Why don't wings have dimples 2

Last night the Mythbusters put about a 2 inch layer of clay a a sedan
and drove it at 65 mph and recorded their mileage. They put dimples
in the clay, like a golf ball, and repeated the same test and got 11%
better mileage. This again begs the question why don't wings have
dimples, especially for STOL type aircrat, where you still have
laminar flow? Heck, hexagonal dimples might reflect radar better for
all I know?
For conversation
Chris

Dancing Fingers wrote:
Last night the Mythbusters put about a 2 inch layer of clay a a sedan
and drove it at 65 mph and recorded their mileage. They put dimples
in the clay, like a golf ball, and repeated the same test and got 11%
better mileage. This again begs the question why don't wings have
dimples, especially for STOL type aircrat, where you still have
laminar flow? Heck, hexagonal dimples might reflect radar better for
all I know?

The Mythbusters tested an automobile that wasn't aerodynamically shaped at
the tail end. They showed wind tunnel flow lines that indicated the
reduction in drag was due to keeping the flow separating a tiny bit farther
downstream. A wing, unlike most autos, is already shaped so that the flow
separates as far down stream as reasonably possible.

Since dimpling causes extra friction, at some point the gain from delaying
flow separation equals and then exceeds the loss due to the extra dimpling
friction.

(There are vortex generators that do something similar to dimpling for STOL
wings. Basic idea is to help keep flow from separating as far down the wing
as possible at high angles of attack.)

The thing of it is that the car they used, as I remember, also had a
very aerodynamic shape and it still helped.

Dancing Fingers schreef:
The thing of it is that the car they used, as I remember, also had a
very aerodynamic shape and it still helped.

Hm. Define "very aerodynamic"? Perhaps "more aerodynamic than most cars"
which would still be far from the average aeroplane?

On Oct 22, 12:38*pm, jan olieslagers
wrote:
Dancing Fingers schreef:

The thing of it is that the car they used, as I remember, also had a
very aerodynamic shape and it still helped.

Hm. Define "very aerodynamic"? Perhaps "more aerodynamic than most cars"
* which would still be far from the average aeroplane?

I recall a salesman trying to sell a bunch of hail damaged airplanes
by saying they went faster because the dents produced a "golf ball
effect". I don't think he got many buyers.

Aerodynamisists have been trying to create "surface treatments" to
improve laminar flow for a century. So far, the mirror smooth
surfaces of sailplanes are best.

There is a guy, a Dr. Sinha, (http://sinhatech.com/) claiming to have
something called a "deturbulator" which is a sort of flexible tape
stuck on wing surfaces. If it works, and survives the rigors of
actual flight operations, it might be a big deal. I wouldn't hold my
breath.

"bildan" wrote in message
...
Aerodynamisists have been trying to create "surface treatments" to
improve laminar flow for a century. So far, the mirror smooth
surfaces of sailplanes are best.

There is a guy, a Dr. Sinha, (http://sinhatech.com/) claiming to have
something called a "deturbulator" which is a sort of flexible tape
stuck on wing surfaces. If it works, and survives the rigors of
actual flight operations, it might be a big deal. I wouldn't hold my
breath.

Actually turbulator tape is fairly common stuff on sailplanes In comes in a
zig-zag pattern or with dimples. Scroll halfway down this page to see the
stuff: http://www.wingsandwheels.com/page29.htm
Google "sailplane turbulator tape" for more info.

Vaughn

On Oct 22, 11:35*am, Dancing Fingers wrote:
The thing of it is that the car they used, as I remember, also had a
very aerodynamic shape and it still helped.

That may be so. But a "very aerodynamic" car is like a lightweight
brick or a comfortable electric chair. It's all relative.

Thanks, Bob K.

Dancing Fingers wrote:
The thing of it is that the car they used, as I remember, also had a
very aerodynamic shape and it still helped.

Disagree - it was not "aerodynamic" on the downstream side.

Do you recall when they showed the wind tunnel and water tank tests? Do you
recall seeing that there was an area behind the car with turbulent air?
Well if the car had been elongated so that the body tapered off such that
it filled that area of turbulence, then I suspect they would have gotten
different results.

Check out the following site:

http://www.grc.nasa.gov/WWW/K-12/rocket/shaped.html

Notice that the prism has a slightly lower drag coefficient than the flat
plate. As that web site states "Comparing the flat plate and the prism, and
the sphere and the bullet, we see that the downstream shape can be modified
to reduce drag."

"Jim Logajan" wrote in message
.. .
Dancing Fingers wrote:
The thing of it is that the car they used, as I remember, also had a
very aerodynamic shape and it still helped.

Disagree - it was not "aerodynamic" on the downstream side.

Do you recall when they showed the wind tunnel and water tank tests?
Do you
recall seeing that there was an area behind the car with turbulent
air?
Well if the car had been elongated so that the body tapered off such
that
it filled that area of turbulence, then I suspect they would have
gotten
different results.

Check out the following site:

http://www.grc.nasa.gov/WWW/K-12/rocket/shaped.html

Notice that the prism has a slightly lower drag coefficient than the
flat
plate. As that web site states "Comparing the flat plate and the
prism, and
the sphere and the bullet, we see that the downstream shape can be
modified
to reduce drag."

I wonder if they would have gotten the same results by attaching a foil
to direct some of the air down from the trunk to make the turbulent area
smaller. Back in the '70s an uncle of mine attached one to the rear of
his station wagon to keep the rear window cleaner. He swore it helped
his gas mileage as well.

--
We have met the enemy and he is us-- Pogo

Anyolmouse

I'm just wondering if any aero student, for their senior or masters
project, ever built a model with dimples and put it in a wind tunnel.
Remember the Mythbusters didn't expect this results based on the dirty
car which got worst mileage. This would suggest that a deliberately
designed airfoil, with certain embedded geometric shapes, could effect
drag under certain conditions.


Chris