what's happening with car aerodynamics?

[gorobei]

Back in the 1980's somebody did a study on car aerodynamics and engine cooling. The end conclusion was that the front grille didn't need to be all that big to cool off the engine, the open space under the engine affected the temperature more than the radiator grille size.

As a result we saw a lot of foreign market/Japanese designs with flat slanted hoods, thin wedge-shaped front ends and minimal or no front grille(i.e. 84 honda prelude, toyota trueno, isuzu impulse, vw scirocco, etc.) Presumably this was a result of the 70's gas shortage and attempts to improve millage thru better aerodynamics.

Today we have large blunt front ends with significant grilles on most models coming from almost all passenger car makers(ignoring trucks and sportscars).

Is this a result of retro-fashion/aesthetics, crash impact absorption, stop and go traffic engine heat, or new aerodynamic studies?

 

[CycloWizard]

The basics of aerodynamics haven't changed. My guess is that it's for marketing purposes rather than functionality. Think about it this way: if you walk into a dealership and see two cars, one that is butt ugly and one that looks pretty nice, which one will you go over and check out? The good looking car, of course. Then, it doesn't matter that the ugly car gets 10 mpg more, since you would never even give it a second thought.

 

[Markbnj]

Aesthetics. Gas hasn't been as cheap in terms of the bite it takes from our incomes since rationing ended in 1946. People buy what they think looks good.

 

[dkozloski]

You're looking at the cars from the wrong point of view. Everybody assumes long and low is the desired shape. To appreciate the aerodynamics of the current cars you must observe straight down from above. The proof of the pudding is the low drag coefficients of the current crop. Millions are spent on wind tunnel time by the designers and engineers to develope cars that are both attractive and efficient. They've succeeded pretty well.

 

[Peter]

... but still, the ego grilles are a big aerodynamic waste. Drag is (drag coefficient) x (frontal surface), and it's the latter parameter where these cars screw up, while current materials allow impressive results on the former. Besides, your observation is very much correct on the US market, but much less so everywhere else. You guys are still paying much less than half for your fuel than we do, and that shows in the attitude toward consumption.

By the way, the first mass production aero wedgies came twenty years earlier than you think. Take the Citroën DS for example, presented and shipped in 1955.

http://www.citroenet.org.uk/ph...eques/ds/dsphoto1.html

 

[dkozloski]

The same "grill" argument was made about the big radial engine fighter planes of WWII. It was just as invalid then as it is now. The air piles up in front and forms it's own shape. Theodore von Karmen preached for years that designers needed to be working on the back end of the car before anybody listened to him. I remember a lecture by a notable person in the flyng world about aircraft that began, "As strange as it may seem, the blunt edge goes forward". The wedge shape described earlier was pursued in the search for down force and esthetics not drag reduction. There was a great lesson in motor vehicle aerodynamics to be learned last weekend when Michael Waltrips car was burning furiously and creating great volumes of smoke but still moving at a high rate of speed at the track in Fontana. The visible churning of the fire at the rear of the car was textbook von Karmen.

 

[Peter]

True, front end shape doesn't matter as much as one might think - but surface area size does.

 

[dkozloski]

Originally posted by: Peter
True, front end shape doesn't matter as much as one might think - but surface area size does.

The frontal area includes the entire cabin area of the car, not just the nose.

 

[Throckmorton]

They aren't screwing up frontal area-- they are screwing up shape, which is the only thing that affects drag coefficient. I don't get why people, including car magazine writers, think frontal area is the area of the front end of the car. Airflow doesn't care what you percieve to be the grill and headlights. It cares about the shape of the car, and the area of the profile when viewed from the front/rear.

 

[dkozloski]

Originally posted by: Throckmorton
They aren't screwing up frontal area-- they are screwing up shape, which is the only thing that affects drag coefficient. I don't get why people, including car magazine writers, think frontal area is the area of the front end of the car. Airflow doesn't care what you percieve to be the grill and headlights. It cares about the shape of the car, and the area of the profile when viewed from the front/rear.

