What does "effective translational lift" (ETL) mean for a rotorcraft or multirotor transitioning to forward flight?
Why →
ETL (Effective Translational Lift) is experienced as forward airspeed increases to approximately 16–24 knots. At that point, the rotor moves through undisturbed air rather than its own recirculating downwash. Rotor efficiency improves noticeably: the same thrust requires less power, or the same power generates more lift. For multirotors, this manifests as reduced motor current draw and improved endurance during slow forward flight vs. hover.The trap →
Maximum endurance speed is a separate, airspeed-specific concept. ETL is not altitude-based. The key is the 16–24 knot airspeed threshold where recirculating downwash is left behind. Below ETL, the rotor constantly fights its own dirty air.SOURCE → FAA Helicopter Flying HandbookCHECKED APR 21ACS IV.A.K1MED