[thedoc (OP)]

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?
Asked by Kevin Fitch


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[evan_au]

My understanding is that dimples on golf balls reduce drag to let the fly farther from the same whack.

It is true that the dimples increase the range of the golf ball. But counter-intuitively, the dimples promote turbulence which actually increases drag under some circumstances, compared to a smooth ball.

The golf ball is hit with a backspin, and the Magnus effect gives the ball lift, allowing a longer range. The angular momentum of the spinning ball and the kinetic energy of the golf ball is expended to give the ball more flight time.

Would dimples on planes make them more aerodynamic?

The cost of fuel is a major cost in running a commercial aircraft; since dimples increase drag, dimples would increase running costs for high-speed cruising.

However, there are some special cases where high-speed cruise is not the major goal, and the Magnus effect has been used in experimental planes to provide increased lift at low speeds, by placing a motor-driven cylinder in front of the wings. One prototype replaced the wings by a motor-driven cylinder.

correction from quandry...

[quandry]

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.
Using dimples is really only a useful aerodynamic process in objects that have constraints on shape (e.g. balls). For objects that have flexibility in shape (e.g. aircraft) it is far more efficient to use the shape to reduce drag (e.g. streamlining).