Twist Control of an Extremely Flexible Rotor Blade for Micro-Aerial Vehicles

摘要

A flexible rotor blade is composed of a thin airfoil stabilized by centrifugal forces acting on a tip mass. Previous experiments show poor Figures of Merit due to a large nose-down spanwise twist induced in the blade. This paper describes three different approaches to passively tailor the spanwise twist distribution for improved efficiency. Firstly, extension-torsion composite material coupling is analyzed and it is shown that the centrifugal force is not large enough to balance the nose-down twist due to the propeller moment. The second concept makes use of the propeller moment acting on the tip mass located at an index angle to produce an untwisted blade in hover. A flexible blade whose tip mass is a tungsten rod oriented perpendicularly to the blade span and whose longitudinal axis makes an angle of 22 degrees with respect to the blade chord is fabricated and tested in hover. The result is an untwisted 18-inch diameter rotor whose maximum Figure of Merit is equal to 0.51 at a blade loading of 0.14. This rotor is found to be stable for any collective pitch angle greater than 11 degrees. In a third approach, addition of a trailing-edge flap at the tip of the flexible rotor blade is investigated. This design is found to have a lower maximum Figure of Merit than that of an identical flexible rotor non-equipped with a flap. However, addition of this control surface resulted in a stable rotor for any value of collective pitch angle.