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Relative windspeed at the advancing blade tip

Aviation Mechanic Airframe (AMA) practice question 2 of 11 on Rotorcraft Fundamentals. ACS element AM.II.N.K6 · 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 with a main rotor blade tip speed of approximately 300 knots is moving forward at 100 knots. What is the relative windspeed at the tip of the advancing blade?

  1. A200 knots, the difference of the speeds
  2. B400 knots, the sum of the two speeds
  3. C300 knots, unchanged by forward flight
Show the answer and explanation

Correct answer: B. 400 knots, the sum of the two speeds

  • A — incorrect 200 knots is the relative windspeed on the retreating side, where forward speed subtracts from blade tip speed.
  • B — correct On the advancing side the helicopter's forward speed adds to blade tip speed, so 300 knots plus 100 knots gives a relative windspeed of 400 knots.
  • C — incorrect Blade tip speed alone applies only with no forward motion; once the helicopter moves forward the advancing and retreating sides no longer see the same windspeed.

BACKGROUND

When the plane of rotation of the rotor blades is tilted and the helicopter begins to move forward, an advancing blade and a retreating blade become established. The relative windspeed is greater on the advancing blade than on the retreating blade, so greater lift is developed on the advancing blade and it rises, or flaps. When blade rotation carries that blade to the retreating side, the extra lift is lost and the blade flaps downward. The handbook works the numbers with a blade tip speed of approximately 300 knots and a forward speed of 100 knots: the advancing side sees 400 knots of relative wind while the retreating side sees only 200 knots. This difference in speed is what is meant by dissymmetry of lift.

STUDY TIP

Add forward airspeed to tip speed on the advancing side and subtract it on the retreating side, then remember that the gap between the two is dissymmetry of lift.

KEY POINTS

  • Forward flight creates an advancing blade and a retreating blade.
  • Advancing tip windspeed is tip speed plus airspeed; retreating tip windspeed is tip speed minus airspeed.
  • At 300 knots tip speed and 100 knots forward, the two sides see 400 and 200 knots, the difference known as dissymmetry of lift.

SOURCES

  • FAA-H-8083-31B ch01 "Semirigid Rotor System", Figure 1-97 (sources/ama-airframe/ch01.txt)
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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.