On Wed, 9 Jul 2008 04:26:33 -0700 (PDT), Dan G
wrote:

I'm crewing this season, and no doubt my pilot will be expecting me
out at 7am with Mer in hand.

My question is (and I'm sure it's been debated before, I did search,
but want fresh input) how worthwhile is polishing?

I can understand keeping the fuselage as slippery as possible to
reduce skin drag, but I've heard that glider wings would still work as
well as possible if their surface was 40 grit sandpaper! Also take a
look at a golfball -- it's dimpled specifically in order to reduce
drag, again implying that a smooth surface may not be perfect.


There's a major difference between blunt bodies like a golf ball and
slender ones like an airfoil. Drag on the former is mostly pressure
drag, while drag on the latter is dominated by skin friction.

If the boundary layer on a golf ball never separated, with the flow
from all sides rejoining smoothly on the back side, there would be no
pressure drag at all...but in real life, on a smooth ball, it will
separate by the time it gets halfway around. Consequently, the back
side of the ball is in a low pressure area and there is lots of
pressure drag.

The dimples create small-scale turbulence on the surface, adding
energy to the airflow, which delays the separation of the boundary
layer to about the 3/4 point. That keeps high-pressure air on part of
the backside, reducing the pressure drag. The dimples also increase
the skin friction, but since that is small on a sphere, it's a
positive tradeoff.

Skin friction drag depends heavily on surface roughness: every time an
air molecule impacts the forward face of a bump, it extracts a little
bit of energy from the aircraft.

rj