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Main rotor blade droop with the rotor stopped

Aviation Mechanic Airframe (AMA) practice question 5 of 11 on Rotorcraft Fundamentals. ACS element AM.II.N.R1 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhy do the main rotor blades of most helicopters droop considerably when the aircraft is at rest and the rotor is not turning?

  1. AThe flapping hinge and blade flexibility allow it
  2. BCentrifugal force pulls the tips down at rest
  3. CThe collective has been left in the full down position
Show the answer and explanation

Correct answer: A. The flapping hinge and blade flexibility allow it

  • A — correct The flapping hinge, together with the natural flexibility found in most blades, permits the blade to droop considerably when the helicopter is at rest and the rotor is not turning over.
  • B — incorrect Centrifugal force acts outward from the tip only while the blades rotate, and in flight it stiffens the blade rather than lowering it.
  • C — incorrect Collective position sets blade pitch angle; it does not bend the blades down along their span.

BACKGROUND

The flapping motion of a rotor blade is the result of the constantly changing balance between lift, centrifugal, and inertial forces, and this rising and falling of the blades is characteristic of most helicopters. The flapping hinge, together with the natural flexibility found in most blades, permits the blade to droop considerably when the helicopter is at rest and the rotor is not turning over. During flight, the necessary rigidity is provided by the powerful centrifugal force that results from the rotation of the blades. That force pulls outward from the tip, stiffening the blade, and it is the only factor that keeps the blade from folding up. An offset flapping hinge sits away from the center of the hub, and the offset multiplied by the force produced at the hinge creates a moment at the hub that is useful for control.

STUDY TIP

Remember that a parked rotor is a limp rotor: only rotation supplies the centrifugal force that makes a blade stiff, so expect droop whenever the rotor is stopped.

KEY POINTS

  • Flapping hinges and natural blade flexibility let blades droop when the rotor is stopped.
  • In flight, centrifugal force pulling outward from the tip supplies the rigidity.
  • That centrifugal force is described as the only factor keeping the blade from folding up.

SOURCES

  • FAA-H-8083-31B ch02 "Offset Flapping Hinge" (sources/ama-airframe/ch02.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.