The current capability for evaluating airfoil characteristics has developed over a period of some 25 years, during which time some twenty airfoils have been tested in the Aircraft Research Association's transonic wind-tunnel dynamic facility. The availability of this test facility has been a key element in the development of a rotor design philosophy, which is to avoid excessive excursions into retreating blade stall and seeks to exploit the benefits of thin airfoils in the blade tip region, thicker, high lift airfoils inboard of the tip, reflex camber airfoils with compensating nose-up pitching-moment further inboard, and thick sections to satisfy bending stiffness criteria over the inner blade region.
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