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Old 05-25-2021, 09:42 AM   #1 (permalink)
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Why don't vehicles have aerofoils to partially take the weight of the car?

In the Tahoe hybrid thread the car would only ever get 60mpg(US) due to rolling resistance, in the insight thread half of the drag was rolling resistance.

Theoretically rolling resistance is proportional to force applied, so a lighter car would have lower rolling resistance.

A car that generates significant lift has lower rolling resistance at higher speed.
Low drag shapes in ground effect often have high lift coefficients.

I am imagining a car, a bit like an ekranoplan, but never leaves the ground so the wheels always drive the car. Steering angle sensors, wheel slip sensors and brake pedal sensors cause the lift to stop and spoilers to create downforce pop up. Turning at speed can be assisted by aerodynamic aids.

As long as it is aerodynamically stable, in all directions, can dump lift as necessary I see no reason why this wouldn't reduce rolling resistance.

Obviously this is really complex but looking at cars like the huayra, certainly possible. Is it the case though that by the time significant rolling resistance reduction occurs, the speed is so high that rolling resistance is a tiny fraction of total drag?

 
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Old 05-25-2021, 10:39 AM   #2 (permalink)
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why don't

Quote:
Originally Posted by AeroMcAeroFace View Post
In the Tahoe hybrid thread the car would only ever get 60mpg(US) due to rolling resistance, in the insight thread half of the drag was rolling resistance.

Theoretically rolling resistance is proportional to force applied, so a lighter car would have lower rolling resistance.

A car that generates significant lift has lower rolling resistance at higher speed.
Low drag shapes in ground effect often have high lift coefficients.

I am imagining a car, a bit like an ekranoplan, but never leaves the ground so the wheels always drive the car. Steering angle sensors, wheel slip sensors and brake pedal sensors cause the lift to stop and spoilers to create downforce pop up. Turning at speed can be assisted by aerodynamic aids.

As long as it is aerodynamically stable, in all directions, can dump lift as necessary I see no reason why this wouldn't reduce rolling resistance.

Obviously this is really complex but looking at cars like the huayra, certainly possible. Is it the case though that by the time significant rolling resistance reduction occurs, the speed is so high that rolling resistance is a tiny fraction of total drag?
I'll try a link to a video with drag charts germane to your question. The proportion of the road load due to R-R compared to the aero portion really falls with increased velocity. So it is part of decision making.
The other thing I'd mention, is liability, and the ability for the system to fail-safe. You don't want the county coroner's office involved with your car.
If you've seen race cars abruptly take to the air at 24- Hours of Le Mans, you'll understand how spontaneously aerodynamic phenomena can occur.
The ECU would have to be programmed to recognize precursor anomalies, and signal lift-kill at the speed of light, in order to protect the occupants and surrounding motorists. Servos, solenoids, pneumatics, hydraulics, or what have you, would have to react instantaneously to the prompt from the ECU output. Things can go south in an instant.
I speculate that the risk assessment, so far, to any venture like this, has been shot down by corporate liability attorneys. Too much pain, for whatever perceived gain.
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Old 05-25-2021, 10:48 AM   #3 (permalink)
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Cars get REALY unstable when aerodynamic lift happens.
But there are vehicles that operate on aerodynamic lift alone for most of the travel distance.
We call them "planes", some of wich can be pretty efficient, especialy when you also use thermals to gain altitude.
Gliders can gain up to 10m/s when circling in thermals above powerplants.
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Old 05-25-2021, 10:52 AM   #4 (permalink)
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The B2 and X29 has proven we can do aerodynamic controls real time with predictive algorithms, but there's no brick walls or jumping telephone poles to avoid at altitude.

Avoiding the suprise deer is difficult, worse when the response is slower because aircraft don't turn really fast. Come to think of it cars are only marginally more responsive at speed.

I exceed 10 meters/sec up once, really scared me thought I would break the plane. Never from a power plant since they are restricted airspace here.

