All the investigations above concentrated on the boundary layer and the associated friction drag. Of course the main idea behind winglets on conventional airplanes is to reduce the induced drag, and all the discussion about pressure distributions and boundary layer effects would be incomplete and questionable, if the different configurations would show very different induced drag figures. On the other hand, the main reason for the usage of winglets on tailless airplanes is not the possible reduction of the induced drag, but the search for the most effective arrangement of vertical fins to achieve directional stability - the influence on drag is an additional benefit for tailless planes.
Lift coefficient versus (induced) drag, as calculated by the panel
method.
The induced drag polar above shows, that all three winglet configurations show almost identical results in term of induced drag. There seems to be a small advantage for the last configuration, but this is close to the limits of the numerical method. All configurations outperform the free wing tip model - as expected, the drag reduction is getting larger with increasing lift coefficient. The graph does not include the friction drag, which reduces the benefit of the winglets due to the additional surface. Taking friction into account, favours the classical wingtip at low lift coefficients, but the total drag of the winglet configurations will still be lower at higher lift coefficients. For the flying wing, the winglets make a vertical fin obsolete, whose friction drag would have to be added to the drag of the conventional wing tip configuration.
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