This document discusses a methodology for estimating control derivatives of a helicopter in ground effect from flight test data. It presents theories of momentum and blade element for rotor behavior. Flight test data was collected at various altitudes both in and out of ground effect. Polynomial models were fit to torque coefficient data to characterize effects of thrust, collective pitch, and altitude. Results showed the derivative of thrust with respect to collective pitch was 10-20% greater in ground effect, important information for helicopter simulators.