Abstract: Aerodynamic characteristics of a simplified aircraft model pitching with constant pitch rate were studied in a water tunnel. Dynamic force measurement and flow visualization were made simultaneously. The three-component forcemeasurement balance was internal type and fixed to the sting, and the flow over the wing was visualized by hydrogen bubble method. In the pitch-down process with reduced pitch rate k larger than 0.11, negative drag phenomena were detected at certain positive angles of attack. Analysis of the force measurement and corresponding visualization results show that during the pitching down process, the interaction between the attaching flow of higher speed near leading edge and the stagnating fluid in the separation region causes an increase of pressure on upper surface of the wing, which causes the negative drag at a positive angle of attack. The magnitude of negative drag depends on the reduced pitch rate and the start angle of attack.