Re: [Paddlewise] [PaddleWise] flutter

Matt wrote:

>>>Among a very good description of the cause and cures for flutter peter...
<clip>What size would you suggest the holes be [to prevent flutter]?

thanks Matt.  I would try two rows of four holes down the center, just to
the side of the stiffening rib, as close to the center line as practical
without weakening the blade too much.  5/8 inch diameter or so should do it,
I would put perhaps four in each row about 3/4" apart at the center of the
widest part of the blade.  A slot would work better but would be harder to
do.

The idea is to allow some of the "bleed" fluid from the high pressure side
to the low pressure side to help trap and stabilize the vortex on the back
so it does not wander back and forth.  Bleed fluid create a kind of "fence'
that both strengthens and hold the vortex in plance.

This is done in a number of applications and works quite well, for example
if you look closely at the leading edge extensions (or strakes) on either
side of the F-18 fighter's fuselage you will see similar slots.  It just so
happens that my first job was at Northrop right after the original
development of the YF-17 fighter, which became the F-18 in co-development
with McDonnell-Douglas, where they used this to stabilize the large vortexes
designed to shed off these strakes.  It was to make the aircraft better
controlled a extreme angles of attack during a dogfight.  They were quite
proud of the design of this strake, it was one of the first successful
application of intentional vortex formation by the flying surfaces [though
all surfaces that work within a fluid do this to some extent, this was a
design developed intestinally to take advantage of it].

On a paddle, it would not only prevent flutter, it could hypothetically
prevent stall, allowing you to get a larger maximum thrust out of the paddle
(though at the expense of slightly higher drag).  this could be very useful
for a WW or surf paddle where maximum transient thrust is routinely
necessary and blade drag is not an issue.  Though on a seakayak you do not
use max thrust very often, except perhaps in surf launching or accelerating,
and the drag penalty would have to be weighed against the max trust
advantage.

Peter

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