Rolling Rotors: The Dilemma of Asymmetric Lift

When either a helicopter rotor or autogiro rotor operates in forward flight with the rotor plane passing edgewise through the air, the blades encounter an asymmetric velocity field [see Fig. 2.1]. The asymmetry of the onset flow and dynamic pressure over the disk produces aerodynamic forces on the blades that are a function of blade azimuth. For blades that are rigidly attached to the shaft, the net effect of these asymmetric aerodynamic forces is an upsetting moment on the rotor and on the aircraft. This was Cierva’s first dilemma when developing the autogiro. As described in Chapter 4, the distribution of lift and induced inflow through the rotor will affect the inflow angle ф and the angle of attack at the blade sections and, therefore, the detailed distribution of aerodynamic lift and drag forces over the rotor. This subsequently affects the blade flapping response and so the aerodynamic loads on the blades. This coupled behavior is a complication with a rotating wing that makes its thorough analysis relatively difficult, a fact well appreciated by Cierva and a point alluded to throughout this book.

Cierva’s first Autogiro, the C-l, was built in 1920 and had a coaxial rotor design. He was to build two more machines, both with single rotors, before he achieved final success with the C-4 (Fig. 12.6) in January 1923. The problem of asymmetric lift between the advancing and retreating blades was initially approached using the idea of a contrarotating coaxial design such that the lower rotor would counteract the asymmetry of lift produced on the upper rotor, thereby balancing out any moments on the aircraft. However, when flight tests began it was found that the aerodynamic interference between the rotors resulted in different autorotational rotor speeds and this spoiled the required aerodynamic moment balance. Cierva considered the possibility of mechanically coupling the rotors to circumvent the problem, but this was quickly rejected because of the obvious mechanical complexity and significant weight penalty. Despite its failure to fly the C-l proved that the rotors would freely autorotate when the machine was taxied with sufficient forward speed.

The next Cierva design was the “compensating” rotor, which was tested in a three-bladed form on the C-3 in 1921 and in a five-bladed form on the C-2 in 1922. This idea used blade twisting in an attempt to compensate for the undesirable characteristic of asymmetric lift (i. e., by using nose-down twist on the advancing blade and nose-up twist on the retreating blade). Photographs of these two machines [see C. A. Cierva (1998)] show a series of cables attached to the trailing-edges of the blades, with the idea that the blade twist could be changed in a cyclic sense as the blades rotated about the shaft. However, while the basic principle was correct, the concept proved impractical and both the C-2 and C-3 were only to achieve short hops off the ground. Perhaps the use of cyclic blade feathering (as opposed to blade twisting) might have been more successful, but it was not to be until 1931 that Wilford in the United States demonstrated this concept on an autogiro – see Wheatley (1935), Larsen (1956), and Gustafson (1971).