Dimples On Model Aircraft Could Greatly Extend Range

[Bret Cahill]

Have they tried dimples on radio controlled aircraft? The size and
speed could designed around the magic Reynolds number = 100,000 where
the coefficient of drag drops precipitously.

Dimpling could vastly extent the range of large and slow as well as
small and fast radio controlled aircraft.

A competitive cyclist is the right size and speed for Nre = 100,000 so
dimple suits can work. Same for golf balls.

Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
except on the runway.

From fluid mechanics the Reynolds number is the ratio of inertial
forces/viscous forces.

N re = Diameter X velocity X density of fluid/viscosity of fluid.


Bret Cahill

[Gregory Hall]

"Bret Cahill" > wrote in message
...
> Have they tried dimples on radio controlled aircraft? The size and
> speed could designed around the magic Reynolds number = 100,000 where
> the coefficient of drag drops precipitously.
>
> Dimpling could vastly extent the range of large and slow as well as
> small and fast radio controlled aircraft.
>
> A competitive cyclist is the right size and speed for Nre = 100,000 so
> dimple suits can work. Same for golf balls.
>
> Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
> except on the runway.
>
> From fluid mechanics the Reynolds number is the ratio of inertial
> forces/viscous forces.
>
> N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
>
> Bret Cahill



We competitive cyclists use dimples already on our disk wheels. And some
skinsuits incorporate them. But they don't look like the dimples on a golf
ball. They are shaped differently and they are shallow.

Check out Zipp disk wheel dimples here:
http://www.zipp.com/wheels/detail.php?ID=33

Over a 40K ITT this rear wheel can give you a 30-40 second advantage over
smooth disk wheels.

--
Gregory Hall

[[email protected]]

> > Have they tried dimples on radio controlled aircraft? � The size and
> > speed could designed around the magic Reynolds number = 100,000 where
> > the coefficient of drag drops precipitously.
>
> > Dimpling could vastly extent the range of large and slow as well as
> > small and fast radio controlled aircraft.
>
> > A competitive cyclist is the right size and speed for Nre = 100,000 so
> > dimple suits can work. �Same for golf balls.
>
> > Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
> > except on the runway.
>
> > From fluid mechanics the Reynolds number is the ratio of inertial
> > forces/viscous forces.
>
> > N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
> > Bret Cahill
>
> We competitive cyclists use dimples already on our disk wheels. And some
> skinsuits incorporate them. But they don't look like the dimples on a golf
> ball. They are shaped differently and they are shallow.
>
> Check out Zipp disk wheel dimples here:http://www.zipp.com/wheels/detail.php?ID=33
>
> Over a 40K ITT this rear wheel can give you a 30-40 second advantage over
> smooth disk wheels.

A model plane about the size of a cyclist would benefit most from
dimples if it only went cycling speeds, 20 - 25 knots.

Smaller aircraft would need to be designed to go faster inverse with
size.

A golf ball sized aircraft would have to go 200 mph for dimples to
work.


Bret Cahill

[Gregory Hall]

> wrote in message
...
> > Have they tried dimples on radio controlled aircraft? ? The size and
> > speed could designed around the magic Reynolds number = 100,000 where
> > the coefficient of drag drops precipitously.
>
> > Dimpling could vastly extent the range of large and slow as well as
> > small and fast radio controlled aircraft.
>
> > A competitive cyclist is the right size and speed for Nre = 100,000 so
> > dimple suits can work. ?Same for golf balls.
>
> > Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
> > except on the runway.
>
> > From fluid mechanics the Reynolds number is the ratio of inertial
> > forces/viscous forces.
>
> > N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
> > Bret Cahill
>
> We competitive cyclists use dimples already on our disk wheels. And some
> skinsuits incorporate them. But they don't look like the dimples on a golf
> ball. They are shaped differently and they are shallow.
>
> Check out Zipp disk wheel dimples
> here:http://www.zipp.com/wheels/detail.php?ID=33
>
> Over a 40K ITT this rear wheel can give you a 30-40 second advantage over
> smooth disk wheels.

\ A model plane about the size of a cyclist would benefit most from
\ dimples if it only went cycling speeds, 20 - 25 knots.
\
\ Smaller aircraft would need to be designed to go faster inverse with
\ size.
\
\ A golf ball sized aircraft would have to go 200 mph for dimples to
\ work.

I wonder if anybody has thought of putting the appropriate dimples on the
surface of propellers? Seems like reducing drag there would increase RPM and
reduce HP required.

A bicycle wheel spins much faster than 20-25 knots apparent to the air it
interfaces with. At 30 knots, for example, the surface of the wheel might be
moving closer to 100 knots apparent to the wind.

--
Gregory Hall

[[email protected]]

> > > Have they tried dimples on radio controlled aircraft? ? The size and
> > > speed could designed around the magic Reynolds number = 100,000 where
> > > the coefficient of drag drops precipitously.
>
> > > Dimpling could vastly extent the range of large and slow as well as
> > > small and fast radio controlled aircraft.
>
> > > A competitive cyclist is the right size and speed for Nre = 100,000 so
> > > dimple suits can work. ?Same for golf balls.
>
> > > Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
> > > except on the runway.
>
> > > From fluid mechanics the Reynolds number is the ratio of inertial
> > > forces/viscous forces.
>
> > > N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
> > > Bret Cahill
>
> > We competitive cyclists use dimples already on our disk wheels. And some
> > skinsuits incorporate them. But they don't look like the dimples on a golf
> > ball. They are shaped differently and they are shallow.
>
> > Check out Zipp disk wheel dimples
> > here:http://www.zipp.com/wheels/detail.php?ID=33
>
> > Over a 40K ITT this rear wheel can give you a 30-40 second advantage over
> > smooth disk wheels.
>
> \ A model plane about the size of a cyclist would benefit most from
> \ dimples if it only went cycling speeds, 20 - 25 knots.
> \
> \ Smaller aircraft would need to be designed to go faster inverse with
> \ size.
> \
> \ A golf ball sized aircraft would have to go 200 mph for dimples to
> \ work.

> I wonder if anybody has thought of putting the appropriate dimples on the
> surface of propellers? Seems like reducing drag there would increase RPM and
> reduce HP required.

I'm not certain dimples would make much difference in a well designed
airfoil wing or prop or fusalage. Maybe something that had an awkward
shape, i. e., a strut, would benefit the most.

