Aeromechanical performance of compliant aerofoils

Abstract

The aeromechanics of compliant aerofoils are studied. Several experimental techniques including hot-wire anemometry, particle image velocimetry, high speed photogrammetry and strain gauge force measurements are used. Tests are performed at a chord based Reynolds number of Re= 4x10^4 and angles of attack between 0º and 25º . They explore the impact of the geometry of the leading- and trailing-edge supports as well as the rigid- ity of the aerofoil on the aeromechanics and aerodynamics of membrane aerofoils. Tests on latex membrane wings subjected by four different types of supports are performed. Firstly, the study focuses on the structural performance by evaluating detailed measure- ments of membrane deflections and lift and drag forces. It will be shown that the use of lower bending stiffness supports results in noticeable deformations, both static and dy- namic, especially at mid-to-high incidences. Moreover, the conjunction of hot-wire results with photogrammetry imagery of the membrane deformation indicates that the membrane vibration is coupled with the vortex shedding. This, when coupled with a low-stiffness rectangular cross-section leading- and trailing-edge, results in large amplitude vibrations affecting the membrane, the support and the wake. Hence, a more detailed study of the vortex shedding and the wake attributes is presented. The findings indicate that for low angles of attack the wake characteristics are highly affected by the leading- and trailing- edge geometry; as incidence increases the wake characteristics become less dependant on the support's geometry, eventually reaching a point in which they are fully independent of it and closely resembling a fully stalled rigid aerofoil. Finally, the effects of the aero- foil rigidity are analysed. Tests of varying thickness but constant Young's modulus on unidirectional carbon fibre composite plates are performed. Results show that the Weber number is a crucial parameter when defining the properties and performance of the wing. Furthermore, the study will show that lift and drag forces are higher for membrane wings than for composite plates and that the dynamic motions of the composite plates increase as the plate thickness is decreased resulting in earlier wing stall and worse post-stall be- haviour than membrane wings. The results of this study should provide valuable insight for future use of membrane wings in micro air vehicles.