Investigation of an Extremely Flexible Stowable Rotor

This paper describes the analysis, fabrication and hover testing of a rotor with extremely flexible carbon-fiber composite rotor blades. The flexibility of the rotor blades is such that they can be rolled into a compact volume and stowed in the rotor hub. While the benefits of a variable-diameter, stowable rotor for full-scale, high-speed helicopters is well known, the present study focuses on application to a micro-helicopter. An 18 inch diameter rotor having untwisted, constant chord blades with a circular arc airfoil profile, is discussed. A comprehensive rotor aeromechanics code is used to analyze the behavior of the flexible rotor blades and to investigate the effect of a tip mass on the rotor stability. The hover performance of a two-bladed rotor with flexible blades is compared to that of a rotor with rigid blades having an identical planform and airfoil section. Stable operation of the flexible rotor in hover was achieved by positioning the tip mass at a suitable chordwise location. A maximum Figure of Merit of 44% was measured for the rigid rotor blades and 22% was measured for the flexible rotor blades. In general, the high twist induced in the flexible rotor blades resulted in a lower thrust and decreased e±ciency compared to the rigid rotor blades. Future plans for increasing the e±ciency for the flexible rotor blades and expanding the test envelope into forward flight are described.