This paper studies aerodynamic effects of ground on regressive lag mode damping during ground resonance. The experimental investigation is performed on a scaled helicopter model built to simulate the ground resonance scenario. The study involves both stationary as well as dynamic (oscillating) ground conditions. Experiments are conducted by measuring lead-lag damping at different rotor speeds (in the ground resonance regime) for different collective inputs. Results show that ground and its dynamics have a significant effect on regressive lead-lag mode damping during ground resonance. Copyright © 2019 by AHS - The Vertical Flight Society. All rights reserved.