attn: resident aerodynamicist

just wanted to throw some learnin' back an forth.

i was messing around with a sunroof today and was wondering the extent that the open sunroof increases lift, lowers air pressure in the car, and how much the spring loaded flap in the front reduces this effect. obviously it doesnt increase lift drastically, since its used on cars (duh) but how much worse will a car stick to the road if the flap thing is gone or held down? and what is the difference between that and having a side window open, not in terms of drag but in terms of lift/downforce?

also, i was talking to a kid in the 240 section about gas mileage, and i couldnt think of any instances where aftermarket aero reduces drag (im talking wind tunnel tested stuff, not super samurai big mouth hideousness). i kind of figure vortex generators help, but would a small lip spoiler (front or back) help or not? how about a splitter? side skirts? smooth undertray?


any thoughts would be cool id like to toss some theory and truth back and forth.

If you just open the sunroof (no flap) the difference would be minimal. It can suck air through the ventilation system and let it out the top which reduce lift, but compared to the rest of the airflow over the car its minimal. The flap does very little as well. If you notice its not very heavily sprung on some cars, meaning that any change in airflow it provides is very minimal.

As with planes, there are three axis, each of which have opposing forces. Its the same with cars. Adding lift (either in a positive or negative way) increases drag. If you have a neutral wing on the trunk (not providing any up or down force) it will add a little drag, but if you start altering the lift by angling the wing, you change the amount of drag you place on the air flowing over it.

The same goes for rear bumper venting. By letting the high pressure air escape from the rear of the car you can increase downforce, but you have to think of the whole thing; all you're really doing is altering the shape of the whole wing that is the car. Additional downforce is negative lift which creates drag. The secret with all aerodynamic setups is to find the crossover point. In racing you'll often see them adjust the wings and foils to balance drag and downforce. More downforce means more traction but less top speed and vice versa.

right but are there little tricks you can do to increase the drag coefficient of the car? again i mention vortex generators since by my understanding they allow the air to follow the shape of the car with less turbulence. im sort of trying to learn some real world aero tricks. would a smooth undertray reduce drag? i ask because it seems like it would do both, (increase and decrease drag, from reducing the pressure of underbody air which increases drag, but the smoother surface would reduce it) but i dont know which one would be more pronounced.

right but are there little tricks you can do to increase the drag coefficient of the car?

You mean decrease the drag coefficient?

Sure, start by smoothing out every join, block as much air from going through the grill and bumper as you can and smooth the underside.
Those moon disc hub caps would surely help too.

Ride height is another interesting one. Lowering your car probably causes more drag.

^^^yeah thats what i mean. i can see how lowering a stock car would increase the drag in sort of same way as smaller exhaust increases backpressure, but i also know that sealing off the underside of the car can reduce it.

i mention vortex generators since by my understanding they allow the air to follow the shape of the car with less turbulence.

Vortex generators generally increase turbulance. Their advantage is that they delay flow seperation, thus they decrease drag if used in the right place.

i kind of figure vortex generators help, but would a small lip spoiler (front or back) help or not? how about a splitter? side skirts? smooth undertray?

All of these help, if used correctly. They can all add downforce, they can all remove downforce, they can all add drag, and they can all remove drag.

Often adding downforce will add drag, mind you. Watch out for that.

One nice thing is that as you lower your car, you reduce the lift the basic shape makes and eventually it starts making downforce. Making downforce and making lift both cause drag, so you might think it is a compromise, but it doesn't have to be: If your car is currently making lift, and you drop it until it stops making lift, but doesn't start making drownforce, then you will effectively increase the load on the tyres while also reducing drag.

It will ruin your suspension geometry in most cases though, so I don't reccomend it unless you know what you are doing.

One nice thing is that as you lower your car, you reduce the lift the basic shape makes and eventually it starts making downforce. Making downforce and making lift both cause drag, so you might think it is a compromise, but it doesn't have to be: If your car is currently making lift, and you drop it until it stops making lift, but doesn't start making drownforce, then you will effectively increase the load on the tyres while also reducing drag.

Eh?

That's the equivalent of fitting a wing to create lift, then fitting another to create downforce and cancel it out.
Both result in more drag, you can't get lift or downforce without paying for it in thrust.

Eh?

That's the equivalent of fitting a wing to create lift, then fitting another to create downforce and cancel it out.
Both result in more drag, you can't get lift or downforce without paying for it in thrust.

As you know, the cars body itself is a giant wing.

When quite high, it creates lift.

As you lower it, it starts creating drag.

If you put it in the middle of those two positions, it will create nothing. This means less drag.

If it was originally a lot higher, then that also means the 'less drag' position will mean less lift as well.

As you know, the cars body itself is a giant wing.
Agreed.

When quite high, it creates lift.
The shape of the top surface creates lift all the time.

As you lower it, it starts creating drag.
As you lower it, you can exclude airflow underneath, creating a low pressure area underneath which adds a downforce component.

If you put it in the middle of those two positions, it will create nothing. This means less drag.
This is where I don't agree.
The top surface is always creating lift, adding downforce to counter it does not reduce drag. This isn't the same situation as changing the angle of air air-foil.

Agreed.


The shape of the top surface creates lift all the time.


As you lower it, you can exclude airflow underneath, creating a low pressure area underneath which adds a downforce component.


This is where I don't agree.
The top surface is always creating lift, adding downforce to counter it does not reduce drag. This isn't the same situation as changing the angle of air air-foil.

