Effects of Speed on Coupled Sweep and Camber in Morphing Wings

Abstract

Like birds, bats, and insects, a benefit of morphing aircraft is their ability to adapt to a variety of flight conditions by changing the geometry of their wings, unlike their traditional counterparts which remain designed and optimized typically for a single flight condition. Here we study the coupled effect of multi-scale morphing, namely sweep and camber, for varying velocities. Nine different wing configurations are considered consisting of combinations of three sweep angles and three airfoil profiles. The three sweep configurations include minimally, moderately, and highly swept planforms. The airfoil profiles considered include a NACA 0012 airfoil, a conventionally cambered airfoil, and a reflex cambered airfoil. The study is based on numerical simulations conducted using a Reynolds-averaged Navier-Stokes (RANS) turbulence model for low-Reynolds-number flow. The results for the simulations are presented and discussed with a particular focus on gliding behavior during steady level flight. From the numerical results, there is a clear indication that there are considerable benefits in the proposed multi-scale morphing, modifying the wing and the airfoil shapes at varying velocities.

Bats actively modulate membrane compliance to control camber and reduce drag

“…at faster speeds. Variable-camber wings, optimized for minimising drag, decrease camber with speed (Woods et al, 2014;Gamble et al, 2017), as do bats (Fig. 1D,E von Busse et al, 2012), and possibly owing to speed-dependent patterns of plagiopatagiales activation (Cheney et al, 2014).…”

“…In steady level flight, the resultant aerodynamic force from lift and drag is oriented vertically, because thrust balances drag and the vertical force provides weight support. In general, increasing camber when low lift coefficients are required results in a lower lift-to-drag ratio (Gamble et al, 2017;Woods et al, 2014;Barbu et al, 2018), tilting the aerodynamic force vector backwards. To return to equilibrium, the bats would require greater thrust.…”

“…Although the plagiopatagiales should expand the flight envelope of bats in the upper range of speeds through tensing the membrane and enhancing flight efficiency, they should not be required for efficient flight at very low speeds. As speed increases and q −1 decreases, membrane wings can maintain a high lift-to-drag ratio by increasing either tension or stiffness to adopt a less-cambered configuration (Hays et al, 2012;Waldman and Breuer, 2017;Gamble et al, 2017).…”

Raptor wing morphing with flight speed

“…In studies of camber-morphing wings, increased camber is consistently beneficial at low speeds or high angles of attack, but, as speed increases and the lift coefficient requirement reduces, camber should decrease for efficient flight [ 9 , 24 , 25 ]. As changing camber with speed is beneficial, it is perhaps not surprising that it occurs in birds, but the mechanism by which camber changes is not clear, as there is relatively little muscle to control wing feather deflection actively.…”

“…Wing sweep alone, with minimal area change, allows wings to operate at high lift-to-drag ratios over a wide range of speeds [ 6 ]. When changes in wing sweep are combined with either wing area [ 7 ], tail area [ 8 ] or camber profile [ 9 ], the flight envelope can be enhanced further [ 9 ]. Additionally, tail morphing alone can also reduce the cost of flight through changing pitch or tail spread [ 10 ] because the tail plays an important role in modulating the lift distribution over the bird [ 11 ].…”