I was wondering if you could create an airfoil for your car (on the airplane wing principle) which would give your car a little ‘lift’ when travelling at freeway speed - not enough lift to go airborne of course or lose control, but maybe enough to relieve some of the weight and therefore increase the gas mileage.
Would this work, in theory and/or practice?
If so:
How much weight could you relieve while retaining safe control?
How big of a ‘wing’ would be needed?
How much gas mileage savings could be realized? Is fuel efficiency directly proportionate to weight reduction?
I assume this wouldn’t work, becuase if it would, it would have been done by now. But I’m interested to see the discussion here.
As it would also reduce the contact/friction of the tires with the road, this would reduce your forward propulsion, which wouldn’t be any good for fuel efficiency or performance I’d imagine.
It isn’t the weight of the car that’s the problem, it’s the mass. F=ma and all that. All you’d be doing with your airfoil scheme is (possibly) reducing road/tire friction. As someone pointed out, that’s not a good idea for the drive wheels and definitely not a good idea when you need to brake.
At highway speeds, you’re not really accelerating very much anyway, basically you’re just fighting drag and friction. So adding an airfoil is going to probably increase your gas consumption.
All wings generate drag in addition to lift. The more lift, the more drag (for a given wing).
The tires generate “lift” to keep you from falling through the road with very little drag.
Weight hurts your milage in three ways:
1)The tires have to carry the extra weight, so flex more, and generate some drag. The wing would help you here if it were not for the point made above.
Accleration. Getting more mass up to speed takes more energy. Wing is no help at all with this.
Hills. Hauling more weight up a hill takes more power. The wing is no help here. As any pilot will tell you, climbing takes power regardless of weither the ground or the air is supporting the vehicle.
Don’t feel bad, you just had to reality check your engineering with other folks… happens to everyone.
You’d be adding weight, too, btw, another bugaboo.
Weight doesn’t actually hurt gas mileage noticeably, providing you maintain a steady speed, don’t count time spent getting up to speed, and don’t go up any hills. It’s the wind resistance that you fight at highway speed, mostly.
The cruising gas mileage was nearly the same on I77 in a Lincoln Continental I drove vs a Ford Taurus… same frontal area, same chassis, Lincoln was 600 lbs heavier…
Not even the mass matters in steady state. Just when accelerating or declerating. F=ma but when you’re at a constant highway speed the only F involved is mechanical friction, rolling resistance, and air resistance. (A reduction in weight might provide a very small reduction in rolling resistance, I imagine.)
Yes, additional thrust is necessary to get the lift, so you’re worse off.
How about patching the wing into the braking system and the traction control to exert downward pressure when extra traction is needed and folding back to reduce drag when not needed? Make the car lighter to begin with?
Race cars have airfoils to increase the pressure on the tires…the faster the car goes the harder the airfoils press it to the track. This makes them less likely to skid.
pmwgreen, I suppose you could add a retractable airfoil, but the trouble is that the faster you’re going the more grip you want, which means more drag. The more grip the airfoil gives you the more drag it creates, and at low speeds and low drag the airfoil won’t give you any extra traction. So there’s no need to retract it at low speeds, because it doesn’t have any effect at low speeds, besides adding weight. But retracting it at high speeds doesn’t make sense either, because that’s when you need the extra traction.
But highway cars don’t really need spoilers, the ones bolted on highway cars are purely ornamental. And a retractable airfoil would add even more weight than a fixed airfoil.
Related question: I recall that a racecar used at Indy used a retractable airfoil, or at least one that flapped when the car’s brakes were applied during turns. I also think it was later banned because it distracted following drivers, though it was acknowledged to be effective for Indy racing. Does anyone know anything more about this?
Look, we just want our flying cars, dammit! It’s the 21st century, where are our flying cars? Here it is halfway through 2006 and we’re still driving on the GROUND.
We have flying cars, they are called airplanes. You can fly an airplane to work if you wish. But why would a machine for flying through the air be crossbred with a machine for driving on the ground? You’d get a machine that doesn’t do either job very well. So you don’t want a flying car, you want a flying machine. Which we have. If you want to install a runway at your house for your light airplane, and convince your place of employment to install a runway, and the local grocery store, and etc, go ahead.
CJJ*, I’m pretty sure you’re right about one of the turbine-powered Indy cars (either Granatelli’s in 1967, or the Lotus 56 in 1968) had a deployable aerodynamic brake, but I can’t find any cites on it.
There was also the Mercedes-Benz 300 SLR in 1955, where the rear deck angled upward under braking (hinged at the rear, visible here), and the Chaparral 2E, where the rear wing would change its angle to increase drag when it was needed.
From what I’ve read, the ban wasn’t because of distraction, but for safety. If you were going full tilt toward a corner (as racers are wont to do) expecting to have a certain amount of braking force available, and the linkage to the aero-brake doesn’t work, something very, very bad is going to happen.
And all the talk about a retractable airfoil is interesting, but irrelevant to the situation in the OP. It wouldn’t matter if you could make a wing with zero mass. You would take some pressure off the tires, and lessen the frictional losses of rolling resistance and tire flex, but the increase in drag (parasitic and induced) would more than make up for it.
Here are 2 answers, one for Key Lime Guy and one for CJJ*.
There is an airflow device that will help gas mileage without increasing drag. It’s an airdam, one of those hang-below-the-front-bumper things. The area under the car must be included in the frontal area; so much air gets crammed under there that there’s a high pressure zone with a lot of turbulence. If you add a flap under the bumper to scoop some of that air around the sides, you reduce the pressure beneath the car. That reduces drag a bit, and it helps a bit with the airflow through the radiator. It will help with gas mileage, but don’t expect a miracle. Maybe 5%.
Now, the moving airfoil. One of Jim Hall’s early Chapparal cars on the CanAm circuit had a big wing on struts above the rear of the car. On the straightaways, the wing was nearly horizontal, and it made some downforce. On braking, the wing rotated to nearly vertical, creating drag for slowing down. It worked very well, and was used for only a short time. Hall made another car with low plastic skirts all around and a big fan in the rear deck. The fan was powered by its own small motor, and it sucked air from underneath the car. It also sucked up sand and little rocks from the track surface, which rained down on other cars. The downforce was astounding. That was outlawed “for safety.”
No, airplanes are airplanes. A flying car would be about the size of a normal car, hold 2-5 passengers, and could do VTOL. So it would not need runways, you’d just use normal parking spots. They would probably be DOT street-legal as well.
I guess the main problems is safety, just how do you safely route thousands of flying vehicles? The answer is probably let computers do the flying, eventually.
You want to take a car and add wings and call it a flying car. Except that makes about as much sense as a sword that can also fire bullets. Yes, such things were actually made back in the black powder days…but they made no sense. If you want a sword, get a sword. If you want a pistol, get a pistol.
So if you need a VTOL passenger aircraft you can get one, but it’s going to be called a “helicopter”. And it’s going to be much more expensive than a car.
You want a safe vehicle not much more expensive than a car, that can hold 2-5 passengers, have VTOL, and still allow you to drive down to the neighborhood grocery store? Not going to happen. Impossible. You can have a (relatively) cheap airplane, you can have an expensive helicopter, but you can’t cram all that into an automobile chassis, much less cheaply and safely.
There’s a reason airplanes look like airplanes, not like cars. There’s a reason helicopters look like helicopters, not like airplanes or cars. And neither of them are cheap, neither of them are fail safe. Why would you expect a consumer VTOL airplane to be cheap and safe, when the only VTOL aircraft in the world are hideously expensive and horribly dangerous?
