Influence of High-Lift Devices on Trim and Pitch Stiffness

Conventional airplanes utilize a wide range of aerodynamic devices for increasing Cimax. These include various forms of trailing edge elements (plain flaps, split flaps,

slotted flaps, etc.), leading edge elements (drooped nose, slats, slots, etc.) and purely fluid mechanical solutions such as boundary layer control by blowing. Each of these has its own characteristic effects on the lift and pitching moment curves, and it is not feasible to go into them in depth here. The specific changes that result from the “con­figuration-type” devices, i. e. flaps, slots, etc., can always be incorporated by making the appropriate changes to hnwb, Cmm. wb and CLwh in (2.2,4) and following through the consequences. Consider for example the common case of part-span trailing edge flaps on a conventional tailed airplane. The main aerodynamic effects of such flaps are illustrated in Fig. 3.4.[8]

1. Their deflection distorts the shape of the spanwise distribution of lift on the wing, increasing the vorticity behind the flap tips, as in (a).

2. They have the same effect locally as an increase in the wing-section camber, that is, a negative increment in Cmac and a positive increment in CLwh.

3. The downwash at the tail is increased; both e0 and Эе/Эa will in general change.

The change in wing-body Cm is obtained from (2.2,4) as

д= ACmacwh + ACLJh – hnJ (3.3,1)

The change in airplane CL is

ACl = A CLwh – а, у Де (3.3,2)

and the change in tail pitching moment is

ACmt = a, VH Де (3.3,3)

When the increments ДСШа< л and ACUh are constant with a and Ah, hh is negligible, then the only effect on CLa and Cma is that of Эе/Э a, and from (2.3,18) and (2.3,21a) these are

The net result on the CL and Cm curves is obviously very much configuration depen­dent. If the Cm — a relation were as in Fig. 3.4c, then the trim change would be very large, from a, at Sf = 0 to a2 after flap deflection. The C, at a2 is much larger than at a, and hence if the flap operation is to take place without change of trim speed, a down-elevator deflection would be needed to reduce atrim to a, (Fig. 3.4c). This would result in a nose-down rotation of the aircraft.