The premise that flapping flight can offer aerodynamic advantages over conventional aircraft has increased interest in bio-inspired flight. Despite the advanced understanding of a rigid flapping wing’s flow dynamics, the topic of flexible wings is yet to be fully explored.
An investigation on passively deforming flapping wings in hover has been conducted to study the influence of chordwise flexibility on the aerodynamic performance. Flat plate wings with a chord length of 52mm and an aspect ratio of 2.8 were prepared: One rigid wing (stainless steel) serves as a reference case and six flexible wings made of PETG and Polymer VPS that differ in stiffness. The leading and trailing edges were reinforced to disregard spanwise deformation.
A wing mounting mechanism was designed to limit the degree of freedom of the wing while still allowing the chord to camber and the leading and trailing edges to adapt to the wings deformation. Mechanism and wing were attached to a mechanical flapper to conduct direct force measurements and particle image velocimetry at half span. The fluid-structure interactions during the in-flight deformation of the flexible wings was detected by means of image post- processing.
The kinematics of flapping wings in hover are mimicked, the wing performs forward- and backward motion undergoing a symmetric rotation at Reynolds number Re=365. The leading edge vortex (LEV) in insect flight is the most important aerodynamic feature, augmenting lift production and was primarily studied.
The rigid wing and the stiff PETG wings display a similar flow structure throughout a half cycle as they do not undergo significant deformation. A leading edge vortex emerges as the wing pitches, attaches and grows to cover 2/3 of the chord during the purely translational phase of the half cycle. The vortex lifts itself from the chord after the maximum stroke velocity has passed. During the pitch up the vortex breaks down and disolves. After the lift-off the vortex’s proximity to the chord is smaller for stiff wings, resulting in a higher maximum lift coefficient clmax of up to 4%.
The flexible polymer wings exhibit in-flight deformations during the entirety of the flapping cycle. The chord cambers to an arc shape and the distance between the leading and trailing edge is reduced. The effective angle of attack and trailing angle decreased significantly to accommodate the chords deformation. Wing cambering has a major impact on the unsteady flow field surrounding the wing. The leading edge vortex attaches to the chord and grows to cover its entire length and stays attached as the vortex breakdown begins once the wing pitches up, the leading edge vortexes lift-off does not occur. The prolonged attachment results in an increase in lift and circulation, and the flexible wings perform up to 40% higher in terms of efficiency.
Furthermore, the influence of stroke kinematics and Reynolds number was studied. The stroke velocity has a dominating impact on lift and drag, as it is reflected in their progression through- out a flapping cycle. An increase in the Reynolds number leads to an increase in lift production and efficiency for all but the most flexible wing. The thinnest polymer wing performs increasingly worse the higher the Reynolds number, and exhibits a similar aerodynamic performance as the rigid wing at Re=731.
Overall it is seen that a deformable chord has a considerable effect on the aerodynamic performance of a flapping wing during hover. Up to a degree, increased flexibility creates a higher clmax as the leading edge vortex encompasses the entire chord length and keeps the vortex in closer proximity to the wing surface.