Rear Wing Endplates in F1: An Extensive Analysis

Origin

Endplates were originally engineered and patented by Frederick W. Lanchester in 1897, the founder of aerodynamics, as a solution in aviation. At the time, his ideas were still very conceptual and too advanced for the time as even motoric flight was not achievable then. His solutions concerning wing and endplate were, and are meant to control vortices and limit their associated drag called 'induced drag'. Later, further studies pointed out lift was improved as well because vortices decrease the effective span of the wing, so measures against such a vortex would make the loss in lift smaller. There is more to this lift increasing characteristics, which will be discussed further down this article.

It's difficult to pinpoint the first car using reversed lift (downforce) to enhance performance. The earliest recorded car to have used this was the opel RAK 1 built in 1928: a rocket propelled car which had so much torque it needed downforce to keep the car from taking off. The first true race car to have used downforce was the chaparral 2E in 1963.

In 1968, Colin Chapman introduced wings and downforce on cars in F1, with the legendary Lotus 49B.

Until that point, nobody in F1 considered lift as a means to produce grip (with the exception of a brief moment a few years earlier where Mclaren tried to fickle with it but quickly abandon this important avenue). At their introduction, rear wings were attached to the unsprung part of the cars high up in the air, but after big accidents which almost caused fatalities, the wings had to be attached to the sprung part of the car: the chassis. A year later, endplates started to feature on the wings on the 49C, as Colin Chapman started to get more expertise on the matter of downforce.

He increasingly realized F1 has similar induced drag issues as the aviation industry. Endplates up until this day still serve that very same purpose of reducing drag (among other purposes which came later on).

Size and general shape of the endplate: evolution

In 1969, the year when endplates got introduced, the endplates generally were ellipse shaped, following the shape of the aero foil. In the years after, when teams were looking for optimization, endplates would become more and more rectangular, especially where the wing tip would meet the endplate. The shape was always in function of the wings. The latter got more and more complex and split up in several elements at a very high rate. Some weirder endplate designs, like the Ensign N173, would come and go where endplates were integrated into the bodywork in front:

But generally speaking, endplate size and shape was solely adapted to the requirements of the rear wing. One evolution that did make it through the test of time was using the endplates as structural support for the wing, by extending the length down to the -in the soon to be introduced- beam wing, or the floor. This was a lighter solution that supported the rear wing, and also reduced blockage underneath the wing by removing or reducing the size of the support pylons, increasing efficiency. Extending the endplates that much down also helped out with controlling tyre wake. Tyre wake is the phenomenon where the rotating tyre creates turbulence which disturbs existing airflow patterns under the rear wing and on top of the diffuser, which reduces downforce and aero efficiency. The Renault RS01 and Brabham BT45B pioneered extended endplates in 1977. It took quite a few years before the rest of the grid took over this idea. In the latter half of the 80’s however, after the ban on ground effect solutions, cars ran such big rear wings the distance between endplate and beam-wings/floors got that much reduced, that most teams made the jump and extended them down and started using them as supports:

Everybody also started to converge to the same general shape of the rear wing endplate due ever tighter regulations on rear bodywork and rear wing. The 90’s saw very little to no general or unique developments. The 2000’s and the current decade however saw several developments which will be discussed in separate sections below.

In 2004, endplates got a minimum surface area regulated to increase advertising space.

In 2014, the beam wing got banned. In the year before, teams have made efforts to make the endplate structurally strong enough to remove the support pylons. However, with the beam wing gone, this was no longer viable as the endplate becomes too long to be rigid enough. Today, the rear wing rests in most cases on a single pylon and with the endplate sitting on the floor/diffuser with a few lighter structures. This setup is aerodynamically and regarding weight the best compromise.

General interactions between endplate and rear wing

As discussed in the first paragraph, endplates control vorticity to decrease drag and increase downforce. This is very often mentioned in similar articles. However, most of these articles fail to point out that the interaction between the endplate and the aerofoils making up the rear wing, causes several vortices to be formed. This is not a choice presented to the teams, it is simply how the endplate influences the airflow over the wing. Since these vortices are there, teams try to manipulate them to gain a bigger benefit or smaller disadvantage out of them. The illustration below shows the 2 most important vortices:

The red and blue lines show the normal flows on top and beneath the wing, which curl up to a vortex at the intersection between the endplate and the wing tip. At that moment, high pressure flow spills into the low pressure. With the orientation of the illustration in mind, this is an anti-clockwise vortex. This vortex will be referred to as a wingtip vortex in further sections of the article.

