Experimental analysis of the sweepback angle effect on the thrust generation of a robotic penguin wing.

Penguins have evolved excellent swimming skills as diving birds, benefiting from their agile wings. This paper experimentally analyses the effect of the wing sweepback angle on thrust generation using a robotic penguin wing. A developed wing mechanism that can realize penguin-like flapping and feathering motion was employed for actuating five alternative wing models with different sweepback angles from 0° to 50°. Force measurements under a steady water flow were conducted for both fixed and flapping states for all wing models. The results showed that small sweepback angles of 30° or less in the fixed state led to a steep lift curve and that a moderate sweepback angle of 30° produced the largest lift-to-drag ratio. In the flapping state, the smaller-sweepback wings generated a larger net thrust for the same wing motion, whereas the larger-sweepback wings produced more thrust given the same Strouhal number. It was also found that larger-sweepback wings more easily achieved the maximum net thrust in terms of less angle-of-attack control. On the other hand, the hydrodynamic efficiency was not greatly affected by the sweepback. Regardless of the sweepback, the trend of the efficiency increasing with increasing flow speed indicated that the penguin wings can be more suitable in high-speed locomotion for higher hydrodynamic efficiency.