While in cruise flight in a semirigid (two-bladed) rotor helicopter, a pilot abruptly pushes the cyclic forward, creating a momentary low-G (near-zero or negative G) flight condition. What is the primary hazard of applying lateral cyclic input during this condition?
- ALateral cyclic becomes ineffective as rotor thrust is reduced, and tail rotor thrust can roll the fuselage into the rotor system, risking mast bumping
- BRetreating blade stall will immediately occur due to increased load factor
- CThe main rotor RPM will decay rapidly, requiring immediate autorotation
- DThe tail rotor will stall, causing loss of anti-torque control
Show answer & explanationAnswer & explanation
Correct answer: A. Lateral cyclic becomes ineffective as rotor thrust is reduced, and tail rotor thrust can roll the fuselage into the rotor system, risking mast bumping
In a low-G condition, main rotor thrust is greatly reduced, making the rotor system less responsive to lateral cyclic input. Meanwhile, tail rotor thrust continues acting on the fuselage, causing it to roll rapidly to the right (for a counterclockwise rotor) relative to the relatively unresponsive rotor disc. Applying lateral cyclic in an attempt to correct this can result in excessive rotor flapping and mast bumping, potentially leading to catastrophic rotor-mast separation.
Why the other options are wrong
- B. Retreating blade stall relates to high-speed high-load conditions, not a low-G pushover.
- C. Rotor RPM decay is not the defining hazard of low-G; the danger is loss of lateral control and mast bumping.
- D. Tail rotor stall is not the primary concern in a low-G pushover; fuselage roll relative to the disc is.
Low-G Condition / Mast Bumping
A dangerous condition in semirigid rotor helicopters where an abrupt forward cyclic push reduces rotor thrust, causing loss of lateral cyclic effectiveness and a fuselage roll that can lead to the rotor hub striking the mast (mast bumping).
- Caused by abrupt forward cyclic (pushover) creating near-zero G
- Lateral cyclic becomes ineffective because rotor thrust is reduced
- Recovery: apply gentle aft cyclic first to restore rotor thrust before correcting roll
Memory trick: Push forward hard, and the tail rotor swings you into the mast.