LIFT AND PITCHING MOMENT OF THE BODY AND NACELLES

The influences of the body and nacelles are complex. A body alone in an airstream is subjected to aerodynamic forces. These, like those on the wing, may be represented over moderate ranges of angle of attack by lift and drag forces at an aerodynamic center, and a pitching couple independent of a. Also as for a wing alone, the lift-а re­lation is approximately linear. When the wing and body are put together, however, a simple superposition of the aerodynamic forces that act upon them separately does not give a correct result. Strong interference effects are usually present, the flow field of the wing affecting the forces on the body, and vice versa.

These interference flow fields are illustrated for subsonic flow in Fig. 2.10. Part (a) shows the pattern of induced velocity along the body that is caused by the wing vortex system. This induced flow produces a positive moment that increases with wing lift or a. Hence a positive (destabilizing) contribution to Cma results. Part (b) shows an effect of the body on the wing. When the body axis is at angle a to the stream, there is a cross-flow component V sin a. The body distorts this flow locally, leading to cross-flow components that can be of order 2У sin a at the wing-body in­tersection. There is a resulting change in the wing lift distribution.

The result of adding a body and nacelles to a wing may usually be interpreted as a shift (forward) of the mean aerodynamic center, an increase in the lift-curve slope, and a negative increment in Cmac. The equation that corresponds to (2.2,3) for a wing – body-nacelle combination is then of the same form as (2.2,3), but with different val­ues of the parameters. The subscript wb is used to denote these values.

~~ CmaCwh Сь„ь(Ь Ь-п„ь)

= Cmalw„ + awhawb(h – KJ

where awh is the lift-curve-slope of the wing-body-nacelle combination.