Charlie,

I did get some info from Paul Lamar on the rotary list, including some
graphs of torque peaks for the 2-rotor.

However, I just looked at some torque graphs of piston engines and even a
six-cylinder will go below the zero axis into negative territory -- although
the amplitude below zero is less than for a 4-cylinder piston.

Only when you get to an 8-cylinder piston does the wave stop going into
negative territory.

I'm not really clear from Paul's graph whether the 2-rotor goes negative or
not. I would think it does, since Paul describes it as similar to a
6-cylinder piston, in terms of torsional excitation.

It is clear of course that the torque pulses from the single-rotor will be
even more uneven and will go deeper into negative territory.

Still, as you pointed out, the cure to torsional excitation is to dampen it,
not to build a stronger transmission. It should also be noted that one way
to avoid torsional vibration is not to run the engine continuously at the
rpm where excitation occurs (as with the non-counterweighted Lycoming).

With a single-rotor engine, I would think that if you dampen the harmonics
at the offending rpm, you should not need a heavy gearbox designed for the
power of the bigger engine.

Also as far as poly-v belts are concerned, the perception is that they are
not as efficient as cog belts, but this just isn't so. A properly tensioned
poly-v belt is as efficient as a cog belt, between 95 and 98 percent.
(Goodyear and other belt makers have info on this on their websites).

This is as efficient as most gearboxes -- or even slightly better.

Also the system should be properly tensioned so there is no slippage at full
power. The only time slippage will occur is when torsional excitation causes
a big torque pulse. Of course, there is no reason to run the engine at that
rpm anyway (whatever it may be on a rotary, but probably below 2000 rpm).

As far as heat is concerned, both cog belts and poly-v belts generate
similar amounts of heat -- the two or three percent of power that is lost
goes to heat. So does a gearbox.

The belt manufacturers have made great strides in poly-v technology in
recent years. This is definitely not just a bunch of v-belts strung
together.

Regards,

Gordon.



"Charlie" wrote in message
.. .
Gordon Arnaut wrote:
Yes, the Egg redrive has no failures yet, from what I know.

However, he seems to have taken the "build it strong as hell" approach
and doesn't use any kind of damping, such as elastomers, sprag clutch,
etc. He is also using a heavy flywheel that helps to smooth out the
torque spikes.

The result is quite a heavy unit. Still his FWF package is competitive
with Lyc on a power-to-weight basis. Not bad at all.


Regards,

Gordon.


"Corky Scott" wrote in message
...

On Thu, 28 Jul 2005 22:08:10 -0400, "Gordon Arnaut"
wrote:


Your story is just another reminder that gearboxes are one of the big
bugaboos of any auto engine conversion -- and torsional vibration (or
resonance) is always the culprit. I know that in the Subaru community
there
is not really a box that I would consider completely trustworthy.

Really? Not even Eggenfellner's? I haven't heard of any failures of
his design yet, but I haven't been actively following Subaru
conversions.

Corky Scott



Sorry for coming in a little late on this; I usually frequent the
Flyrotary list & Rusty mentioned that this thread was alive over here. The
non-existent email address is there because I got tired of a steady diet
of spam.

Several things come to mind about the previous few messages in this
thread, from the stuff I've read in about 10 years of following
Powersport, then Tracy Crook's development trials & tribulations. This is
from memory & I never claim to have a good memory. :-)

Gearbox strength for 1rotor vs 2rotor: The big deal about a 1rotor is that
the torque curve actually reverses (goes negative) with a 1 rotor, like a
4cyl 4stroke piston engine. With a 2 rotor, the torque curve never
actually reverses so the gear box isn't stressed as much in the torsional
resonance dept. even though there's twice the power. If you frequent Paul
Lamar's list I'm sure he will be happy to show you the torque curve for
the 2rotor. IIRC, the torque curve for a 1rotor looks like a 4cyl piston
engine, going negative between each positive torque peak. If the system
resonates & you continue to excite it without damping the resonance, no
amount of strength will keep it from breaking.

The 1st incarnation of Powersport are the guys in the northwest with the
rotary powered RV-4 that had such horrendous torsional resonance problems
*on a dyno*. Current thinking is that they had a problem with resonance on
that particular dyno with that particular engine/dyno coupling (it was
built to test V-8's) They also had severe problems getting their P-port
engine to idle properly. Others have had no problem at all getting them to
idle smoothly. The developers had racing V-8 backgrounds & some of that
stuff doesn't transfer well to the rotary. Their internal tooth ring gear,
designed to keep the gearbox 'tight', like Rusty mentioned, is very heavy,
very expensive, & if it isn't heavy enough will actually loosen up as rpm
comes up & the ring gear tries to stretch. Kind of self-defeating. The
'tight' vs 'loose' issue is really an issue of moving resonant frequency
above the operating rpm range or moving it below the operating range.
'Tight' moves it up; 'loose' moves it down. Manual transmission cars are
'loose', moving resonance below normal operating rpm. You've probably
experienced the automotive version of torsional resonance if you've put a
manual trans car in 2nd or 3rd & let the idling engine try to pull the
car. If the engine continues to run, the car will move forward in big
surges. That's the resonant frequency of the drive train. I don't remember
Powersport ever having a problem with broken props or gearboxes; my memory
is that they went straight from their dyno problems to the big internal
spur gear. They did have a gearbox failure when competing in time-to-climb
at SNF because they were using nitrous & over stressed a bearing in the
gearbox. I think they were producing somewhere between 350-400hp (13B
without turbo) when that happened.

Damping torque pulses with belt slippage: inefficient & produces a lot of
heat.

I think Corky mentioned the nightmare of an intake manifold on RX-7 13B's;
fortunately a much simpler & lighter intake works fine for aircraft since
low rpm torque isn't needed.

Eggenfellner: I believe they've recently had the 1st failure of one of
their gearboxes.

Charlie
(Rusty's 'hangar away from home' for the next hurricane)