Drag coefficients(shape) are getting lower and lower every year. Where the real skill comes into play is that the shapes are still pleasing to the eye. Eyeball examination of shape as an aerodynamic tool is problematic. The truth is that aeodynamics of automobiles is progressing nicely. Drag is gradually decreasing and milage increases. I'm afraid the big conspiracy theory is a bust. There is a wealth of information on automobile aerodynamics on the web and in general outlines steady improvement.

 

[dkozloski]

Aerodynamics is a black art. The next time you're in an a jet airliner look out the windows and you are bound to see odd pieces of angle aluminum stuck in strange places to adjust airflow. If you get a chance, look at all the strange appendages and the bulbous shape of a C-17. There is more to this subject than meets the eye.

 

[AeroEngy]

As Stated before the Area in question is not just the grill size. It is the entire cross sectional area of the object. One of the main sources of drag is a result of flow separation on the trailing edge which causes a low pressure region behind the object. To minimize this you either need smooth curves at the trailing edge to help the flow stay laminar and "connected" to the surface shape which allows the air to rejoin together smoothly. Or if that is impossible or impractical add something to "trip" the air flow. This induces some turbulence to help the flow curve over the trailing edge to reduce the size of the low pressure region.

When messing around with aerodynamics and wind tunnel testing sometimes small changes can have dramatic and unexpected results. It is a general rule that (for pilots anyway) that if it looks good it will fly good. This is why it is almost more of an art than a science.

Just my two cents.

 

[gorobei]

I get that the front of any aeroform isn't as critical as it used to be, as anything traveling through a medium creates its own pressure wave in front that functions like the prow of a ship. But I would think a certain amount of incline would be necessary. I remember a tv special talking about the 3rd gen soviet fighter designs.(mig29 su35) The gist was that it only needed to be smooth and contoured up front for high performance and could be rougher in the rear.(maybe only applies to vehicles generating their own thrust?)

I thought aerodynamic broke the factors into two i.e.: head-on wind resistance(frontal cross section) and rear drag(laminar flow pockets that generate negative pressure behind the car)
The lan-evo 9 has vortex generating fins to create turbulence to maintain laminar flow over the rear(reducing the drag pocket over the rear windscreen, don't know about the tailgate.) The shark skin suits that swimmers use and the new micro chevrons treatment for aircraft hulls can push the flow away from the surface, reducing the contact between high and low speed air. All these things suggest that the back end can be as ugly and blocky as you want as long as you tweak the air coming over the top.

The thing is, planes don't have to have crash absorbing deformation zones in front. So for cars, have there been studies that say that a blunt front end with its pressure wave is better than a slanted hood and windshield? Or are engineers just making the best of the NTSA crash survivability requirements(for longitudinal crumple zones)?

 

[dkozloski]

I have seen several lists of representative vehicles that show a steady downward trend in drag coefficients of automobiles over the last several decades. There are commuter cars of the present that have lower drag than the exotic race cars of just a few decades ago. Check with WiKi. Coefficients in the 0.2s are common. Top speeds of common American cars such as my '06 Cadillac STS are in the 165 MPH range. Compare this with exotics of 30 years ago. This is not just from horsepower increases. Todays car bodies are pretty slippery.

 

[AeroEngy]

Originally posted by: gorobei
I get that the front of any aeroform isn't as critical as it used to be, as anything traveling through a medium creates its own pressure wave in front that functions like the prow of a ship. But I would think a certain amount of incline would be necessary. I remember a tv special talking about the 3rd gen soviet fighter designs.(mig29 su35) The gist was that it only needed to be smooth and contoured up front for high performance and could be rougher in the rear.(maybe only applies to vehicles generating their own thrust?)

High Speed fighter (especially supersonic) can't be compared to low speed automobiles. However, you are right you don't want a totally square front end. You want the streamlines from the center of your frontal area to closely approach the surface of the vehicle. If you have a very blocky front end you will get very turbulent regions on the edges the streamline will separate from the surface. This effectively increases the frontal area, by an effect called displacement thickness. However, you can still achieve this with relatively blunt curves on the front surface. Just look at the leading edge of airfoil or the nose of a subsonic plane and you will see what I mean.
You are also correct about the vehicles generating thrust via exhaust gas out the rear surface. This dramatically effects how the air flows over the vehicle and can eliminate some of the rear surface drag problems like in cars.