This is a good point. I read the less-drag thing from a reputable source and obviously didn't think it through well enough.

However, it is possible that depending on the height of the bumper/height of the underbody etc, that as you lower the car, less air gets in underneath it to cause drag against all the components down there. If you lower it too far, you wind up having to speed all the air up to create the vaccuum. There is probably some sweet spot.

That is my guess.

This is a good point. I read the less-drag thing from a reputable source and obviously didn't think it through well enough.

However, it is possible that depending on the height of the bumper/height of the underbody etc, that as you lower the car, less air gets in underneath it to cause drag against all the components down there. If you lower it too far, you wind up having to speed all the air up to create the vaccuum. There is probably some sweet spot.

That is my guess.

I'd agree with that. The underside of a car contains some very messy shapes as far as airflow goes.

true... you can decrease drag with a smooth underpan, but the turbulence under the car tends to create a low pressure area which counteracts the low pressure on top of the car created by the wing shape.

Its all a trade off. You could increase MPG or lower drag by installing a belly pan, but it comes at the extreme risk of taking serious weight off the tires at highway speed.

Like has been said... the car is shaped like a wing. The distance the air has to travel over the vehicle is greater than the distance it has to travel under the vehicle. This is a concept that must be accepted with current car design parameters. Aerodynamic add-ons are simply there to counteract those forces. Many production cars dance around this neutral lift territory. The worst car I ever drove as far as lift was a Plymouth K car. At 60 mph, I could actually yank the steering wheel and it had very little effect. Contrast that with a C4 corvette with one of the nicest aerodynamic balances ever. Under 80 mph, it was slick and provided nice neutral lift. Over 80, the air going over the rear deck stalled which allowed the turbulent undercar air to provide nice suction. The benefits were that it had very little trade offs at normal human speed, but if you wanted to open it up it was almost as if the aerodynamics changed for you.

Turbulence = low pressure.
Longer distance = low pressure.

The secret is to balance the two for the application.

IMO it is stretch to say a car is wing shaped, just because upper surface is longer than the bottom. But it doesn’t matter because you don’t need an airfoil to create lift, considering the fact that even a flat plate can create lift given some angle of attack.

The problem is that air is displaced as the vehicle moves and as it follows the car it is pulled downward at some point along the body. The reaction force creates lift on the body.

It would be easy to get rid of lift on the upper surface of the vehicle if you allowed the flow to separate in the area that lift is being created. But there will be large drag penalty associated with doing this.

So a better solution is probably using the techniques already mentioned to create a downforce elsewhere on the vehicle.

IMO it is stretch to say a car is wing shaped, just because upper surface is longer than the bottom. But it doesn’t matter because you don’t need an airfoil to create lift, considering the fact that even a flat plate can create lift given some angle of attack.

The problem is that air is displaced as the vehicle moves and as it follows the car it is pulled downward at some point along the body. The reaction force creates lift on the body.

It would be easy to get rid of lift on the upper surface of the vehicle if you allowed the flow to separate in the area that lift is being created. But there will be large drag penalty associated with doing this.

So a better solution is probably using the techniques already mentioned to create a downforce elsewhere on the vehicle.

The same logic applies when the air is pushed up by a car. And yes, in profile, the body goes up just as much as it goes down.

a car is an airfoil in itself. it is longer on top than it is on the bottom. by adding wings, one can counter-act the lift created by the shape of the vehicle at the expense of drag. by smoothing the underbody and creating a rake from front to back one can counter-act the "downforce" of the top of the vehicle due to "ground effect" on the bottom. This actually reduces drag as the air under the vehicle is moving faster (aka has a lower pressure) (basic bernouli's principle). the air on top of the vehicle is not acting in ground effect so therfore has less of an influence as the air under the car. this is amplified by adding side skirts to prevent air coming in from the sides. if the side skirts have a nice sharp edge on the bottom they will also induce a vortex witch will allow the air to stick better to the underbody, especially at higher AOAs (steeper rakes). so bassically, the top of a car will always create lift. that needs to be balanced by or overcome by downforce created by the underbody or wings. underbody is drag free. wings are not.

The same logic applies when the air is pushed up by a car. And yes, in profile, the body goes up just as much as it goes down.

True, although it doesn’t necessarily follow the same slope (in magnitude anyway). As well, this doesn’t imply that the forces some how cancel, there is still lift at some point and downforce at another. Both of these points are likely to be on opposite ends of the body.

The point I was trying to make was that saying the upper surface of the car is longer than the bottom therefore it is an airfoil so it creates lift is an over simplification of both airfoils and vehicle aerodynamics.

For airfoils the upper surface doesn’t need to be longer to create lift. If you turn a classic wing shape upside down it can still create positive lift given enough angle of attack. Otherwise how would a plane be able to fly upside down?

Likewise, the upper surface of a car, while longer than the bottom, is not a smooth curve. So the flow over it will be complex and vary from transition to transition. Certainly, there are generalizations to be made, but they are just that.

For airfoils the upper surface doesn’t need to be longer to create lift. If you turn a classic wing shape upside down it can still create positive lift given enough angle of attack. Otherwise how would a plane be able to fly upside down?

Actually... when they fly upside down with a high angle of attack, the stagnation point moves down (in side view, with plane upside down), such that the effective 'top' is still longer than the effective 'bottom'.