I may recant.

A golf ball goes 4 times further with dimples but a golf ball isn't
aerodynamic in the first place.

> A bicycle wheel spins much faster than 20-25 knots apparent to the air it
> interfaces with. At 30 knots, for example, the surface of the wheel might be
> moving closer to 100 knots apparent to the wind.

It's just double the speed of the hub.


Bret Cahill

[[email protected]]

In rec.aviation.marketplace Gregory Hall > wrote:

> I wonder if anybody has thought of putting the appropriate dimples on the
> surface of propellers? Seems like reducing drag there would increase RPM and
> reduce HP required.

While reducing drag would be a goal, fixed propeller systems are designed
to keep the tip velocity under mach 1.

For constant speed props, the RPM is whatever you set it to, again
under mach 1.


--
Jim Pennino

Remove .spam.sux to reply.

[[email protected]]

In rec.aviation.marketplace wrote:

> I'm not certain dimples would make much difference in a well designed
> airfoil wing or prop or fusalage. Maybe something that had an awkward
> shape, i. e., a strut, would benefit the most.

If dimples were an "improvement", why do airplane makers bother with
smoothing out the dimples from flush rivets?


--
Jim Pennino

Remove .spam.sux to reply.

[daestrom]

wrote:
> In rec.aviation.marketplace Gregory Hall > wrote:
>
>> I wonder if anybody has thought of putting the appropriate dimples
>> on the surface of propellers? Seems like reducing drag there would
>> increase RPM and reduce HP required.
>
> While reducing drag would be a goal, fixed propeller systems are
> designed to keep the tip velocity under mach 1.
>
> For constant speed props, the RPM is whatever you set it to, again
> under mach 1.

True, but reducing Hp requirements would still be an advantage.

[[email protected]]

On Nov 4, 4:26*pm, "daestrom" >
wrote:
> wrote:
> > In rec.aviation.marketplace Gregory Hall > wrote:
>
> >> I wonder if anybody has thought of putting the appropriate dimples
> >> on the surface of propellers? Seems like reducing drag there would
> >> increase RPM and reduce HP required.
>
> > While reducing drag would be a goal, fixed propeller systems are
> > designed to keep the tip velocity under mach 1.
>
> > For constant speed props, the RPM is whatever you set it to, again
> > under mach 1.
>
> True, but reducing Hp requirements would still be an advantage.

There's an outfit that markets a perforated tape for
propeller leading edges. The perfs act like dimples. They claim
performance improvements with their stuff, of course. See http://www.dimpletape.com/

Dan

[[email protected]]

In rec.aviation.marketplace daestrom > wrote:
> wrote:
>> In rec.aviation.marketplace Gregory Hall > wrote:
>>
>>> I wonder if anybody has thought of putting the appropriate dimples
>>> on the surface of propellers? Seems like reducing drag there would
>>> increase RPM and reduce HP required.
>>
>> While reducing drag would be a goal, fixed propeller systems are
>> designed to keep the tip velocity under mach 1.
>>
>> For constant speed props, the RPM is whatever you set it to, again
>> under mach 1.
>
> True, but reducing Hp requirements would still be an advantage.

What part of "While reducing drag would be a goal," did you not
understand?

--
Jim Pennino

Remove .spam.sux to reply.

[[email protected]]

In rec.aviation.marketplace wrote:
> On Nov 4, 4:26Â*pm, "daestrom" >
> wrote:
>> wrote:
>> > In rec.aviation.marketplace Gregory Hall > wrote:
>>
>> >> I wonder if anybody has thought of putting the appropriate dimples
>> >> on the surface of propellers? Seems like reducing drag there would
>> >> increase RPM and reduce HP required.
>>
>> > While reducing drag would be a goal, fixed propeller systems are
>> > designed to keep the tip velocity under mach 1.
>>
>> > For constant speed props, the RPM is whatever you set it to, again
>> > under mach 1.
>>
>> True, but reducing Hp requirements would still be an advantage.
>
> There's an outfit that markets a perforated tape for
> propeller leading edges. The perfs act like dimples. They claim
> performance improvements with their stuff, of course. See http://www.dimpletape.com/
>
> Dan

Glaringly missing from that site is any mention of a STC.

--
Jim Pennino

Remove .spam.sux to reply.

[Vaughn Simon]

> wrote in message
...
> There's an outfit that markets a perforated tape for
>propeller leading edges. The perfs act like dimples. They claim
>performance improvements with their stuff, of course. See
>http://www.dimpletape.com/

A related product is called Turbulator Tape. It is used mostly used on
smooth glider wings to control the separation bubble. A Google search of the
term will yield several hits.

Vaughn

[[email protected]]

> > I'm not certain dimples would make much difference in a well designed
> > airfoil wing or prop or fusalage. �Maybe something that had an awkward
> > shape, i. e., a strut, would benefit the most.
>
> If dimples were an "improvement", why do airplane makers bother with
> smoothing out the dimples from flush rivets?

The Reynolds number is over 100,000 for anything bigger than a drone
going over 10 mph.


Bret Cahill

[[email protected]]

On Nov 4, 4:29�pm, wrote:
> On Nov 4, 4:26�pm, "daestrom" >
> wrote:
>
> > wrote:
> > > In rec.aviation.marketplace Gregory Hall > wrote:
>
> > >> I wonder if anybody has thought of putting the appropriate dimples
> > >> on the surface of propellers? Seems like reducing drag there would
> > >> increase RPM and reduce HP required.
>
> > > While reducing drag would be a goal, fixed propeller systems are
> > > designed to keep the tip velocity under mach 1.
>
> > > For constant speed props, the RPM is whatever you set it to, again
> > > under mach 1.
>
> > True, but reducing Hp requirements would still be an advantage.
>
> � � � � � There's an outfit that markets a perforated tape for
> propeller leading edges. The perfs act like dimples. They claim
> performance improvements with their stuff, of course. Seehttp://www.dimpletape.com/
>
> � � � � � � Dan

If the Reynolds number isn't between 50,000 - 120,000, it's a scam.


