TABLE OF CONTENTS The jet engine presents a separate set of problems for the flight test engineer when attempting to measure jet thrust or power. In many cases it may not be necessary to measure either one of these variables. Instead, the flight data may be referenced to some indication of jet thrust such as "referred rpm", N/ 
, or fuel flow. The pilot does not need to know what thrust the engine is developing in order to operate the airplane as long as the performance information has been referenced to some related variable. However, we always have the problem in flight testing of knowing if our engine is developing the standard thrust for a given rpm or fuel flow. There are also times when we want to determine airplane drag and to do this we need to determine thrust. Before we examine the methods of in-flight thrust measurement, let us review the basic equations for the thrust of a jet engine.
A jet engine produces thrust by taking a mass of air in the intake, adding energy, and accelerating it out the tailpipe. The gross thrust developed by this process can be expressed by the equation:
| (7.1) |  |
where
| F G | = gross thrust |
| Q e | = mass flow through tailpipe |
| V ex | = the air mass exit velocity |
| A e | = the exit area |
| P e | = the exit static pressure |
| P A | = ambient static pressure |
A measurement of gross thrust does not consider forward...
