FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsHard
A fixed-displacement engine-driven hydraulic pump delivers 6 gallons per minute of fluid while being driven at 4,800 rpm. Approximately what is the pump's displacement per revolution, in cubic inches? (1 gallon = 231 cubic inches)
- A0.58 cubic inches
- B1.16 cubic inches
- C0.14 cubic inches
- D0.29 cubic inches
Show answer & explanationAnswer & explanation
Correct answer: D. 0.29 cubic inches
Convert flow rate to cubic inches per minute: 6 gal/min x 231 in³/gal = 1,386 in³/min. Divide by pump speed: 1,386 in³/min ÷ 4,800 rpm = 0.28875 in³/rev, which rounds to approximately 0.29 cubic inches per revolution.
Why the other options are wrong
- A. This is double the correct displacement, a common conversion error.
- B. This is four times the correct displacement value.
- C. This value results from dividing by roughly double the correct rpm figure.
Pump Displacement Calculation
Pump displacement per revolution equals volumetric flow rate divided by pump rotational speed, after converting flow to consistent volume units.
- 1 gallon = 231 cubic inches
- Displacement (in³/rev) = flow (in³/min) ÷ speed (rpm)
- Fixed-displacement pumps have constant displacement per revolution
Memory trick: Gallons to inches, then divide by spins — that's the pump's gulp per turn.