FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesHard
A turbojet engine has a mass airflow rate of 200 lbm/sec. The inlet (freestream) velocity is 400 ft/sec and the exhaust jet velocity is 1,200 ft/sec. Using F = (W/g)(Ve − Vo), what is the approximate net thrust produced (neglecting pressure thrust)?
- A9,938 lb
- B1,600 lb
- C4,969 lb
- D800 lb
Show answer & explanationAnswer & explanation
Correct answer: C. 4,969 lb
F = (W/g)(Ve − Vo) = (200/32.2)(1,200 − 400) = 6.211 × 800 ≈ 4,969 lb. This is the basic momentum thrust equation used to estimate net thrust from mass flow and velocity change.
Why the other options are wrong
- A. This is double the correct thrust value, a common calculation error.
- B. This is double the velocity change but omits the mass flow term.
- D. This equals only the velocity change, not multiplied by mass flow/g.
Net Thrust Calculation
Net thrust equals mass flow rate (in slugs/sec) multiplied by the change in velocity between exhaust and inlet: F = (W/g)(Ve − Vo).
- W = mass airflow in lb/sec
- g = 32.2 ft/sec²
- Ve = exit velocity, Vo = inlet velocity
- Pressure thrust term often added for full gross thrust
Memory trick: Thrust = mass × speed gain, divided by gravity's pull