Bret Cahill

[[email protected]]

In rec.aviation.marketplace wrote:
>> > I'm not certain dimples would make much difference in a well designed
>> > airfoil wing or prop or fusalage. �Maybe something that had an awkward
>> > shape, i. e., a strut, would benefit the most.
>>
>> If dimples were an "improvement", why do airplane makers bother with
>> smoothing out the dimples from flush rivets?
>
> The Reynolds number is over 100,000 for anything bigger than a drone
> going over 10 mph.
>
>
> Bret Cahill

Ice cream has no bones.


--
Jim Pennino

Remove .spam.sux to reply.

[bbrought]

On Nov 4, 9:04*pm, Bret Cahill > wrote:
> Have they tried dimples on radio controlled aircraft? * The size and
> speed could designed around the magic Reynolds number = 100,000 where
> the coefficient of drag drops precipitously.
>
> Dimpling could vastly extent the range of large and slow as well as
> small and fast radio controlled aircraft.
>
> A competitive cyclist is the right size and speed for Nre = 100,000 so
> dimple suits can work. *Same for golf balls.
>
> Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
> except on the runway.
>
> From fluid mechanics the Reynolds number is the ratio of inertial
> forces/viscous forces.
>
> N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
> Bret Cahill

You have a fundamental misunderstanding of aerodynamics. There are
several mechanics that produce drag, and the two involved here are
pressure drag due to seperated flow and skin friction drag. First, on
a bluff body, such as a golf ball (and a cyclist for that matter), the
majority of the drag is pressure drag due to the flow seperating as it
cannot negotiate the steep adverse pressure gradient towards the rear
of the object. Pressure drag is much higher - sometimes one or more
orders of magnitude - than skin friction drag.

Skin friction drag comes from the shear inside the boundary layer,
where the airspeed drops from approximately the free-stream velocity
outside the boundary layer to zero where it actually touches the
surface. This comes in two forms - laminar and turbulent. The skin
friction drag due to a laminar boundary layer is once again much lower
than that due to a turbulent boundary layer.

The reason dimples work on a golf ball is due to the fact that a
turbulent boundary layer, although having more drag than a laminar
boundary layer, tends to stay attached through much steeper adverse
pressure gradients than laminar boundary layers. The dimples force the
flow to transition from laminar to turbulent, which means it stays
attached for longer and you therefore end up reducing the pressure
drag as a smaller region of flow eventually seperates. The drag
savings therefore is because there is less seperated flow, not because
a dimpled surface causes less skin friction than a smooth one. Many
bluff bodies can benefit from this.

When it comes to streamlined bodies, such as an airplane wing, the
situation is very different. When an airfoil is well designed (I'll
get back to low Reynolds number airfoils on which I have done quite a
bit of work over the years) the flow is almost completely attached at
the typical local angle of attack that the wing sees at speeds between
loiter and maximum speed, which is of course where the low drag
matters. Since there is virtually no seperated flow (there is usually
a tiny bit right at the trailing edge), there is no extra benefit to
be had from dimpling. In fact, if you dimple the whole wing you are
going to transition to a turbulent boundary layer early and you are
actually goint to increase the total drag.

Low Reynolds number airfoils are slightly different. The Reynolds
numbers of interest for small - not micro - UAVs is typically between
about 40,000 on tail surfaces to about 500,000 on the wing. At these
Reynolds numbers you sometimes get what is called a "seperation
bubble". While still laminar, the flow seperates, but then it
transitions to turbulent off the surface and then re-attach as a
turbulent boundary layer that remains attached all the way to the
trailing edge if properly designed. These seperation bubbles are
sometimes unavoidable, but good airfoil design can minimize their size
and therefore their drag. In some instances, a small strip can be used
to force the boundary layer to transition to turbulent just ahead of
the point where the flow would have seperated, to prevent the
formation of the seperation bubble. If well designed and placed, you
end up with a nice low drag laminar boundary layer over the forward
part of the airfoil, and then a higher drag turbulent boundary layer
towards the rear but without the seperation bubble. The overall drag
is usually only reduced over a small part of the flight envelope and
only if designed and placed properly - and it only really works at
Reynolds numbers below about 200,000. Again dimples would be too crude
to lead to an overall improvement, as you will once again end up with
a fully turbulent boundary layer while you could have benefitted from
keeping some of the flow laminar.

Finally, your equation:
> N re = Diameter X velocity X density of fluid/viscosity of fluid.

That 100,000 you used is for Reynolds number based on diameter as in
your equation above, which is indeed valid for a sphere or cylinder.
However, a wing's Reynolds number is based on the local chord (it
changes along the span if the wing is tapered):

Re = chord X velocity X density of fluid / viscosity of fluid.

Because we are talking about a completely different situation on a
streamlined body such as a wing, that magic Reynolds number of 100,000
that you quoted for a sphere is simply not relevant.

[Eeyore[_2_]]

Bret Cahill wrote:

> Have they tried dimples on radio controlled aircraft?

Oh it's Bret. IGNORE his stupidity.

You need laminar flow wings for best airliner etc efficiency.

Graham

[Eeyore[_2_]]

wrote:

> In rec.aviation.marketplace wrote:
>
> > I'm not certain dimples would make much difference in a well designed
> > airfoil wing or prop or fusalage. Maybe something that had an awkward
> > shape, i. e., a strut, would benefit the most.
>
> If dimples were an "improvement", why do airplane makers bother with
> smoothing out the dimples from flush rivets?

Or in Airbus's case not using rivets at all as far as possible ! I think
Boeing are catching up on that one too btw.

Graham

[Eeyore[_2_]]

bbrought wrote:

> On Nov 4, 9:04 pm, Bret Cahill > wrote:

A load of nonsense as is usual for him.


> You have a fundamental misunderstanding of aerodynamics.

And just about most things else too.

Graham

[cavelamb himself[_4_]]

Even the smoothest skins will still have flow separations leading to
turbulent flow.

While the problem can be addressed, it takes a bit more than dimples
to stick the boundary layer down tight over the entire wing.

Consider the X-21A program.
It worked exceptionally well, but unfortunately proved to be unmaintainable.