I thought aerodynamic broke the factors into two i.e.: head-on wind resistance(frontal cross section) and rear drag(laminar flow pockets that generate negative pressure behind the car)
The lan-evo 9 has vortex generating fins to create turbulence to maintain laminar flow over the rear(reducing the drag pocket over the rear windscreen, don't know about the tailgate.) The shark skin suits that swimmers use and the new micro chevrons treatment for aircraft hulls can push the flow away from the surface, reducing the contact between high and low speed air. All these things suggest that the back end can be as ugly and blocky as you want as long as you tweak the air coming over the top.

I think you are talking about skin friction which is effected by the viscous shear of slow moving (actually zero) air at the surface and high speed air slightly above the surface (boundary layer). By changing the surface type rough verse smooth you can change the characteristics of the flow and tailer how it travels over curved surfaces. Hence why golf balls are dimpled. The sphere is a terrible aerodynamic shape but by adding dimples the flow becomes turbulent increasing skin friction but dramatically reducing the flow separation and reducing the size of th rear low pressure region.

The thing is, planes don't have to have crash absorbing deformation zones in front. So for cars, have there been studies that say that a blunt front end with its pressure wave is better than a slanted hood and windshield? Or are engineers just making the best of the NTSA crash survivability requirements(for longitudinal crumple zones)?

I am sure there is some give and take with requirements from the government. However, I think the Automobile manufactures in general do pretty well(with passenger cars anyway) in reducing drag. With exception for those boxy SUV like the Scion and Honda element. They make me cringe just thinking about how much drag is generated by those huge boxy sharp rear surfaces.

Sorry for the wall of text. Not may Aero Question are usually on this forum

 

[lyssword]

I heard flatter front-ends are safer for pedestrians' legs when they are hit.

 

[Peter]

Legs aren't the problem with pedestrians. It's the head hitting the hood, and the hard stuff right under the hood. Car makers are starting to add "active hood" tech, lifting the hood up before the impact so the head only makes contact with the comparatively soft sheet metal of the hood, not the rock hard tech underneath it.

 

[imported_Truenofan]

http://en.wikipedia.org/wiki/A...bile_drag_coefficients
may be a pointless post, i just figured this would help out the argument by showing which has the best and worst.

edit: i like on the bottom, "a smooth brick. 2.1"

 

[gorobei]

interesting how the honda insight keeps showing up. I heard they were bringing them back to production.

Also, one of the reasons for the 2007 mini cooper redesign was to meet the pedestrian impact safety requirement in UK/EU. (the other was cost, those glass C-pillars were costing them a fortune.) The hood needed to be taller and longer.

 

[zig3695]

cars and trucks dont create enough drag to matter much at the speeds we go, on an individual level. where we will see the change is semi trucks, where those will become long and lean, looking to squeeze every penny for mile they can. todays large flat mack trucks create terrible drag, and really do make a difference on the entire performance of the vehicle.

 

[KillerCharlie]

Originally posted by: zig3695
cars and trucks dont create enough drag to matter much at the speeds we go, on an individual level. where we will see the change is semi trucks, where those will become long and lean, looking to squeeze every penny for mile they can. todays large flat mack trucks create terrible drag, and really do make a difference on the entire performance of the vehicle.

That is incorrect. The force required to propel the vehicle is essentially just the drag plus the rolling resistance from the tires. These two forces are typically in the same ballpark.

The rolling resistance roughly estimated as vehicle weight times rolling coefficient. The rolling coefficient on a typical car is 0.015 - the force is 1.5% of the vehicle weight. On a 3000 pound car, that's 45 pounds.

Drag, usually written as drag coefficient times reference area times dynamic pressure, is usually the same order of magnitude. For the Corvette, the CD*A is about 6.5. The dynamic pressure at 60 mph is about 9.2 psf. That means the drag is about 60 lbs.

In fact, in many cases, semi-trucks have a lot more rolling resistance than drag - meaning that drag on your car is a bigger part of performance than it is for a semi.

