2023 Volume 18 Issue 3 Pages JFST0032
In this study, the ground effect in flapping flight was investigated by using a two-dimensional flapping mechanism that make the flapping angle and twisting angle of the wings moving in a sinusoidal function. The experiment employed a height-adjustable ground platform to test the aerodynamic force variation with/without ground effect. The results indicated that when the distance to the ground is more than double the wing’s chord length, the lift coefficient remains unaffected by the ground for various Reynolds numbers and flapping amplitudes. Conversely, when the distance to the ground is less than double the chord length, the lift coefficient increases dramatically. The Reynolds number also plays a crucial role in the ground effect, as the lift coefficient grows less than 10% when the distance to the ground is the closest in the low Reynolds number (Re=5000), but increases by over 25% in the high Reynolds number (Re=20,000). Although the increase curve of the lift coefficient about each movement is similar for different flapping wing amplitudes, the lift coefficient decreases when the minimum distance is double the chord length. The analysis of the position of the center of pressure on the wing's surface showed that the wing with ground effect shifts the center of pressure toward the wing root and trailing edge, indicating an increase in the effective angle of attack. PIV measurement verified that the ground effect in the middle wing stroke increased the effective angle of attack. This study expanded our understanding of the ground effect theory in flapping aerodynamics under various flight conditions and provided valuable references for the take-off and landing manipulation of flapping-MAV.