X-21A
http://users.dbscorp.net/jmustain/x21.htm


First Flight: April 18, 1963
Mission: Full sized test bed for testing "Laminar Flow Control" (also
referred to as boundary layer control) theory
Major Accomplishments: Proved that while Laminar flow control was
possible, it was not feasible with existing technology.
Power Source: General Electric J79-GE-13, 9,400 lb thrust max.
Wing Span: 93' 6"
Length: 75' 3'
Weight (Loaded): 83,000 lbs
Maximum Achieved Speed: 560 mph
Maximum Achieved Altitude: 42,500 ft

Additional Information: Only two X-21's were built, and were actually
highly modified Douglas WB-66D's. The X-21 was flown to test the
"Laminar Flow Control" theory. The basic concept is that the exterior
surface of the aircraft can be designed to create a slight suction
during flight. Slots are incorporated in the aircraft's surface to
produce the suction. Though the concept works, environmental
considerations including rain, dirt, dust and other particulates
required excessive maintenance on the aircraft.

The last known location of both X-21's was Edwards AFB, where they had
been gutted of most instrumentation and left out of doors to deteriorate.

[BobG]

On Nov 5, 4:10*am, bbrought > wrote:
> pressure drag due to seperated flow and skin friction drag. First, on

> majority of the drag is pressure drag due to the flow seperating as it

> drag as a smaller region of flow eventually seperates. The drag

> Reynolds numbers you sometimes get what is called a "seperation
> bubble". While still laminar, the flow seperates, but then it

> trailing edge if properly designed. These seperation bubbles are

> the point where the flow would have seperated, to prevent the
> formation of the seperation bubble. If well designed and placed, you

> towards the rear but without the seperation bubble. The overall drag
============================================
Remember that separate has a rat in it.

[bbrought]

On Nov 5, 3:16*pm, BobG > wrote:
> On Nov 5, 4:10*am, bbrought > wrote:
>
> > pressure drag due to seperated flow and skin friction drag. First, on
> > majority of the drag is pressure drag due to the flow seperating as it
> > drag as a smaller region of flow eventually seperates. The drag
> > Reynolds numbers you sometimes get what is called a "seperation
> > bubble". While still laminar, the flow seperates, but then it
> > trailing edge if properly designed. These seperation bubbles are
> > the point where the flow would have seperated, to prevent the
> > formation of the seperation bubble. If well designed and placed, you
> > towards the rear but without the seperation bubble. The overall drag
>
> ============================================
> Remember that separate has a rat in it.

Dammit, I always get it wrong:) English is my second/third language
and it seems you eventually reach an age where you just stop
improving...

[[email protected]]

> > Have they tried dimples on radio controlled aircraft? � The size and
> > speed could designed around the magic Reynolds number = 100,000 where
> > the coefficient of drag drops precipitously.
>
> > Dimpling could vastly extent the range of large and slow as well as
> > small and fast radio controlled aircraft.
>
> > A competitive cyclist is the right size and speed for Nre = 100,000 so
> > dimple suits can work. �Same for golf balls.
>
> > Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
> > except on the runway.
>
> > From fluid mechanics the Reynolds number is the ratio of inertial
> > forces/viscous forces.
>
> > N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
> > Bret Cahill
>
> You have a fundamental misunderstanding of aerodynamics. There are
> several mechanics that produce drag, and the two involved here are
> pressure drag due to seperated flow and skin friction drag. First, on
> a bluff body, such as a golf ball (and a cyclist for that matter), the
> majority of the drag is pressure drag due to the flow seperating as it
> cannot negotiate the steep adverse pressure gradient towards the rear
> of the object. Pressure drag is much higher - sometimes one or more
> orders of magnitude - than skin friction drag.
>
> Skin friction drag comes from the shear inside the boundary layer,
> where the airspeed drops from approximately the free-stream velocity
> outside the boundary layer to zero where it actually touches the
> surface. This comes in two forms - laminar and turbulent. The skin
> friction drag due to a laminar boundary layer is once again much lower
> than that due to a turbulent boundary layer.
>
> The reason dimples work on a golf ball is due to the fact that a
> turbulent boundary layer, although having more drag than a laminar
> boundary layer, tends to stay attached through much steeper adverse
> pressure gradients than laminar boundary layers. The dimples force the
> flow to transition from laminar to turbulent, which means it stays
> attached for longer and you therefore end up reducing the pressure
> drag as a smaller region of flow eventually seperates. The drag
> savings therefore is because there is less seperated flow, not because
> a dimpled surface causes less skin friction than a smooth one. Many
> bluff bodies can benefit from this.
>
> When it comes to streamlined bodies, such as an airplane wing, the
> situation is very different. When an airfoil is well designed (I'll
> get back to low Reynolds number airfoils on which I have done quite a
> bit of work over the years) the flow is almost completely attached at
> the typical local angle of attack that the wing sees at speeds between
> loiter and maximum speed, which is of course where the low drag
> matters. Since there is virtually no seperated flow (there is usually
> a tiny bit right at the trailing edge), there is no extra benefit to
> be had from dimpling. In fact, if you dimple the whole wing you are
> going to transition to a turbulent boundary layer early and you are
> actually goint to increase the total drag.
>
> Low Reynolds number airfoils are slightly different. The Reynolds
> numbers of interest for small - not micro - UAVs is typically between
> about 40,000 on tail surfaces to about 500,000 on the wing. At these
> Reynolds numbers you sometimes get what is called a "seperation
> bubble". While still laminar, the flow seperates, but then it
> transitions to turbulent off the surface and then re-attach as a
> turbulent boundary layer that remains attached all the way to the
> trailing edge if properly designed. These seperation bubbles are
> sometimes unavoidable, but good airfoil design can minimize their size
> and therefore their drag. In some instances, a small strip can be used
> to force the boundary layer to transition to turbulent just ahead of
> the point where the flow would have seperated, to prevent the
> formation of the seperation bubble. If well designed and placed, you
> end up with a nice low drag laminar boundary layer over the forward
> part of the airfoil, and then a higher drag turbulent boundary layer
> towards the rear but without the seperation bubble. The overall drag
> is usually only reduced over a small part of the flight envelope and
> only if designed and placed properly - and it only really works at
> Reynolds numbers below about 200,000. Again dimples would be too crude
> to lead to an overall improvement, as you will once again end up with
> a fully turbulent boundary layer while you could have benefitted from
> keeping some of the flow laminar.
>
> Finally, your equation:
>
> > N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
> That 100,000 you used is for Reynolds number based on diameter as in
> your equation above, which is indeed valid for a sphere or cylinder.
> However, a wing's Reynolds number is based on the local chord (it
> changes along the span if the wing is tapered):
>
> Re = chord X velocity X density of fluid / viscosity of fluid.
>
> Because we are talking about a completely different situation on a
> streamlined body such as a wing, that magic Reynolds number of 100,000
> that you quoted for a sphere is simply not relevant.

