Experimental aerodynamic study of tandem flapping membrane wings

A systematic series of wind-tunnel tests was conducted on an ornithopter configuration consisting of two sets of symmetrically flapping wings, located one behind the other in tandem. It was discovered that the tandem arrangement can give thrust and efficiency increases over a single set of flapping wings for certain relative phase angles and longitudinal spacing between the wing sets. In particular, close spacing on the order of 1 chord length is generally best, and phase angles of approximately 0 +/- 50 deg give the highest thrusts and propulsive efficiencies. Asymmetrical flapping was also studied, which consists of the two sets of wings rocking relative to one another 180 deg out of phase. It was found that the performance of such an arrangement is poor, relative to the best performing symmetrical tandem flapping.