another instuctors question

[dynamite dean]

Thankyou to those that helped me last time.

I have another question could somebody please tell me why aircraft with a forward centre of gravity have higher stalling speeds than aircraft that have an aft CG

thankyou. :) :)

[AffirmBrest]

Generally speaking, for an aircraft to be statically stable, the C of G is usually located forward of the Wing Centre of Pressure, with the horizontal tail providing a down force to balance the 'see-saw'.

This tail down-force obviously detracts from the lift produced by the wing, and means the wing has to work slightly harder to balance the sum of weight and tail down-force; ie. fly at a slightly higher angle of attack.

The further forward the C of G is, the higher the tail down-force required and so for the same speed with two different CG positions, the wing AoA will be higher for the fwd CG case.

This leads to the stalling AoA being reached at a higher speed for the fwd CG than for the aft case.

Incidentally, an interview question I was asked many moons ago was whether a fwd or aft CG was more fuel efficient - from the above it is apparent that an aft CG requires a lower wing AoA for the same speed and so less induced drag = less fuel burn.

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...proceeding below Decision Height WITH CAUTION...

[This message has been edited by AffirmBrest (edited 16 November 2000).]

[Prof2MDA]

And it's not just the effect on the wing that you have outlined nicely. The horizontal stab generates more drag the more it has to "work" to keep the nose up. This drag, known as "trim drag" also must be added into the equation. The higher AoA may also lead to higher parasite drag due to less than ideal fuselage angle, etc.