I caught that in a post yesterday but I'm glad someone gave a more
detailed treatment. Usually I have to correct my errors myself.

Still there may be some situation where an airfoil might conflict with
a structure, either because of cost or other considerations.


Bret Cahill

[Bill Ward]

On Wed, 05 Nov 2008 00:10:29 -0800, bbrought wrote:

> On Nov 4, 9:04Â*pm, Bret Cahill > wrote:
>> Have they tried dimples on radio controlled aircraft? Â* The size and
>> speed could designed around the magic Reynolds number = 100,000 where
>> the coefficient of drag drops precipitously.
>>
>> Dimpling could vastly extent the range of large and slow as well as
>> small and fast radio controlled aircraft.
>>
>> A competitive cyclist is the right size and speed for Nre = 100,000 so
>> dimple suits can work. Â*Same for golf balls.
>>
>> Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
>> except on the runway.
>>
>> From fluid mechanics the Reynolds number is the ratio of inertial
>> forces/viscous forces.
>>
>> N re = Diameter X velocity X density of fluid/viscosity of fluid.
>>
>> Bret Cahill
>
> You have a fundamental misunderstanding of aerodynamics. There are several
> mechanics that produce drag, and the two involved here are pressure drag
> due to seperated flow and skin friction drag. First, on a bluff body, such
> as a golf ball (and a cyclist for that matter), the majority of the drag
> is pressure drag due to the flow seperating as it cannot negotiate the
> steep adverse pressure gradient towards the rear of the object. Pressure
> drag is much higher - sometimes one or more orders of magnitude - than
> skin friction drag.
>
> Skin friction drag comes from the shear inside the boundary layer, where
> the airspeed drops from approximately the free-stream velocity outside the
> boundary layer to zero where it actually touches the surface. This comes
> in two forms - laminar and turbulent. The skin friction drag due to a
> laminar boundary layer is once again much lower than that due to a
> turbulent boundary layer.
>
> The reason dimples work on a golf ball is due to the fact that a turbulent
> boundary layer, although having more drag than a laminar boundary layer,
> tends to stay attached through much steeper adverse pressure gradients
> than laminar boundary layers. The dimples force the flow to transition
> from laminar to turbulent, which means it stays attached for longer and
> you therefore end up reducing the pressure drag as a smaller region of
> flow eventually seperates. The drag savings therefore is because there is
> less seperated flow, not because a dimpled surface causes less skin
> friction than a smooth one. Many bluff bodies can benefit from this.
>
> When it comes to streamlined bodies, such as an airplane wing, the
> situation is very different. When an airfoil is well designed (I'll get
> back to low Reynolds number airfoils on which I have done quite a bit of
> work over the years) the flow is almost completely attached at the typical
> local angle of attack that the wing sees at speeds between loiter and
> maximum speed, which is of course where the low drag matters. Since there
> is virtually no seperated flow (there is usually a tiny bit right at the
> trailing edge), there is no extra benefit to be had from dimpling. In
> fact, if you dimple the whole wing you are going to transition to a
> turbulent boundary layer early and you are actually goint to increase the
> total drag.
>
> Low Reynolds number airfoils are slightly different. The Reynolds numbers
> of interest for small - not micro - UAVs is typically between about 40,000
> on tail surfaces to about 500,000 on the wing. At these Reynolds numbers
> you sometimes get what is called a "seperation bubble". While still
> laminar, the flow seperates, but then it transitions to turbulent off the
> surface and then re-attach as a turbulent boundary layer that remains
> attached all the way to the trailing edge if properly designed. These
> seperation bubbles are sometimes unavoidable, but good airfoil design can
> minimize their size and therefore their drag. In some instances, a small
> strip can be used to force the boundary layer to transition to turbulent
> just ahead of the point where the flow would have seperated, to prevent
> the formation of the seperation bubble. If well designed and placed, you
> end up with a nice low drag laminar boundary layer over the forward part
> of the airfoil, and then a higher drag turbulent boundary layer towards
> the rear but without the seperation bubble. The overall drag is usually
> only reduced over a small part of the flight envelope and only if designed
> and placed properly - and it only really works at Reynolds numbers below
> about 200,000. Again dimples would be too crude to lead to an overall
> improvement, as you will once again end up with a fully turbulent boundary
> layer while you could have benefitted from keeping some of the flow
> laminar.
>
> Finally, your equation:
>> N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
> That 100,000 you used is for Reynolds number based on diameter as in your
> equation above, which is indeed valid for a sphere or cylinder. However, a
> wing's Reynolds number is based on the local chord (it changes along the
> span if the wing is tapered):
>
> Re = chord X velocity X density of fluid / viscosity of fluid.
>
> Because we are talking about a completely different situation on a
> streamlined body such as a wing, that magic Reynolds number of 100,000
> that you quoted for a sphere is simply not relevant.

Now THATS what Usenet should be like! Thanks for an interesting,
informative post.

[alexy]

wrote:

>> A bicycle wheel spins much faster than 20-25 knots apparent to the air it
>> interfaces with. At 30 knots, for example, the surface of the wheel might be
>> moving closer to 100 knots apparent to the wind.
>
>It's just double the speed of the hub.

Maybe he was riding into a 40 knot headwind? <g>
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

[Ken S. Tucker]

Hi Bret

On Nov 4, 11:04 am, Bret Cahill > wrote:
> Have they tried dimples on radio controlled aircraft? The size and
> speed could designed around the magic Reynolds number = 100,000 where
> the coefficient of drag drops precipitously.
>
> Dimpling could vastly extent the range of large and slow as well as
> small and fast radio controlled aircraft.
>
> A competitive cyclist is the right size and speed for Nre = 100,000 so
> dimple suits can work. Same for golf balls.
>
> Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
> except on the runway.
>
> From fluid mechanics the Reynolds number is the ratio of inertial
> forces/viscous forces.
>
> N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
> Bret Cahill

I design/build model gliders as a hobby, (not an expert :-).
The dimples on golf balls are primarily to operate aero-
dynamically when spinning, like baseball seams,
http://en.wikipedia.org/wiki/Baseball_(ball)
As Mr Brought posted so well, they react with turbulent flow.

