Why don't wings have dimples 2

[Dancing Fingers[_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

[Jim Logajan]

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.)

[Dancing Fingers[_2_]]

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

[jan olieslagers[_2_]]

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?

[Bob Kuykendall]

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.

[Capt. Geoffrey Thorpe]

"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.

[Jim Logajan]

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."

[cavelamb[_2_]]

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.

[Peter Dohm]

"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

[bildan]

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.

[Anyolmouse]

"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

[Dancing Fingers[_2_]]

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

[vaughn[_2_]]

"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

[Wayne Paul]

"vaughn" > wrote in message ...
>
> "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
>

It should be noted the exact placement location of "sailplane turbulator tape" varies dependent on the airfoil. The first turbulator tape I place of my HP-14 ruddervators was the "dimpled" variety. (It is less expensive then the zig-zag.) I couldn't see much improvement. That wasn't the case when I upgraded to zig-zag. I only have turbulator tape on the ruddervators. Adding turbulators to the wing airfoil has shown only marginal performance improvement, so I haven't bothered.

Wayne
http://tinyurl.com/N990-6F

[Harry K]

On Oct 23, 5:22*am, "Anyolmouse" > wrote:
> "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- Hide quoted text -
>
> - Show quoted text -

Yeah, in the ?50s? the 'Kamm back' was tried on several makes. Hope
that "kamm" is the correct spelling.

Harry K

[Harry K]

On Oct 23, 5:22*am, "Anyolmouse" > wrote:
> "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- Hide quoted text -
>
> - Show quoted text -

Ooops that was not a foil, it was a 'chopped' off back. And I think
it was in the 60s.

Harry K

[cavelamb[_2_]]

Dancing Fingers wrote:
> 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

It's not that simple and straight forward.

What SIZE dimples?
Diameter?
Depth?
Located WHERE?

[Dancing Fingers[_2_]]

> It's not that simple and straight forward.
>
> What SIZE dimples?
> Diameter?
> Depth?
> Located WHERE?

I thought that was the point of research project? Why do golf balls
have dimples in the size and geometry that they do and go from there.
Didn't the the old German Stork have a very rough surface in the nose
area? May thy knew something that we overlook?

[Bob Kuykendall]

On Oct 24, 3:28*am, Dancing Fingers > wrote:

> I thought that was the point of research project? *Why do golf balls
> have dimples in the size and geometry that they do and go from there.
> Didn't the the old German Stork have a very rough surface in the nose
> area? *May thy knew something that we overlook?

Your general thesis that there is some ancient mystery to aerodynamics
that is not well understood by modern aero engineers and not well
modeled by modern CFD software is humorous but not enlightening.
TANSTAAFL and all that.

"Capt. Geoffrey Thorpe" covered it pretty well when he wrote:

> 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).

That's pretty much all there is to it. Where you can get laminar flow,
you hang onto it as long as you can because it offers the lowest drag.
Where the laminar flow starts to stagnate and threatens to form a
separation bubble, you trip it over into turbulent with a feature such
as a vortex generator, a turbulator, a dimple, or whatever, because
turbulent flow has less drag than separated flow.

Thanks, Bob K.