Abstract:

A knowledge of minimum leading-edge pressures has long been required for aircraft stressing purposes. Previous theoretical methods resulted in infinite pressures at the leading edge, and the use of a high proportion of vacuum pressure always carried the risk of unnecessarily high structural loads estimates, resulting in over-design, except perhaps for high-speed, high-performance aircraft. The calculation of the drag curve beyond the onset of non-linear characteristics requires an accurate prediction for the minimum pressure coefficient, its development with increase of incidence and its development across the span of a wing. Similarly, the onset of disturbances to the directional characteristics of an aircraft require an accurate prediction for the minimum pressure coefficient and its subsequent development, which may also result in disturbances to the lateral characteristics. Because wind-tunnel testing is almost invariably carried out at low Reynolds numbers, it is important to be able to predict the separate effects of Reynolds number and Mach number for all these applications, to enable extrapolation to full-scale conditions.

ESDU 20002 provides three methods to predict the minimum leading-edge pressure coefficient for symmetric aerofoil sections. Each method is formulated to provide a prediction for associated lift coefficient and increment in axial force. Method 1 is used to predict values for wing sections with a Reynolds number, based on the leading-edge radius, up to 1,000. Method 2 is used to predict values for a wing section with a Reynolds number, based on the leading-edge radius, between 1,000 and 13,300, and Method 3 is used to predict values for a wing section with a chord-based Reynolds number up to 45×10⁶. This Item does not include a means to predict the leading-edge pressure coefficient subsequent to achieving the minimum value, which would also allow the prediction of the change of axial force coefficient, after the minimum has been reached. It is intended to carry out data analysis with a view to providing such a method in the future. based on the work of Gault (NACA tech. Note 3963).

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