Car Aerodynamics

The surface underneath the car, the road, and the higher velocity of the air relative to the car in this space will cause a lowering of pressure beneath it, and so there will be no net lift force.

Vehicles are tested in wind tunnels to prove this. Cars are not known for taking off like aeroplanes.

You wrote, "Cars are not known for taking off like aeroplanes."

Think again!

Going Airborne

I'll bet that left a mark somewhere, and I don't mean in his Nomex shorts.

Well posted.

When I looked on Google there were many examples of cars flying......I think that the shape DOES play a strong role.

We are lucky that most road cars are run at both relatively heavy and at relatively lower speeds, otherwise it would happen more often....

Over the years I can remember several cars that I drove that the steering went light when driven fast......Original Ford Fiesta was one (1981 Germany) and an earlier Austin crap saloon (UK) as well as a similar crap Chrysler (UK) car (both around 1970)........you didn't have to be psychic to feel the balance change for the worse in any of them....I am sure that many other models were also afflicted.....

Modern German cars at least, even though I sometimes drive at well over 200 KMH (I deliver cars for a dealership sometimes), never give me that "light front" feel!!

Though the later model BMWs with so called "run flat" suspension and "run-flat" tyres with 50% wear or so are downright dangerous when taking autobahn curves at 200 KMH or more.....they jump and jive over every white line and slight unevenness.....the only way to stop that is to use normal (not "run-Flat") tyres instead we find.....BMW has made some serious errors in design, but in the USA, you will probably never notice unless driving illegally fast (what a nice surprise for an uneducated driver!), maybe a reason for crashing a BMW over there?

The problems that BMW have are well known, but not accepted by BMW.......yet!

As a long time BMW owner but a design engineer for much longer, I can say that for a car labeled "The Ultimate Driving Machine" there are startling design deficiencies that come out of Munich, even in top of the line models. Some of them are violations of basic good engineering practice too.

The fuel pump failures of the last few years are a case in point -- cars would lose nearly all power without warning, yet the Engineering VP of BMW USA said they didn't consider it a problem. It was only a piece by ABC News that got them to come around and eventuall a recall was issued. Still, for a VP of Engineering to say that sudden loss of power was "not a problem" is pretty shocking. If I was passing on a two lane road and suddenly lost power, I might have a head-on collision and I certainly consider that to be a problem.

Fortunately, most of the common failures are not that dramatic....but the inconvenience and maintenance cost factor are considerable. I must say that when a BMW is in top shape and running well only a supercar like a Ferrari or Aston Martin (or more recent Mercedes Benz) can match it. But it takes some real insider knowledge and preventive maintenance to keep a higher mileage BMW from eventually leaving you on the side of the road.

Yeah, BMW has a history of not admitting to design failures, no matter how well documented.

Sudden loss of power reminds me of finding the electronic governor in my Suburban, passing a truck on a two lane three of us broke left to go around, problem is we needed just a few more mph, then the governor kicked in and dropped me back to 5mph below governor limit - yikes!

Tell that to Mark Webber.

Bazzer

Mercedes and Porsche surely wish it was that simple:

http://www.youtube.com/watch?v=O9EU8JH03gE&feature=youtube_gdata_player

http://www.youtube.com/watch?v=ZXZaAuyuYmQ&feature=youtube_gdata_player

;-)

It is both and a little more. Ground effects will be another factor.

How those forces balance out depends on the shape, velocity, and distance from the road surface.

GA Both apply - this is why race cars use air dams in the front to keep air from getting under the car and spoilers or wings to provide a rear downward force.

Let's not consider the ground effect here.

My question is , purely related to the car shape alone. By Bernoulli law , there will be up-lift, by Newton law since the air flow is striking on a front sloping surface, the reactive force will act downward. Contradictory , isn't it. But we know airfoil will generate uplift, so the safe conclusion is that , the Bernoulli effect is stronger than Newton effect ?

"But we know airfoil will generate uplift"

Really? If this is so how can an airplane with symmetrical wings fly?

Interesting and valid question, can you tell us?

It's called angle of attack.

So, the lift is due to angle of attack, which cause a reactive force effect.

So, can we conclude that the airfoil uplift is due to Bernoulli effect is not so valid ?

Or, is it that when the airplane flying normally , it is due to Bernoulli effect, if it is flying upside down , it is due to Newton effect ?

NO!

All of these forces are always there. Depending on the angle of attack, the relative air velocity and the shape of the airfoil the magnitude and direction of these forces will be different. But they will always be there.

" So, the lift is due to angle of attack".

Have you ever put your hand out the window of a vehicle going 60MPH and pretended it was an airplane wing? And turned it up and down?

As the car is an unsymetrical wing shape, why the question???????

