c f k M P Pm R RAE v 1Y ama 1 Q' nnll P w Peter G. Wilhy Corrrrrltr~,,r Westlarid Helicoprer,~ Lir~riled, Yeo~,il, Sos~ersel, Et~glnnd, UKThe current capability for evaluating airfoil characteristics has developed over a period of some 25 years, during which time some twentv airfoils have heen tested in the Aircraft Research Association's transonic wind-tunnel dvnamic facilitv. 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. Design optimization demands knowledge of dynamic stall characteristics of the full range of airfoils and the ability to model these characteristics within the rotor design codes. This is possible only with the aid of a test capability that provides near full-scale Reynolds numbers and covers the full Mach number range in a single facility. Questions are raised about the validity of steady tests for establishing the steady stall angle of attack, which is needed in the dynamic stall models, and techniques for evaluating steady stall angle of attack from dynamic tests are demonstrated by example. They show clearly that it is not possible to assess the relative merits of different airfoils on the basis of steady measurements alone.

SummaryThe paper first describes an approximate method for calculating inviscid flows round arbitrary aerofoils at sub-critical Mach numbers, based on second-order theory with empirical improvements to give better agreement with exact theory; several comparisons are shown. This method is then used as the basis of an iterative procedure for calculating the effect of the boundary layer on the surface pressures and overall forces; several comparisons are given with recent experimental results.

Tonight we commemorate the achievements of Juan de la Cierva, the great pioneer of rotary aviation. His development of the autogyro stimulated the development of the early rotor theories and paved the way for the modern helicopter. In turning the autogyro into a practical aircraft, Cierva proved to be an innovator prepared to take an unconventional approach. He was at the same time pragmatic in that he really wanted to build a helicopter but realised that the required technology was not yet mature, and settled on the autogyro as representing what was an achievable concept. His concept was thus unconventional, dramatic and achievable — an admirable combination. However, in his first attempts to fly an autogyro he rapidly came up against the problem that faces all rotory wing designers — which is that the advancing blade has a much greater lifting potential than the retreating blade, due to the difference in dynamic head. This led him to introduce the flapping hinge that allowed the rotor to achieve roll balance as well as providing the backward tilt of the rotor that is necessary for autorotation. Having made this step forward, he realised that the introduction of blade camber would allow the retreating blade to achieve higher angles of incidence before stalling, but in so doing he built in a potentially dangerous control characteristic.

No abstract