Drag of a polished car?

I don't polish my cars at-all and I can still normally get a good year between washes.

Mind you, they do spend most of their time looking a bit on the filthy side.

Maybe it would be noticeable - if you were driving through an atmosphere consisting entirely of dusters.

Cheers,

Colin.

My cars always feel like they are running better/quieter etc. after a good polish and clean. It is entirely subjective, but it's almost as if the car appreciates the cosseting. It's all in the mind I reckon but you get the feel it's had a service (a good one) after a polishing session. ;-)

-- Stuart

There was once this outtake, I think it was. They tried to place a bikini girl on the bonnet of a car in a showroom. I think it was a new Jag. The combination of a creamed up girl and the polished bonnet didn't go well together. No matter how much they tried to position her, the girl always slid off the bonnet and down onto the floor.

I have read somewhere, or been told or something, that those pylon racers in Leftpondia get a bit of extra speed by polishing their planes. What we don't know is whether the military polish those stealth planes that fly ever so fast.

I have a book about WWII fighters somewhere which had a bit about some trials involving polishing aircraft.

IIRC a polished Spitfire is about 7mph faster in level flight...

...that's about 2%, or equivalent to 60-70bhp extra.

Polishing your Spitfire is worth the effort.

Boosting my cars top speed by 7mph is equivalent to a 43bhp power hike, even a 2% increase would need an extra 20bhp...

...time to get that wax out!

And as something of a paradox, having a rough surface can be useful to maintain smooth airflow over the object travelling at speed. Something about creating a high pressure air pocket over the object - then the object has a very smooth "air" surface when travelling at speed. Haven't got a clue as to where to start googling for supporting data, though.

Golf balls.

No, that's about creating a pressure differential so the ball stays up longer.

Biggles

The message from Alistair J Murray contains these words:

I've never heard it called that before.

The message from Mike Dodd contains these words:

Try "laminar flow".

In article , Biggles wrote: [rough surfaces being faster than smooth sometimes, and where to start research on the phenomona]

How does that work then, where there's holes on the bottom too?

The pits in a golf ball create a turbulence pattern, the outside edge of this turbulence creates an "artificial skin" of air, which, being air, is going to be more slippery (ie. create less friction) than the actual surface of the ball.

Similar sort of thing happens in high-spec convertable cars with air conditioning.

"Aircon in a convertable" sounds absolutely bloody ludicrous until you find out that the lip on the top of the windscreen part creates a turbulence pattern which acts as an "air roof", keeping the conditioned air within the cockpit of the car, instead of just being sucked out of the top.

Fascinating stuff.

John

Backspin - the top of the ball is moving with the airflow, the bottom against, hence you get higher pressure underneath the ball and lower on top, which gives lift. Same principle as an aircraft wing, but using the movement of the ball rather than shape of the wing to create lift. The dimples increase the lift.

I've never heard of that argument for the existence of dimples on a golf ball but I guess it's possible.

Biggles

Backspin - generates lower pressure on top than underneath, and hence lift.

Yes, I think you are right. There seem to be two factors which dimples have an effect on - lift and drag. However, a golf ball benefits from dimples because it is spherical, and therefore not very aerodynamic. More aerodynamic shapes don't benefit from having rough surfaces. But you can make use of the effect for other purposes such as the one you mention.

Biggles

The message from John Duffey contains these words:

Backspin. A properly [1] struck golfball has considerable backspin. The dimples allow the ball to grab some of the air and pull it from above, creating lower pressure there, to below, creating higher pressure there. This allows the ball to loft far further than a plain ball would have done.

Note, this is a trade-off between drag and lift, as is the case with all aerodynamic systems. In the case of the golf ball it turns out that it's better to have some lift in exchange for some drag. A javelin, for example would almost certainly work best with least drag.

The way I think of it, is a matt surface causes a thin layer of air (or water in the the case of boats or surfboards) to stick to the vehicle creating a non stick surface. Air or water pockets caused by large steps in the surface are a bit different.

In message , Biggles writes

Works for sharks:

Lots more, google for

shark hydrodynamic drag denticles -swimwear -swimsuit

The message from Willy Eckerslyke contains these words:

Try googling for "turbulators".

Best bloody place for her IMO. If I ever catch some rented bint draped on our bonnet (she'd have to be small) I'd be most unimpressed!

ISTR that it's been measured at around a 1% reduction in drag.