Kevin Fitch asked:
Why aren't boats and planes dimpled? My understanding is that dimples on golf balls reduce drag to let the fly farther from the same whack. Wouldn't it make sense to use the same trick to reduce drag on vehicles?
Dave Ansel had a go at answering Kevin's question...
Dave - Thatís a really, really interesting question. So golf balls have dimples because if you had a perfectly smooth golf ball and you hit it really hard, what happens is you get really big swirls of air coming off the back of it, the air gets round the first half of the ball it detaches and you get these big swirls out the back of it and that causes a lot of drag. Now if you have the dimples and what it does is it creates small swirls, which actually tend to mean the air detaches from the ball later, and that means that you get less drag overall so the ball goes faster and further. Now, planes normally are in a very different regime. Theyíve carefully designed their wings so you donít get very much turbulence off the top and, in fact, when you do get this kind of turbulence itís the detachment of the air flow (itís called a stall), thatís normally something which you want to try and avoid and so mostly when youíre flying dimples would just give you a bit more drag and make it worse. However, if youíre about to stall, having dimples can be useful and, in fact, planes do have things like dimples (called vortex generators) and these are little fins attached to the front of the wing and they can delay the stall a bit so that the planes can fly slower, and land slower, and land on shorter airfields, and things like this. But most of the time you donít need them and all planes have them.
Dimples on golf balls increase surface turbulence, but not drag. Drag is caused by the width of the turbulent area behind the ball. Surface turbulence causes the airflow over the surface to adhere slightly longer to the surface thereby reducing the width of the turbulent tail and therefore reduces drag.