SkyDrill

Main rotor rpm from a transmission reduction ratio

Aviation Mechanic Airframe (AMA) practice question 8 of 11 on Rotorcraft Fundamentals. ACS element AM.II.N.K2 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA helicopter engine turns at 2,700 rpm and the main rotor transmission provides a 6:1 reduction. What is the resulting main rotor speed?

  1. A300 rpm, which is one ninth of engine speed
  2. B2,700 rpm, the same as engine speed
  3. C450 rpm, one sixth of engine speed
Show the answer and explanation

Correct answer: C. 450 rpm, one sixth of engine speed

  • A — incorrect 300 rpm is the figure that results from a 9:1 reduction of the same 2,700 rpm engine speed, not from a 6:1 reduction.
  • B — incorrect The primary purpose of the main rotor transmission is to reduce engine output rpm to optimum rotor rpm, so rotor speed cannot equal engine speed.
  • C — correct Dividing 2,700 rpm by the 6:1 reduction gives 450 rpm, exactly the example the handbook uses for main rotor transmission reduction.

BACKGROUND

The primary purpose of the main rotor transmission is to reduce engine output rpm to optimum rotor rpm, and the amount of reduction differs from helicopter to helicopter. The handbook works the arithmetic with an engine turning 2,700 rpm: a rotor speed of 450 rpm requires a 6:1 reduction, while a 9:1 reduction would turn the rotor at 300 rpm. In helicopters with horizontally mounted engines, the transmission also changes the axis of rotation from the horizontal axis of the engine to the vertical axis of the rotor shaft. Most helicopters use a dual-needle tachometer or a vertical scale instrument to show both engine and rotor rpm. The rotor rpm indicator is normally used only during clutch engagement, to monitor rotor acceleration, and in autorotation to hold rpm within limits.

STUDY TIP

Treat the reduction ratio as simple division of engine rpm, and practice both directions: rotor speed from a ratio, and the ratio implied by a pair of speeds.

KEY POINTS

  • The main rotor transmission reduces engine output rpm to optimum rotor rpm.
  • 2,700 engine rpm through a 6:1 reduction gives 450 rotor rpm; a 9:1 reduction would give 300 rpm.
  • With a horizontally mounted engine the transmission also turns the axis of rotation from horizontal to vertical.

SOURCES

  • FAA-H-8083-31B ch02 "Transmission System — Main Rotor Transmission" (sources/ama-airframe/ch02.txt)
Practice the Aviation Mechanic Airframe (AMA) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON ROTORCRAFT FUNDAMENTALS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.