5.1 Introduction

In flight testing the determining of engine power is performed for essentially two purposes:

1. To determine the engine power as installed in the airframe.

2. To measure the drag of the airplane with the propulsive system operating.

Because the aircraft's induction, exhaust, and cooling system affect engine power, and engine accessories (such as generators, alternators, or hydraulic pumps) also require power, the manufacturer's tests on a test stand will not give the power of the engine installed in the airframe. In flight testing, then, we must devise methods to measure the actual installed power so we can in turn use this number to define airplane drag. Because the airplane and engine function as a unit we cannot divorce the effects of one on the other. Even though these interaction effects may be small, they can cause data errors when they are ignored. They may also be the cause of poor performance and may be an area to examine when performance does not meet predictions. Some of these interaction effects for propeller-driven airplanes are as follows:

1. effects of cowling shape and blockage on propeller efficiency

2. propeller slipstream effects on airplane drag

3. effects of engine cooling drag on airplane performance

4. effects of engine cooling and fuel distribution on range, cruise, and climb performance

5. effects of engine exhaust thrust on apparent power required

6. effects of induction and exhaust system on power available

Some flight test methods, at least in part, take care of some of these interaction effects and we will discuss...