A wing that generates lift is unsymetrical......like a car is!!

I can edit a post but not delete it?

Yes, you can, but you must enter code: OICU812 when prompted. Caps are required.

We're not going to talk about the flying underpowered uncontrollable wingless rotorcraft again, are we? Or how come airplanes can fly upside down?

You pointed out rightly something puzzling. If air planes flies because of the airfoil uplift, how come airplane can fly upside down ? Hope someone can explain this puzzle.

See post #16.

With enough power, you can get a barn door to fly.

LOL!!!

If you fail to consider ground effects, then the problem becomes strictly a wing or airfoil.

At that point you must also consider angle of attack.

Sorry I could not reply sooner. The two forces (both present and dependent on vehicle shape, forward speed and prevailing air turbulence) are indeed opposing and because the Newtonian drag & down force is on the front and the Bernoullian lift is over the entire shape, this imparts a torque on the vehicle tending to push down on the front and lift the rear (reduces traction for rear drive vehicles). So the rear can slide outward when entering into a turn. In racing they therefore use spoilers or wings to impart a downward force on the rear to counter act the normal rear lift effect.

This rear lift effect alone is not why vehicles can go airborne at high speeds. When air turbulence (or contact with another vehicle) upsets the balance of the two forces and air can get under the vehicle, the forces then become additive upward at the front and at a minimum you loose some steering control. If the vehicle speed is sufficient....viola, you have lift off!

The physics formulas for this air flow are complex (hence air tunnel simulations), but the concepts and relationships are reasonable understood without getting into the calculations. Lot of GA posted here to help - hope we answered your questions sufficiently.

This might have some effect on a 500 pound vehicle.

This has an effect on a 2000 pound vehicle. The Porsche GT1 has a curb weight of 2090 pounds. Long before there will be sufficient lift to launch a vehicle, tire traction will diminish because tire traction is proportional to the normal force on the tire.

Bernoulli's equation for aerodynamic lift on airplanes and car bodies is over-rated. It is angle of attack and the force induced by changing the movement of air.

how can a "LAW" of nature be over rated?

Over-emphasized in most folks educations would be more to the point.

See post #25 by Graycav. He gives a better explanation than I can.

The FAA Pilot Written Exam had a question for many years on why wings generated lift. To get the question graded as correct, you had to any with Bernoulli's equation, even though aerodynamics has advanced to the point that the experts know it has a minor effect on producing lift on a wing. My flight instructor told me to give the technically "incorrect" answer to get credit on the computer scored test. Some myths die hard.

Bernoulli was a good researcher, and his work has applications in fluid flow, but this effect is not the major cause of "lift".

Planes can and do fly upside down. Aircraft designed to do this better than those not designed to fly upside down do usually have more symmetrical airfoils. The car leaving the road was in close turbulence from the car in front of it. The net effect of the shaped airfoil is lift, but this is summed over the whole curve. There is a strong downforce at the front of the curved shape (from the change of momentum of the gas molecules), but the sum of the forces has a net upforce. The wing on the car in front is designed to add downforce at the rear of the leading car to add traction for the drive wheels, but wing's effect is to move the air upwards in front of the second car. This reduces the downforce on the second car at the front of the car since the air is already moving upward, so the torsional couple of forces of the drive wheels at the pavement level and the air drag at the body level lifted the front end. The rest, as they say, is history.

assuming that are surfces are aero-dymatically designed "smooth surfaces with no openings or restrictions" the longer air flow path, or the top of the model, will create a low pressuse area causing lift, if we dismiss ground effects, a tear drop shape has neutral aero-dynamic

neither scientific "laws" contradicts the other.

I Grew up in the sixties/seventies and would pour over Motor magazines to get the info on the latest developments in the auto industry,In one of the articles I can remember an expert in aero dynamics saying that if the auto industry continued to develop aero dynamics to its logical conclusion then all cars would look identical, How right was he.

Bazzer

Had a '62 Mercury Meteor long time ago-dropped in a 289 & T-10 4 spd. Stock 2.88:1 rear and driving by the tach had it to about 140MPH once and the steering got VERY light and felt like it was floating! only did this once. Loved the 289's-one of the best engines Ford ever made. Could beat it all day every day -float the valves now and then and never hurt it.

A very good answer!

If you say it is the Bernoulli effect that provide the lift, how do you explain, as many here have asked, plane that can fly upside down ? Or plane with symmetrical wing can also flies. The wing can't have "Bernuoulli curve" on both side of wing, right?

If on the other hand , the air plane lift is mainly due to angle of attack, then we would expect the wing angle of attack must be constantly changed with different speed in order to maintain same altitude, is this the case with air craft?

