Dimples On Model Aircraft Could Greatly Extend Range

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

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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

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

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

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.

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.

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

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

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

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.