Aerodynamic Parameter Measurement Using the Wind Driven Manipulator: Inverse Force Measurement on Wings

Mm Pitching moment generated by the model Inverse force measurement using a wind-driven dynamic manipulator is explored. Using system identification techniques, the WDM offers a method of measuring forces and moments on wind tunnel models during rapid, multi-axis maneuvers. Previous work has shown the measurement of dynamic stability parameters for small disturbances using the WDM. System identification has been used to construct simulations of WDM behavior in order to match desired and actual maneuver trajectories. Here the nonlinear behavior of a pitch manipulator is measured over a range of model loads and attitudes. A look-up table constructed from these data is used to measure quasi-steady forces and moments on a rectangular wing and a delta wing. The WDM is used to hold models steady in a wind tunnel under feedback control, at various attitudes. Each model is taken through a range of attitudes, and the control input needed for equilibrium is recorded. By repeating the experiment with 3 model-mounting positions, a set of 3 simultaneous equations is generated for the lift, drag and pitching moment at each angle of attack. The technique is successful in measuring the lift of a rectangular wing and a delta wing. Measurement resolution was inadequate to resolve drag and pitching moment accurately in these tests. Methods for measuring these are discussed. Continuous force measurement from repeated maneuvers is discussed. lb Length of model attachment boom (WDM pivot to model quarter chord) q Wind tunnel freestream dynamic pressure Wm Model weight INTRODUCTION In the development of aircraft capable of executing rapid multi-axis maneuvers, measuring forces and moments during such maneuvers is a problem of interest. Modern fighter aircraft must venture into flight regimes where the aerodynamics involve substantial uncertainties 1,2 . These regimes are explored in dynamic wind tunnel tests where the model is moved through portions of multi-axis maneuvers at scaled rates, either using robotic arms 3,4 , or rotary balances 5,6 . These devices typically use stiff arms to hold the model at precisely known attitudes, and hydraulic or electric actuators to move the model through desired motions, with such an abundance of power and mechanical stiffness that the displacements and resistance caused by the forces acting on the model are negligible. The forces and moments themselves are generally measured directly using strain gages and accelerometers, attached at the model mount, or to the balance. From the forces and moments, the stability derivatives, which are parameters for a state model of the aircraft dynamics, can be computed. From these state model parameters, control laws are synthesized 7 . The apparatus for achieving these is generally massive and heavy, and causes significant flow interference 8 . Specialized equipment is needed for particular motions. NOMENCLATURE θ WDM Boom Angle α WDM Wing Relative Angle of Attack (α + θ = WDM wing aerodynamic angle of attack) Mmodel Moment generated by attached model MWDM Net moment generated by the WDM The Wind Driven Dynamic Manipulator 9 (WDM) is a device which uses the energy in the wind tunnel free stream to produce the moments necessary to drive the model through desired motions. The device is intrinsically light, unobtrusive and flexible, achieving accurate trajectories using feedback control and iterative refinement rather than by brute force. As shown in Fig. 1, a 3-d.o.f. (pitch-yaw-roll) WDM 10 has 4 control wings, each driven by a servo motor about an axis through its quarter-chord. The forces on the wings are Lm Lift generated by the model Dm Drag generated by the model † Graduate Student, School of Aerospace Engineering ‡ Professor, School of Aerospace Engineering ¥ Research Engineer, Physical Sciences Incorporated Copyright c 1997 by R. Ames, N. Komerath, J. Magill. Published by the American Institute of Aeronautics and Astronautics by permission.