I think the Reynolds number is actually a quantum effect
because air is particles, so flying insect wing design is
quite different from an average birds, due to scaling.

I should mention some mysteries, such as the roughness
of shark skin and the unusual nature of feathers that are
rough at a smaller size, that have quite different Reynolds,
that seem to contribute to improved gliding performance,
thus supporting your suggestion.

Interesting subject.
Ken

[Ken S. Tucker]

On Nov 5, 1:08 pm, wrote:
> On Wed, 5 Nov 2008 12:52:12 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> > wrote:
> >I should mention some mysteries, such as the roughness
> >of shark skin and the unusual nature of feathers that are
> >rough at a smaller size, that have quite different Reynolds,
> >that seem to contribute to improved gliding performance,
> >thus supporting your suggestion.
>
> These are not mysteries and have been studied and initially implemented as
> riblets.

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).

The turbulent air very near the surface, where laminar air
flow interfaces with the surface, uses "riblets" to convert
the turbulence into the equivalent of a microscopic "ball
bearings" type phenomena.
Apparently it works, so who knows maybe cars of the
future will feel rough to the touch, and need a special
wax for a reduction of 10% on Cd.
Ken

[Ken S. Tucker]

On Nov 5, 2:40 pm, wrote:
> On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker"

> > wrote:
> >On Nov 5, 1:08 pm, wrote:
> >> On Wed, 5 Nov 2008 12:52:12 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> >> > wrote:
> >> >I should mention some mysteries, such as the roughness
> >> >of shark skin and the unusual nature of feathers that are
> >> >rough at a smaller size, that have quite different Reynolds,
> >> >that seem to contribute to improved gliding performance,
> >> >thus supporting your suggestion.
>
> >> These are not mysteries and have been studied and initially implemented as
> >> riblets.
>
> >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).
>
> Forgive me, I have no idea what SM board is.

styrofoam sm board, the blue stuff, it has a very
interesting texture.

> >The turbulent air very near the surface, where laminar air
> >flow interfaces with the surface, uses "riblets" to convert
> >the turbulence into the equivalent of a microscopic "ball
> >bearings" type phenomena.
> >Apparently it works, so who knows maybe cars of the
> >future will feel rough to the touch, and need a special
> >wax for a reduction of 10% on Cd.
>
> Not quite. As far as riblets go, it is my understanding that the height
> and spacing of the riblets is specified according to the boundary thickness
> such as to prevent the growth of turbulent bursts which causes an exchange
> of low momentum fluid near the surface with higher momentum fluid from
> above. This momentum exchange being a loss/drag mechanism. The other point
> of importance is the orientation of the riblets along streamlines.

Yes, that's seems clear...but NOT simple :-).

> The fundamental studies were done at NASA Langley Research Center in the
> late 70s/early 80s. The first open use of riblets was on the boat Dennis
> Connor (sp?) used to win back the America's Cup, being applied via a
> special 3M tape. The wind tunnel test articles from that study reside in
> the basement of the building next door to the one the branch I work
> resides. I my be wrong but It's possible the same the facility used in the
> study was also used for the work leading to the Speedo LZR Racer suit.

Nifty,
Ken

[Ken S. Tucker]

On Nov 6, 1:44 pm, wrote:
> On Thu, 6 Nov 2008 01:54:20 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> > wrote:
> >On Nov 5, 2:40 pm, wrote:
> >> On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> >> Not quite. As far as riblets go, it is my understanding that the height
> >> and spacing of the riblets is specified according to the boundary thickness
> >> such as to prevent the growth of turbulent bursts which causes an exchange
> >> of low momentum fluid near the surface with higher momentum fluid from
> >> above. This momentum exchange being a loss/drag mechanism. The other point
> >> of importance is the orientation of the riblets along streamlines.
>
> >Yes, that's seems clear...but NOT simple :-).
>
> That's a lot different that your original point that it was not well
> understood.

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"

> Also after the original riblet research was performed
> similarities to shark scales/skin were observed.
> http://ntrs.larc.nasa.gov/search.jsp?N=0&Ntk=all&Ntx=mode%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).

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.

Setting aside sharks skin, we may want to have a
look at penguin swimming, that also has very low
resistance.
Regards
Ken

[Eunometic]

On Nov 6, 9:40*am, wrote:
> On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
>
>
> > wrote:
> >On Nov 5, 1:08 pm, wrote:
> >> On Wed, 5 Nov 2008 12:52:12 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> >> > wrote:

>
> The fundamental studies were done at NASA Langley Research Center *in the
> late 70s/early 80s. The first open use of riblets was on *the boat Dennis
> Connor (sp?) used to win back the America's Cup, being applied via a
> special 3M tape. The wind tunnel test articles from that study reside in
> the basement of the building next door to the one the branch I work
> resides. I my be wrong but It's possible the same the facility used in the
> study was also used for the work leading to the Speedo LZR Racer suit.

German researchers concurrently came up with the same conclusions from
studying
shark skin, they've continued with considerable efforts in 'printing'
them into
UV cured paints and other techniques. In any case the riblet effect
has been
known for over 50 years. http://researchnews.osu.edu/archive/riblets.htm

[Eunometic]

On Nov 7, 9:35*am, "Ken S. Tucker" > wrote:
> On Nov 6, 1:44 pm, wrote:
>
>
>
> > On Thu, 6 Nov 2008 01:54:20 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> > > wrote:
> > >On Nov 5, 2:40 pm, wrote:
> > >> On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> > >> Not quite. *As far as riblets go, it is my understanding that the height
> > >> and spacing of the riblets is specified according to the boundary thickness
> > >> such as to prevent the growth of turbulent bursts which causes an exchange
> > >> of low momentum fluid near the surface with higher momentum fluid from
> > >> above. This momentum exchange being a loss/drag mechanism. The other point
> > >> of importance is the orientation of the riblets along streamlines.
>
> > >Yes, that's seems clear...but NOT simple :-).
>
> > That's a lot different that your original point that it was not well
> > understood.
>
> 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"
>
> > Also after the original riblet research was performed
> > similarities to shark scales/skin were observed.
> >http://ntrs.larc.nasa.gov/search.jsp?N=0&Ntk=all&Ntx=mode%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).
>
> 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.
>
> Setting aside sharks skin, we may want to have a
> look at penguin swimming, that also has very low
> resistance.
> Regards
> Ken

Riblets aren't the only way; hair and feathers have a similar effect.

