light twins?

"In the airplane, build an intake tuned to 6000rpm (give or take), build
a stainless steel muffler and your ready to go."

You made the point quite well that I was trying to make humorously. The true test of an engine is not whether it powers a car or not. The real issue, does it work well for a given application. If yes, then it is a good engine for that particular application. The rotary does have its applications, not just cars. And yes, it does work well where a constant rpm is desirable.

Using the analogy of "does it work well in a car" is parmount to rating UL engines by "does it work well in a snow mobile". Pretty darn absurd I would say.

"Gordon Arnaut" wrote

The advantage of the poly-v over a cogged (synchronous) belt is that they
naturally dampen torsional spikes, by means of slippage. Another plus is
that the pulleys can be considerably lighter.

Gordon

Get a clue. Compare efficencies between a cogged belt sized for 100 HP,
and the necessary number of v-belts for the same 100 HP. Next, take that
percentage of power wasted for each, and calculate how much heat will be
produced by each of these two systems.

Should be no problem for a math wiz like yourself.
--
Jim (barking, foaming mad) in NC

First rule of getting out of a hole. Quit digging.

Second rule for getting out of a hole. Leave.

Wow. It's the brain-dead moron who took the beating of his life on the wood
species thread and was too chicken to even respond.

Now he's back here, spitting up his usual crap.

This is a pattern with Moron from NC. He jumps in slinging personal attacks
completely unprovoked and yet he doesn't know the first thing about the
subject.

If I was to ask him what the efficiencies are of cogged and poly-v belts
would he be able to give an answer? Of course not. Just watch and see. Yet
here he is badmouthing a person in a most uncalled for manner.

What a pathetic loser. Gets his pathetic little rocks off by jumping on
these lists and acting out some kind of alter ego -- a poser, a
substance-free poser.

Regards,

Gordon.



"Morgans" wrote in message
...

"Gordon Arnaut" wrote

The advantage of the poly-v over a cogged (synchronous) belt is that they
naturally dampen torsional spikes, by means of slippage. Another plus is
that the pulleys can be considerably lighter.

Gordon

Get a clue. Compare efficencies between a cogged belt sized for 100 HP,
and the necessary number of v-belts for the same 100 HP. Next, take that
percentage of power wasted for each, and calculate how much heat will be
produced by each of these two systems.

Should be no problem for a math wiz like yourself.
--
Jim (barking, foaming mad) in NC

First rule of getting out of a hole. Quit digging.

Second rule for getting out of a hole. Leave.

Charlie,

Thanks for the insightful post. What exactly is the source of the torsional
vibration in the rotary? And how does it compare in severity to a piston
engine?

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)

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)

Gordon Arnaut wrote:
Charlie,

Thanks for the insightful post. What exactly is the source of the torsional
vibration in the rotary? And how does it compare in severity to a piston
engine?

Regards,

Gordon.


The source is the same as in a piston engine. The fuel/air mixture is
has to be compressed right before it is ignited. The ignited mixture
then only provides power for about 60 degrees of rotation.

The negative peek is very small in the two rotor for two reasons.
First, the 8 to 10 pound rotor plus very large cranshaft provides a
significant flywheel effect. And second, the negative pulse is slightly
overlapped by the postive pulse of the other rotor; eg. the pressure
graph is actually the result of the overlay of the graph from two
independant rotors.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Gordon Arnaut wrote:

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).

No. The only cure for torsional excitation is the move the system's
harmonic into a range that will only be seen in low power operations and
then won't be used for very long. Tracy's PSRU moved the excitation
range down to between 500 and 800 rpm (working off the top of my head
here, so the numbers may be off). A well tuned rotary will idle in the
900-1000rpm range (prop at about 1/3rd of that).

Dampening doesn't exist. Elasticity in the system may shorten the
peaks, but you'll be left with a fatter mountain. The energy has to go
somewhere. The problem is that you'll still be hitting the shaft at
it's resonant frequency, causing a vibration in it. Each hit adds a
little to the vibration. If you repeatedly hit anything at it's
resonant frequency, each hit will add to the system vibration until it
either wears out very quickly or comes apart catastrophically. That
applies to reduction units, crankshaft, and control surface skins (yes,
flutter is a form of resonant vibration). Adding some elasticity to the
shaft may make the gearbox last longer, but you won't be able to do much
in those few seconds 8*)

Here's someone who says it much better than I:

http://rotaryaviation.com/PSRU%20Zen%20Part%202.html


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Ernest,

You are right that springs and elastomers do not technically dampen kinetic
energy, they simply store and relase it at a later time (how much later
depends on the frequency at which it is tuned).

However, both springs and elastomers can achieve our objective of clipping
of destructive harmonics if they are tuned to the resonant frequency of the
object that we want to protect from resonance.

Also, fluid dampers do exist for both engines and propellers which actually
do dampen kinetic forces by turning them into heat, just like the shock
absorber on your car.

Drive belts do this as well, including the synchronous (cog) belts which
don't actually slip but which do absorb some energy as heat.

In fact, even a propeller can be tuned efectively to do this, because it is
kind of like a spring -- it flexes at its tips with the power pulses and
then whipsaws back during the lulls. A propeller thus tuned can do the same
thing as a harmonic balancer on an engine.

Of course the trick is in the tuning. More often we are concerned about the
opposite, where they propeller may in fact add to the excitations of the
crankshaft at its resonant frequency. We see this in non-counterweighted
Lycoming four-cylinder engines, which have operating restrictions in certain
rpm ranges.

