Red Bull Air Race 2017: Understanding winglets
Winglets have been highly visible modifications to the Red Bull Air Race planes, and there is much debate – both among and within teams – regarding their design and effectiveness.
There are two primary reasons why an aircraft designer would place a vertical surface on the end of a wing. First, if the wingtip lies some distance behind the centre of gravity of the aircraft (as is typical for canard type aircraft), the vertical surface can provide directional stability and control similar to that of a fin and rudder. This case is inapplicable to Red Bull Air Race planes, since their winglets lie quite close to the centre of gravity, and they are prohibited by race rules from having any moving (ie, rudder-like) parts.
However, a second reason for incorporating vertical surfaces is that they act as an extension of the wing and thus make the main wing more efficient at producing lift, although at a cost of producing extra drag. Vertical (up, down or both) surfaces which use an airfoil and are aerodynamically integrated with the main wing are typically called winglets, while vertical surfaces that are simply flat plates are more properly termed endplates. Endplates are less effective than winglets and are not used on Red Bull Air Race aircraft.
The reason there is no consensus on winglets in the Red Bull Air Race is because winglets represent a tradeoff between two types of drag. Wings produce vortices at their tips which are proportional to the amount of lift being generated. At high speeds, profile drag (ie, drag due to the shape and size of the wing) is large, while induced drag (drag due to these vortices from the generation of lift) is small, and so the benefit of winglets is small, while the drag they generate is significant.
At low speeds though, where the airspeed is near stall and the wing is producing a huge amount of lift – the profile drag is relatively small while the induced drag is much higher – perfect conditions for a winglet. So the question swirling around winglets is: “Do they slow you down more on the fast part of the course than they speed you up in the high-G sections?”
Although winglets definitely work, they tend to be “on-point” devices, which means that they work best under a particular set of conditions. But piloting styles, track layout, weather and the design of the winglet itself can all push the winglet off-point, such that benefits are minimal – or even worse, performance suffers.
When speaking of winglets, we should also point out that even if they do not use vertical winglets, all Red Bull Air Race teams at least use wingtip extensions. Extensions serve the same purpose as winglets – to improve lift and reduce induced drag during high-G maneuvers – but they also suffer from the same disadvantages, namely increased profile drag.
In fact, given the option, most designers would prefer to use extensions rather than winglets, since they offer better performance and are much easier to design. Obviously there is a limit though, since lateral extensions run an increased risk of a pylon strike compared to vertical winglets – plus there is a Red Bull Air Race rule governing max wing span.
Photo © Balazs Gardi / Red Bull Content Pool