Paul,
Thanks for the gear box/speed reducer pics - Interesting - A
few questions -
1 - Good lifetime on the needle bearings ??
2 - Looked like uncaged bearings ??
3 - Presume the 10 lb heavier unit used some other type of
bearings ??
4 - What provisions for lubrication ??
5 - The drawings have all the appearance of a multi-thousand
dollar product - Price range ??
6 - Cooling problems - I would think that there might be some
with uncaged needles ??
7 - Doesn't look like a particularly difficult project from a
machining perspective -
8 - Presuming outer housings are aluminum castings - Patterns
exist ??
Roland Friestad
You must keep the RPM on the needle bearings down.
Particularly on the thrust needle bearings. Those tiny needles are
really zinging around in there. We have a lot of experience on this.
Some people have worn them out. That is the reason for three back to
back. The RPM on any one is one third of the total.
The shaft, surprisingly, does not get any load on the bearings
until the aircraft stalls or spins. Characteristic of planetary gear
sets as all side load is canceled out. Hard to imagine but true.
Those give no problem and they are much lighter, as is the housing,
if they were ball or tapered rollers. Win win situation not to
mention much lower cost all around.
Note the extended point set screws are better and easier to
machine than splines. All back lash is eliminated.
I have an account set up with a local wholesaler of tranny
parts. I make no money on them I just charge cost plus shipping. The
3.17 gear set is just under $200.
The box can be built for under 5 or 6 hundred in parts. It all
depends on what the NC shop charges.
Shares synthetic oil with the engine. BTW the oil stays clean
in a rotary. Uses the engine oil cooler.
Here is some more stuff on the subject from 16 years exp. :)
There will be more on the rubber coupling with the engine.
Paul Lamar
Paul and Roland,
Glad to see motion on this project. Plenty of people would be
happy to pay $2-4K for a competently done redrive in the 100-200 hp
range. The extra cost is for support -- good engineering and
documentation so that people can figure out where their problems are
and how to fix them. The only thing worse than being on a schedule
to deliver things is being on the hook for phone support when stuff
breaks. The planetary system and bearings are entirely known
technology; where there isn't a proven system, and a lot of trouble
remains, is in handling of torsional vibration.
Rotary engine power pulses will be at a higher frequency, which
makes some problems easier and takes some solutions off the table.
It would be worthwhile to make aany new design applicable to a range
of powerplants, not just rotary.
At some engine speed, the frequency of its power pulses will
resonate with the spring constant of its drive shaft, gearbox,
propeller shaft and the inertia of the rotating prop. At this
frequency, each engine pulse meets the reflection of the previous
pulse delayed through the compliance of the system, and causes a
huge increase in torsion throughout (at other frequencies the
reflected pulse doesn't exactly meet the outgoing pulse and
therefore doesn't reinforce.) The engine is pushing one way while
the inertia is pushing back the other way, doubling the torque in
the shaft at the first pulse. The peak torsion can exceed 20 times
each individual torque peak because each successive pulse adds more
energy to the spring formed by the shaft. The key to a successful
redrive design will be to know exactly what's going on and build
something that will never get to a destructively resonant condition.
There are many methods of achieving that, but you won't know you're
there until you have modeled the system mathematically and proved
that your model is accurate by testing.
We need to: (1) design the compliance of the system so that the
resonance, with all values of propeller inertia, lies outside the
ground and flight operating range of the engine and (2) if the
resonant point is reachable in normal operations, include something
to break the resonance (lower the Q of the system) when excursions
exceed a safe value. In normal flight operation the resonance
breaker should not deflect at all. Ross used the springs of a clutch
plate with limited success. Paul has suggested a coupling built with
pieces of silicone rubber, it looks good but as far as I know has
not been tested. Tracy had his own design and wrote clearly about
the design process but without sharing any of the math. There are
commercially available rubber giubo disks used in many European
cars. Tracy called this the "BMW seduction" but it's also available
from Centaflex and commonly used to solve the same problem in
industrial and marine drives. Vassily at Airtrikes successfully uses
Mercedes giubos with his spur gear redrives.
Rotax and Geared Drives (Bud Warren design) use clutches --
Rotax to decouple when torque gets too high, Geared to not engage
until engine speed exceeds the resonant point. In every case you
need to establish how long something will last in worst case
conditions (operating at the critical resonance point) before
something breaks.
