Aerodynamics Fundamentals: Rear Wing Theory
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Rear Wing Theory
06.38
| 00:00 | The rear wing is a crucial component for setting the downforce level of the rear of the car, and the overall drag level of the vehicle. |
| 00:07 | Even though the rear wing is significantly less sensitive to car attitude than the front splitter, its design is critical for car performance. |
| 00:15 | Rear wing assemblies consist of a wing section with an end plate on each end. |
| 00:20 | The wing profile is the shape of the wing if we took a slice through it when looking at it from the side, which is commonly referred to as an aerofoil. |
| 00:29 | Wing performance in terms of drag and downforce is largely dictated by the aerofoil profile, and the curvature of this component can have quite a large effect on the amount of downforce the wing makes. |
| 00:42 | Generally speaking, more curvature in the aerofoil profile will mean more downforce and more drag, while less curved, flatter profiles will be more efficient with less drag , but less downforce as well. |
| 00:55 | Wing downforce behaves in a roughly linear fashion with respect to the forwards -backwards angle of the wing with respect to the flow, which is known as the angle of attack. |
| 01:06 | At lower angles of attack, a linear relationship between the wing angle and downforce holds. |
| 01:12 | However, with increasing angle, the lift-to-drag ratio of the wing gets slowly worse. |
| 01:18 | Eventually the wing will start to encounter flow separation in small amounts, which will lead to downforce of the wing no longer increasing linearly with the angle of attack, even though the drag will continue to increase. |
| 01:31 | With more angle again, the separations will become larger, causing the downforce to start decreasing with increasing angle. |
| 01:38 | This means that generally for peak downforce, we want just a little bit of flow separation near the trailing edge of the wing, but certainly don't want a large -scale separation. |
| 01:48 | It's typically safer from a setup perspective to try and run less angle of attack rather than use the maximum angle we can, and if we want to push the angle of attack to the limits, we really need some sort of flow visualization to check for large-scale separations. |
| 02:05 | There is a device that can enhance the wing's amount of downforce without requiring an increase in angle, which is known as a gurney flap. |
| 02:13 | A gurney flap is a small lip, typically around 10-20mm tall, that sits perpendicular to the wing surface on the pressure side towards the trailing edge of the wing. |
| 02:23 | This device effectively lengthens the wing by creating two vortices behind it, creating more suction underneath the wing and more pressure on top of it. |
| 02:32 | Another way to think about it is it's like adding a little spoiler to the wing. |
| 02:36 | Pressure builds up in front of it, which applies pressure onto the pressure side of the wing, while the upwash from the gurney creates more extraction and hence more suction on the underside of the wing. |
| 02:47 | The important part to understand is that we can add a gurney without increasing the angle of attack of the wing, so if we run out of angle of attack and need more downforce, we can always add a gurney on. |
| 02:58 | In many cases, the gurney can actually make a wing more robust to separation by adding that little bit of upwash at the rear. |
| 03:06 | While the shape of a wing is an important factor, where it's placed and how it's mounted can also have substantial impacts on performance. |
| 03:14 | A general rule of thumb is to overlay the rear wing so that about the forward third sits over the boot and the rearward two thirds sit behind. |
| 03:22 | I would then generally put the front leading edge of the wing in line with the top of the roofline when viewed from the front. |
| 03:28 | This is by no means the optimal setup for every car, but is a great starting point. |
| 03:32 | If our rulesets limit us to smaller than that, then I would just target as far rearwards and up as the rules allow. |
| 03:39 | Typically, rearwards will give us more rearwards aero balance as we're going further up. |
| 03:45 | Going lower with the wing will often provide a bit more downforce and drag as we start to interact with the rear of the car and the car underbody more. |
| 03:53 | However, we do need to be careful here. |
| 03:55 | If we go too low, we'll be in the wake of the cabin and the wing performance will drop substantially. |
| 04:01 | For this reason, we're usually better off playing it safe and going high. |
| 04:05 | This is also the dominant factor in overall car loads not being equal to the loading on the wing itself. |
| 04:11 | The wing may suck against the body of the car and locally have very low drag or even thrust on the wing itself, however the car will be putting on substantial amounts of drag overall. |
| 04:23 | Once we've determined the wing location, we need to address the mounts themselves. |
| 04:28 | There are two primary ways in which a wing can be mounted, either from the bottom surface, which is what's known as a conventional mount, or the top surface, which is what's known as a swan neck mount. |
| 04:38 | There's also end plate mounting, however that isn't applicable for most cars. |
| 04:43 | Between swan and conventional mounts, aerodynamically speaking, swan is universally the way to go. |
| 04:49 | The reason for this is that the flows on the suction side of the wing are highly stressed, with strong adverse pressure gradients, and adding disturbances in the form of mounts causes the flows to separate much earlier than they would if it was just the wing in free air. |
| 05:04 | Swan mounts alleviate this issue and keep the bottom surface of the wing clean, so this is a great strategy here. |
| 05:10 | The only reasons we'd go for conventional mounting are if we couldn't get the structure to work from a strength perspective, if the rules required it, or we needed to maintain a classic look. |
| 05:21 | The final component we need to consider is the end plates. |
| 05:24 | These limit the flow of air from the high pressure side of the wing to the low pressure side of the wing, around the wing tips, and in doing so they make the wing generate more downforce and be more efficient. |
| 05:35 | If we have to make end plates from scratch, a good starting rule of thumb is to do two times the distance below as we would above. |
| 05:42 | So, for example, we could go 100mm above the wing and 200mm below a wing with end plates. |
| 05:48 | Again, this isn't an exact law, but more of a ballpark guide. |
| 05:52 | Let's quickly summarize what we've covered in this module. |
| 05:55 | The rear wing is crucial for setting rear downforce and drag levels. |
| 06:00 | Its performance depends on the air or foil profile, with more curvature providing higher downforce , but also more drag. |
| 06:07 | The wing's angle of attack affects downforce, but too much angle can cause flow separation and reduce performance. |
| 06:13 | A gurney flap can boost downforce without increasing the angle, allowing a wing to create more downforce than it could without one. |
| 06:20 | The placement and mounting of the wing, particularly higher and rearwards, are important for optimizing car balance, improving underfloor performance and avoiding poor airflow to the wing, with swan mounts being preferable for cleaner airflow underneath the wing. |
