PREVENTION OF THE STALL FLUTTER

According to Eq. 1 of § 9.4, the critical stall-flutter speed can be raised by increasing the frequency of free torsional oscillation of the blade, i. e., by increasing the torsional stiffness and reducing the mass moment of inertia.

Stall flutter can be delayed if the airfoil can be prevented from stalling. Thus, in designing propellers, care must be exercised to choose the proper airfoil section, and, if possible, the working angle of attack should be limited to below the stalling angle.

It is shown by Theodorsen and Regier4,36 that the divergence speed of a propeller is an important parameter to consider in connection with the stall flutter (see § 4.8). If the rotational speed of a propeller is so high that the relative wind speed is close to the critical-divergence speed, the
blade will be twisted excessively, possibly beyond the stalling angle, and thus causing stall flutter. Theodorsen and Regier show that, for several models of wind-tunnel propellers tested in the high-angle-of-attack range (with initial settings below the stalling angle), flutter invariably occurs at a speed substantially below the classical flutter speed. The angle of attack of the blade at which flutter occurs appears to be nearly constant and independent of the initial blade setting. Apparently the blade simply

 Да — а і mean ~ ** static stall ————– NPL, R = 1.42 x 105, в = 2.0°, Axis of rotation, mid-chord. ————– NPL, R = 1.42 x 10®, в = 6.0°, Axis of rotation, mid-chord. ————– NPL, R = 2.83 x 10°, в = 2.0°, Axis of rotation, mid-chord. ————– NPL, Д = 2.83х 1O5,0 = 6.0°, Axis of rotation, mid-chord. ”□Blunt wing Д = 0.84×10® MIT j a Intermediate 0 = 6.08° Sharp wing 0 = pitch amplitude Axis of rotation, 37% chord. Ref. NPL: ARCR&M 2048; MIT: NACA TN 2533 Fig. 9.11. Critical reduced frequency.

twists to the stalling angle, and flutter starts. Furthermore, Theodorsen and Regier show that the classical flutter speed and the divergence speed of a propeller are approximately the same because of the centrifugal force effect. The problem of predicting propeller flutter is thus resolved pri­marily into the calculation of the speed at which the propeller will be twisted to stall. This can be done by methods of Chapter 3.

As to the practical design measures to raise the critical stall-flutter speed, we may quote an interesting case reported by Sterne and Brown.7-141