How to create lift using the underside of your car
 

The question is simple, Can we create lift by modding the underside of our vehicle? Not enough lift to flip us over, but enough lift that at certain speeds could affect the engine load thereby making our drive more efficient?

At first I was contemplating two ideas, dimpled bellypan, (Mangus effect).

Or some type of wing or spoiler built under the car or near and between the two tires to take advantage of that turbulent air.

Let hear some ideas, or just call me crazy and ignore this thread .. LOL

I don't think there is a practical solution for reducing rolling resistance through lift. In order to generate enough lift with some type of wing to make a difference the car will have to be going pretty fast - by which point aerodynamic drag should be a bigger factor than rolling resistance. Hang your car from a streamlined helium blimp and you can maybe have a win at around 10 mph - if there's no wind. Surround your car with a hovercraft skirt and add a big blower and you can reduce rolling resistance to almost nothing, but again, not very practical since you have to use a lot of energy to maintain the pressure under the car.

When you generate lift/downforce you also create an equal amount of drag. (Downforce is simply negative lift)

being aerodynamically neutral ( L = 0) is the best. teardrop/non cambered airfoil is best for low drag.

(See attachment, its the NACA 0015 airfoil. Its upper and lower surface are identical. at 0 angle of attack it generates no lift and no drag due to lift. There is still drag but its very low)

Why would you want lift? What would it accomplish?

Serious question.

Less rolling resistance.

When driving I notice that a couple of points off my engine LOD translates into large chunks of FE, so by lifting the nose/front end a hair you could stand to gain quite a bit of FE and not risk disaster.

I think you're a little confused.

a) lift makes for less traction. More speed = more lift = less traction at highway speeds = bad.
b) lift and downforce both create additional drag. You don't get something from nothing. (it is not equal, however, that depends on a lot of stuff)
c) more air under the car means more air along the turbulant underside
d) a wing in turbulance does nothing. Race cars try to smooth the air before their wing, or hang it up above the roofline for clean air.

Sorry if that sounded like grilling... I think you're just trying to stretch a concept a little too far. Yes, lower RR uses less gas, yes lift will lowing your RR, but it also affects other things... negatively.

Cheers!

Make a belly pan and lower your front bumper. You generate less lift, but you'll have less drag.

Rolling resistance with respect to increases in downforce are very minimal when compared to rolling resistance with respect to speed. You'll definitly end up on the wrong end of the stick should you try to increase lift. I'll see if I can get any graphs or info for you out of Hucho's book for reference.

Unlimited Hydroplanes use lift from under the hull to raise them out of the water. They would probably be the best example of what you are trying to achieve, but of course they are not worried about saving fuel. Any type of lift creates drag and may cancel out any gains made in rolling resistance if any.

It's good that your thinking outside the box,however,there already exists a body of empirical scientific data which has already addressed your theoretical inquiry,and no fruit was borne out of investigations.I recommend that you exploit what is already in the bag,then refine the small points from there.Good questions!

Double your speed and your drag increases FOUR FOLD.

Sticking a wing, spoiler, etc on the rear of a car is an attempt to make up for a aero design deficiency. A perfect example of good aerodynamic design (not perfect design) is the soap box derby racer bodies (do a Google Image search). Notice no spoiler, wing, etc.

Lift, whether negative lift or positive, creates drag. And a wing at zero angle of attack still produces parasitic drag, which increases 4X as speed 2X's.

In order to enjoy any benefits from lift, you would first have to lighten the car and build it like an airframe. Even then, to get any reduction in friction you would have to build wings on the car to allow the lift to reduce its weight on the ground by any significant amount.

A car body designed for low drag will usually generate some lift. This causes more drag than leaving the weight on the tires, but less than spoiling it deliberately. Even with efficient wing shapes, aircraft have to fly where the air is very thin to get reasonable economy.

Aircraft fly high for two reasons. Less air with the same air to fuel ration burns less fuel. Higher altitude (colder) the higher the true airspeed.

You DON'T want lift. Trust me, I have an MR2, and the front end aero is pretty poorly designed, and known for creating lift, and being unstable on the highway. The car can get pretty twitchy when you don't have all of the stock underpanels on to reduce lift.

That just means that they have fewer HP available. A throttle does the same thing. Piston engines often have superchargers that would blow the heads off at low altitude, that are used to help burn more gas up higher, for a faster, but less efficient cruise speed. The altitude does help with reducing drag and avoiding weather.
Colder air is denser, which gives both more drag and more lift.

Old aircooled VW's have lift... rounded roof, relatively flat belly pan, in many respects they are like one giant airplane wing.

A drag racing buddy of mine wanted to confirm what many of us thought we already knew. He had a guy video tape his bettle-bodied race car at the top end of the quarter mile, while he went through the traps at 120mph. The observed separation between the top of the tire and the inside fender wheel arch was greater at 120 than standing still, by about two inches! Definately lift. Definately dangerous in a crosswind situation.

