Preliminary results of an in-flight thrust measurement method are presented in order to determine the drag force. Flight experiments are conducted by an off-the-shelf aircraft which has been equipped with an autopilot system and additional sensors measuring the electric motor rotation speed, battery voltage, drawn current, and direct thrust of the motor. Several level cruise flights have been performed in order to measure the thrust, thus the drag force generated by the aircraft. The propulsion system used on the aircraft is first characterized in the wind-tunnel. The direct thrust measurement system which is on-board the aircraft presents a lot of noise coming from vibrations, characteristics of the sensor itself, and also the fuselage wake and propeller interaction. Hence a better estimation of the in-flight thrust and the resultant drag is obtained by sensor fusion of Gaussian signals.