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End-plates
In our description of the wing vortex system, we noted that theory predicts that for a vortex to persist, it must either form a closed ring (as it does in the horseshoe system), or be terminated by a solid boundary. It was reasoned that one method of removing the trailing vortices might be to place solid walls or
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Fig. 4.12 The use of wing-fuselage blending as on this MiG-29 helps to reduce drag due to interference. The use of a lifting fuselage also reduces trailing vortex drag by improving the spanwise distribution of lift |
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Fig. 4.13 Turned-down and cut-off tips are intended to encourage separation of the spanwise flow at the tip. The resulting modification of the tip flow field has been found to produce a reduction in drag |
end-plates at the wing tips. Experiments with end-plates show that they can produce a reduction in trailing vortex (induced) drag. However, it was found that end-plates large enough to have any significant influence on the drag, created lateral stability and structural problems.
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Fig. 4.14 Bent tips on the Aerospatiale Robin |
It should be noted, that end-plates do not in fact destroy the trailing vortices, they merely modify the trailing vorticity in a beneficial way.
Sometimes, an end-plate effect can be achieved by ingenious design, as on the tailplane of the Optica, shown in Fig. 4.9. Auxiliary wing-tip fuel tanks and tip-mounted weapons can also have a marginal end-plate effect, as well as helping to reduce wing bending stresses.
