Why don't vehicles have aerofoils to partially take the weight of the car?

[freebeard]

Cars are more like sail boats than airplanes. They surf the interface between two media.

Rather than biasing the body toward lift, perhaps a dynamic suspension system that reads the weather and the approaching road surface to control ride height (front and rear) at high resolution.

[Isaac Zachary]

Originally Posted by freebeard

Cars are more like sail boats than airplanes. They surf the interface between two media.

Rather than biasing the body toward lift, perhaps a dynamic suspension system that reads the weather and the approaching road surface to control ride height (front and rear) at high resolution.

But that wouldn't improve rolling resistance. Or would it?

One thing we do know, cars can be made much lighter than they are now. Even big cars, trucks and SUVs.

Now that does impose a couple of disadvantages. While I'm not in favor of excessively large vehicles there is a certain safety benefit in having more mass in a vehicle. But prehaps with self driving and maybe even exterior air bags that can detect an impact before it happens and deploy in time to further cushion the force of a collision we could maybe make cars much much lighter than they are now.

The other thing is that if the world is going to transition to battery electric vehicles then, well, batteries are heavy.

[AeroMcAeroFace]

Originally Posted by JulianEdgar

So, one of the lowest drag cars ever created in the world to run on a public roads had driver handling issues with just 14 per cent rear lift in crosswinds, but you're suggesting 50 per cent lift would be fine?

To be clear, the car didn't have driver handling issues, and appears to have completed the entire solar challenge with this lift, what the paper says is a precautionary warning:

The reduction cannot be neglected because it translates into a 1.4% and 14.2% reduction in front and rear wheel load, respectively. Disturbances induced on vehicle behaviour by lift force resulting from crosswind, as well as other disturbances by side force and yaw moment, must be suppressed to a level low enough not to disturb vehicle driving.

But the most important thing is that the lift was 10 times more at the back, that is an imbalance, as quoted imbalance relative to weight distribution is the real issue for stability

If the uplift between the front and rear of the car is different, then the slip-angles generated by the front and rear tyres will not be equal; accordingly this will result in an under-or over-steer tendency instead of more neutral-steer characteristics. Thus uncontrolled lift will reduce the vehicle's road holding and may cause steering instability

As far as I am aware, there has never been a road car of any type, ever created anywhere, where aerodynamic lift was a positive.

Considering most road cars have positive lift, and as a result had no additional aero drag because that is how they were designed. Reduced tyre wear, tyre particulates and tyre noise are all positives.

Then I would disagree, the majority of road cars have aerodynamic lift as a positive, at least from a rolling resistance point of view. You may choose to disregard those as positives or claim that the lift induced drag (which is of course true but the form/style was already there) but we have differing perspectives.

However, there has never been a road car I agree (which wasn't exclusively what I was talking about anyway), to my knowledge, that is specifically designed to significantly lift (other than actually flying cars) I will admit.

As we have seen, solar cars can and do preferentially lift, the maths in Permalink 42 support the concept for road cars too.

But the issue of stability requires careful design of weight distribution, lift coefficients, and pitch sensitive lift coefficients to ensure that the lifting force is either central or slightly nose biased to maintain stability under cruise.

And of course rearward centre of pressure in crosswinds to prevent what happened in the solar car challenge https://ecomodder.com/forum/showthre...tml#post648989

[kach22i]

Originally Posted by JulianEdgar

It's only friction with the ground that provides ALL of a car's propulsive, braking and cornering forces. You can easily experience what a car that is only just in contact with the ground would be like by driving on a surface with almost no friction (like on ice, or aquaplaning with all four wheels on water). Basically, impossible to control.

Alternatively, if you wish the car to be like an ekranoplan, you then need to devise different propulsive, braking and cornering systems to those used in cars. For example, propellers or jet engines for propulsion. With ekranoplan-style braking and turning systems, no normal roads would be suitable, and no normal vehicle spacing would be possible either.

