FAA Powerplant Written Practice Test

In a convergent exhaust nozzle, what is a critical design consideration?

1. The outlet diameter must be too large

2. The outlet diameter must be exactly circular

3. The outlet diameter must not be too large to prevent energy waste

4. The outlet diameter should vary with engine speed

The correct answer is: The outlet diameter must not be too large to prevent energy waste

In a convergent exhaust nozzle, a critical design consideration is that the outlet diameter must not be too large to prevent energy waste. This principle is grounded in the fundamental operation of a convergent nozzle, which is designed to accelerate the exhaust gases exiting the engine. If the outlet diameter is too large, it allows the exhaust gases to expand excessively, leading to a reduction in the velocity of the exhaust jet. This not only diminishes the efficiency of the nozzle but also results in wasted energy that could have been converted into thrust. The design of the nozzle must balance the need to minimize backpressure on the engine while ensuring that the exhaust gases are expelled at optimal velocity for effective propulsion. While factors like the shape of the outlet, such as being exactly circular, can influence performance, it’s the outlet diameter’s direct impact on the exhaust flow dynamics that is paramount for maintaining thrust efficiency. Moreover, although varying the outlet diameter might have some implications for performance in specific applications, a constant outlet diameter optimized for a given design condition is more commonly practiced in convergent nozzles to ensure steady performance across various operational scenarios.