Experimental Study of an Extremely Flexible Rotor for Microhelicopters

Abstract

This paper describes the development and testing of a rotor with extremely flexible blades, focusing on application to a microhelicopter. The goal is to develop rotor blades that are so flexible that they can be rolled into a compact volume and stowed inside the rotor hub. The design of an 18-in-diameter two-bladed rotor with flexible blades is presented. Stiffening and stabilization of the rotor blades are enabled by centrifugal forces acting on a tip mass. The relationship between blade-design parameters, aeroelastic stability boundaries, and hover performance is explored by a systematic set of experiments. It is observed that rotational speed and collective pitch angles have a significant effect on rotor stability. In addition, a large negative spanwise twist is induced in the flexible blades during stable operation, resulting in a decreased efficiency compared with rigid rotor blades having an identical planform and airfoil section. Different approaches to passively tailor the induced twist distribution are investigated. The highest efficiency was measured on a configuration making use of the propeller moment acting on a tip mass located at an index angle to the blade chord. A maximum figure of merit of 0.51 was measured at a blade loading of 0.14, which is comparable to the efficiency of rigid rotor blades.