what's happening with car aerodynamics?

[Throckmorton]

Originally posted by: 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.

Yeah but he said that without drag, acceleration would be independent of weight, which makes no sense. Weight is the biggest factor until you get to high speed, and without drag it would be the only factor.

 

[KillerCharlie]

Originally posted by: Throckmorton

Yeah but he said that without drag, acceleration would be independent of weight, which makes no sense. Weight is the biggest factor until you get to high speed, and without drag it would be the only factor.

I'm terribly sorry - what I mean to say is that the force required depends solely on mass in the absence of drag . (I'm not sure what I was saying or trying to say earlier; it didn't make sense). When you coast (in the absence of drag), your deceleration though does not depend on weight.

 

[KillerCharlie]

Originally posted by: Greymatter6

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.

An egg is still a very poor aerodynamic shape. Whatever direction you orient the egg, the aft end is just too blunt and prone to separation. Typically the front end of the object isn't a big deal since you usually have the favorable pressure gradients preventing separation.

The aft end is always a trade off, however. If you make it blunt, you will have less surface area (less skin friction drag) at a cost of more separation (more separation drag). If you make the aft end of the body too long, you get the opposite (too much skin friction drag for only modest savings in pressure drag). Aftbody design for low drag is always a trade-off between these two things. An egg, as I said earlier, will definitely have too blunt of a rear in most flow conditions.

I am not a car designer, but I am an aerodynamicist for a *large* aircraft company, so I have a feel for aerodynamic design. Designing aerodynamic lines is a highly constrained problem - you want to minimize drag, but at the same time minimize weight, material and construction cost, and maximize the looks (that comes to play even in airplanes). This all has to be done by wrapping your surface around existing hardpoints.

 

[FeuerFrei]

It's all about new pedestrian crash standards. Auto manufacturers are being forced to abandon aerodynamics in the interests of pedestrian safety. Somehow raising the hood level increases survivability of the impact.
Look for further ugly designs thanks to these unnecessary regulations.

 

[ShawnD1]

Originally posted by: Greymatter6
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).

I too wonder why they do this. When I traded my 1992 Ford Tempo for a 2006 Honda Civic, I was shocked with just how bad the visibility is. There is literally no way to see behind you. From the driver seat, looking back is in an upward direction, so anything shorter than maybe 5 feet is completely invisible unless you're bold enough to back up solely based on looking through the rear view mirror, which is ridiculous since driving school specifically tells people not to do that (you're supposed to turn around and look directly out the back, not through the mirror).

 

[Peter]

Originally posted by: FeuerFrei
It's all about new pedestrian crash standards. Auto manufacturers are being forced to abandon aerodynamics in the interests of pedestrian safety. Somehow raising the hood level increases survivability of the impact.
Look for further ugly designs thanks to these unnecessary regulations.

That or what the new big Citroën has - an "active" hood that pops up a bit upon frontal impact.

The thing here is, pedestrians tend to take fatal head injury from the hard stuff right under the hood, not from the comparably soft hood itself.

 

[jagec]

Originally posted by: Peter

Originally posted by: FeuerFrei
It's all about new pedestrian crash standards. Auto manufacturers are being forced to abandon aerodynamics in the interests of pedestrian safety. Somehow raising the hood level increases survivability of the impact.
Look for further ugly designs thanks to these unnecessary regulations.

That or what the new big Citroën has - an "active" hood that pops up a bit upon frontal impact.

The thing here is, pedestrians tend to take fatal head injury from the hard stuff right under the hood, not from the comparably soft hood itself.

From what I heard adults have pretty good survivability with sports cars...they just sort of crumple onto the hood and lose a lot of energy that way...and poor survivability on vans/SUVs where your spine gets twisted sharply as the big front end of the car hits you in your hip and abdomen and lets your head and upper body jerk around towards the windshield. Kids were the opposite. They ended up hitting their heads on the windshield of sports cars after getting flipped up a bit, whereas SUVs hit them uniformly and they went under.

The windshield is NOT your friend in a car accident...whether you are the driver or the pedestrian being struck.

 

[highwire]

Well, I had a slope-snooted Prelude of that era until quite recently, and I really did like its looks. I am now staring out the window at the front end of a neighbor's new Dodge truck and, yes, big and blunt. That seems to be the way styling is going.

I have been doing aerodynamic stuff all my life, first model planes, then bigger ones. I was raised in the Detroit area and several friends from the model aircraft community actually worked at Ford styling. I just never took much interest in it except to note the blunders and misconceptions.

An example is Von Karmen's idea mentioned previously in this thread that the back end was more important than the front. His idea was actually implemented in car styling and gave us some "torpedo" shapes with long sloping rear areas. Besides making the rear end lighter and less stable at high speeds, it also sets up a big high drag vortex system. Well, Von Karmen also thought the jet engine was not a good idea. So much for geniuses always being right.

The major drag mechanisms for cars are separation and vortex generation. By using a generous curvature surrounding a blunt front end, separation and added drag from that source can be reduced to near nothing. That is exactly what they are doing. Good going guys.

I might add that the "blunt" thing bothered airplane designers, too. It's an intuitive thing. Since a cowled radial aircraft engine has that shape, NACA, I think, did a project about 1938 to reduce the drag of radial engines by extending the propeller shaft and making the whole thing much more pointy. It didn't work. The drag was about the same as the original - and heavier. Intuition in this arena is not always reliable.

