Rear Wing Endplates in F1: An Extensive Analysis
Cutouts at the rear top of the endplate
BAR pioneered this in 2000 with their BAR 002 car:
Author: Morio
Back in that era, it had a slightly different function and shape. BAR was a relative new team at the time and when it ran downforce comparisons to other teams, it got the impression the car generated too much drag compared to the competition (this later turned out to be false as it was actually lacking engine power). In order to cut drag, it made a cut out in the rear wing endplate. Rear wings had less restrictions during that time and were typically constructed as a one or two element aero foil in front, with the main 3 element wing behind to maximize downforce. In the space between these 2 wings, the endplate was cut out below the frontal aero foil and on top of the rear aero foil. This allowed to have higher pressure from the outside to be introduced. This reduced profile drag and induced drag, but also lowered downforce.
The cutout would be dropped the following year. Renault would however pick it back up in 2003 for the rear aero foil, as well as for the front aero foil a couple of times. They did this however only during tests to prepare for 2004. Tighter rules in 2004 limited the rear wing as we roughly know it today: a 2 element wing only. This got combined with a bigger endplate then necessary, forced by regulations. More team started to use such cutouts in 2004, and by 2005, everybody was using it.
There are several theories about the cutout:
The tip at the trailing edge of the endplate creates a vortex which rotates clockwise (again, with the orientation of the illustration in mind). This vortex, while not a particular strong one, weakens the wingtip vortex (which runs anti-clockwise) and reduces induced drag as a result. It also manipulates the path of the wingtip vortex as they are counter-rotating, which means they deflect each other:
This is a way to manipulate the path of the tip vortex. Teams do this in different ways. The shape of the cutout determines largely how aggressive these vortices deflect one another. Also note that while the underwing vortex and this trailing tip vortex are too counter-rotating, they do not seem to weaken each other as the underwing vortex does not travel close enough to have a significant effect.
Another theory is that the neutral pressure flow (which has a higher pressure then the flow underneath the wing) that flows along the outside surface of the endplate, flows right back inside at the cutout to mix with the underwing vortex:
This is optimization of the underwing vortex by further encouraging the ambient airflow to pass into the low pressure flow. By cutting out the endplate to right near the rear wing, more ambient airflow will roll into the low pressure flow, as well as putting the vortex closer to the underside of the rear wing, increasing airflow velocity underneath the wing.
The opinion is that this will not influence the adverse pressure gradient (the pressure differential), but will add kinetic energy to it, meaning this strengthens the underwing vortex. As one can see, the red flow, outside the endplate, flows inside at the cutout, mixing with the underwing vortex and strengthening it. This will increase flow velocity downstream the vortex. This will also allow the often aggressively cambered wing to keep its airflow attached and will less likely stall, while maintaining the adverse pressure gradient.
Mind that these theories aren’t mutual exclusive.
Endplate louvres/gills
These are the horizontal cuts in the endplate above the mainplane. First used by Renault in 2004, they are there to bleed off high pressure air from the rear wing to the lower ambient pressure of the neutral airflow outside the endplate. This weakens the wingtip vortex, which reduces induced drag. This is because by leaking high pressure flow into the more neutral pressure ambient air, the pressure differential reduces. This will reduce downforce a tiny bit, but this is more limited to local areas on the wing close to the endplate. Overall, it’s a good boost to aero stability and lift/drag ratio, at the cost of a small bit of peak downforce.
Further note that airflow coming out of these louvres will flow towards the cutout of the endplate. The consequence will that the underwing vortex will be enhanced, which sounds contradictory: On one hand you are trying to achieve a reduction of induced drag by weakening the wingtip vortex strength, and on the other you are increasing induced drag again by strengthening the underwing vortex. The reasoning however is quite simple: this solution increases the lift/drag coefficient. Strengthening the underwing vortex yields more downforce for each point of drag, then the wingtip vortex does. Having solutions which provide a better lift/drag coefficient, opens up further ways to tweak the aerodynamic set up.
As shown on the picture below, louvres started out as a few simple slits in the endplate:
Author: Rick Dikeman
However, coming into 2009 where rear wing regulations got even further tightened, these louvres evolved to more extreme versions:
Author: Jose M Izquierdo Galiot
These slots also got further and further complicated on the inside to ensure the high pressure airflow is as enticed as possible to flow to the outside of the endplate:
Author: Gil Abrantes
In the present, these gills aren’t as extreme because further restrictions on the rear wing made the rear wing much shallower, which reduces the need for these gills. Shallower wings produce both less profile and induced drag.
However, teams like Red Bull and Toro Rosso still used some more radical approaches. Red Bull linked the tyre wake slot (which will be discussed in the appropriate section) to one of these louvres, while Toro Rosso extended these slots completely outwards on the leading edge.