"With Robert Brodkey, professor of chemical engineering, Koeltzsch has
now turned his attention away from sharks, to penguins and seals. He
hopes to determine whether hair makes these aquatic mammals more
hydrodynamic. Initial studies by other scientists have shown that
natural and artificial fibers can reduce drag by amounts that vary
from 1.5 to 50 percent.

Continued research could show whether hair would improve the design of
boat hulls and even airplanes, Koeltzsch said.

"Wouldn’t it be something if, in the future, airplanes had hairy
surfaces?" he asked. "

[Gregory Hall]

"Bret Cahill" > wrote in message
...
> Have they tried dimples on radio controlled aircraft? The size and
> speed could designed around the magic Reynolds number = 100,000 where
> the coefficient of drag drops precipitously.
>
> Dimpling could vastly extent the range of large and slow as well as
> small and fast radio controlled aircraft.
>
> A competitive cyclist is the right size and speed for Nre = 100,000 so
> dimple suits can work. Same for golf balls.
>
> Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
> except on the runway.
>
> From fluid mechanics the Reynolds number is the ratio of inertial
> forces/viscous forces.
>
> N re = Diameter X velocity X density of fluid/viscosity of fluid.
>
>
> Bret Cahill
>


Do her dimples make her faster?

http://underscorebleach.net/content/misc/pics/back-dimples/p2/arched.jpg

--
Gregory Hall

[Ken S. Tucker]

Hi Eunometic and guys.

On Nov 7, 2:59 am, Eunometic > wrote:
> On Nov 7, 9:35 am, "Ken S. Tucker" > wrote:
>
>
>
> > On Nov 6, 1:44 pm, wrote:
>
> > > On Thu, 6 Nov 2008 01:54:20 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> > > > wrote:
> > > >On Nov 5, 2:40 pm, wrote:
> > > >> On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> > > >> Not quite. As far as riblets go, it is my understanding that the height
> > > >> and spacing of the riblets is specified according to the boundary thickness
> > > >> such as to prevent the growth of turbulent bursts which causes an exchange
> > > >> of low momentum fluid near the surface with higher momentum fluid from
> > > >> above. This momentum exchange being a loss/drag mechanism. The other point
> > > >> of importance is the orientation of the riblets along streamlines.
>
> > > >Yes, that's seems clear...but NOT simple :-).
>
> > > That's a lot different that your original point that it was not well
> > > understood.
>
> > 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"
>
> > > Also after the original riblet research was performed
> > > similarities to shark scales/skin were observed.
> > >http://ntrs.larc.nasa.gov/search.jsp?N=0&Ntk=all&Ntx=mode%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).
>
> > 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.
>
> > Setting aside sharks skin, we may want to have a
> > look at penguin swimming, that also has very low
> > resistance.
> > Regards
> > Ken
>
> Riblets aren't the only way; hair and feathers have a similar effect.
>
> "With Robert Brodkey, professor of chemical engineering, Koeltzsch has
> now turned his attention away from sharks, to penguins and seals. He
> hopes to determine whether hair makes these aquatic mammals more
> hydrodynamic. Initial studies by other scientists have shown that
> natural and artificial fibers can reduce drag by amounts that vary
> from 1.5 to 50 percent.
>
> Continued research could show whether hair would improve the design of
> boat hulls and even airplanes, Koeltzsch said.
>
> "Wouldn’t it be something if, in the future, airplanes had hairy
> surfaces?" he asked. "

I did read the link you posted previously thanks.
I thought about tennis balls (hairy balls :-) too.
Regards
Ken

[[email protected]]

On Nov 7, 3:59*am, Eunometic > wrote:
> On Nov 7, 9:35*am, "Ken S. Tucker" > wrote:
>
>
>
> > On Nov 6, 1:44 pm, wrote:
>
> > > On Thu, 6 Nov 2008 01:54:20 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> > > > wrote:
> > > >On Nov 5, 2:40 pm, wrote:
> > > >> On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker"
>
> > > >> Not quite. *As far as riblets go, it is my understanding that the height
> > > >> and spacing of the riblets is specified according to the boundary thickness
> > > >> such as to prevent the growth of turbulent bursts which causes an exchange
> > > >> of low momentum fluid near the surface with higher momentum fluid from
> > > >> above. This momentum exchange being a loss/drag mechanism. The other point
> > > >> of importance is the orientation of the riblets along streamlines.
>
> > > >Yes, that's seems clear...but NOT simple :-).
>
> > > That's a lot different that your original point that it was not well
> > > understood.
>
> > 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"
>
> > > Also after the original riblet research was performed
> > > similarities to shark scales/skin were observed.
> > >http://ntrs.larc.nasa.gov/search.jsp?N=0&Ntk=all&Ntx=mode%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).
>
> > 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.
>
> > Setting aside sharks skin, we may want to have a
> > look at penguin swimming, that also has very low
> > resistance.
> > Regards
> > Ken
>
> Riblets aren't the only way; hair and feathers have a similar effect.
>
> "With Robert Brodkey, professor of chemical engineering, Koeltzsch has
> now turned his attention away from sharks, to penguins and seals. He
> hopes to determine whether hair makes these aquatic mammals more
> hydrodynamic. Initial studies by other scientists have shown that
> natural and artificial fibers can reduce drag by amounts that vary
> from 1.5 to 50 percent.
> > Continued research could show whether hair would improve the design of
> boat hulls and even airplanes, Koeltzsch said.
>
> "Wouldn’t it be something if, in the future, airplanes had hairy
> surfaces?" he asked. "

None of this is new. When I was into waterskiing way back in
the '80s the best slalom skis had varied textures along the bottom.
Toward the rear the surface was just a little rough, like 1000 grit
sandpaper, which reduced drag on the water. Nearer the front, the
surface was much smoother so that the skier, just by flattening the
ski on the water, could slow down quickly.
I've noticed that when we paint an airplane with the really
shiny smooth urethane paints, it seems to lose a little cruise speed.

