Drag Coefficient wrt speed
 

I remember reading somewhere that drag coefficient depends on the speed at which it is measured.

And this makes sense - for example, see how airplanes are designed. The higher the speed, the less deviation away from a flat angle is allowed.

Although it's indeed possible to conserve fuel by slowing down, this seems to me to be a quick fix. Better to optimize the highways we have and reduce the time it takes to get from place to place and also design around that rather than plan for congestion.

https://www.speedbikebgl.de/pix/sbc04_002.jpg

To this end it would be nice to see Cd/ speed curves from the auto manufacturers. I doubt that the econobubbles are going to be that good above 50mph, whereas something very low and sporty could likely have a best fuel economy at 80 mph.

It's something to think about.

the problem is not really aerodynamics. The problem is speed. Aerodynamics isn't going to help a lot for fuel economy when the car is driving 100mph. Right now freeways are getting built for 100mph speeds because in the future that is what's going to happen. We need more gears. 4 speeds and 5 speeds are not cutting it anymore.

Toyota is already working on 7 and 8 speed automatics because as the speed increases we need to increase fuel economy with it.

The manufacturers are more concerned about looks than aerodynamics. It's looks that sell the car, not aerodynamics.

True. Although the higher gas prices go as a percentage of disposable income, the more interest people will have in the subject and the more cars it will sell.

why would more gears help though? you still need the power AND aerodynamics to get up there. the amount of gears allows a better blend of acceleration and top speed. you can still have a higher speed with less gears.bmw already had them, last year, and automatic trannys suck.

anyway the bottom line is that there is no need to try to change the subject of this thread. aerodynamics are necisary and it is very to consider the fact that a cars coefficient of drag is in fact a speed dependant variable. good food for thought, try chewing.

to mira: kudos on the intreguing topic. it makes me feel that the number one concern in aerodynamics at any speed is reducing the frontal area.

Mercedes is shipping their ML (or whatev) SUVs with 7 speeds now, WOAH, watch out 5speed autos...

That's all I'm clever enough to add.

Essentially that's what it comes down to.

Really, the greatest length that would be realistic in a car is the length of a Ford Expedition, roughly 5.2m. I don't see a workable solution to extend that during travel, although perhaps it is possible if the car was constructed properly. There would have to be an extensible shell, coupled with indicators to do it properly. It would also require a portion of the sides to be windowless to pull it off correctly. I've done a very rough plan view to give an idea. Think of it unfolding much like one of those Mercedes convertables do, at speed like the whale tail of a Porsche does.

https://i5.tinypic.com/143g6t4.jpg

Such a contraption could make the effective length of a car longer, perhaps bringing the 5.2m of the Expedition up to perhaps a maximum highway 7m.

The other constraint is that it needs to be wide enough at the base to enable adequate cornering.

So, I suppose we eventually get something that looks more and more like an elongated (and probably taller) horseshoe crab:

https://www.udel.edu/PR/UDaily/2004/hcrab404lg.jpg

Essentially this shape is where we are heading. After the wheels are covered, the underside is covered, the grille is basically just a tiny horizontal mouth, the wheels are as narrow and tall as possible with LRR tires, this should ultimately be our destination.

https://i5.tinypic.com/143hrf4.jpg

I suppose the only thing is that in most of the world, the econobubble shape will still predominate because older cars cannot easily be legislated off the road.

I think that one way the government can effectively legislatively provide incentives is to target mpg AND emissions.

Although now there are increasing market forces to bias us in this direction anyway. More if Bush (or his handlers) decide to invade Iran. He might ironically be the best president for the environment the US has ever produced, for all the wrong reasons!

Another thing we will need to see is a more reclined, sportscar seating type of arrangement. Which should not be a problem because a longer car will enable a lower drag coefficient.

More gears help with fuel economy too. If i want to drive like everybody else I would be in 3rd most of the time because there is no power in 4th till about 75mph. The average person pushes the pedal harder if they want more accelalration because some cars like mine the gears a very long. With more gears you'd waste less gas because you'd have to press the gas less. Plus you make final drive higher with more gears and the gears longer. You don't understand this concept because you don't drive a 4 speed automatic everyday. More gears is good.

i understand more gears is good but thats not what we are talking about.

remember we are not talking about acceleration. we are talking about a maintained speed. the only thing that would matter is the one gear ratio. dont tell me i wouldnt understand a concept. i understand both physics and statistics as well as the topic of this thread.

back on topic: ive heard that its important to reduce the surface area as well as the frontal area. if a shape become too drawn out, it will not help much for the cd so its really difficult to strike the right compromise... man i wish i had a wind tunnel...

you do - stick your hand out the window while driving down the highway!

