FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsEasy
A student pilot is told that a helicopter accelerating through approximately 16-24 knots during takeoff will experience a sudden, noticeable increase in lift and a corresponding nose-up pitching tendency without any change in collective or cyclic. What aerodynamic phenomenon causes this?
- ATranslating tendency
- BEffective translational lift
- CCoriolis effect
- DGyroscopic precession
Show answer & explanationAnswer & explanation
Correct answer: B. Effective translational lift
Effective translational lift (ETL) occurs as the rotor system outruns its own recirculated turbulent downwash and begins receiving relatively undisturbed air, dramatically improving rotor efficiency and producing extra lift, often felt as a shudder and a slight nose-up pitch.
Why the other options are wrong
- A. Translating tendency is the sideways drift caused by tail rotor thrust, unrelated to airspeed lift gains.
- C. Coriolis effect relates to blade acceleration/deceleration during flapping, not overall lift increase.
- D. Gyroscopic precession is a 90-degree delayed response to a rotor input, not a lift phenomenon.
Effective Translational Lift (ETL)
An increase in rotor efficiency that occurs around 16-24 knots as the rotor system moves out of its own recirculated downwash into relatively undisturbed air.
- Occurs around 16-24 knots forward airspeed
- Rotor disk tilts to gain purchase on clean, undisturbed air
- Pilot may feel a slight shudder and nose-up pitching tendency
Memory trick: ETL = Escaping The (turbulent) air, Lift jumps!