FAA Powerplant Written Practice Test

In a convergent exhaust nozzle, what must not be too large to avoid wasting energy?

1. The inlet diameter.

2. The outlet diameter.

3. The nozzle length.

4. The overall length of the exhaust duct.

The correct answer is: The outlet diameter.

In a convergent exhaust nozzle, the outlet diameter must be appropriately sized to effectively convert the thermal energy of the exhaust gases into kinetic energy, maximizing thrust while minimizing energy losses. If the outlet diameter is too large, it can lead to a drop in exhaust velocity and reduced efficiency. This loss of velocity means that less kinetic energy is imparted to the exiting gases, which in turn decreases the thrust produced by the engine. The nozzle is designed to accelerate the exhaust gases as they pass through it, achieving higher velocities at the exit. A large outlet diameter can disrupt this acceleration process, resulting in lower pressure and less efficient thrust generation. Therefore, maintaining an optimal outlet diameter is crucial to avoid wasting energy and ensuring that the engine operates efficiently.