Re: [Paddlewise] Flutter

John Winters wrote:
> Some years back while tank testing paddles I tried a number of techniques
> for reducing or eliminating flutter. The one that worked best involved
> gluing a 1" wide band of driveway de-icing salt along the perimeter of
the
> power face of the blade. For some reason nothing else I tried worked as
> well. The paddle had an interesting "soft" feel to most paddles that some
> did not like. It also looked strange.

A lot of research on flutter has been done by aircraft makers for over 100
years, the focus and cause on aircraft parts are similar as on paddles
(except fortunately on paddles it is seldom life threatening).  Even with a
century of research and massive supercomputers it is still something
difficult to predict reliably, the difficulty with it on aircraft is it
comes on very suddenly and can become very large and violent to the point of
actually failing structure or losing control of the aircraft.

It is a complex interaction of the shape, dynamic fluid pressure, the
flexing of the structure (in the case of the paddle, it would include the
shaft, the blade and the amount of "give" in the paddle's hands and arms)
which causes both the speed and angle of attack of the surface to change
relative to the fluid.

A more stiff surface will not allow as much pressure change, and greatly
reduce the risk of flutter, hence the reason a tight grip on the paddle
shaft helps.  Also the shape of the edge of the blade is very critical, a
sharp 90 degree edge that would make a "clean" break for the fluid would
help too so the point the fluid flow detaches does not wander over the back
of the blade.

By putting turbulators on the edge of the surface can also work well.  You
are putting more energy into the boundary layer which would stabilize the
flow (by keeping it attached to the surface) reducing separation and
thereby prevents stalls and the rapid pressure changes across the surface.
Which causes the rapid changing directions of the blade.  You can see the
turbulators on many commercial airplanes, little blades or pins out on the
wing or tail in front of the control surfaces.  They could work on a paddle
the same way.

This can actually reduce drag, improve controllability and safety, but at
the
expense of the slightly higher drag of the turbulators.  So a careful and
proper design theoretically would prevent flutter without the turbulators,
but even with big budgets and computers, we are not that good yet.  So we
live with vortex generators and turbulators in sensitive areas of flow.

Another interesting way to stabilize flow and prevent flutter is by putting
slots or vents in strategic locations the would both energize the flow and
trap the vortex so it does not wander over the surface.  this is part of the
reason the slots on used on the leading edge of aircraft wings, and on some
modern swim fin designs.  Properly shaped and carefully placed slots on the
surface of a paddle blade could also help prevent flutter too, but also the
expense of other aspects of the paddle performance, not to mention adding
cost.

So to prevent flutter on a paddle, you could hypothetically: 1) make it
stiffer and hold it tighter, which would and weight or be tiring, 2) add
turbulators to the edges 3) cut strategically located slots on the blade 4)
change the shape of the paddle blade (but these last tow would require go
though a lot of trial and error blades before you could find optimum
configuration).  On an existing paddle trying different ways to hold the
blade (i.e. at an angle to the direction of movement, a la Greenland
technique), or try the turbulators.

This could be done with temporary attachment of sand paper or grit with
rubber cement or wax to try it.  What I would try, which is MUCH less
obvious, is to take a triangle file or a hack saw blade and cut very tiny,
shallow "teeth" around the edges of the blade at the widest part for about 8
or 9 inches on each side.  This too you can test with some think tape over
the edge that you cut little notches in with a sharp exacto knife.  If it
helps, remove the tape and file tiny notches in the blade edge.  They would
be most effective if the were more pronounce on the high pressure ("power")
face of the blade.

If you do not like them you can always sand them off, or try reshaping the
edges at the same time.

Good luck,

Peter



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