I'm no flight engineer, but from what I've learned about Bernoulli's principles I'd guess that a basic car should be able to generate some lift with minor modification. Basically if you cause air to move faster on the upper side of your car than it does on the lower side of your car, you will create a lower pressure above than below. The result: lift. Since cars already are pretty flat on the bottom, and less so on top, the design isn't too far off from an airfoil, depending on the car.

So, if you were to put a full flat belly pan on the bottom of your car and completely eliminate air flow resistance, the air underneath you would remain relatively static. If you raise your bumper/air dam height just a little, you could actually compress air between the ground and your car (and change your angle of attack) which would increase the air pressure. The air flowing over the top of your car will have an increased distance to travel and will flow much faster than the air underneath your car, creating a low pressure area. If you streamline the top of your car to reduce turbulence, the flow will be smooth, fast and uninterrupted. If your car slopes down toward the back in a boattail fashion it should maintain this flow. If your car ends abruptly like most hatches, the turbulence behind the car will prolly create drag which may cancel out some of the lift.

A spoiler is usually put on race cars to interrupt the smooth flow of the air, reduce lift and add down force. Since your goal isn't to take sharp corners at 100+mph, you prolly won't need as much down force. However, if your car is light to begin with, expect to be pushed around by wind gusts when you are at speed. Also, sharp high speed maneuvering may be a little more difficult. If your car is decently heavy, it may just take some pressure off the wheels.

I remember reading an article in Car and Driver many years ago about the effectiveness of spoilers on common sports cars (not race cars). They found that most spoilers didn't make much of a difference at normal to excessive driving speeds with the exception of the Porsche 911, which became effective at around highway speed. If you look at the shape of that car when the spoiler is down, you can see how it might be able to generate some lift at speed.

Again, I'm no flight engineer, but the theory kinda makes sense, doesn't it?

actually, adding a bellypan will increase downforce thanks to the venturi effect - the car would become like a tube with open ends of greater area than the middle. however doing so will also reduce drag by a substantial amount because typical car underbodies are catastrophically unsmooth

everybody here needs to look at the autospeed articles where they tuft-tested several different cars

For some particular car, can anybody give an example of the speed at which rolling resistance is equal to aero drag? I know that it takes a LOT longer for my car to go from 10 to 0 than it does to go from 50 to 40.

the speed
 

CAR and DRIVER used to publish road horsepower,broken into aero and friction and tire losses,at 30-50,and 70-mph,for their in-depth road tests.I believe there is a descrepancy do to the optical speed sensor slung off the side of the car.I'm awaiting a return communication from C&D to see if Daytron Messteknik,makers of the speed sensor offered a correction factor.---------------------- The old rule of thumb,was that at about 50-mph,aero and RR were about the same.If I hear back from them,I'll let everyone know.

Lift and the average car
 

As someone who has owned a few cars with lift problems, I'd like to say I don't think it's a very desirable property of anything you plan on driving regularly at highway speed. Minimizing drag by shooting for zero downforce, perhaps, but with actual lift, at speed the car begins to lose traction. It also attains an unappealing "dancing" quality in the rear that generally feels unstable. In wet weather and snowy conditions it can get a bit scary, actually. If you notice, VW has had some problems with rear end lift on the new Beetle. There is a warning in the owners manual that if you have the model with the automatically deployed roof spoiler, adding any other aero devices can make the car dangerously unstable.

Also, positive lift would have a detrimental effect in relation to neutral lift. The lift is caused by drag, and so you would be trading drag for a decrease in friction. I'm willing to bet you'd find they cancel each other out. Lift isn't free after all.

If the car has a high front end and low in rear, (ie; carrying a heavy load in the trunk),
it will have lift. One problem is, as you go faster, you end up with a LOT of lift..
Once those front wheels start floating along and you get hit by a cross-wind,
say hello to the EMTs.. ;)

You're crazy lol...

To generate a relatively efficient means of lift you must have an aerofoil shape. If you look at most cars these days, they come pretty close. Either way, they all contain the same basic ingredients to create a fair amount of lift. These ingredients are to have a flat surface on the bottom and a curved surface on top. Air going over the top of the car creates a low pressure vs. the air going underneath (no change in pressure). This causes an upward force and most of our cars *DO* create a marginal amount of lift albeit nowhere nearly enough to render it airborne HOWEVER!!!!! Make any car go fast enough and it WILL take off. Have you ever driven so fast that the car began to feel very light and bouncy??? I have and it is scary. With this said, I don't think that purposely trying to create additional lift on your car would be a very good idea.

Sorry if this has been already discussed, as I've not read the whole thread.

That said, you'd want to generate negative (i.e., downward) lift, to keep the car firmly planted on the road, or at least reduce positive lift.

Bernoulli's principle being what it is, accelerated flow beneath the car would reduce pressure there, generating negative lift. Think of an inverted airplane wing.

Accelerated flow and reduced drag could best be done via smooth belly panels, often discussed in this forum.