IHS ESDU Example of similarity methods applied to aircraft drag data from flight test 08010

ESDU 08010 provides an example of the application of similarity methods to assist in the correlation of in-flight measurements of aircraft drag coefficient. Similarity rules are used to identify the form of the variables used in the process of correlation.The aircraft used to obtain the drag measurements is a heavy, subsonic, low-level attack type powered by two side-mounted low-bypass-ratio turbo-fan engines (fixed geometry intakes). An outline of the flight-testing from which the data were drawn is included. The flight-tests formed part of a programme to determine the changes in lift, drag, pitching moment and downwash induced by fitting a range of stores under the wing considered. Measurements were taken at eleven Mach numbers.An equation for drag coefficient, CD , is derived in terms of a chosen similarity function using these data. The equation contains three independent unknowns, all of which are correlated by a similarity parameter. The correlations are shown to accord with the measured data.