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Dimples On Model Aircraft Could Greatly Extend Range



 
 
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  #21  
Old November 5th 08, 05:11 PM posted to sci.energy,rec.aviation.homebuilt,rec.aviation.military,rec.aviation.marketplace,sci.engr.mech
bbrought
external usenet poster
 
Posts: 4
Default Dimples On Model Aircraft Could Greatly Extend Range

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...
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  #22  
Old November 5th 08, 05:13 PM posted to sci.energy,rec.aviation.homebuilt,rec.aviation.military,rec.aviation.marketplace,sci.engr.mech
[email protected]
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Posts: 10
Default Dimples On Model Aircraft Could Greatly Extend Range

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



  #23  
Old November 5th 08, 05:53 PM posted to sci.energy,rec.aviation.homebuilt,rec.aviation.military,rec.aviation.marketplace,sci.engr.mech
Bill Ward
external usenet poster
 
Posts: 2
Default Dimples On Model Aircraft Could Greatly Extend Range

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

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.


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


  #25  
Old November 5th 08, 09:52 PM posted to sci.energy,rec.aviation.homebuilt,rec.aviation.military,rec.aviation.marketplace,sci.engr.mech
Ken S. Tucker
external usenet poster
 
Posts: 442
Default Dimples On Model Aircraft Could Greatly Extend Range

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
  #26  
Old November 7th 08, 07:27 PM posted to sci.energy,rec.aviation.homebuilt,rec.aviation.military,rec.aviation.marketplace,sci.engr.mech
Gregory Hall
external usenet poster
 
Posts: 26
Default Dimples On Model Aircraft Could Greatly Extend Range


"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/.../p2/arched.jpg

--
Gregory Hall


  #27  
Old September 24th 09, 02:15 AM
Chris Wells Chris Wells is offline
Senior Member
 
First recorded activity by AviationBanter: Oct 2005
Posts: 106
Default

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.
 




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