I don't use the terms "damper" or "harmonic" because there is
no damping going on, and this is mostly at the fundamental
frequency, not a harmonic. Damping of a resonance requires
resistance, in which the excess energy is dissipated as heat, and
that's not happening here. Any of these devices would burn up in an
instant if they were actually absorbing any significant part of the
engine's power. And "dampen" means to make wet, not attenuate, so
it's not a "dampener." An elastomeric member or spring breaks a
resonance by compressing, shifting the resonant frequency
momentarily so the resonant point is no longer where it was.
My background is in EE and acoustics. I design and make
acoustic transducers for a living, so resonance, damping and
compliance are a little bit familiar. I have studied redrive design
for some time (I am still looking for an engine/redrive that will
work for a low speed propeller design project) and have met a few of
the problems, but know only that I am certainly not an expert in any
of this -- but I can now tell when "expert" opinions aren't
supported by physics.
David Josephson
Let me know what you come up with. A lot of people have been
fooling with this for a long time. Gear and spline back lash
probably enters in to it and that changes over time. So too rotor
bearing clearance. Two nine pound rotors have a significant effect
on the polar moment of inertia. Prop weight also varies. Beware of
the rotary's high RPM when looking at open rubber couplings. I think
that had something to do with the RotoMax crash.
Then there is the question of shock load on the gears. Seems
like a bit of rubber is called for there. Some Brit engineer said
"An ounce of rubber is worth a pounds worth of engineering :)
Silicone rubber is tough and high temperature. Ideal for this use.
It seems no matter what we do the resonance happens at 1800
engine RPM.
No problem idling the rotary engine above 1800 RPM as the prop
is only going 600 RPM roughly. I was rather surprised to find the
same resonance RPM on the RotoMax engine with an entirely different
gear box, engine design and coupling.
When I shut off the O-470 in my Cessna 182 the whole airplane
shutters as it passes through resonance RPM.
Paul Lamar
I'd make provisions for a hydraulically operated prop and
governor pad if I was to do a new PSRU product.....make it an
option-----even Geared Drives is going to make a 0 offset geared
unit now, with their clutch and their hydraulic prop and gov...........
Marc W.
I knew you were going to say that Marc :)
In order to use a hyd prop the whole design must be changed to
pressure
fed plain bearings. The vast majority don't need a 50 pound hyd
prop. The power to weight of the rotary airplane is so good they
leap off short runways :) Cost is also one of the negative issues.
Paul Lamar
There are also a few electrically actuated CS props available. MT
makes certified versions with composite blades.
http://www.mt-propeller.com/en/entw/pro_elec.htm
There is no need to re-invent the wheel to add a hyd gov unit. The
MT composite props are so light that the biggest complaint is the
aft moving CG. I fly a Decathlon with an MT composite prop, and the
biggest downside I can see is the aft CG, and the low inertia.
Removing the 60+ lb Hartzell and replacing it with the 40 lb MT made
a noticeable difference especially when flying solo from the front
seat. A friend with an RV4 with an IO 360 recently replaced his
aluminum prop with a fixed MT. Overall he is happy, but his CG moved
aft enough that a 200 lb passenger makes his plane out of CG. Also,
when you pull the prop back it stops almost instantly and will even
stop windmilling at best glide speed, requiring a dive to get it
spinning again. The plus side is instant acceleration. The low
inertia allows the engine to go from idle to WOT almost as fast as
you can push in the throttle.
Another plus is that if you damage a blade, they will send you a
single blade so you dont have to replace the whole prop. In the
event of a prop strike, the blade pretty much shear off. You still
have to tear down the engine, but most times engine damage is minimal.
Overall, I like these props.
Kevin Alderman
Hi, I am new to the group, and thanks for my being allowed to
participate. Is the gearbox design that is being talked about in
this e-mail feed this one, by Paul Lamar,
http://www.rotaryeng.net/High-tech-complete-PSRU.jpg.
Thanks, Steve Carlisle
In theory that is lighter than the conventional and traditional prop
flange and shaft.
The drawback is it takes a high HP NC lathe and the prop shaft is
made from very
expensive and very costly 4340 solid billet bar. LOTS of waste
chips :)
Paul Lamar
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