One interesting thing the drag does is make the vehicle's performance highly dependent on weight. If there was no drag, your car's acceleration would be almost completely independent of weight.

 

[Throckmorton]

Originally posted by: KillerCharlie

Originally posted by: zig3695
cars and trucks dont create enough drag to matter much at the speeds we go, on an individual level. where we will see the change is semi trucks, where those will become long and lean, looking to squeeze every penny for mile they can. todays large flat mack trucks create terrible drag, and really do make a difference on the entire performance of the vehicle.

That is incorrect. The force required to propel the vehicle is essentially just the drag plus the rolling resistance from the tires. These two forces are typically in the same ballpark.

The rolling resistance roughly estimated as vehicle weight times rolling coefficient. The rolling coefficient on a typical car is 0.015 - the force is 1.5% of the vehicle weight. On a 3000 pound car, that's 45 pounds.

Drag, usually written as drag coefficient times reference area times dynamic pressure, is usually the same order of magnitude. For the Corvette, the CD*A is about 6.5. The dynamic pressure at 60 mph is about 9.2 psf. That means the drag is about 60 lbs.

In fact, in many cases, semi-trucks have a lot more rolling resistance than drag - meaning that drag on your car is a bigger part of performance than it is for a semi.

One interesting thing the drag does is make the vehicle's performance highly dependent on weight. If there was no drag, your car's acceleration would be almost completely independent of weight.

No, if there was no drag, acceleration would still depend on the ability of the drivetrain to accelerate the mass, which is the biggest factor in acceleration as it is. If there was no drag, it would take no energy to maintain the momentum of a car, so you could go infinite distances without using energy like you would in space.

 

[QuantumPion]

thundercougarfalconbird!

 

[jagec]

Originally posted by: Throckmorton

Originally posted by: KillerCharlie
That is incorrect. The force required to propel the vehicle is essentially just the drag plus the rolling resistance from the tires. These two forces are typically in the same ballpark.

The rolling resistance roughly estimated as vehicle weight times rolling coefficient. The rolling coefficient on a typical car is 0.015 - the force is 1.5% of the vehicle weight. On a 3000 pound car, that's 45 pounds.

Drag, usually written as drag coefficient times reference area times dynamic pressure, is usually the same order of magnitude. For the Corvette, the CD*A is about 6.5. The dynamic pressure at 60 mph is about 9.2 psf. That means the drag is about 60 lbs.

In fact, in many cases, semi-trucks have a lot more rolling resistance than drag - meaning that drag on your car is a bigger part of performance than it is for a semi.

One interesting thing the drag does is make the vehicle's performance highly dependent on weight. If there was no drag, your car's acceleration would be almost completely independent of weight.

No, if there was no drag, acceleration would still depend on the ability of the drivetrain to accelerate the mass, which is the biggest factor in acceleration as it is. If there was no drag, it would take no energy to maintain the momentum of a car, so you could go infinite distances without using energy like you would in space.

Acceleration is and always will be F=ma...fully dependent on weight. Drag comes into play by reducing the available surplus force the engine is generating...it takes a certain amount of power just to keep the car going at a given speed, and that increases with speed, reducing your acceleration. If you had frictionless bearings and were driving in a vacuum, your acceleration would be constant...or at least, if you had an infinitely variable transmission...and no weight loss from fuel consumption...and a supply of air to burn it...well, let's just say it won't be happening anytime soon.

 

[Greymatter6]

Originally posted by: gorobei
In the 1980's somebody did a study on car aerodynamics & engine cooling. The end conclusion was that the front grille didn't need to be all that big to cool off the engine, the open space under the engine affected the temperature more than the radiator grille size.

As a result we saw a lot of foreign market/Japanese designs with flat slanted hoods, thin wedge-shaped front ends and minimal or no front grille(i.e. 84 honda prelude, toyota trueno, isuzu impulse, vw scirocco, etc.) Presumably this was a result of the 70's gas shortage and attempts to improve millage thru better aerodynamics.

Today we have large blunt front ends with significant grilles on most models coming from almost all passenger car makers(ignoring trucks and sportscars).

