Strut-Mounted Model

A third configuration of a wind-tunnel mount is a strut system, as illustrated in Figs. 4.8 and 4.9. The two linearly elastic struts have the same extensional stiffness, k, and are mounted at the leading and trailing edges of the wing. The model is mounted in such a way as to have an angle of attack of ar when the springs are both unde­formed. Thus, as before, the angle of attack is а = ar + в. As illustrated in Fig. 4.9, the elastic part of the pitch angle, в, can be related to the extension of the two struts as

c

The sum of the forces in the vertical direction shows that

L – W – Щг + 52) = 0

The sum of the moments about the trailing edge yields

Mac + L(c – Xac) – W(c – Xcg) – kc5i = 0 (4.21)

Again, using Eqs. (4.3) and (4.4) for the lift and pitching moment, the simultaneous solution of the force and moment equations yields

As usual, the divergence condition is indicated by the vanishing of the denominator, so that

kc

SCLa (1 – 2 Xf)

It is evident for this problem as specified that when the aerodynamic center is in front of the mid-chord (as it is in subsonic flow), the divergence condition cannot be eliminated. However, divergence can be eliminated if the leading-edge spring stiffness is increased relative to that of the trailing-edge spring. This is left as an exercise for readers (see Problem 5).