Aerodynamics of leading-edge and trailing-edge control surfaces at low Reynolds number

Small biological flyers and micro air vehicles (MAVs) operate in a low Reynolds number flight regime, close to the ground (within the atmospheric boundary layer) where the flow is known to be highly turbulent. Existing MAV designs with conventional control surfaces mounted on wing trailing-edges actuated with commercially available actuators have not been able to achieve sufficient control authority and rapidity to keep MAVs flying straight and level in a turbulent flow. The use of control surfaces hinged at the leading-edge of a wing is investigated as a potential solution to improving MAVs control authority and speed of response. Concerns with the unstable nature of such leading-edge control surfaces mean that they have not been used on larger manned aircraft. However the relatively low loads on fixed- wing MAV, coupled with the requirement to operate controls at much higher frequencies than manned aircraft, make them potentially useful for rapid manoeuverability and turbulence mitigation. Thus, there exists a gap in the current body of literature regarding the static and dynamic lift response of leading-edge control surfaces. In this thesis, the findings from experimental and analytical investigations into the following research questions are presented and discussed: 1. What are the time-averaged and time-varying forces and moments of a statically deflected leading-edge control surface in smooth flow and how do these compare against a conventional trailing edge hinged control surface? 2. What are the time-averaged and time-varying forces and moments of a dynamically deflected leading-edge control surface in smooth flow and how do these compare against a conventional trailing edge hinged control surface? 3. How well can a low-order theoretical model evaluate force production on an airfoil due to rapidly actuating leading-edge control surfaces? Two flat-plate airfoils, one with a leading-edge control surface (LECS) and another with a trailing-edge control surface (TECS) were manufactured and tested in the RMIT University Aerospace Wind Tunnel, Australia and the Hao Liu Laboratory Low-speed Wind Tunnel in Chiba University, Japan. Surface pressure measurements were made from the two airfoils to study the static and dynamic forces and moments produced at variations of airfoil angles of attack, control surface deflection angles and actuation rates in fully attached and massively separated flow conditions. Likewise, particle imagery velocimetry (PIV) experiments were also conducted to provide further insight into the flow mechanics associated with both static and dynamic test cases. Analysis of the statically deflected case revealed that the lift and drag forces produced from deflections of the LECS is complex and nonlinear when compared to the static lift response of TECS. TECS deflections correlated linearly with the amount of CL produced across control surface deflection angles of ±40 degrees, beyond which further deflections did not lead to increased CL. CL produced from LECS deflections increased with deflection angles up to a specific deflection, beyond which CL decreased with further deflections. For instance, in the case of 0 degree airfoil angle of attack, CL was linearly proportional with LECS deflections up to ±20 degrees . Further deflections beyond this resulted in decreasing the CL. TECS was found to be generally more effective (change in CL per degree of control surface deflection) in the time-averaged forces produced over the airfoil. LECS was found to be advantageous in the case of time-varying characteristics. The lift characteristics of both LECS correlated well to the results from a simple potential flow model at low airfoil angles of attack and control surface deflections. Dynamically actuated TECS and LECS were found to generate instantaneous lift response immediately upon control surface motion. Unsteady aerodynamic effects were observed for non-dimensional actuation rates above 0.11 with TECS and 0.05 with LECS. Unsteady effects included the production of large CL; well above steady-state values. The transient CL peaks were up to one and a half times greater than the static values with TECS and up to three times greater with LECS. The magnitude of the transient CL peaks in the case of TECS increased linearly with an increase in control surface actuation rate. With LECS deflections, a clear relationship between the magnitude of the transient CL peak and reduced frequency was not observed. Furthermore, Theodorsens classic unsteady aerodynamic model was found to over-predict CL and under-predict the duration of the unsteady effects for both LECS and TECS deflections. A modified model is proposed to accurately capture the unsteady lift response with a rapidly actuated LECS and compared against experimental results.