Is this a result of retro-fashion/aesthetics, crash impact absorption, stop and go traffic engine heat, or new aerodynamic studies?

I also have thought about this and agree with your observations (retro for example). As for the crash aspect, I have simply "heard" about human impacts and why cars are becoming more blunt nosed.
Perhaps boxy front ends are possible with the drag coefficients attained? In other words "frontal resistance" is off-set with the raised rear designs to assist in *vehicle dynamics.*
I have been very disappointed with the raised rear of cars where it seems you are more prone to backing over a kid (less rear visibility).

*More interest may be placed in handling through in more forward pressure and reduced drag however.*
I had a 71 Volkswagen bug and was interested in finding a way to improve the aerodynamics and small changes had bad (and scary) effects on handling.
Therefore you may need to plow through more air (bigger/boxier grille), given how slippery your rear is--for a particular level of handling. (This is conjecture, in that I did not persist in researching the Bug aerodynamics too much. In extrapolation to a blunt bodied massive (heavy) vehicle to me is pointless if you wish to go fast (over 130 mph for example). Therefore, my opinion is that car aerodynamics is currently in regress i.e. boxes like Dodge
Click to a Link on Vehicle aerodynamic regress

Stop & go traffic (heat) and aerodynamics do not seem to play a part...

Originally posted by: dkozloski
The same "grill" argument was made about the big radial engine fighter planes of WWII. It was just as invalid then as it is now. The air piles up in front and forms it's own shape. Theodore von Karmen preached for years that designers needed to be working on the back end of the car before anybody listened to him. I remember a lecture by a notable person in the flyng world about aircraft that began, "As strange as it may seem, the blunt edge goes forward". The wedge shape described earlier was pursued in the search for down force and esthetics not drag reduction. There was a great lesson in motor vehicle aerodynamics to be learned last weekend when Michael Waltrips car was burning furiously and creating great volumes of smoke but still moving at a high rate of speed at the track in Fontana. The visible churning of the fire at the rear of the car was textbook von Karmen.

I think considerable mis-perception exists in this argument in a huge scale of speed that is far removed from highways speeds. Specifically, the blunt shapes of the subsonic speed realm and the rakish/angular lines of super/hypersonic flight.

Originally posted by: AeroEngy
High Speed fighter (especially supersonic) can't be compared to low speed automobiles. However, you are right you don't want a totally square front end. You want the streamlines from the center of your frontal area to closely approach the surface of the vehicle. If you have a very blocky front end you will get very turbulent regions on the edges the streamline will separate from the surface. This effectively increases the frontal area, by an effect called displacement thickness. However, you can still achieve this with relatively blunt curves on the front surface. Just look at the leading edge of airfoil or the nose of a subsonic plane and you will see what I mean.
I think you are talking about skin friction which is effected by the viscous shear of slow moving (actually zero) air at the surface and high speed air slightly above the surface (boundary layer). By changing the surface type rough verse smooth you can change the characteristics of the flow and tailer how it travels over curved surfaces. Hence why golf balls are dimpled. The sphere is a terrible aerodynamic shape but by adding dimples the flow becomes turbulent increasing skin friction but dramatically reducing the flow separation and reducing the size of th rear low pressure region.

The thing is, planes don't have to have crash absorbing deformation zones in front. So for cars, have there been studies that say that a blunt front end with its pressure wave is better than a slanted hood and windshield? Or are engineers just making the best of the NTSA crash survivability requirements(for longitudinal crumple zones)?

I am sure there is some give and take with requirements from the government. However, I think the Automobile manufactures in general do pretty well(with passenger cars anyway) in reducing drag. With exception for those boxy SUV like the Scion and Honda element. They make me cringe just thinking about how much drag is generated by those huge boxy sharp rear surfaces.

I agree with AeroEngy's assessment in curving corners. This is evident in overall economical cars in an "egg-shape." This "egg-shape" is actually an ideal shape (and is in aerodynamic terminology also but I cannot find it. Karmann or something...) aerodynamically. Although I do not know about it's dynamics/handling through Highway speeds, it's aesthetics, etc.