Dimples On Model Aircraft Could Greatly Extend Range

On 9 Nov, 23:22, "Highflyer" wrote:
"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.

Eh?
Virtually all RAF combat aeroplanes in WWII were painted in a matt
finish. Why on earth would you paint a Spit so it was hard to see at
night? They did not fly at night - or do you have loads of pics of
Spits in a matt black finish?

Guy
SNIP

Highflyer wrote:


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.


You're thinking of Special Night, the extremely matt "anti-searchlight"
black finish specified for night fighters from 1940, and the underside
of bombers from 1939.

Standard camouflage paints at this time had been improved from the
pre-war, biplane era standard by the use of more finely ground pigment
to give a smoother surface. The paints were still matt, but of a sheen
we could call eggshell. These are the "Type S" paints.

Special Night was applied in two stages, an undercoat of smooth Night
and a topcoat of Special Night. In January 1942, de Havilland performed
speed trials with a Mosquito before and after the application of the
Special Night topcoat, and discovered a loss of 26 mph. Special Night
was replaced by "smooth" Night on Mosquito night fighters within a
couple of months.

In August 1942 the Night Fighter scheme was redefined to be Medium Sea
Grey overall with a camouflage pattern of Dark Green on the upper
surfaces. This followed complaints from the squadrons that the night
fighters could be detected as dark shapes on most nights, and that a
lighter colour would be better.

The only single engined night fighters in RAF squadron service during
the "Night" period were Hurricanes and Defiants.

Hi m

On Nov 9, 2:26 pm, wrote:
On Thu, 6 Nov 2008 14:35:21 -0800 (PST), in sci.engr.mech "Ken S. Tucker"

wrote:

Well I think we're nit-picking sematics, my quote,

"My understanding is the theory is NOT _well_ understood,
but is evolving, along with applications, by experimental
feed-back, aka trial & error, (I'm using SM board). "

Note the word "theory"

What theory do you mean? It seems it's well understood how to design the
riblets to minimize drag. If you mean the theory of the underlying fluid
dynamics then I wouldn't hold your breath. We still don't have adequate
turbulence models to begin with.

Yes, well turbulence is a problem.

Also after the original riblet research was performed
similarities to shark scales/skin were observed.
http://ntrs.larc.nasa.gov/search.jsp...de%20matchall&...

Yes! Thanks for those links.
Those papers are experimental results and testing,
AFAIK, there is NO generally accepted theory of the
"riblets effect", though it appears to be evolving.
(If you have a ref to a General Theory of Riblets, I'd
would appreciate a link).

Not sure by what you mean by this. The requirements to design the riblets
are known. What more do you want?

We build and fly models (wingspan ~ 24", speed 20 mph)
what riblet would you recommend? We could glue a
sandpaper to the surface of one wing, balance, and
test fly to observe yaw anomally.

I'm guessing: At a molecular level the riblets control
the turbulent interfacing between fluid and surface
and inhibit the integrated formation of macroscopic
turbulence, such as Eddy's. That micro control is
certainly a quantum relation between molecules in
the fluid and the interacting solid surface, whereby
the micro turbulences are quantized.

Fluids as it effects most every situation we deal with, including this one,
deal with the macroscopic properties and not the microscopic or molecular
properties of the media. There is no quantum relationship between the fluid
and the surface in continuum flow which we are addressing here. That only
becomes important in rarefied gas dynamics.

That is where turbulence begins, a vacuum like back
suction.
Pardon the poopy ascii, of a circulation,

Good Air === (going above wing)
/\----------
| |
| |
o--------\/
Surface

"o" is pulled up and cycles around.
Regards
Ken