Dan

[Gregory Hall]

> wrote in message
...
On Nov 7, 3:59 am, Eunometic > wrote:
> On Nov 7, 9:35 am, "Ken S. Tucker" > wrote:
>
>
>
> > On Nov 6, 1:44 pm, wrote:
>
> > > On Thu, 6 Nov 2008 01:54:20 -0800 (PST), in sci.engr.mech "Ken S.
> > > Tucker"
>
> > > > wrote:
> > > >On Nov 5, 2:40 pm, wrote:
> > > >> On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S.
> > > >> Tucker"
>
> > > >> Not quite. As far as riblets go, it is my understanding that the
> > > >> height
> > > >> and spacing of the riblets is specified according to the boundary
> > > >> thickness
> > > >> such as to prevent the growth of turbulent bursts which causes an
> > > >> exchange
> > > >> of low momentum fluid near the surface with higher momentum fluid
> > > >> from
> > > >> above. This momentum exchange being a loss/drag mechanism. The
> > > >> other point
> > > >> of importance is the orientation of the riblets along streamlines.
>
> > > >Yes, that's seems clear...but NOT simple :-).
>
> > > That's a lot different that your original point that it was not well
> > > understood.
>
> > 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"
>
> > > Also after the original riblet research was performed
> > > similarities to shark scales/skin were observed.
> > >http://ntrs.larc.nasa.gov/search.jsp?N=0&Ntk=all&Ntx=mode%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).
>
> > 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.
>
> > Setting aside sharks skin, we may want to have a
> > look at penguin swimming, that also has very low
> > resistance.
> > Regards
> > Ken
>
> Riblets aren't the only way; hair and feathers have a similar effect.
>
> "With Robert Brodkey, professor of chemical engineering, Koeltzsch has
> now turned his attention away from sharks, to penguins and seals. He
> hopes to determine whether hair makes these aquatic mammals more
> hydrodynamic. Initial studies by other scientists have shown that
> natural and artificial fibers can reduce drag by amounts that vary
> from 1.5 to 50 percent.
> > Continued research could show whether hair would improve the design of
> boat hulls and even airplanes, Koeltzsch said.
>
> "Wouldn’t it be something if, in the future, airplanes had hairy
> surfaces?" he asked. "

| None of this is new. When I was into waterskiing way back in
| the '80s the best slalom skis had varied textures along the bottom.
| Toward the rear the surface was just a little rough, like 1000 grit
| sandpaper, which reduced drag on the water. Nearer the front, the
| surface was much smoother so that the skier, just by flattening the
| ski on the water, could slow down quickly.
| 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

[N:dlzc D:aol T:com \(dlzc\)]

Dear Gregor Hall:

"Gregory Hall" > wrote in message
...
> Depending upon the size of the aircraft paint can add
> a considerable weight burden.

The external tank of the space shuttle used to be painted. They
stopped for exaclty this reason.

> However, when the paint is really smooth it can

.... ...
[i]
> drag along with it an increasingly thicker layer of turbulent
> air building toward the aft end of the aircraft.

.... better to say that it allows for earlier boundary layer
separation, essentially increasing the cross sectional area of
the shape.

> Dragging this thick turbulent boundary layer causes increased
> drag which seems counter-intuitive to
> smoothness.

It sure does.

> 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.

NASA had an aircraft where they sucked air into the upper surface
of the wing, to try and create a "laminar" boundary layer. I
think it took more power than they got extra-lift / reduced-drag.

David A. Smith

[Gregory Hall]

"N:dlzc D:aol T:com (dlzc)" > wrote in message
...
> Dear Gregor Hall:
>
> "Gregory Hall" > wrote in message
> ...
>> Depending upon the size of the aircraft paint can add
>> a considerable weight burden.
>
> The external tank of the space shuttle used to be painted. They stopped
> for exaclty this reason.
>
>> However, when the paint is really smooth it can
>
> ... ...
>[i]
>> drag along with it an increasingly thicker layer of turbulent
>> air building toward the aft end of the aircraft.
>
> ... better to say that it allows for earlier boundary layer separation,
> essentially increasing the cross sectional area of the shape.
>
>> Dragging this thick turbulent boundary layer causes increased drag which
>> seems counter-intuitive to
>> smoothness.
>
> It sure does.
>
>> 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.
>
> NASA had an aircraft where they sucked air into the upper surface of the
> wing, to try and create a "laminar" boundary layer. I think it took more
> power than they got extra-lift / reduced-drag.
>
> David A. Smith
>

Probably reduced lift as well. Seems to me sucking air into the upper
surface of the wing would degrade the low pressure caused by the Bernoulli
effect because it would tend to slow down the sped-up air traveling across
the top surface of the wing.

http://www.av8n.com/how/htm/airfoils.html

--
Gregory Hall

[Morgans[_2_]]

"N:dlzc D:aol T:com (dlzc)" > wrote

> NASA had an aircraft where they sucked air into the upper surface of the
> wing, to try and create a "laminar" boundary layer. I think it took more
> power than they got extra-lift / reduced-drag.

I was reading about that one, just a few days ago.

It said one of the largest problems was keeping the vent holes open, from
bugs and stuff, and rain changing the laminar flow.
--
Jim in NC

[Highflyer]

"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 )

[Maxwell[_2_]]

"Gregory Hall" > wrote in message
...
|
| "Bret Cahill" > wrote in message
| ...
| > Have they tried dimples on radio controlled aircraft? The size and
| > speed could designed around the magic Reynolds number = 100,000 where
| > the coefficient of drag drops precipitously.
| >
| > Dimpling could vastly extent the range of large and slow as well as
| > small and fast radio controlled aircraft.
| >
| > A competitive cyclist is the right size and speed for Nre = 100,000 so
| > dimple suits can work. Same for golf balls.
| >
| > Nre = 100,000 for widebodies going 0.5 knots so dimples won't work
| > except on the runway.
| >
| > From fluid mechanics the Reynolds number is the ratio of inertial
| > forces/viscous forces.
| >
| > N re = Diameter X velocity X density of fluid/viscosity of fluid.
| >
| >
| > Bret Cahill
| >
|
|
| Do her dimples make her faster?
|
| http://underscorebleach.net/content/misc/pics/back-dimples/p2/arched.jpg
|

No, I think it is more likely her dimples would make you faster.

[guy]

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

[Alan Dicey]

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.

[Ken S. Tucker]

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?N=0&Ntk=all&Ntx=mode%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