Aerodynamic notes.. aero principles

Converting the car for ground effects is one of the not so big challenges currently facing us and sth MVS Racing are used to working on. I got some great notes from Phil on Lotus Esprit Forum and had a chat with Martyn this morning..

If the racing regs allow you to extend aero mods beyond the body, you should. The idea is that you have a high pressure zone on top of the spoiler pushing down, and a low pressure zone below it pulling down. If the spoiler is inside the line of the body, the low pressure zone is pulling the boot-lip up by an equal amount to the downward pull on the spoiler, so they effectively cancel each other out, wasting half of the aerofoil's potential.By extending the spoiler rearwards, the low pressure zone now acts directly between the spoiler and the road. It's a double win too, because the leverage due to the downforce is increased. The leverage idea also holds true at the front.

The second principle is that you need to allow as little air as possible under the car. This means that the front splitter needs to be as low as possible, so you'll need much stiffer suspension to prevent bottoming out and to keep body roll from "breaking the seal" just when you need it most. Side skirts help stop air sneeking in from the sides.

Third principle (which your pic suggests you're aware of) is that the aerofoil works best with end plates to prevent spillage of the air around its sides. Extend them upwards as well as down. Group C cars usually extend them forwards too to prevent leakage into (and turbulence within) the low pressure zone via the notch between the flying butress and the end plate.

Fourth principle (which also reduces drag) is to avoid any sudden changes in shape because they will cause turbulence. This is why NACA ducts are shaped the way they are. For example, your diffuser will be most effective if it curves upwards rather than being a straight line. This reduction in turbulence will also make the low pressure zone behind the car more effective at pulling the spoiler down.

Fifth, the rear of the (smooth) undertray should ideally be higher than the front to reduce the pressure along the length of the car. Without sideskirts, this isn't too effective, but every little helps.You'll notice that some of the points also make another point more effective, so it pays to keep the whole car in mind. This will also ensure yo keep the car balanced - you don't want to make the rear so effective that you take off like Mercedes did at Le Mans!

Phil