THE flow of air relative to a blade element during translational flight of a sustaining rotor periodically varies, the speed normal to the blade tip being Rω + V sin ψ, i.e., Rω + V in the transverse upstream position and Rω − V in the opposite position downstream (Fig. 40). This dissymmetry of relative air speed produces a corresponding dissymmetry of lift, unless the blade angle is sinusoidally varied or feathered (see ref. 41), and if there were no flapping hinge at the blade root the dissymmetry of lift would produce a rolling couple increasing with forward speed. The rolling couple could, of course, be balanced by controlling the rotor lift vector laterally with respect to the aircraft's centre of gravity or by aileron control, but the flapping hinge, besides suppressing this couple, relieves the blade from excessive bending at its root.
Bennett, J.A.J. (1940), "Rotary‐Wing Aircraft: The Fifth Article of a Series Summarizing the Current State of Knowledge", Aircraft Engineering and Aerospace Technology, Vol. 12 No. 5, pp. 139-146. https://doi.org/10.1108/eb030641
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