.