Interesting episode of Mythbusters last night

[kfowler]

I saw a great episode of Mythbusters last night. The myth was that a dirty car actually got better fuel economy than a clean car. The myth was based on the dimples of a golf ball vs. a smooth surface.

They started by having a PGA pro hit a golf ball with no dimples. I think the average was around 183 yards. He thin hit a normal golf ball and averaged around 280 yards. The interesting part was I always assumed that the dimples helped improve lift therefore causing the ball to go further. They were saying that the dimples reduced drag.

Their next test was to take a car and cover it in smooth clay molding. They drove around a 1 mile track at 65 mph and recorded the fuel economy. They then added dimples to the clay molding to resemble a golf ball and conducted the same test. It looked really cool.

I was shocked at the results. I was expecting the smooth car to actually get better or same gas mileage as the dimpled car. Since I believed that the dimples provided lift, I didn't see how the car would benefit. The dimpled car actually got 11% better fuel economy than the smooth car. The drag on the dimpled car was significantly less than the smooth. Wow!!!!

BTW, they also tested a really dirty car and it got the worst fuel economy. So you really need to add dimples to improve your gas mileage.

[becksfan2185]

I saw that too... great episode! The car with the dimples was pretty wild looking... who knows, maybe the car comanies will take notice and use some dimple technology to improve mileage?! With all the talk of hybrids and new ways to improve gas mileage, I have never thought about changing the body of the car.

[msd3075]

When you get into fluid mechanics, normal intuition and "logic" don't usually hold true (ex. dimples will reduce drag).

I just remember being in Fluid Mechanics back when I was in school and reading the book and all the equations and how almost every single one of them was based upon empirical data and not on any type of derivation at all. You'd read the explanation of each equation, and it'd basically say, "We really don't know what's going on and can't really explain it all that well, but this funky looking equation with seven different fudge factors in it kinda fits what should be going on most of the time. If that's not close to what you are seeing, try this other crazy equation with thirteen different random fudge factors." That was when I first realized I needed to stop reading the textbooks.

[Jay-Bird]

People with dimples are faster runners.

Here's a mythbusters episode for ya:

http://www.snotr.com/video/256

[bruce]

That's it,I'm getting the hammer and going to customize my truck,this gas is too expensive That's amazing how that works.

[Where's the Cart Girl?]

That was a pretty good episode. I like how the other "required" the guys to get shitfaced on different kinds of alcohol to test hangovers. Tough work there....

What shocked me more than the golf ball car though is how ridiculously pregnant Kari is. Good God.

[kfowler]

That was a pretty good episode. I like how the other "required" the guys to get shitfaced on different kinds of alcohol to test hangovers. Tough work there....

Yeah I had no idea she was pregnant. That caught me off guard. The "Beer before liquor, never been sicker" myth was hilarious. I loved when they started jumping all over things.

[zeg]

What sort of car was it?

I wonder because you'll notice that things like aircraft are not dimpled. I presume that, if you've got a fixed not-particularly-aerodynamic form, dimpling it can help. If you're free to design your surfaces to control air flow, it could be the case that it's less of a boost -- if your wings already follow the airflow lamina, there will be a lot less drag to cut. I seem to recall hearing about smaller-scale surface textures being used in some cases, but I dunno...

[Fourputt]

What sort of car was it?

It looked like a late 90's Ford Taurus. And the gain was a bit more than 11%. That isn't just a curiosity... that's a significant improvement. Likely the only reason nobody has actually done it is the fear that no one would buy a car that looked that strange. But if gas gets to $5 a gallon, they might find a market...

[kfowler]

Likely the only reason nobody has actually done it is the fear that no one would buy a car that looked that strange.

I'm sure that's a major piece of it. I'm also not sure how cost effective it would be to stamp body skins with lot's of dimples.

The plane idea is really interesting.

[beisenhauer]

Actually, a friend of mine in college, who was an aeronautical engineering major, bought a hail-damaged car. 1) It was cheaper, and 2) he knew it would get better gas mileage.

[bunkerputt]

Dude! All you had to do was watch the Simpson's episode where Homer puts speed holes in his car. That dude knows what he is doing...

[iacas]

They started by having a PGA pro hit a golf ball with no dimples. I think the average was around 183 yards. He thin hit a normal golf ball and averaged around 280 yards.

183 yards? Wow.

We actually reviewed some dimple-less golf balls. The review is here . Those things went NOWHERE. Maybe 183 included roll? I hit those things as far in the air with a 9I as I did with a driver. I'd buy a dimpled car. 11% is a LOT of mileage savings... wow.

[kfowler]

183 yards? Wow.

It looked like they did include roll.

[Fourputt]

Interesting note: The principle that a rough surfaced ball flies farther than a smooth ball was discovered accidentally back in the days of the gutta percha ball. Players used to have to remold the ball back into shape after use, because the rubber was so malleable that the ball would be out of round and quite beat up after play. A player with some sort of scientific background, through laziness or time constraints, failed to repair his ball when it should have been done, and he observed that the out of shape ball was consistently flying farther than usual, and farther than the balls of his opponents. He started doing some testing and found that the ball's aerodynamic properties were significantly improved by roughing up that smooth surface.

That was the start of the long and still ongoing experiment to find the most effective dimple form and pattern for a golf ball.

[msd3075]

What sort of car was it?

Actually, a friend of mine in college, who was an aeronautical engineering major, bought a hail-damaged car. 1) It was cheaper, and 2) he knew it would get better gas mileage.

When you get into drag/lift questions concerning airflow, you can't compare two different situations and just assume that the same type of results will be seen. Different situations have different coefficants of drag, Reynolds numbers, type of flow (laminar or turbulant), and so on. You can't simply just punch a ton of dents in a surface and expect the coefficient of drag to decrease. The "dents" have to be the correct size, shape, and arrangement for you so see any type of positive gain.

Also, at least in terms of the plane, you have to look at how any change in design affects BOTH drag AND lift. It won't be much of a plane without lift.

[zeg]

When you get into drag/lift questions concerning airflow, you can't compare two different situations and just assume that the same type of results will be seen. Different situations have different coefficants of drag, Reynolds numbers, type of flow (laminar or turbulant), and so on. You can't simply just punch a ton of dents in a surface and expect the coefficient of drag to decrease. The "dents" have to be the correct size, shape, and arrangement for you so see any type of positive gain.

Definitely. My thinking about the aircraft was that it seems unlikely that dimpling one up would make a 10% difference in mileage or they'd all be dimpled-- the aesthetic concerns that would make this difficult for a car would generally not apply to planes. So there's probably some reason-- either they've already gained that mileage through more careful design, or it interferes with lift as you suggest.

There was an obituary for Richard Whitcomb in the paper last week. He invented/designed the little winglets that angle upward at the ends of large aircraft wings. According to the obit, these increase fuel efficiency by as much as 7% by reducing turbulence. Pretty amazing...

[lobsterzilla]

i believe it went from 26.0 mpg to 29.8 mpg . .. that's a pretty huge increase honestly... I as well was extremely impressed