Cavities Improve the Power Factor of Low-Reynolds-Number Airfoils and Wings

The poor aerodynamic power factor of low-Reynolds-number airfoils greatly limits the flight time of small-scale drones. Although the power factor can be improved by maintaining a turbulent boundary layer over the wing, this is challenging to achieve at Reynolds numbers below 50,000. We present a design strategy combining high airfoil camber and a cavity carved into the suction side of the airfoil to generate a turbulent, attached flow. The camber of a low-Reynolds-number AG14 airfoil is increased from 2 to 7%; whereas the cavity position, length, and depth are derived based on spatial linear stability theory of the separated shear layer over the cavity. Lift and power factors of the modified airfoils are characterized computationally using XFOIL. The higher camber cavity airfoils show a 100% gain in power factor over a smooth AG14 at Reynolds numbers of 30,000 and 10,000. Wind-tunnel testing of wings with the cavity airfoils showed power factor improvements of 30 and 80% at Reynolds numbers of 30,000 and 10,000, and hot-wire measurements confirmed that the cavity triggers a turbulent boundary layer. Finally, it is suggested that flow control may be required to avoid laminar reattachment of the shear layer and to extend the use of the cavity design to even lower Reynolds numbers.