Hi Paul, A couple of pics of the gearbox bits so far. I am going to
see if the gearbox can be made so as to be able to change the ratio
to 2.85 or 3.17 relatively easily. Cheers Chris. in NZ
Looking good Chris. Good idea. Since the stock red line on the Mazda
RX8 is 9000 RPM the 3.12 makes the most sense. At 1700 prop RPM that
gives 8400 Engine RPM which is still below the 300 HP possible with
a Mazda p-port at 9000 RPM. My guess is an easy 280 HP. That puts
the Mazda rotary 100 pounds lighter than any piston aircraft engine
of the same power and about the same weight as a turbo prop engine
with half the fuel burn. With aluminum end housings it would be 50
pounds lighter still. Of course a turbo compound version would
destroy the rest of the light aircraft engine industry and give more
power at altitude than a turbo prop.
I wonder when they are going to wake up :)
Paul Lamar
Hi Paul, Here are a few more pics of gearbox parts, I am going to
make it so it can be used for pusher or tractor configuration.
Cheers Chris.
Beware those rubber doughnut couplings. They have about a 5000 RPM
limit. That could have been the demise of the Rotomax. The crash
occurred because the engine disconnected from the prop. They are
also a bit too stiff.
What brand is it?
Paul Lamar
It is a centaflex-A. The guys at Bravini in Auckland where I bought
it reckon it should go higher. Note it is a four bolt one, size 22
as opposed to the three bolt one in your picture. I'll give it a try
and keep an eye on it. I'm going to make the engine cowlings like a
car bonnet (hood) so that it will be real easy to open for each
preflight. Cheers Chris.
Size 22 is too small.
200 foot pounds from the rotary is 271 newton meters. That is
average. Peek is twice that. For oscillating torque you must triple
the rating. That means you will need at least a number 30 and that
max RPM is 4000.
Also the temperature must be taken into consideration. The box runs
up to 200F or 93 C. For 90C you must double the torque factor. They
are not made with high temperature (500 F) silicon rubber.
On top of all that they are too stiff.
Frankly IMHO these joints are dangerous as they give no warning of
total failure.
Billet couplings or welded couplings are fail safe in that they go
into a metal to metal state if all the silicon rubber gets out of
there some how so the prop keeps turning no matter what.
Paul Lamar
The axial allen head bolts break, I have seen it a few times in
marine applications. The hub just sits and spins, no big deal in a
boat, just radio for a tow !
Steve Carlisle
The old cyclical bending mode. Bend metal back and fourth long
enough and it will fatigue and crack. At 6 or 7000 RPM it does not
take long.
It starts happening around a million cycles or 142 minutes. About 2
and a half hours :)
What is bad about it is the bolt threads are a built in crack to
start.
The same thing happens in a rotary tension bold if you don't damp
the vibrations with silicon rubber.
Paul Lamar
I was also thinking about this cyclical bending mode of the extended
point set screws used to attach prop hub and male coupler component.
I have been trying to research material properties of ‘typical’
extended point set screws to try to overdesign the [number of set
screws] for the coupler. Can you assist me in researching this with
some key words or resources? Thanks.
Gene Kahn
Not much in bending. Mostly shear. In addition they have a
compressive pre load which helps.
I have no idea whether any R&D has been done on this. They are very
simple. One could calculate the area of the extended point and then
calculate the shear stress. It is not quite that simple as the stress
distribution is not uniform. It is pretty easy to set up and test for
maximum torque capabilities on the assemble. Regular set screws have
been around for well over 100 years probably. All this does is
increase the torque capabilities
Paul Lamar
Perhaps "fatigue limit" of the set screws might be helpful? Heck, that
is something that could be observed experimentally on a workbench using
a vise to hold a small aluminum block with a set screw. The protruding
tip of the set screw is challenged by cycles of increasing shear force
until the elastic limit is observed. The elastic limit would be
observed using reflected laser light from a small handheld pointer onto
a remote scale, and
Gene Kahn
They are not in a situation that is conducive to fatigue.
Fatigue happens on parts subject to reverse bending.
The e-shaft as opposed to a crankshaft is a good example.
Cracks in e-shafts are unheard of. Cracks in crankshafts
are common.
Paul Lamar
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