1792, he made a series of experiments with a toy helicopter named "China Flying Gyro". 1804, he wrote his first paper on flight principles. Kelly proposed in the paper that modern aircraft should adopt the mode of fixed wing+propeller instead of imitating the vibrating wing of birds. He described the outline of modern aircraft in detail, and pointed out that proper stability is produced by obtaining some angles when manufacturing the wing surface, which is the dihedral angle of modern aircraft. He also pointed out that the tail must have vertical and horizontal rudder surfaces and the aircraft must be streamlined. He also studied the relationship between speed and lift, wing load and how to reduce the weight of aircraft.