Slots/holes below the rear wing
Red Bull pioneered this in 2014. Red Bull was in search for more aero efficiency and especially less drag. This was their answer to it. These radial slots appear to have the same function as the louvres, but with the airflow reversed. They inject neutral flow into the low pressure area underneath the wing, on the inside of the endplate.
This drops the famous pressure differential. This leads to reduced vortex strength, reducing induced drag, by quite a bit actually. However, downforce will go down considerably as well, more so then with the louvres/gills. This is because the low pressure flow is more important than the high pressure flow on top of the wing.
An alternative theory however, is that these slots purposely inject flow into the underwing vortex we already discussed. The opinion is that this can increase vorticity and increase the pressure differential, which could result into the wing being worked so hard, that it stalls above a certain speed the car travels, with the pressure differential getting too high for flow to stay attached. It has to be noted that injecting too much high pressure flow will instead lead to reducing the vortex strength as the pressure differential gets reduced.
Both cases will lead to a reduction in downforce and drag. What exactly happens, is not entirely clear as it depends on how easily the wing is able to stall and how much high pressure flow gets injected. A personal opinion on the matter is that these slots rather weaken the vortex, as trying to purposely stall the wing at certain speeds can lead to inconsistencies where the wing stalls too early or too late, as well as the airflow reattaching too early or too late. However, the performance gains with stalling are bigger if teams are able to detach and reattach the flow reliably.
It has to be pointed out that this can sound contradictory to what is explained in the section of the endplate cutout: on one hand you have a solution that decreases the chance on stalling and also increases flow velocity, while the other solution either promotes stalling and/or decreases flow velocity. However, putting these solution like that side by side is probably misleading. The reason why is because flow patterns change in function of the how fast the car is traveling. The cutout for instance can very well be a lower speed solution to increase low speed downforce. On such speeds, the radial slots we are discussing might not be able to draw in as much flow, simply because the pressure differential is not as high. On high speed, these radial slots will work better. It will still hurt low speed downforce, but teams who have employed these solution were almost always in a position where they needed more speed on the straights and even high speed corners. You’ll find this solution much sooner on a McLaren than a Mercedes. The latter has enough engine power to compensate for the drag.
Endplate leading edge slot
These vertical holes/slots along the leading edge of the endplate are a relative new solution, introduced in 2012 by Williams. It took a few years before other teams started to pick this up, but nowadays every team uses this. They are situated in the rear tyre wake, which is turbulent airflow. The inside of the endplate usually has to deal with a lot of turbulent airflow, which will increase boundary layer build up. The thicker the boundary layer becomes, the more turbulent and slow moving it becomes, which has negative consequences for both the diffuser and rear wing. Introducing a slot there will allow this flow to speed up again, which in turn promotes laminar flow. This will allow the top of the diffuser and the underside of the rear wing to work harder because these devices have to deal less with turbulent airflow. At first, teams were wary about injecting turbulent airflow underneath the rear wing, which would imping the downforce created from there, so if a team ran them, the slots were quite limited in size and kept away from the underside of the rear wing. However, teams figured out that careful sculpting of the slot allows the turbulent flow to be straightened to be much more laminar. This also aids the aero devices which are in close proximity of the wheel wake, like the brake ducts as airflow around the endplate becomes less turbulent due being sucked through the slot.
In the present, these holes/slots extend much further along the leading edge of the endplate, and often times teams runs 2 slots to keep airflow nicely attached to the endplate.
Red Bull went quite extreme with these slots in 2015, extending the slot completely along the leading edge of the endplate, connecting to a louvre, similar to this:
It’s quite a bit of head-scratching what they tried to achieve. One explanation could be they try to redistribute high and low pressure flow all across the endplate leading edge, reducing drag and cleaning up boundary layer flow. Note that at the top half of the endplate, above the mainplane, flow goes from the inside to the outside of the endplate, while below the mainplane this is vice versa.
Endplate strakes
Introduced by Lotus in 2013, these bits are more about fine-tuning the existing airflow patterns over the surface of the endplate. The boundary layer at the lower half of the endplate has the tendency to flow downwards. Initially strakes were used to emphasize this flow. However, trying to encourage the boundary layer flow into the vortex allows the rear wing to be worked harder, so all the strakes are bent upwards nowadays. It improves the aero efficiency of the overall wing, which makes usage of, for instance holes, below the wing, more viable. They essentially are bits to fine tune the flow and support the other flow structures, especially the upwash flows, encouraging both the ambient pressure flows and the boundary layer flows to upwash. More upwash across the trailing edge of the endplate will increase the interaction between the rear wing and diffuser, which we discussed earlier.
Credits to:
-Andy Urlings for general content and illustrations
-Charlotte (cmF1) for grammar, spelling and style review
-Steven de Groote for final review
-Wikimedia Commons as database for reuseable images
-A special thanks to a friend who wishes to stay anonymous, who helped immensely with the content and who’s input was undeniably crucial.
I also wish to dedicate this article to the victims of the 22/03/2016 terrorist attacks at the Brussels metro station and the Brussels airport.