The green and yellow lines show a different kind of vortex. The yellow line is originally the same flow as the blue line: low pressure flow underneath the wing. However, it splits path at one point and instead of forming the same wingtip vortex, it merges with the ambient pressure flow outside the endplate, at a different point across the trailing edge of the wing. Let’s call this vortex our underwing vortex.

It is important to note this particular vortex significantly increases downforce and drag. Downforce is increased because the vortex increases flow velocity underneath the wing, further lowering pressure and increasing the differential. Drag is increases because of induced drag. With the orientation of the illustration in mind again, this is, just like our wingtip vortex, an anti-clockwise vortex.

Note that influencing the underwing vortex has implications on the former wingtip vortex as well. The wingtip vortex is correlated to how hard you work the wing, and that is on its own correlated to how strong the yellow/green vortex is. Therefore, weakening or strengthening the underwing vortex, will result in weakening or strengthening the wingtip vortex. The reverse, the wingtip vortex having influence on the underwing vortex, is true as well, although the impression is that this is less. This is because the wingtip vortex is primarily a result of how hard the wing is working, while there are other factors having significance on the flow outside the endplate and forms the underwing vortex Also note these vortices both run anti-clockwise, which means these vortices are co-rotating. Although images of F1 cars in action suggested these vortices don’t merge, they do tend to travel slightly towards each other, which is a characteristic of co-rotating vortices. Reason why they don’t merge is probably because the underwing vortex is not stable enough to maintain vorticity across a longer distance. Again, this is not by choice, but because how an endplate interacts with the rear wing. Because their rotational directions are the same, they also have the tendency to strengthen each other directly (next to interaction/correlating with the rear wing). In the current guise of the F1 technical regulations, the green/yellow vortex stays more on the inside and raises significantly higher above the wing, while the red/blue one will be pushed further outside and lower, following the edge of the endplate. The path and strength of these vortices can be manipulated, but to repeat: the rotational direction is not by choice and cannot be changed.

The complexity of how those vortices individually work and work interdependently, makes life harder for engineers to come up with the best aero efficiency for the rear wing of their team. The general goal always is the most downforce for the least drag, but downforce is at a premium with the current strict rules. The next section will discuss how rear downforce is produced not only by the rear wing, but as part of a system. This is essential to explain some of the endplate solutions.

Endplate slats

These pieces of bodywork are extensions on the rear bottom of the endplate and on top/after the diffuser. It actually makes use of a loophole in the current regulations which grants a 50mm width window to place them, similar to the maximum allowed thickness of the endplate. Toyota was the first team to use this local area of endplate surface to create slat-like gills: Author: F1fanatic

They wouldn’t be featured much throughout the 2010 season, but McLaren picked it back up during the 2011 and turned it into a trend where every team would end up using it, up until this very day.

The slats/vanes are twisted outwards, encouraging airflow to follow this pattern. This aids in expanding airflow exiting the diffuser by encouraging upwash/outwash of flow passing over the top of the diffuser. This flow will create a low pressure wake for the flow inside the diffuser to expand into. The quicker airflow can expand coming out of the diffuser, the harder the diffuser can suck through airflow under the floor, overall increasing downforce from the diffuser and floor. The next illustration shows how specifically the slats expand flow exiting the diffuser:

As one can see, the slats bend airflow passing through them outwards (blue arrows). This flow usually has a bit lower pressure then the flow coming out of the diffuser (red). This means diffuser flow is pulled towards the flow coming through the slats, which expands the diffuser outwash (purple). This event is often called “making the diffuser bigger than is” because the expansion is bigger than diffuser dimensions suggest.

However, these slats are also part of a bigger airflow structure altogether, where the flows of the wing, diffuser, slats and before 2014 the beam wing work together in one homogenous system with the purpose of increasing rear downforce:

The blue flow represents flow from underneath the rear wing. Because it follows the surface of the wing, it has an upwash profile. This leaves underneath it a low pressure wake. The green flow, representing the flow from the beam wing, top of the diffuser and the slats, gets accelerated and expands into the low pressure wake. Underneath the green flow, another low pressure wake is formed. This will allow for the red diffuser upwash to expand into. Increased upwash expansion will allow the diffuser to extract more airflow from the floor, which ultimately benefits diffuser and floor downforce. So this system allows for more downforce production then it's aero devices separately.

Since 2014, the beam wing has been banned which makes diffuser upwash much more difficult to connect with the flow coming from underneath the wing. The beam wing did produce downforce on its own, but was more essentially a stepping stone between the diffuser and rear wing flows, connecting the 2 and essentially making both devices work harder. It has to be stressed that the slats in the endplate have been used when the beam wing was still allowed. Still, since 2014 the teams are more dependent on solutions like these slats to have the airflow being pushed up. The end goal is still to connect diffuser and rear wing flows despite the absence of the beam wing.