Finally, if you wish a car to be able to change from being just in contact with the ground (or not at all in contact with the ground) to being a conventional car when friction with the ground is required to turn, brake or accelerate the car, then cars will need to be enormously complex (because they will need to be two different types of vehicles) and roads will need to be huge and traffic spacing equally huge (because of the finite time it would take to change from one mode to the other).

Sounds like a really bad idea to me.

Good post Julian.

I am a long time hovercraft enthusiast, and probably the only one in the forum that has attempted to operate their hovercraft on an asphalt road with a crown and attempt to drive straight and turn a curve (about 23 years ago).

The goal of the experiment goal was to get a feel for what some casters and or wheels might add to the control at low parking lot speeds, and wanted to establish a baseline stating point.

I made sure their were no parked cars to hit, and no traffic, and that was a wise decision as I had my hands full to say the least.

If you have ever hit black ice in your car and spun out while changing lanes or sliding off the crown of the road you just might be able to relate to the hovercraft experiment I did.

In a word, "Lateral Stability" or Longitudinal Control" become factors long before braking and cornering when the forces of momentum caused by mass rear their ugly heads.

You have heard the expression that the only thing between you ad the road are your tires, well it's true.

In short, I was sliding all over the place with only my ability to lean my body weight around to counter the forces of a friction-less platform.

If there is a magic ratio of friction to mass/weight which is in-between that of a 400 lb hovercraft + 175 lb operator, and an automobile I personally have no faith in finding it if safety and control are at all considerations.

.................................................. .........................

I also did an experiment on my Porsche 911 about 15 years ago that may be of interest.

I was storing my car in the summer in my neighbors garage that was bigger and more dry than my own garage. However, the single wide narrow concrete drive had a steep incline with a double step in it because of a public sidewalk.

The scraping pads on the front suspension points would scrape terribly no matter the speed and approach taken, it as an acoustic wreck and worthy of a horror film soundtrack for strange noises like nails on a chalkboard 10X.

As a temporary solution/experiment for that single summer I fitted a row of a half dozen or so fixed angle casters, the supporting flanges of which were weak enough to fail long before damaging the front underbelly pan of the car.

In short, the casters fixed the scraping problem, however when I hit the sidewalk (slowly at snails pace) they would lift the car just enough as to lose steering despite the front wheels still making some contact.

The car would ease down the drive with little steering control, it was scary as the approach angle had to be perfectly straight.

Another person in the Pelican Parts forum (JMPro as I recall) placed long hard neoprene rollers/ cylinders attached to the front suspension points. On a lowered 911 the front end would dive down just enough as to contact the rollers to the ground and thereby raise the front wheels off the ground just a little, more like take a little weight off them as with my car, and again they lost some braking and steering as the car lifted. The project development did not continue after that experiment as it was deemed an unsafe concept.

As for my casters, the next summer I used my own drive and a temporary fabric and steel tube carport. I left the casters on as they were not hurting anything. That is until I went down a wavy brick road near a stone church in Ann Arbor, a large swell followed by the raise in the road bottomed out my car. I felt the car lift for a moment and then drop.

Later an inspection revealed that some of the casters did make contact at speed about 20 mph verses the 1/2 mph they were intended for and were bent and or sheered off completely, presumably left at the gutter in front of the church.

..................................................

Conclusion: Anything that lifts weight off the car be it at 1/2 mph, 20 mph or 200 mph leads to compromised vehicle control.

[AeroMcAeroFace]

Originally Posted by kach22i

In a word, "Lateral Stability" or Longitudinal Control" become factors long before braking and cornering when the forces of momentum caused by mass rear their ugly heads.

If there is a magic ratio of friction to mass/weight which is in-between that of a 400 lb hovercraft + 175 lb operator, and an automobile I personally have no faith in finding it if safety and control are at all considerations.

Conclusion: Anything that lifts weight off the car be it at 1/2 mph, 20 mph or 200 mph leads to compromised vehicle control.