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Old 05-25-2021, 11:06 AM   #5 (permalink)
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I've also had the displeasure of flying an aerodynamicly unstable plane once because I made a misstake with the ballast.
It was an Astir, I strapped like 25 kg of ballast to the seat because I'm way too light for this plane.
Well, idiot teenager me strapped the weight the the backrest, wich is almost perfectly in the center of gravity, so I ended up some 25 kg tail-heavy on my first flight with the plane.
Couldn't hold speed at all, was either getting increasingly faster or slower and twitchy as hell.
Managed to land it somehow, even though this plane has some realy significant grounf effect.
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Old 05-25-2021, 11:36 AM   #6 (permalink)
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I would think the induced drag would outweigh any benefits in fuel economy gained by lowering the rolling resistance.
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Old 05-25-2021, 11:53 AM   #7 (permalink)
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I have seen the Le Mans cars take off, but they were aerodynamically unstable in the pitch direction, that could be corrected if designed properly. I was more focussing on really low drag cars where the rolling resistance is more significant.

One of the proposed solutions for the Le Mans cars was suspension connected wings, that would alter their angle of attack as the suspension compressed. I see no reason why that couldn't be done here to maintain a consistent level of "hover". Wheel goes light or comes off the ground, the suspension droops, pushing the wing angle of attack way down into negative lift.

With stiff enough suspension, the rebound rate would be more than fast enough. No servos needed for pitch control. (Although cornering and braking servos or hydraulics would be neccessary)

Quote:
I speculate that the risk assessment, so far, to any venture like this, has been shot down by corporate liability attorneys. Too much pain, for whatever perceived gain.
I think you are right there. And I do agree than commercially it would never be possible. But for something like a solar car with lots of batteries or similar, where the aerodynamic drag is very low, but the rolling resistance is high and the car travels really fast.

Quote:
Cars get REALY unstable when aerodynamic lift happens.
But there are vehicles that operate on aerodynamic lift alone for most of the travel distance.
We call them "planes", some of wich can be pretty efficient, especialy when you also use thermals to gain altitude.
Gliders can gain up to 10m/s when circling in thermals above powerplants
But that is only because they aren't designed to lift. Planes go too high and so can't be wheel powered, but something like an ekranoplan is a better comparison

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I would think the induced drag would outweigh any benefits in fuel economy gained by lowering the rolling resistance.
You may be right, but considering generally, very low drag cars are shaped like an aerofoil to reduce drag, but then have to add spoilers to counteract the lift, it seems possible to me.

In the never ending template debate, which we won't resurrect, it was revealed how high the coefficient of lift was for this "optimum" shape.
 
Old 05-25-2021, 12:17 PM   #8 (permalink)
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Doing some calculations it seems that aerofoils of over 100 times lift to drag are common, and given a rolling resistance of 0.01 the difference between lifting using wings and using wheels are identical, so theoretically adding wings would have no effect on total drag.

But with greater than 100 times lift to drag then it is potentially beneficial to add wings.
And if the car is already shaped like an aerofoil, such as a solar car, then most of the lift induced drag would already be there in the shape of the car.

Those are in free air and ground effect may change those things.

But the car has to exist, making that shape have lift would give very little extra drag because the shape is already there, already generating drag.

(I do know about the ultra low rolling resistance tyres that they use on solar cars, but you can't buy those and they only last for 200 miles)
 
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Old 05-25-2021, 12:17 PM   #9 (permalink)
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I would think the induced drag would outweigh any benefits in fuel economy gained by lowering the rolling resistance.
I was looking at the 2015 Yukon Denali, tested by CAR and DRIVER.
This is a little larger SUV than the Tahoe, and a curb weight of 6,060-pounds.
EPA test weight would be 6,360-pounds.
For it's frontal area estimate, and using Cd 0.36, aerodynamic road load @ 70-mph is 67.72% of overall load ( 29.798- horsepower, leaving 32.27% for rolling-resistance ( 14.2- horsepower )
Should there be one particular airfoil design, superior to all others, which, say, produced an evenly-distributed 3,180-pounds lift, then rolling resistance could be cut in half, and the lift-drag table for that airfoil would indicate the required power to create that magnitude of lift. Then it would be a matter of comparison, to establish your cost-benefit ratio.
If you drove into a chain-reaction rear-end collision, involving a breached cooling system, and had to maneuver on top of a glycol-wetted road surface, I can't imagine a sensor package which could identify this as a threat, reacting spontaneously, such that it could avoid catastrophe.
Product-liability attorneys would be all over that. A 'rainmaker'.
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Old 05-25-2021, 12:22 PM   #10 (permalink)
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I always thought it dumb to increase cornering ability on a Formula 1 car by creating massive downforce, most of which isn't needed at high speed straights where the majority of downforce is created. I'd rather see a rudder that dynamically moves to fly the car around the track.

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