Which factor affects thrust the least?

• A. Humidity
• B. Temperature
• C. Compression RPM
• D. Altitude

Answer: (A)

Thrust is largely determined by how much air the engine can move and how much energy it adds to that air. The amount of air the engine ingests (and hence the mass flow) depends mainly on air density, which changes with altitude and temperature, and on how fast the compressor is turning (compression RPM). Higher altitude or higher temperature makes air less dense, so the engine moves less air and produces less thrust. Increasing compressor RPM directly increases the mass flow and the pressure ratio, boosting thrust.

Humidity, meanwhile, changes air density only very slightly. moist air is a bit lighter than dry air, but the density change is small compared with what altitude or temperature do. So the effect of humidity on thrust is minor compared with the other factors, making it the factor that affects thrust the least.