Drag Coefficient wrt speed

[Mighty Mira]

Originally Posted by thisisntjared

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...

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.

[thisisntjared]

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?

[Mighty Mira]

Originally Posted by thisisntjared

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.

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:



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.

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

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

[thisisntjared]

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.

[Mighty Mira]

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).

[Mighty Mira]

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