ESDU Engineer

Wing/High-Lift Device Aerodynamics

Pitching moment curve of wings with leading-edge and trailing-edge high-lift devices deployed at low speed

Beginning in 1994, ESDU International began a systematic extension of its Data Item coverage of the effect on lift and pitching moment of high-lift device deployment. The Data Item methods are largely empirical, being based on a wide range of wind-tunnel data. Mr J.R.J. Dovey, the current Chairman of the ESDU Aerodynamics Committee, was largely responsible for their development. Starting with the lift coefficient increments for aerofoil and wing at zero angle of attack and at maximum lift, the initial body of work culminated with the prediction of the lift curve to maximum lift with devices deployed, including a modelling of the progressive reduction in lift slope with increasing angle of attack.

With that part of the work completed, attention was directed to the effect of device deployment on pitching moment, combining the previously established lift coefficient increments with empirically determined centres of lift. As before, progress was made through the predictions for aerofoils and wings with different types of leading-edge and trailing-edge devices, with extensive comparisons with wind-tunnel data. The conclusion of the second phase of the work is marked by the issue of Item No. 03017 which provides the pitching-moment curve to high lift.

In particular, the method of Item No. 03017 includes a treatment of two sources of non-linear behaviour which alter the change in centre of lift due to angle of attack with devices deployed from that associated with linear characteristics. The first of these results from the shortening of the high-lift device planform chord due to its rotation angle. The second is associated with a progressive shift forward of the centre of lift as angle of attack is increased, ascribed to reduction of the effectiveness of the rear part of the wing due to the thickening of the boundary layer as angle of attack or flap angle are increased, prior to flow separation. The effect is most pronounced with large combinations of angle of attack and flap angle at low Reynolds number. For a wing without high-lift devices this effect is small enough to be ignored but the characteristic can dominate the estimate of the centre of lift at large angle of attack and/or large flap deflection angle, to such an extent that for cases that might instinctively have been assumed to be stabilising, the net result can be destabilising.

The method is applicable to wings with aspect ratios greater than 4. Leading-edge devices include drooped leading-edges, plain leading-edge flaps, sealed and vented slats and Krüger flaps. Trailing-edge devices include plain and split flaps, single-, double- and triple-slotted flaps. A technique is included for the treatment of a wing with different device types over different spanwise extents. The method is recommended for use up to at least CLmax–0.3 and its satisfactory employment towards maximum lift is discussed.

Item No. 97002 provides a summary ot the Data Items available on high-lift aerodynamics.

Bob Gilbey is the Group Head responsible for the Aerodynamics Series and he can be contacted at bgil@esdu.com.