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, 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 in message
...
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

Because wings aren't shaped like golf balls or cars.

The dimples in a golf ball help reduce drag by making the boundry layer
turbulant which helps it flow just a little further around the back side
and reduce the area behind the ball where the air flow is seperated
(compared to a laminar boundry layer). Also, the dimples work only across a
limited range of Reynolds numbers (less than 3*10^5).

The airflow around a wing isn't generally seperated (except near a stall)
so it just ain't a gonna help.

I would question the results they got with the car as well, but that's just
me.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

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."

Delve into the aerodynamics of blunt bodies, as automotive
aerodynamics is called.

Actually, dimples on the wings might actually help in certain
conditions.

So would tapering the thickness of the skins(!).

But both of these are impractical from a manufacturing stand point.

Might check into "vortex generators" for more insight into current
approaches as to getting the boundary layer energized.

"Dancing Fingers" wrote in message
...
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

Something along this line has been discussed here from time to time, and we
did not all agree at that time.

However, even ignoring the possibiliyt of measuring errors in the
Mythbusters tests, an automobile does not need to produce lift in order to
operate--and may work best if all lift can be eliminated.

The most classic case of dimpling, of which I am aware, is a golf ball.
There, the dimples provide a relatively clean breakaway of the airflow--so
that, even thought the cross section of the wake appears larger, the total
energy level of the wake is reduced.

There have also been some potentially interesting experiments involving
propellers, although I have never personally seen the sort of conclusive and
complete data that a really well designed experiment should be able to
provide. For example, there was an initially interesting article in
Experimenter several years ago--but the experiment was not sufficiently well
planned, and the aircraft was not sufficiently instrumented, to provide
really conclusive findings. (Actually, changes in the relationship of
engine RPM to airspeed without accurate monitoring of manifold pressure and
fuel flow can be extremely misleading--but that is a subject for nearly
endless future threads.)

So, the bottom line is that the results of dimpled wings could be
interesting--and there has been some successful experimentation of gliders.
But seemingly small changes in the installation be better or worse, might
have a softer or more abrupt stall, etc.

Peter

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.