A wing without end plates allows the low-pressure air below the wing to bleed over into the high-pressure air on top of the wing. This interaction creates vortices, which reduce downforce and create drag. The middle of the wing still works well, but you get progressively less downforce and more drag at the ends. For a quick video on why a wing needs endplates, see this video by Kyle.Engineers.
End plates separate the flow between the top and bottom of the wing, effectively reducing drag and increasing downforce. The end plate has to be large enough to keep these two pressure zones from colliding.
In the following image, notice how different wing shapes have similar high-pressure areas above the wing, but very different low pressure shapes below the wing. Indeed, the shape of the end plate should be similar to the pressure zone shape. Note that the low pressure side (suction) is more important than the high pressure side, and so end plates must extend further below the wing.
Take a look at the wing shapes above:
- The first one (on the left) is a wing with a NACA profile around 4410. (4 degrees of camber, max camber at 40% of the chord length, thickness of 10% of chord length). It’s like a less aggressive 9 Lives Racing wing.
- The second one looks like a skateboard deck. I’ve seen a lot of DIY wings in 24 Hours of Lemons (skateboard decks, snowboards, and just a piece of curved wood), and I love the spirit. Mostly I don’t see them with end plates. Do it!
- The third one is a symmetrical airfoil. It doesn’t make a great wing for a car, but is good for stanchions and other places where you need to hold something up with little drag.
DIY single-element end plate
I use aluminum sheet for end plates (recycled street signs are a good source of aluminum), but you could use any sheet metal, carbon fiber, plywood, etc. The endplate needs to be relatively stiff and light.
Different racing organizations have rules on end-plate size, and for simplicity, you can make a rectangle or square of whatever the maximum size is. Some people will cut a notch on the top trailing edge to lessen the trailing vortex here, and you can shape the bottom to match your low pressure zone (see first image).
In an article in Racecar Engineering from 2008, Simon McBeath CFD tested end plates of various depths in 25mm increments to 300mm on a 300mm (12″) chord wing. I’ll convert these to inches and pounds and summarize them here. All end plates were identical in shape, except for the depth below the wing.
|Depth||Downforce lbs||Drag lbs||L/D Ratio|
The first thing you might notice in the table is that there’s about a 10% difference in L/D ratio between the best and worst. That’s pretty significant. However, there’s less than a 3% difference in L/D ratio between a 3″ end plate and anything else, and the 3″ and 6″ end plates were almost identical in L/D ratio. So as long as you have something on the end of the wing, you’re good.
Notice that the highest downforce and best lift/drag ratio is with the 5″ deep end plate. The author goes into a long investigation about why this is, but it’s too complex of a relationship to go into here. It’s kind of a magic number, and trying to find this on your own would be folly, because on either side of 5″, the numbers are worse.
If you throw out the 5″ outlier, then the 12″ depth end plate is best, which conveniently matches the chord of the wing. So a good rule of thumb here is make your end plates the same depth as the chord of your wing. Bigger end plates are also good for stability, since they move the center of pressure rearwards.
Pro tip: Lay a straight edge across the chord of the main wing, and use that same angle for the top of the end plate. This makes it easy to set and adjust your wing angle using the top of the end plate.
If you look at F1 end plates you’ll notice slots above and below the wing, a leading-edge slat, strakes along the sides, and a gurney flap at the trailing edge. Yes, all of this on the end plate! Gurney flaps on the sides of the end plate increase drag and downforce, but most of the other tricks are used to tame vortices. Mitigating vortices reduces drag, but usually results in a loss of downforce as well.
This is all a bit overkill on a street-based car with an off-the-shelf wing. If you reflect on MacBeath’s CFD, there was only 3% difference in downforce between a 3” endplate and a 24” one, and I don’t know if any of us could feel the difference between 100 and 103 lbs of downforce.
I personally wouldn’t bother with these modifications, because we are going a lot slower than an F1 car. But it’s good for conversation, and fun to build stuff.
End plates for dual wings
Last summer I raced in the 24 Hours of Lemons race at Thompson, and saw some good aero, and a lot of bad. Lemons cars have wings largely for looks, it doesn’t really matter that some of them were a slab of plywood set at an angle. Among these quasi-aero devices were a lot of cheap eBay/Amazon wings that would have worked, but were done poorly.
Case in point: on one orange Chevy Lumina (winner of the IOE), the wing was on backwards. I enquired about this, and apparently the wing came pre-assembled with the pointy part of the wing facing forward! That’s just dumb from the “factory” but shame on the team for not correcting it. Or maybe it was intentional? This is Lemons, it’s hard to tell.
At the race I saw a lot of dual wings with absolutely ridiculous end plates that had big holes or cutouts on the underside. As you saw from the first image, the underside of the wing is what matters! Moreover, they had the upper wing mounted so far away from the main wing that it defeated the purpose of a dual wing setup.
I’ve seen a lot of terrible end plates that are more for show than go. They have most of the surface area of the endplate at the back of the wing, or cutouts below that would let the pressure zones collide. It would be easy to correct the function of these wings by building your own end plates.
DIY dual-element end plates
So if you have a crappy dual-element wing with crappy end plates, and you want to make it work better, build your own end plates. Again, let’s start by looking at the pressure zone below the wing.
It’s not intuitive, but the suction side is more important than the pressure side of a wing. This is apparent in the numbers: the blue is 3x the value of the red. Notice that the low-pressure zone extends below the wing by almost the length of the chord of the main wing. Meaning, if you have a 10″ chord wing, you’re going to need at least a 10″ deep end plate. Also notice that the low pressure zone extends in front of the wing, but not much at the trailing edge.
In Competition Car Aerodynamics, McBeath examines what happens with end plates of different sizes. At first he uses no end plate (ep0), and then end plates of increasing size. The larger the end plate, the more downforce and less drag.
|End Plate||Downforce||% Increase||Drag||% Decrease|
OK, so if bigger is better, how big is too big? There is a height at which end plates start creating more drag, and a diminishing return on downforce. But I don’t want to give away all the secrets, so please buy the books on my Resources page and learn yourself some aero.
Here’s how I’d DIY myself end plates:
- Start with a 12″ x 12″ piece of sheet metal. Use a street sign if you’re Lemons, otherwise plain aluminum will do.
- Put most of the surface area at the front and below the wing (as pictured in the drawing, above).
- Lay a straight edge across the chord of the main wing, and use that same angle for the top of the end plate. This will help you set and adjust your wing angle.
- After mounting the main wing as above, mock up where you want the holes for the secondary wing. I would put a single mounting hole in front that acts as a pivot and drill two or three holes at the rear. Don’t exceed 35 degrees. I don’t trust adjustment slots because they can shift out of whack, and so I go with holes instead.
- Make the gap between the wings about a half inch in height, and overlap the upper wing on top of the lower wing by about a quarter inch. This should create a convergent gap between the wings, meaning the front opening is larger than the rear. This will accelerate the air going through the gap.
- Set the lower wing angle almost flat (zero degrees). Most wings will have the highest lift-drag ratio in this vicinity.
- Start the upper wing at 25 degrees and if you need more downforce, use the 35 degree hole. Don’t exceed 35 degrees with the upper wing. If you still need more downforce, rake the entire wing a few degrees.