"Stealth Pilot" wrote in message
...
On Sat, 13 Dec 2008 15:38:06 -0800 (PST),
wrote:

I use a balancing stand made from shelving angle. The angle is
about 1.5" x 1.5" and has slots that allow you to bolt together just
about anything. I ran a file across the critical edges, draw-file
fashion, to level and smooth them. It works fine.

I found, with the wooden prop off my Jodel, that it was out of
balance *chordwise*, not spanwise. The maker had drilled the center
hole off by about 3/32" and the bolt hole circle is radiused from
that, so the whole prop was/is orbiting the crank axis just a little.
My shelf-angle stand is tall enough that I can support a prop
vertically as well as horizontally to catch such imbalances.

Of course, we now have a Chadwick-Helmuth electronic balancer that
does the analysis with the engine running. Washers and small hardware
are attached at the specified place on the spinner backplate. Makes a
big difference. Saves instruments and radios.

Dan


dan you've mentioned a *very* important point. when making a prop, cut
the blank to outline shape then drill the hub holes.
the hub holes and the back face of the prop then become the datums to
work from. that way you get the prop evenly disposed about the hub
hole and boss.
if you dont have a prop that is symetrical about the centre you have a
clock mount for the wall.

the chadwick-helmuth 'little finger extended while drinking tea'
balancer may have achieved static and dynamic balance for you but what
of the asymetric thrust??
it would be an interesting prop to watch you fly behind. (while
standing on the ground) must make some interesting noises.

for veedubber; the critical thing to find out with any balancer that
is home made is whether a very light weight added to a tip moves the
prop. I was amazed that mine moved with quarter of a post it note.
indeed experimentation has shown that balancing a prop in a room full
of still air is far more important than knife edges.

Stealth Pilot

I agree with you regarding the proper proceedure, and also believe that a
multi-blade propeller that is not concentric may contain additional hidden
flaws--such as voids--and likely does. Such a blatant lapse in workmanship
and inspection should clearly be viewed as a "warning shot across the bow!"

However, I also suspect that far too much can be made of the eccentric
thrust problem.

Over the decades, there have been a number of experiments with single blade
propellers--primarily on helicopters--with an opposing counterweight. While
my intuitive reaction was to question the probable bending force applied to
the crank shaft--or drive shaft--I have never actually heard of that being a
problem.

Therefore, I can only presume that the lack of popularity is due to other
factors, such as:
1) Reduced capacity for any given disk area.
2) Poor streamlining at speed in airplane propeller applications.
3) Strange appearance.

OTOH, it does make variable pitch ridiculously simple!

Peter

BTW, I still can't quite accept the idea either.