In the past few years most teams ran up to 12 of these slats. However, the current trend has them in a different format:

Mercedes introduced this solution back in 2014. They replaced the slats with vertical/diagonal serrations in the endplate. The pieces of bodywork in between the serrations are then further sculpted to encourage airflow to expand. Note how they interact with the leading edge slot and strakes on the endplate:

The more turbulent airflow is first straightened into more laminar flow, and then bended upwards. It’ll clean up the boundary layer on the outside of the endplate, reducing drag. We will discuss the leading edge slot further below.

Also note these serrations are placed higher than the previous incarnation of the strakes. The reason why has probably to do with the absence of the beam wing as well as the expanded bodywork at the rear, which ejects turbulent hot air from the engine.

Not all teams follow this trend. Red Bull was the only one copying this solution last year. For now, only McLaren has joined this group as well.

As discussed, these slats are part of a system which aims to improve overall rear downforce. However, the system is built around a static environment. What happens if one these devices change? More specifically, how do these structures behave when the Drag reduction System comes into play and what solutions does the endplate provide to counter unwanted effects?

Endplate interaction with DRS

Before we go into detail what DRS actually does for the endplate, some aspects have to be explained first. When DRS opens, the angle of the flow changes, with a hefty reduction in rear wing upwash:

Red pictures the flow when DRS is closed, blue/purple when open. This will have a knock-on effect on the upwash flow from the endplate slats/serrations, and ultimately on the diffuser upwash, which further reduces drag then just the profile and induced drag of the wing. Pat Symonds even mentioned during 2012 that for specific circuits like Monza, they effectively want to stall the diffuser at a specific speed of 130mph. The way this was done, is by putting the ride height low enough in the first place to make the diffuser more sensitive. This is because the diffuser generates vortices on the inside near its fences. These vortices are very similar to our underwing vortices. However, if the fences get too close to the ground, the vortices inside the diffuser start to choke and destabilize, leading to reducing the diffuser’s ability to extract the flow underneath the floor. Once DRS is engaged, the reduction in wing upwash and consequently the reduction of the low pressure wake and diffusers upwash, will further destabilize the flow inside the diffuser. Combined with decreased or even removed vorticity inside the diffuser, this will detach the flow inside the diffuser, stalling it.

It should be noted this was in 2012, when DRS was allowed to be used during qualifying whenever the driver wishes and cars were allowed to have a beam wing. The rules have been changed since then: DRS is only allowed to be used twice during the qualifying lap and the beam wing is removed, inevitable leading to DRS having less effect on the diffuser. There are arguments in favor that diffuser stalling through DRS, still happens, and arguments against. This will not be discussed here as the article’s topic does not yield towards it. A more in depth explanation will be given in an article more related to this.

To relate this all back to the endplate: the reduced upwash and knock-on effects on diffuser flow, creates issues when DRS closes again. The flows are not inclined to immediately reattach, which gives issues in braking zones due the compromised aerodynamics slowing the car down less. While this might look at first sight to be a sole concern for the rear wing, rear wing regulations are that much restricting, there is little to no room for solutions in the area. Instead, teams look for solutions at the endplate.

One particular solution is interesting:

Since a few years, teams have been exploiting the thickness of the endplate to sculpt it out where the aerofoil’s edges are supposed to merge into the endplate. The regulations provide a thickness of 50mm for the endplate. In reality, the effective endplate around the wing is made as thin as possible, and the space is filled up with a number of small aerofoils. Next to increasing the effective wingspan of the main plane and upper flap, these bits are important in reattaching flow after DRS is disengaged. During the time DRS us open, upwash at the main wing reduces significantly, these bits will keep the same upwash as if DRS is closed. This is despite the general flow underneath the main plane having a pulling force on the small flows over and under these winglets. There are numerous slots involved, encouraging flow attachment to the camber of the sum of these winglets:

When DRS closes, flow coming through the slotgap between the main plane and upper flap will be pulled towards the slots in between the winglets. Since these latter slots follow the same camber as the upper flap, flow will be encouraged towards the surface of the upper flap, allowing for a quicker flow reattachment:

Once the flow is reattached underneath the complete wing, the upwash is restored as well as the drag properties, needed to slow down the car during braking. Again, this will cause a knock-on effect where the low pressure also reappears, which in turn draws in the flow from the diffuser, leading to a further restoration in drag and downforce.

Note that a wing with less camber will more quickly restore these airflow structures. The more aggressive the camber, the more difficult airflow reattaches.

With solutions designed to support our downforce producing system out of the way, let’s take a look at solutions aimed more specifically at the rear wing.