So when we talk about torsional resonance issues, there are two aspects: one
is the engine itself, and specifically the crankshaft, which shoulders the
torsional forces. The other aspect is what the engine is connected to,
whether a propeller, a gearbox, or both.

We know of course that each object has a natural frequency at which it
resonates -- the point at which external excitations cause harmonics to
build. The object's mass, stiffness and damping determine how it will
respond to an excitation force.

Obviously having smoother excitations -- such as cylinders or rotors firing
at more frequent intervals -- will result in less extreme instantaneous
torque excursions, and the engine will run smoother. Dow that mean it will
be free of torsional vibration? No. Even an electric motor has torsional
vibration because the magnets arranged around its circumference which propel
the rotor are like firing pulses -- a motor with 12 magnets will be smoother
than a motor with two magnets.

However the engine can be as smooth as butter, but if the shaft on which it
is riding is very thin, it will still flex -- and at some point will reach
its resonant frequency, and if left there for the harmonics to build up --
will break.

So that's where mass and stiffness comes into play. Look at tuning forks,
the smaller ones will vibrate at a higher frequency. Even a small excitation
will cause a lot of flex in the thin tuning fork.

It's the same with crankshafts or other shafts -- such as those in a
gearbox, or a propeller -- which are exposed to torsional forces.

For example, most V-8 engines come with a harmonic balancer, even though
they have four power pulses for each crankshaft rotation. That's because
there is enough flex in the crankshaft that the crank can begin to resonate
at some rpm within the operational range.

But look at the four-cylinder Subaru opposed engine. It has never needed a
harmonic balancer. Why? It's crankshaft is quite massive and very stiff, so
its resonant frequency is below the engine's operating range. (It is stiff
because it has five main bearings, so each crank throw is supported by two
main bearings, one on each side; plus the crank journals are quite massive).

Although I'm not yet fully up to speed on the rotary, it is quite clear that
it is similar to the Subaru in that it does not need a torsional dampening
device. That e-shaft is quite rigid and it's throw is short enough that it
will not flex the way a crankshaft will flex. So more stiffness equals a
lower resonating frequency.

So why be concerned with torsional vibration? It's because we are putting a
gearbox and propeller on this engine. Just because the rotary does not
resonate itself does not mean it won't set a gear shaft into resonance.
That's the point of the damping at the engine-gearbox coupling.

When I asked what the source was of torsional issues with the rotary, I did
not have a full grasp of the dynamics with the rotary engine -- until I
watched some helpful animations.

However, as you can see, torsional vibration is not strictly a function of
power pulses. It is a function of the many things inside the engine --
including moments of inertia, stiffness of torque shafts, etc -- plus just
as many variable in whatever it is the engine may be connected to.

Regards,

Gordon.




"Ernest Christley" wrote in message
. com...
Gordon Arnaut wrote:

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).

No. The only cure for torsional excitation is the move the system's
harmonic into a range that will only be seen in low power operations and
then won't be used for very long. Tracy's PSRU moved the excitation range
down to between 500 and 800 rpm (working off the top of my head here, so
the numbers may be off). A well tuned rotary will idle in the 900-1000rpm
range (prop at about 1/3rd of that).

Dampening doesn't exist. Elasticity in the system may shorten the peaks,
but you'll be left with a fatter mountain. The energy has to go
somewhere. The problem is that you'll still be hitting the shaft at it's
resonant frequency, causing a vibration in it. Each hit adds a little to
the vibration. If you repeatedly hit anything at it's resonant frequency,
each hit will add to the system vibration until it either wears out very
quickly or comes apart catastrophically. That applies to reduction units,
crankshaft, and control surface skins (yes, flutter is a form of resonant
vibration). Adding some elasticity to the shaft may make the gearbox last
longer, but you won't be able to do much in those few seconds 8*)

Here's someone who says it much better than I:

http://rotaryaviation.com/PSRU%20Zen%20Part%202.html


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Gordon Arnaut wrote:
For example, most V-8 engines come with a harmonic balancer, even though
they have four power pulses for each crankshaft rotation. That's because
there is enough flex in the crankshaft that the crank can begin to resonate
at some rpm within the operational range.

Actually, V-8s have a harmonic balancer because they would otherwise
have a first order imbalance. The physics explanation is pretty long
and doesn't make a lot of sense anyway, but it's because the
crankpins are 90 degrees apart (inline fours don't have this
imbalance because the pins are in pairs 180 degrees apart, but they
have second order imbalance instead... that is what a pair of balance
shafts cures) and the mass on the ends of those crankpins (rods and
pistons) flinging around are at different distances along the
crankshaft. Also, the harmonic balancer on a V-8 is two weights, one
on each end of the crankshaft. A lot of people don't realize there
are two weights, not just the one on the front of the engine.

Harmonic dampers are a different animal. They will smooth out power
pulses on any engine configuration. Harmonic balancers have nothing
to do with power pulses and everything to do with complicated
vibration of large pieces of metal moving back and forth in different
directions and different places.

I think the terms balancer and damper are confused with each other
because they look almost the same- a big part attached to the front
of the crankshaft to make the engine smoother.

Gordon Arnaut wrote:
Ernest,

You are right that springs and elastomers do not technically dampen kinetic
energy, they simply store and relase it at a later time (how much later
depends on the frequency at which it is tuned).

However, both springs and elastomers can achieve our objective of clipping
of destructive harmonics if they are tuned to the resonant frequency of the
object that we want to protect from resonance.



You don't listen to or read the work of others, and you like to read
your own writing way to much.


http://rotaryaviation.com/PSRU%20Zen%20Part%202.html




--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."