Four-wing flapping flight - effects of wing phasing and material flexibility on performance in hover and forward flight

Abstract

Previous researches on dragonfly aerodynamics are based on rigid wings, while natural dragonfly wings are flexible with varying spanwise and chordwise stiffness. Flexibility in single paired insect wings has shown enhanced flight efficiency. The effect of wing flexibility in tandem wings aerodynamics is relatively unknown and is explored in this study.

Firstly, rigid wing simulations in hover using rectangular shaped tandem wings are carried out for phases 180° to -135° in an interval of 45°. A detailed analysis to understand vortex dynamics is carried out for phase 0°, 45° and -90°. Next, flexible wing simulations are carried out in all phases. The flexible tandem wings in all the phases show an increase in the mean lift with improved hover efficiency. However, wing flexibility also increases the difference in the mean lift between the tandem wings in the three selected phases which may lead to flight instability in hover.

Next, the effect of wing flexibility on dragonfly shaped wings with the spanwise and chordwise stiffness distribution as found in real dragonfly wings is studied at phase 180° in hover. Wing flexibility improves the lift generated by the single wings and by each tandem wing in phase 180°, with little change in the hover efficiency. The difference in the mean lift of the flexible tandem wings is lower which may lead to a more stable hover flight However, even in flexible tandem wings, each wing produces lowe1 lift than the respective as single wing suggesting that tandem arrangement is detrimental for lift.

Lastly, in forward flight at phases 90°, 0° and 180°, based on the kinematics, location of axes, pivot point, the tandem wings in all phases produced drag rather thrust However, the mean drag is almost 2 orders of magnitude lower than the peak drag, suggesting a minor change in the peak can bring substantial change in the mean drag The flexible tandem wings produced higher lift except in phase 180°. Phase 0° had large force variations which is detrimental for flight stability and control. Phase 90' gave the best lift economy with smaller difference in mean lift than phase 180°.