Active flow control as a technique to improve fixed-wing and rotary-wing aerodynamics of uavs

Increasing the aerodynamic efficiency of unmanned aerial vehicles (UAVs) is critical to improving the flight time and range of these vehicles. In this paper, active flow control techniques are presented to increase the flight performance of fixed-wing tail-less UAVs using a conceived dual micro-tab device driven by piezoelectric materials, and the 'figure of merit' (FM) of propulsive rotors for large multi-copter drones. A novel reconfigurable UAV concept known as 'Switchblade' is presented as a research platform on which to investigate new active flow control techniques for flying wing UAVs. For propulsive rotors, a flow control scheme is proposed where 60 synthetic jet actuators are utilized on a large-scale propulsive rotor and the jets' velocity is increased from the blade root to the blade tip in order maintain constant levels of momentum coefficient per blade module. The three-bladed rotor measures 2.58 m in diameter and contains a NACA 0012 airfoil with zero blade twist distribution. The rotor was tested at speeds of 250, 500, 750 and 1,000 revolutions per minute (RPM) and blade pitch angles of 2, 5, and 8 degrees. Rotor thrust and power were measured using a high-capacity load cell and current sensor, and the streamwise flow was measured using phased-locked laser Doppler velocimetry (LDV) techniques at two measurement planes near the blade root and near the blade tip along the upper surface of the airfoil. It was found that applying flow control near the blade root does not improve aerodynamic performance because the boundary layer profiles are attached and stable. However, near the blade tip the boundary layers undergo transition to turbulent flow combined with an increasing degree of adverse pressure gradient with rotor speed and blade pitch angle. Under these conditions where the normalized streamwise velocity profiles show significant velocity deficit in the near-wall flow, synthetic jets provide some momentum to delay separation as quantified by the boundary layer shape factor. There is also good agreement between the fluid dynamics over the blade tip module and the overall figure of merit of the rotor, where the FM and thrust coefficient increased by up to 12.7% and 4.1% respectively at 500 RPM and a blade pitch of 5 degrees.