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Aerodynamic Performance of Low Form Factor Spoilers

Harley, Christopher Donald

[Thesis]. Manchester, UK: The University of Manchester; 2011.

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Abstract

The development of low form factor flight controls is driven by the benefits of reducingthe installed volume of the control device and/or minimising the change in external geometry, with particular application to flight control of low observable aircraft. Forthis work, the term „low form factor‟ does not refer to the aspect ratio of the controldevice rather the overall installed volume. This thesis compares the use of low formfactor geometric and fluid devices on a NACA 0015 aerofoil section through two-dimensional numerical analysis and low speed wind tunnel experiments. The geometricspoiler is implemented as a small (boundary layer scale) variable height tab oriented normal to the local surface, referred to as a Micro Geometric Spoiler (MiGS). Thefluidic spoiler is implemented as an air jet tangential to the local surface acting in theforward direction, referred to as a Counter-Flow Fluidic Spoiler (CFFS). Twochordwise spoiler locations were considered: 0.35c and 0.65c. Numerical analysis was undertaken using a commercial CFD code using an unsteady solver and k-omega shear-stress-transport turbulence model. Experimental forces and moments were measured via an overhead force balance, integrated surface pressures and pressure wake survey. Device performance is assessed against the magnitude of control achievable comparedto macro scale spoilers and trailing edge controls (effectiveness), the ratio of aerodynamic output to control input (efficiency or gain), the shape of control response curve (linearity), and the degree of control cross coupling. Results show that the MiG and CFF spoilers work by a similar mechanism based oninducing flow separation that increases the pressure ahead of the spoiler and reducesthe pressure downstream. Increasing control input increases drag and reduces lift, however the change in pitching moment is dependent on chordwise location.Chordwise location has a significant effect on effectiveness, efficiency, linearity and separability. Forward MiGS location gives the largest drag gain however the controlresponse is strongly nonlinear with angle of attack and there is a significant undesirablecoupling of drag with pitching moment. Aft MiGS location significantly improvescontrol linearity and reduces pitching moment coupling however the drag gain is much reduced. For the CFFS, the control linearity with respect to control input and angle ofattack is good for both forward and aft locations, with the aft location giving the largest gain for lift and drag. The control response trends predicted from numerical analysis aregood, however a calibration factor of around ½ has to be applied to the control inputmomentum to match the experimentally observed gains. Furthermore numerical control drag polars under predict the change in lift with change in drag at low blowing rates. Through the use of a CFFS device on both the upper and lower surfaces of a wing section it is possible to generate control drag inputs fully decoupled from both lift and pitching moment, thus potentially simplifying device control law implementationwithin an integrated yaw control system.