Anything you do to the car compromises control in some way, whether it is steering, braking, adding downforce, removing downforce, adding lift or taking passengers. It is about the safety envelope, as long as you are within that you are okay.

The magic ratio as you say would depend on so many things, cornering radius, car speed, weight distribution, moment of inertia, headwind speed, crosswind speed, centre of pressure, coefficients of front lift and rear lift, coefficients of sideways lift on the front and also rear of the car under yaw conditions, turbulence and many other factors.

That ratio also changes constantly, it wouldn't be wise to have a lifting car in a strong crosswind, or when cornering heavily, or in turbulent conditions. But when the conditions are right, it may be possible to do so, to get closer to the edge of the envelope.

Whilst maths supports the concept, and it is theoretically possible to design a car that can: have variable amounts of lift based on conditions, is stable under mild cornering when lifted, has a centre of pressure that doesn't cause the car to be unstable, has front and rear coefficients of lift that are balanced, has the ability to pitch down and create negative lift under crosswind or heavy cornering/braking, it would be not only difficult to do, but the design priorities would very rarely line up such that it would ever be worth it.

Partial lift has been employed by solar cars, but these cars had high front lift and low rear lift in a straight line and low front lift and high rear lift in a cross -wind. The aerodynamic and form priorities of a solar car would not easily align with the aerodynamic and form priorities of a partially lifting body car.

Designing a car that could be clever enough to recognise the times/parameters when high lift would be beneficial and where high lift would be unsafe and go into reduced/no/negative lift would be difficult, and this isn't considering all the fine aerodynamic balances that would need to be maintained at various speeds.

Do I think this concept is useless or flawed? No, of course not, I started this debate to find out why it hadn't been done, because I thought it was possible. Do I think that this concept is practically applicable? Given the right conditions, priorities and restrictions yes I do. Do I think that those will ever arise? No, I don't.

I have explored as far as I wanted to with this topic and so I will not be likely to comment further unless new information is presented. I wanted a debate, I got a debate.

[aerohead]

In Elseviers', JSAE article on the 1996 HONDA Dream-3, tandem solar race car, the team leader reported that the car had no stability issues in crosswind.
Their design parameters were, velocity = 100 km/h, and a crosswind velocity of 21.6-km/h.
Front lift= + 2.1-kg ( 0.7% of total weight )
Rear lift = 21.4-kg ( 7.13% of total weight )
No rear fins ( GM Sunraycer ) were required
Dream's race weight was reported at 300-kg ( 660-pounds )

[freebeard]

Originally Posted by kach22i

If you have ever hit black ice in your car and spun out while changing lanes or sliding off the crown of the road you just might be able to relate to the hovercraft experiment I did.

Been there and done that. Here's my theorization:



One 4ft donut (braced for impact) under the nose and stern wheels.

[aerohead]

Originally Posted by JulianEdgar

So, one of the lowest drag cars ever created in the world to run on a public roads had driver handling issues with just 14 per cent rear lift in crosswinds, but you're suggesting 50 per cent lift would be fine?

Anyone can construct in a 'thought experiment' a series of scenarios where anything is possible, but I'd much prefer to look at the real world - what has been achieved, and what the problems were.

As far as I am aware, there has never been a road car of any type, ever created anywhere, where aerodynamic lift was a positive.

Overall lift, or lift specific to the front or rear axle?

[aerohead]

Originally Posted by AeroMcAeroFace

Well it was more to start a debate about whether lift is in itself bad. I don't believe it is.

If 10kg of lift is going to upset a car, then driving with very little fuel will have the same effect. The issue there is not lift, it is something else.

But I really want to discuss about the practical possibilities when lift could be used to reduce rolling resistance. Not all the applications where it can't, because I know there are many applications where it can't.

I am assuming road tyres, for a simple sort of "practical" solar car. Road tyres have a rolling resistance of 0.0008-0.0015

For example, a solar car that uses a symmetrical aerofoil (E475) with no angle of attack as the main body/panel carrier.

