Free-Tip Rotor Wind Tunnel Test Results

A new rotor blade tip design called the free-tip has been proposed as a means to improve forward flight performance characteristics and reduce oscillatory loads. The free-tip design incorporates a tip that is free to pitch independently of the rest of the blade. Pitching about an axis forward of the quarter-chord, the tip weathervanes into its local wind, thus reducing angle of attack perturbations and the resulting oscillatory lift loadings. A nearly constant nose-up pitching moment is applied mechanically to the tip so that the tip, to maintain pitching equilibrium, produces nearly steady positive lift around the azimuth. A wind-tunnel test of a small-scale, 5.1 m diameter model rotor was conducted to obtain comparative forward flight performance and oscillatory loads data with the tips free and fixed. The free-up was shown to reduce power in trimmed flight over a wide range of advance ratio and thrust; at an advance ratio of 0.3 and C(sub L)/sigma of 0.08 the reduction is 12%. Oscillatory flapwise bending-moments and oscillatory pitch link loads are also reduced, but the oscillatory in-plane bending moments increase.