The Case for the Convertible Rotor

Extract

The rotor performance is computed for the two modes of VTOL flight, hover and cruise. In the hover mode the rotor must provide sufficient thrust to support the whole aircraft weight, whereas in the cruise mode only a small fraction of the thrust is required. On the other hand, the flow through the rotor is relatively small in hover, but large in cruise. These different conditions in the two modes of operation demand variable blade geometry in terms of blade twist, as well as aerofoil camber. The high thrust in the hover mode must be obtained with a minimum of blade area to avoid excessive drag in the cruise mode. To achieve this a fairly high tip speed must be combined with high lift coefficients. In the cruise mode on the other hand, tip speed and lift coefficients must be reduced considerably. This leads to a high advance ratio.