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.

[Brian Whatcott]

On Tue, 04 Nov 2008 21:05:03 GMT, 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?


Good question. Golf ball dimples make an advantageous shape
for a SPINNING ball. Wings can often use vortex generators, or
something not unlike dimples - to energize the boundary layer.

Brian W

[Brian Whatcott]

On Wed, 05 Nov 2008 01:05:02 GMT, wrote:


>> 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.


People with light aircraft often notice a black plastic tape applied
to the prop's leading edges as minor erosion protection.
As far as I can recall, it's not an STC item.

Brian W

[[email protected]]

In rec.aviation.marketplace Brian Whatcott > wrote:
> On Wed, 05 Nov 2008 01:05:02 GMT, wrote:
>
>
>>> 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.
>
>
> People with light aircraft often notice a black plastic tape applied
> to the prop's leading edges as minor erosion protection.
> As far as I can recall, it's not an STC item.
>
> Brian W

See this:

http://www.mcfarlaneaviation.com/Products/?CategoryID=135&ID=58031136&PartNumber=FP1001&

Read the second paragraph.

STC yes; 337 no.


--
Jim Pennino

Remove .spam.sux to reply.

[[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

[Brian Whatcott]

On Wed, 05 Nov 2008 02:35:01 GMT, wrote:


>>>> 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.
>>
>>
>> People with light aircraft often notice a black plastic tape applied
>> to the prop's leading edges as minor erosion protection.
>> As far as I can recall, it's not an STC item.

>See this:
>
>http://www.mcfarlaneaviation.com/Products/?CategoryID=135&ID=58031136&PartNumber=FP1001&
>
>Read the second paragraph.
>
>STC yes; 337 no.


Ouch! If they want $74 for a tape, you bet it's STCed
even as a minor mod
(sorry for the cynicism - thanks for the URL)

Brian W

[[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.

[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

[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

[Chris Wells]

Haven't any of you people seen a competition glider? Turbulator strips are used constantly, and are proven to work, on wings, props, turbines & in combustion chambers. When they're in the right spot, they help the air "stick" to a surface that's bending away, and reduce drag. I've seen them on r/c planes as well.