 

[Throckmorton]

Originally posted by: highwire


I might add that the "blunt" thing bothered airplane designers, too. It's an intuitive thing. Since a cowled radial aircraft engine has that shape, NACA, I think, did a project about 1938 to reduce the drag of radial engines by extending the propeller shaft and making the whole thing much more pointy. It didn't work. The drag was about the same as the original - and heavier. Intuition in this arena is not always reliable.

Why do planes have pointy noses then? Does the blunt nose thing not work at higher speeds?

 

[KillerCharlie]

Originally posted by: Throckmorton

Originally posted by: highwire


I might add that the "blunt" thing bothered airplane designers, too. It's an intuitive thing. Since a cowled radial aircraft engine has that shape, NACA, I think, did a project about 1938 to reduce the drag of radial engines by extending the propeller shaft and making the whole thing much more pointy. It didn't work. The drag was about the same as the original - and heavier. Intuition in this arena is not always reliable.

Why do planes have pointy noses then? Does the blunt nose thing not work at higher speeds?

What airplanes are you referring to? Fast subsonic commercial planes (Boeing/Airbus) have rather blunt noses. When you're subsonic, more bluntness is typically better - otherwise you might get some separation at higher angle of attack. Supersonic aircraft have pointed noses for reduction of wave drag.

As an aerodynamicist, though, we honestly spend very little time on the nose (especially for subsonics). It's not very difficult to design a decent nose. Much more time and energy is spent on the wings and aft-body.

 

[Throckmorton]

Originally posted by: KillerCharlie

Originally posted by: Throckmorton

Originally posted by: highwire


I might add that the "blunt" thing bothered airplane designers, too. It's an intuitive thing. Since a cowled radial aircraft engine has that shape, NACA, I think, did a project about 1938 to reduce the drag of radial engines by extending the propeller shaft and making the whole thing much more pointy. It didn't work. The drag was about the same as the original - and heavier. Intuition in this arena is not always reliable.

Why do planes have pointy noses then? Does the blunt nose thing not work at higher speeds?

What airplanes are you referring to? Fast subsonic commercial planes (Boeing/Airbus) have rather blunt noses. When you're subsonic, more bluntness is typically better - otherwise you might get some separation at higher angle of attack. Supersonic aircraft have pointed noses for reduction of wave drag.

As an aerodynamicist, though, we honestly spend very little time on the nose (especially for subsonics). It's not very difficult to design a decent nose. Much more time and energy is spent on the wings and aft-body.

A 747 has a pointy nose compared to a Chrysler 300. It's not like it's flat or even dome shaped. By the way, why did they design the 747 with the bulge at the top for the second deck? It looks like it adds a lot of drag for a small increase in seats.

 

[PlasmaBomb]

The bulge at the top looks reasonably aerodynamic, I believe that area is typically for first class so those people pay a lot for their seats.

If you look again it is situated directly behind the cockpit, which has to be there. So blending it into the fuselage reduces the skin drag and weight of the plane compared to making the whole fuselage the same diameter.

 

[KillerCharlie]

Originally posted by: PlasmaBomb
The bulge at the top looks reasonably aerodynamic, I believe that area is typically for first class so those people pay a lot for their seats.

If you look again it is situated directly behind the cockpit, which has to be there. So blending it into the fuselage reduces the skin drag and weight of the plane compared to making the whole fuselage the same diameter.

I'm not quite sure what you're talking about...

The upper deck was put on the 747 to get the cockpit higher. I believe that was because they wanted the entire main deck available for cargo space for a freighter version. That's what allows for the nose cargo doors. However I'm not even sure if most cargo operators even use the nose cargo door.

 

[bullbert]

Aerodynamics or not, the manufactors are only going to continue to produce what sells well. Consumers are generally stupid. Yes, I said it. STUPID! (Speaking for my fellow American consumers. I'll let the Euro & Asian members comment later, but we are only talking US market cars, correct?) Over the last 20 years, speed limits have risen (back to what they were 30 year ago) but less aerodynamic cars are selling better. Gasoline pump prices (before taxes) are 5+ times higher than they were 9 years ago, but less MPG cars are selling better.

Interesting you should mention the 1980s. Between Carter and the Fed Congress of that time (late 1970's), the Fed's auto regulations were still making an impact into the mid-1980s. There were a few cars I test drove in the mid-1980s that get way better gas milage than the Hybrids on the market now. One was the 1984 Honda CRX 1.2L engine. (Or was it a 1.3L?) There were others, but that is the only one I almost bought. Why don't you see such cars on the market now? Because they won't sell well. People are stupid. They want more engine power than Donald Trump. They want more heavy safety and luxury bells and whistles than Paris Hilton has working brain cells. They want more flash than that guy fighting in Mongo.

If they could make POS engines back then with no fuel injestion, no electronic timing, but great gas milage, just imagine how efficient cars COULD be today with a modern control/feedback system on a lesser autoframe. BUT NO! JoeBob want big engine. Biff want big wheels, small sidewalls. Buffy want big flash. Me go buy SUV now. Ug.

 

[Throckmorton]

This thread has reminded me of a Mythbusters I watched a few weeks ago where they were trying to build gliders. The red headed one made a glider that had ridiculous looking wings--- the front was almost vertical. Somehow he expected this wing to work and create lift.