Wing span loads of complex high-lift systems from wake measurements

Abstract

H IGH-lift systems of transport aircraft are very complex, usually consisting of a wing with leading-edge devices and multiple slotted trailing-edge flaps. Power plant installations and associated flap cutouts add to the geometric complexity. It is difficult to make accurate wind tunnel measurements of the loads on such a configuration since model flaps are often too small to allow installation of a sufficient number of surface pressure taps. The purpose of this Note is to show that accurate wing span loads can be obtained from wake surveys without the use of pressure taps. Wake surveys are usually conducted to measure profile drag and vortex drag (e.g., Refs. 1-3) or to gain a qualitative understanding of the flowfield. Such experiments are timeconsuming and expensive since a large number of data points must be acquired to get accurate drag data. Performing detailed wake surveys, however, becomes more attractive if the measurements will yield wing span loads in addition to drag. This Note presents wake data of a twin-engine transport in a takeoff configuration and outlines a method of calculating wing loads based on well-known concepts that relate loading to the strength of trailing vortices. The method has some features in common with that of ElRamly and Rainbird, who measured wing span loads on simple wing geometries. It differs in principle from the method of Orloff, who enriches the measured wake data using lifting line theory in order to find the circulation of a wing section. The method described below calculates wing circulation directly from the measured data without recourse to theoretical enrichment.