When a raceplane is in flight there is high pressure below the wings and an area of low pressure above the wings created by the airflow. The area of high pressure will try to move to the low pressure area because it's unbalanced. This is what generates lift.
The air will move below, along the length of the wings and then upwards over the wings. The winglets disturb this movement and reduce drag, as it stops the two types of pressure meeting. Because of the difference in air pressure, when they meet it disturbs the air and this takes energy away from the raceplane – causing drag. With the winglets this disturbance is minimised and the drag is reduced. When pulling high Gs more pressure is created under the wing and therefore more drag. This is when the winglets will be the most useful.
Flight Operations Manager Dario Costa is an experienced Unlimited level aerobatic pilot himself, so has plenty of insight about aerodynamics. He explains why winglets are the most talked about modification right now.
Winglets are all about reducing drag – but how exactly do they work?
DC: Basically you have two types of drag – profile and induced. These are completely opposite in their action. Profile drag increases with speed, while induced drag is the result of the 'angle of attack' (the angle between the oncoming air/wind and a reference line on the raceplane). The winglet is a barrier to stop the high pressure disturbing the lower pressure, which has a direct impact on the level of induced drag. As the high pressure disperses it will try to enter the area of low pressure, creating what's known as a vortex. The quickest way for these vortices to form is when the air moves over the end of the wing. The winglets lessen the impact, meaning the airflow is smoother and the raceplane can fly faster.
Some pilots have really visible winglets and others have more subtle versions. What's the difference?
DC: There are two types of modifications we're seeing being added to the ends of the wings. There is the classic wingLET, which is used by Matt Hall and Nigel Lamb and then there is the raked wingTIP, which has a lot less vertical profile. So no raceplane has the straight edge we used to see.
In the Red Bull Air Race you can experiment with both types. What the teams are trying to do is make one fixed device that takes into consideration two completely opposite requirements. When flying straight and level, winglets will create profile drag but at a high angle of attack, in the high G turns, they'll reduce induced drag, so it's a balance. So the teams have to calculate how much time the pilot flies at a high angle of attack and then calculate what works best. That's why the pilots have different styles. The pilots also have to adapt because it's like adding a new wing to the aircraft. There are some tracks where there are fewer high G turns and that's when it's better to have the smaller raked wingtips.
As a pilot yourself, what would you have on your ideal raceplane?
DC: If it was me and I had an unlimited budget I would have both winglets and a set of raked wingtips to use for the different tracks. For example, if I was racing in Chiba I would have the raked wingtips but for Ascot I would have the big winglets. A pilot's flying style could influence the type of wingtips or winglets they have. For a pilot with a smooth style who makes very few adjustments in the track, the benefits of reduced induced drag would be outweighed by the additional profile drag. So the choice can be based on the pilot's flying style and the track type, but it will mainly be based on the technical assessment of the actual airflow of the aircraft.