"Gregory Hall" wrote in message
...

snip

| I've noticed that when we paint an airplane with the really
| shiny smooth urethane paints, it seems to lose a little cruise speed.


Depending upon the size of the aircraft paint can add a considerable
weight burden. However, when the paint is really smooth it can drag along
with it an increasingly thicker layer of turbulent air building toward the
aft end of the aircraft. Dragging this thick turbulent boundary layer
causes increased drag which seems counter-intuitive to smoothness. This is
why various small surface patterns (seems to me somebody should try
fractals) often decrease drag - they decrease the extent of the turbulent
boundary layer thus the drag caused by it.

--
Gregory Hall


During WWII they decided to repaint the Spitfire with a flat matt finish
paint so it would be harder to see at night.
It worked, they were harder to see at night. They lost 20mph because of the
increased drag of the matt finish paint.
If you noticed a loss in cruise speed with a slick paint job, I would
suspect your data collection procedures.

Paint can add considerable weight to the aircraft. What in the world makes
you think a smooth surface causes the
thickness of the boundary layer to increase? Have you been mislead by the
installation of Vortex Generators to stir
up the boundary layer?

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )