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
Paul
Is the hang-up here the prop reduction housing?? We cast
aluminum anvils, several hundred a year, 356 primary out of Dallas..50
plus lbs, green sand mold. I could machine the core die out of alum
have them pour, in production probably about $50 before machining,
vastly cheaper than billet..Is the design established enough to get this
going...?
jeff doddridge
What say you Roland. Are you going to build us some gear boxes?
We will need some sooner AND later :)
Jeff can do the housings. Share the risk.
Paul Lamar
Paul says -- What say you Roland. Are you going to build us some gear
boxes ? - Well, maybe - I need to do something for my own project - I
have purchased a Mazda Wankel (cheap) as a learning tool but won't pick
it up until mid November - I'm heading for a homebuilt aircraft meeting
in Pensacola and will pick up the Wankel in Birmingham on my way back to
Illinois - Three other things that come into play
1 - My Bridgeport CNC machines and lathes are up and running but the
MAZAK machining centers are in sort of live storage at the factory of a
friend and need to have wiring hooked up to them - Been unhooked for
about 5 years since health forced sale of my manufacturing business -
Just couldn't bear to sell them at the auction - Health now under
control and want to work on airplanes and machining for the rest of my
life, but selecting my own projects -
2 - I spent a long time making parts, dies and custom CNC machines for
clients and this project is a retirement thing - The last thing I want
to do is have someone "bugging" me for delivery with the "Gotta have it
by next Saturday" sort of thing -
3 - I built some special CNC machines for a world renowned Colt 45
custom gunsmith. He had a 3 year waiting list and required a signed
check without the amount filled in just to be on his list - I had
another client for whom I built a special machine used in making custom
pool cues - Have you ever seen a $30,000 pool cue? - I'm sort of in that
situation now - I don't need the work or the money but would like to
help, but only if I don't get hassled - I don't want to come off as an
arrogant prick or a jerk here, but at age 73 I am in a position and have
the attitude to tell just about anyone to take a hike, except my wife
who is too good a cook - On the other hand, I'm really a pretty nice
guy, usually -
Roland Friestad
In my 16 years with this newsletter the rotary guys are pretty much easy
to do biz with. I'll be your distributor and customer buffer :) You
might only sell one or two a year.
While you are in Pensacola look up Richard Sohn.
Not far away. Builds his own one rotor engines and has his own air strip.
Take 10 east to Mossy Head. Go North on 1087 for about 7 miles and
Richard is on the left. I think it is # 8029. Is that correct Richard.
Richard Sohn
<res12@fairpoint.net>
Paul Lamar
Roland,
Come on to Texas with us.
Easey going!
There is an empty lot at Silver Wings Airport, 4 acres, next door 3800
foot paved runway, secured fencing, etc. You and that good cook you have
and the good cook that I have, I agree a good cook is everything.
Low taxes, put a word in for you with the board
Milo
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