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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. In this article we’ll discuss how winglets function and why they remain such hot topics of debate.
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 center 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 RBAR planes, since their winglets lie quite close to the center 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 (as seen on Formula 1 racecar wings, for example) are more properly termed endplates. Endplates are less effective than winglets and are not used on Red Bull Air Race aircraft.
The reason we don’t see a consensus on winglets in 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. That is why we see no clear cut move either towards or away from tiplets, and even why the tiplets we do see tend to be so different in size, shape and location/angle of tip placement. Ideally, teams would have multiple sets of winglets and choose the optimal ones for each race.
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 (and, there is an Red Bull Air Race rule governing max wing span).
Interestingly, on non-canard aircraft, we typically see winglets appear on mature aircraft that are seeking ever greater performance increases but for some arbitrary reason are limited from extending their wingspan: airport gate size for commercial jets, 15 or 18 meter wingspan limits for #competition gliders, and pylon separation for Red Bull Air Race aircraft. Given that Red Bull Air Race speeds have crept slowly up over the history of the races, it’s not unlikely that we will see a growing consensus on the use or abandonment of tiplets as these higher speeds make the advantage or disadvantage of tiplets more apparent.
What to look for on raceday: Note that some teams have both tiplets and extensions and will use one or the other based on pilot or computer simulation feedback. For teams with tiplets, note the large variety in size, sweepback angle, and position (i.e., forward or rearward) on the wingtip.
About Red Bull Air Race:
Created in 2003, the #redbullairraceworldchampionship will celebrate its landmark 75th race at the 2017 season opener in Abu Dhabi. The #redbullairraceworldchampionship features the world’s best race pilots in a pure motorsport #competition that combines speed, precision and skill. Using the fastest, most agile, lightweight #racing planes, pilots hit speeds of 370kmh while enduring forces of up to 10G as they navigate a low-level slalom track marked by 25-meter-high, air-filled pylons. In 2014, the Challenger Cup was conceived to help the next generation of pilots develop the skills needed for potential advancement to the Master Class that vies for the World Championship.
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