INTRODUCTION Data Items in the group ESDU PERF RP 1/0@ RP 1/1@ RP 1/2 and RP 1/3 (Reference 1@ 3@ 4 and 5) give the standard British methods of performance reduction for piston-engined aircraft with constant-speed propellers (see also Reference 6). Besides ignoring compressibility effects these methods imply certain other assumptions@ for example@ average values are assumed for the variation of propeller efficiency with speed and the relations assumed for the engine performance do not apply to turbo-blown or compounded engines or to engines with charge temperature control. Because of these limitations there are some cases in which it is necessary to use a more general form of the performance reduction equation@ and the present Data Item suggests a method of doing this.