These aerofoils would have at zero alpha, a drag coefficient of 0.01, however changing this to a lifting aerofoil would produce increase the drag coefficient to 0.016, however the lift coefficient would be 1. This gives 166 times the increase in lift, for the increase in drag.

Aerofoils can and do go into negative lift, the NACA 2415 that I am referencing has neutral lift at 2 degrees, and negative lift at anything below that. A car with trailing arm suspension, as is typical on a solar car could lift the back wheels up and thus decrease the lift generated.

We can calculate the lift/drag, of tyres. For every Newton pushing down on the car tyre, there is between 0.008 and 0.015 Newtons pushing backwards. This gives a lift/drag of between 125 and 66. In the example provided above, we get a lift/additional drag of 166. This strongly suggests that lifting the car would reduce the rolling resistance by more than the extra drag created.

However stability would be an issue and a rudder to move the centre of pressure back would probably be necessary, however proportionally this rudder would have a generous 1/20 frontal area of the wing and because it is a symmetrical aerofoil would have half the Cd, this gives a total additional drag to lift ratio of 162/1.

However, all this overlooks a few assumptions, one is that the rolling resistance is 0.008-0.0015, but any team or company that would put this amount of work into making a car as odd as this would have access to the unobtainable ultra low rolling resistance tyres. It also ignores the ground effect, which may invalidate much of the lift/drag figures.

In response to the technical literature about lift, yes I have read many books and papers, and they all pretty much say that lift on a car is generally a bad thing, and of course I agree. However, as aerohead wrote earlier, imbalance of lift is the issue.



But the fact is, that lift itself is not necessarily an issue providing it is balanced between front and rear, such that the load on the tyres does not change balance front/rear, planes do not crash on takeoff, ekranoplans and hovercraft do not crash when lift gets too high. However, on cars, as Julian explains, which are not designed to take advantage of this and have uneven F/R lift coefficients relative to the weight distribution then that is where the problem begins.

Lift on the cars Julian has driven, maybe that is an issue, but I still say that lift itself is not an issue for stability unless the vehicle is not designed to lift. Adding lift and equal distribution of lift is even desirable according to some



No, you are right, they haven't overlooked what I suggest, in fact they implement it, not all the time but they do.

Ah, yes the honda dream, I have read that paper, the concern was the lift in a crosswind and subsequent instability, and However, this all comes back to the issue and probably real reason why this isn't implemented, is that solar cars have access to ridiculously low lift/drag (Crr) tyres, so there is no way that lifting the body would reduce total drag. At least that is what I thought until I reread the papers

The honda dream was pitched up 0.4 degrees to reduce drag during the race, and the sunswift iv was pitched to generate 10kg lift to decrease drag.

However, I will accept that at high speeds the stability of both of these were an issue, the honda dream had to be pitched down 0.4 degrees to remain completely stable at high speed at the expense of rolling resistance, same with the sunswift iv.

Although it isn't entirely clear whether the downwards pitch increased drag through change in angle of attack or reduced lift. So I may be wrong here.

Attachment 30724

Attachment 30725

sources:
https://core.ac.uk/download/pdf/32425613.pdf

https://doi.org/10.1016/S0389-4304(98)00019-8

*At Bonneville, for instance, cars have no suspension.
*The SCTA does not want the vehicle's body to alter inclination ( rake ) during a run.
* Whatever their static inclination is, it's the same at top speed.
* Any coefficient of lift or lack thereof, would not become a dynamic variable.
* None of this can protect a driver from 'flight' if they ignore wind tunnel data demonstrating absolute lift for their specific vehicle.
* Some teams have a higher threshold for risk than others.
* You could win, you could lose.

[JulianEdgar]

Originally Posted by AeroMcAeroFace

I have explored as far as I wanted to with this topic and so I will not be likely to comment further unless new information is presented.

One reason that lift causes increases drag is that more of the tyres are exposed.

Cars with high lift have both increasing Cd with speed and increasing CdA.

I don't think that has been mentioned in this thread.