Dimpled cars and aircraft (Mythbusters)

Last night’s Mythbusters (Oct 21) tested the hypothesis that a dirty car gets better mileage than a clean one. It doesn’t.

However, the crew then built a car with a (golfball-like) dimpled surface. It was a lovely thing to see. They covered a car with about 1" of clay, and tested it with a smooth surface and with dimples.

The smooth clay car (with a running start) got mpg equal to a normal painted vehicle. The textured car did 11% better.

Approximately 26mpg vs 29mpg.

Eleven percent is a major improvement, and I’m sure the texture could be fine-tuned for better results.

I think I’ve read about WW2 fighter aircraft with textured surfaces, but a quick google didn’t turn up anything.

My General Question is "Why aren’t we seeing hybrid cars, race cars, and aircraft with textured sheet metal?

My educated WAG: Increased manufacturing costs to include dimples >> improvements in fuel economy from said dimples.

I thought the same thing. I can imagine that it might be too expensive for standard production cars, but since when is expense a factor in race cars? If it can make the car go faster or need fewer pit stops, it seems like they’d be all over it.

I really didn’t expect the results, because I had figured that if something that simple would work, they’d already have dimpled race cars.

I can see several places in the manufacturing process that dimpling would add significant costs. Pressing the sheet metal wouldn’t be one of them, thats just an alteration to the dies.

But, while it reduces air drag, it may also reduce structural strength - meaning they’d have to thicken the metal to get the same strength. The added weight may offset the reduced drag.

For race cars? I don’t know why they don’t. The only purpose of the skin is to reduce drag (and add advertising space).

For aircraft, it may be a strength/weight thing or a cost of manufacturing thing. For all I know the B-2 and F22 may have micro-dimples in their skin. Even when I was in the AF, they never really let anyone close enough to discern such details.

I DID find it an interesting result (and probably a more worthwile use of time than watching Grant throw up). It would also be interesting to know of any more extensive studies undertaken by the automotive manufacturers. Manufacturing a dimpled sheet metal car body WOULD present problems, but how about a lightweight plastic cladding with dimples in it? What are the manufacturing problems in applying the idea to fiberglass bodies?

Wait, isn’t a car covered in 1" of clay heavier than the same car with 1" less clay?

How did they handle that fact in their MPG ratings?

Part of the reason may be that the Mythbusters got much better results than other people who have tried this:

http://www.popularmechanics.com/automotive/new_cars/4316702.html

If what the Mythbusters did can be replicated, you might see it in the future.

They got the car up to test speed before recording fuel consumption.

They were testing freeway speeds, testing during acceleration would involve too many variables.

They tested with the 1 inch of clay as a smooth surface, then cut dimples in the existing clay surface on the car. To keep the weight the same, they took the clay they cut out and put it inside the car. They also observed that the car with the smooth clay surface got about the same results as the car without clay - they only measured the mileage at a constant 65 mph, and noted that the extra weight probably DID mean that it took more gas to accelerate to 65 in the first place.

The problem with a race car with slippery skin is that you still stick a huge drag inducing wing on it to increase downforce, so there is little point in trying to make the body more efficient.

And the OP is probably thinking of aircraft with corrogated metal skins, best seen in the Junkers Ju52/3m. These provided extra strength at the expense of drag. I don’t know of any ww2, or ww1, aircraft with textured skins to improve drag. The designers of the time focused on getting everything as smooth as possible.

Agreed on making dimpled sheet metal would cost no more than the usual un-dimpled metal. Just a change the the press and voila…good to go.

I disagree on the structural strength though. I do not believe there is significant structural strength provided by the sheet metal at all. The strength is all in the frame. The sheet metal is window dressing for the most part. Look at a dune buggy as an example. All frame. No need for sheet metal to provide strength.

Cycling has been on the dimpled stuff bandwagon for quite a few years.

Zipp puts dimples on their wheels to reduce drag : http://www.zipp.com/wheels/detail.php?ID=149
Here is a helmet with dimples:
http://www.bikesportmichigan.com/shop/product.php?productid=16682

Race car designers will go to spectacular lengths for the tiniest improvement in performance. The wings do their job, but if some other part of the car would benefit from lower drag, they’ll consider it.

But a Formula 1 car looks pretty weird already. There are so many vanes and various doodads that it’s hard to believe the airflow isn’t going exactly where the designer wants it. I’d love to know if they’ve considered it, though.

A possible reason why dimpling isn’t used on aircraft or race cars: they are already designed for great wind tunnel results.

While dimples help on a round golf ball and cars with average aerodynamic efficiency, maybe they wouldn’t have much effect on a Prius or a P-51 Mustang.

The OP might be thinking of the laminar flow airfoil (wing) design, first used intentionally in the P-51 Mustang fighter (apparently the B-24 Liberator bomber had some laminar flow characteristics as an accident of design), and also used in the P-63 Black Widow. This was a function of wing cross-sectional shape, not dimpled texture, but there are offhand references to it in history books that can be confusing.

If were assume the same weight of a car’s steel sheet metal for dimples vs non-dimpled dimpled skin would probably severely compromise the rigidity of the steel panel and probably affect the crash test safety of the car.

For a car -think side impact.

For airframe - the skin is very much part of the structural strength.

There was also this http://en.wikipedia.org/wiki/Zimmerit , but it was a defensive measure for tanks, not for increasing their speed.

A sharkskin exterior has been used by racing sailboats for many years. The decrease in water drag has been demonstrated clearly. The bonus is that they show a resistance to barnacles too.

I think I’m going to spend some time in my garage with a ball-peen hammer tonight. :wink: