Tailplane 'Lift'
In S&L flight, any change in nose up pitch would tend to increase the lift of the wing, hence further increasing the angle of attack. This pitch up tendency is countered by the similar pitch up affecting the tailpane, hence lifting the tail and restoring the S&L flight situation. If the tail produced lift in a downwards direction, such a pitch up of the wing would result in further pitch up and hence lead to an unrecoverable stall.
Near the cruise condition most of the lift is generated by the wings, with ideally only a small amount generated by the fuselage and tail. We may analyse the longitudinal static stability by considering the aircraft in equilibrium under wing and tail lift, and weight. The moment equilibrium condition is called trim, and we are generally interested in the longitudinal stability of the aircraft about this trim condition.
(insert diagram of a/c with large arrow pointing up above the wing and a small arrow pointing up above the tailplane.)
Equating forces in the vertical direction:
W = Lw + Lt
where W is the weight, Lw is the wing lift and Lt is the tail lift.
The nature of stability may be examined by considering the increment in pitching moment with change in angle of attack at the trim condition. If this is nose up, the aircraft is longitudinally unstable; if nose down it is stable. Differentiating the moment equation with respect to α:
It is convenient to treat total lift as acting at a distance h ahead of the centre of gravity.
The total lift L is the sum of Lw and Lt so the sum in the denominator can be simplified and written as the derivative of the total lift due to angle of attack.
I'm sure this was covered some time ago but has since disappeared.
Last edited by rubik101; 8th Mar 2010 at 05:59.