FAA Aviation Mechanic Powerplant (AMP)PropellersHard
A propeller blade station is located 20 inches from the center of rotation and has a blade angle of 20 degrees at that station. What is the approximate geometric pitch at that station? (tan 20° = 0.364)
- A22.9 inches
- B72.8 inches
- C45.7 inches
- D36.4 inches
Show answer & explanationAnswer & explanation
Correct answer: C. 45.7 inches
Geometric pitch equals the circumference at that radius multiplied by the tangent of the blade angle: GP = 2πr × tan(angle). Circumference = 2π(20) = 125.66 inches. GP = 125.66 × 0.364 ≈ 45.7 inches.
Why the other options are wrong
- A. This results from using only half the circumference in the calculation.
- B. This doubles the correct answer, likely from a squaring or radius/diameter error.
- D. This is just the circumference multiplied incorrectly, omitting the full 2π factor.
Geometric Pitch
The theoretical distance a propeller would advance in one revolution if moving through a solid, based purely on blade angle geometry at a given station.
- Formula: GP = 2πr × tan(blade angle)
- Differs from effective pitch due to slippage in air
- Calculated at a specific blade station radius
Memory trick: Circle the radius, then tilt it with tangent to get geometric pitch