Propeller gap destruction test

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "John Richardson" <jprich-at-up-dot-net> 

Hello,

I've been intrigued by the simplicity of Terry Blake's gap idea for some 
time, mainly because of the simplicity of the design, and recently had some 
correspondence concerning any problems he may have had.  My biggest fear 
was tungsten hitting tungsten, and possibly making an interesting hobby 
into a deadly one.  I constructed a bullet proof box around this gap, and 
proceeded to do things that you wouldn't normally do to see what the 
results might be, and to determine what kind of minimal safety measurements 
one should pursue in order to avoid injury.  The first couple of tests were 
done by setting the stationary electrodes close enough to just clip when 
the rotor was turned by hand.  I enclosed all four sides, got a long 
extension cord, and plugged it in from a distance.  After doing this 
several times, I checked it out, and could see where it was hitting, but 
still no shrapnel.  I set the electrodes a bit closer, so that there was a 
more pronounced clipping, and plugged it in again.  Certainly could hear 
it, but upon inspection, there was no damage.  I am assuming that the 
plastic for the rotor arbor has enough elasticity to allow it to give as 
the tungstens hit each other.  So far so good.  The next phase of my coil 
project involved setting up the control panel, and setting various trannies 
on the platform to determine what power source I wanted to use and what the 
best layout would be.  FYI, the platform is a 2 by 2 foot piece of plywood 
with a perimeter of 2 by 4s.  Pretty stout.  I finally decided on a 12/120 
and set it in it's place.  Prior to this, satisfied with my safety runs, I 
had set the gap distance nice and close, figuring that this phase of the 
project was out of the way.  Moving to the workbench to proceed with 
something else, I plugged the gap in to let it run for a while, planning to 
check to see if it would get hot running completely enclosed.  No sooner 
had I done it than I heard shrapnel flying.  Turns out the extreme weight 
of the 12/120 caused enough flex in the plywood base to cause the gap 
spacing to get to a point of severe interference, and a nice big shard flew 
off.  I think this gap style has lots of potential, but play it safe.  Even 
after testing this design, it was the unknown factor of the tranny flexing 
the base to cause a mishap.  Could have been disasterous if everything 
wasn't enclosed.  Also, for those building such things with a minimal of 
tools, take lots of time to make sure that holes are centered and true, 
etc.  There is potential here for a decent gap on the cheap, but enclose it 
all the way around, use good craftsmanship, and you'll have a decent 
RSG.  Judging from what I have learned, I would suggest to keep the gap 
space a little wider than usual, and to place the stationary electrodes 
behind the flying electrode, so that even on the one in a million chance 
that the rotor tungsten started to creep, it can't hit the 
stationaries.   And don't forget that good ole Murphy likes to help with 
your projects too!

John Richardson