ACS Alignment

FAA-S-ACS-15 — Private Pilot, Rotorcraft–Helicopter · Area of Operation VI. Fundamentals of Flight / Aerodynamics · Task: Aerodynamics — Translating Tendency, Dissymmetry of Lift, ETL

PA.VI.A.K1 — translating tendency & tail-rotor drift PA.VI.A.K2 — dissymmetry of lift & flapping PA.VI.A.K3 — effective translational lift (ETL)

Translating Tendency, Dissymmetry of Lift & ETL

Why the helicopter drifts, rolls, and surges as it starts to fly.

By the end of this lesson you can:

Explain translating tendency (tail-rotor drift) and how it's corrected.
Describe dissymmetry of lift and how blade flapping equalizes it in forward flight.
Define effective translational lift and the speed range where it occurs.
Relate transverse flow effect and gyroscopic precession to the handling changes near ETL.

1 · Translating tendency

In a hover the tail rotor produces a sideward thrust to counter main-rotor torque, and that thrust makes the whole helicopter drift sideways — this is translating tendency (tail-rotor drift). The pilot (and often a slight rigging offset) counters it with cyclic so the helicopter holds position.

2 · Dissymmetry of lift

In forward flight the advancing blade sees higher relative wind than the retreating blade, so it would make more lift — an imbalance called dissymmetry of lift. The rotor equalizes it through blade flapping: the advancing blade flaps up (reducing angle of attack) and the retreating blade flaps down (increasing it), keeping lift balanced across the disc.

3 · Effective translational lift (ETL)

As airspeed increases (roughly 16–24 kt), the rotor moves into undisturbed air and becomes markedly more efficient — effective translational lift. You feel it as a tendency to climb and a slight pitch/roll change, with some airframe vibration smoothing out. Transverse flow effect and gyroscopic precession contribute to the pitch-up/roll you anticipate with cyclic.

4 · Watch

Curated reference clip — “Effective Translational Lift (ETL) in Helicopters,” Helicopter Lessons In 10 Minutes or Less (YouTube). Embedded with the creator's player; we don't host or alter it.

5 · Reference sources

Use the authoritative references

📄 FAA Helicopter Flying Handbook, Ch. 2 — Aerodynamics of Flight 📄 FAA HFH, Ch. 3 — Helicopter Flight Controls

Risk management (the “Consider”): these effects aren't hazards by themselves, but failing to anticipate them is: an un-corrected translating tendency drifts you off a spot, and an un-anticipated ETL pitch-up at low altitude can balloon you into trouble. Fly the controls proactively — small inputs ahead of the aircraft, not behind it.