But seriously get a blower fan and setup a small tunnel.

I've heard that... a long time ago though.

Nothing I've read or seen recently indicates that this is very true.

For example, see the above 80mph bike. If surface area really would have made a difference, they would have shaved off 4 feet. Or look at the bonneville racers. Or look at the sr-71. Or look at a high speed glider.

With a standard car, there is no risk for it becoming "too drawn out". They do the best they can, usually - ending abruptly after a fastback shape. If they could, they'd shape it like a dolphin to get the Cd of 0.05 or whatever it has. But they can't, so they chop it off.

Something leads me to think that what the true drag of a vehicle is is represented by the area at the back where it abruptly drops off (more than 10-20 degrees).

This is why you don't have aircraft ending with an abrupt drop off, they aren't constrained by length and they keep drag to a minimum.

Suffice to say, I see very little evidence that surface area is a significant factor when it comes to automobile design.

Either that, or perhaps you misinterpreted what they said about Surface Area. For example, a typical boxy car has a large surface area. Morph the car into the raindrop shape, and it automatically will have a smaller surface area - the cross section will become circular, and the length will still be smoother than the 3 box style car, and likely have smaller area.

But ultimately, I think the surface area thing is more of a zeroth order guideline than a hard and fast rule.

i was just relaying what i have been told in the past. if a mack truck's trailor is twice its length what happens to the cd? it doesnt remain constant.

airplanes are drawn out because they are reaching much higher speeds, but this isnt also for reducing drag, its also to help establish more stability. so again it is very largely speed dependant.

so now the question begs to be asked where do we 65mph creatures draw the lines?

Yes, that much makes sense. However, say you have a mack truck. But instead of a trailer, it starts going back into a cone shape (at 11 degrees, for example). Say that the first trailer length isn't long enough to get it to a point. However, it gets it to half the height and width it once had.

Now, there will still be the problem of the turbulent flow behind the truck, but after the first trailer, this turbulence is only in 1/4 the area it once was. If you have another trailer and continue the same conical projection, eventually there will be no turbulent area as it files down to a point.

Since the main drag effect comes from the turbulent wake, eliminating the wake will solve most of the problems. i.e. If you can eliminate wake, it will trump other concerns.

A quick google of shapes should give an idea:

https://www.insideracingtechnology.co...es/Shapes2.gif

Note that the streamlined body trumps everything by a wide margin. And also note that even the long cylinder trumps the short cylinder. Aerodynamics is initially a little counterintuitive, but once you familiarize yourself with enough shapes and their drag coefficients, you can soon start to build up a mental model to calculate a first order drag coefficient for a given shape without ever putting it through a wind tunnel.

And note that it is the shape that is important - it scales. Hence, if the frontal area of a shape is given, then you can decrease the Cd by making it look like a streamlined body.

If you look at the ultra high mpg cars such as the UFE-III, the insight, the VW 1L car etc, they all mimic the streamlined body up until a point where they end it sharply at a 90 degree angle. In applications where the length of the vehicle is not a constraint (gliders, hpv bicycle competitions, etc), they go the whole hog and file it down to a point.

Basically, the longest vehicle practical provided that once the 11 degree conical approximation starts, it keeps going.

killer info!! man was i off!! the shorter cylinder has a worse cd than the longer!!

its also very interesting the tha half sphere has a better cd than the full sphere..... weird.

Yep, there's some more useful info here, and here. Note that second diagram. While that says a bullet shape should eventually reach a point of diminishing returns for the length (at 5 times longer than it is wide), that is under the assumption there is a cylindrical projection behind the bullet nose, not a conical projection (which would make it more of a teardrop shape).

https://www.princeton.edu/%7Easmits/B...ag_coeff_2.GIF

https://www.aerospaceweb.org/question...g-cylinder.jpg

A continuation of my thoughts on this subject is found here.