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Geometric pitch calculation

Aviation Mechanic Powerplant (AMP) practice question 1 of 10 on Propellers. ACS element AM.III.M.K1 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA propeller blade has a blade angle of 45° at the 75 percent station, where the blade radius is 24 inches. Using π = 3.14, what is the propeller's geometric pitch?

  1. A151 inches
  2. B75.4 inches
  3. C302 inches
Show the answer and explanation

Correct answer: A. 151 inches

  • A — correct Geometric pitch = 2 × π × R × tangent of the blade angle at the 75 percent station. Because the tangent of 45° is 1, the result is 2 × 3.14 × 24 = 150.7, or about 151 inches.
  • B — incorrect This is the result of leaving the factor of 2 out of the formula and computing only π × R × tangent of the blade angle.
  • C — incorrect This doubles the correct answer, as though the 48-inch diameter had been used in place of the 24-inch radius called for by the formula.

BACKGROUND

Geometric pitch is the distance a propeller should advance in one revolution if there were no slippage, and it is normally expressed in pitch inches. The handbook gives the formula GP = 2 × π × R × tangent of the blade angle at the 75 percent station, where R is the radius at that station. Effective pitch is the distance the propeller actually advances, and geometric pitch minus effective pitch equals slip. Pitch and blade angle are not the same thing, but because pitch is largely determined by blade angle, the two terms are often used interchangeably.

STUDY TIP

Memorize the formula together with the fact that R is the radius at the 75 percent station, not the propeller radius or diameter, and practice it with a 45° blade angle so the tangent drops out.

KEY POINTS

  • Geometric pitch = 2 × π × R × tangent of the blade angle at the 75 percent station.
  • R is the radius at the 75 percent blade station, and the tangent of 45° is 1.
  • Geometric pitch minus effective pitch equals slip.

SOURCES

  • sources/amp-powerplant/ch07.txt "Basic Propeller Principles" (geometric pitch formula)
  • FAA-H-8083-32 ch07, Basic Propeller Principles
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON PROPELLERS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.