Thrust, Flying and Swimming

Figure 12, The Simple Science of Flight,pg 68)

Force and speed for the downstroke of a wing, seen from the side. The aerodynamic force, K, is practically perpendicular to the track of the wing, because the air drag on the wing is quite small. The downstroke generates not only lift (L) but also thrust(T). Because the force and speed triangle have the same proportions, the ration T/L is equal to the ratio w/V between the downward speed of the wing and the forward speed of the bird.

(Figure 13, The Simple Science of Flight,pg 69)

The force balance in horizontal flight. The weight W is balanced by the aerodynamic lift L, and the aerodynamic drag D is overcome by the thrust T. The airspeed is V, and the product of T and V is the work that has to be performed each second ( that is, the power required) in order to maintain the force balance. Most of the time, D is much smaller than L. The smaller D/L, the better.