An Experimental Investigation of the Effects of Trailing-Edge Strips on the Unsteady Aerodynamic Forces on Tabs,
AERONAUTICAL RESEARCH LABS MELBOURNE (AUSTRALIA)
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A fairly common problem on new aircraft is that of achieving good control surface effectiveness. Often boundary layer thickness and interference from other surfaces may cause flow separation on all or part of the control. This reduces control efficiency and can cause serious problems in achieving good control of aircraft response. These types of problems are difficult to predict analytically or from wind-tunnel tests. Usually the first evidence of such difficulties occurs during prototype flying and consequently serious efforts must be made to find a fix which can be introduced fairly simply and cheaply at a late stage in the design. These fixes often involve vortex generators, placed on or ahead of the control, or thickening of the trailing edge of the control. The latter may be simply achieved by attaching a small angle to one or both sides fo the control along the trailing edge. The aim of these and other methods is to improve the effectiveness of the control and hopefully to increase this to its theoretical or design value. While the effect of such modifications is fairly well known for steady flow, little evidence is available of the effect in unsteady flow. There are no theoretical methods of predicting unsteady aerodynamic forces on oscillating control surfaces which have vortex generators or trailing-edge angles. Usually complete reliance is based on classical thin lifting surface theories for predicting flutter speeds and frequencies. The work described was undertaken as a result of an accident to the Nomad aircraft in which flutter occurred as a result of fitting trailing-edge strips to the tailplane tabs.
- Fluid Mechanics