Landing-starting-landing in that order. Airliners dont fully deploy flaps upon take-off :-)

Landing

Taking Off

Landing

Judging by the position of the flaps (which adjusts the overall shape of the wing) this can be seen though the middle photo is a little small to judge.

It seems that most posts miss the fact that the wings of most modern aircraft do not have a fixed shape. Flaps, slats, ailerons, spoilers, etc. all play a part in creating this shape.

Hi again Bravo88, You have to have a look at some pictures of the flow lines and the pressure distributions around wings from computer models or from smoke line tests. It all comes down to the "announcing" effect if you look at an airfoil that has a large angle of attack, you will see that the air ahead of the wing will start moving upwards from a surprisingly low level to turn over the upside of the wing. (if I would manage formatting on this site better I'd paste a couple of pictures from google, since I suck at that I just ask you to search with something like "flow around wing" and look at the pictures).

So, if you now look at the flow lines moving above the wing and the flow lines moving below the wing, you will see that due to the "announcing" effect the lines above the wing are considerably longer than those moving below the wing. This then creates again the effect we know from Bernoulli, where higher path lengths creates higher air velocities and lower pressures.

you can look at it as though the "announcing" effect creates a much larger apparent object moving through the air (delimited by the zones where air flow differs from the direction of movement) especially in front of the object.

Now, substitute the (hopefully asymetric) airfoil you had before by a symmetric profile and you will realize that due to the "announcing" effect creating a large apparent object in front of the wing, you still get more air moving over the top of the wing than over the bottom. In essence, when the symmetric airfoil is at a positive angle of attack, the apparent shape is still an asymmetric airfoil with a higher path lengths above the wing than below the wing.

You can spin the thought further and turn an asymmetric profile upside down (what would then be a negative angle of attack, like you would see it on a normal airplane flying inverted). You can see that if you draw your flow lines that also to a lesser extend, you can still create longer pathways over your apparent object's top. Therefore the Bernoulli effect can sustain an airplane with almost any kind of airfoil like shape in the air. If the airfoil is symetric it will do it just as well heads up or down. If it has asymmetric fouls it will do much better heads up but it is still capable of achieving lift heads down.

Like it has been pointed out, it comes down to the angle if attack AOA. If you have asymmetric airfoil, you will need a very low AOA to create lift. As a matter of fact, the neutral line is often at a slightly negative AOA for asymmetric wings. As you increase the AOA the lift you create increases nearly linearly at first, until the wing starts to stall. When the wing stalls, the flow lines can not follow the profile of the wing anymore. A lot of turbulence. It is very difficult to still increase lift then and drag increases very rapidly. At this point the models get really messy and all the nice laminar calculations go down the drain.

anyway, if you have familiarized yourself now with how the "announcing" effect creates an apparent object in the air and on how this gets thicker with increasing AOA, you should be able to see that all of the above comments about whether it is AOA or Bernoulli creating lift is quite one and the same thing. The higher the AOA the larger the apparent wing, the larger the Bernoulli effect, the bigger the lift.

This last part should answer the last part of your question. Yes, an airplane certainly does change AOA during its flight depending on speed, altitude and lift needed.

in general you can put that Lift=1/2*roh*A*Cl*v2 whee roh is the air density. Cl the lift coefficient, A the surface of the wing and v2 the velocity squared. Like I said before in a certain range you can assume that lift increases proportionally with AOA. We can therefore approximate Cl by Cl=C*AOA where C would be a constant for the wing geometry. With this you get:

Lift=1/2*roh*A*C*AOA*v2

with this you can play through the cases now: suppose level flight and yo decide to go faster. Well, you airplane will not need more lift so if speed increases you have to decrease AOA. If you fly higher up, air density decreases so you have to increase speed or AOA. since you can not increases AOA too much without stalling all that is left is flying faster.

you also see all the additional options you have if you need to increase lift: make a bigger or more efficient wing or both (factors C and A). now I am sure you have flown before and have remarked how whe wings unfold the high lift devices before landing which create both a larger ( A ) and more voluminous ( C) wing profile.

there is a final remark to this: if you have ever observed how an airplane pilot makes a turn, you will see that he heals the airplane over to the side and then pulls back a bit on the stick. All this last part does is increase AOA to give the airplane the extra force it needs to change course in addition to keeping the plane flying. Contrary to what the layman may think, the vertical rudder is of very little importance to flying a turn.

Cheers

That's great!.I'll be your fan from now!-

Back to the original question, your observation shows how wrong is that generic way to use the Bernouili analysis.On the other hand, Bernouilli comes from Newton's laws in very simplified cases.I see others mistakes here remarked several times by somebody but nobody shares the idea: a simmetrical wing lift exist! and is dependent of angle attack.-