# How many people are interested in a GOOD safe psru for the rotary?

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#### daveklingler

##### Well-Known Member
Sorry, I don't have any magic for you on single rotors Wankels. They would be pretty heavy for the power produced. Several reasons:

Really big counterweights are necessary;
Firing order vibration would be huge, requiring one or both of, torsional pendulums and very long stroke "soft" isolation system.

With only one rotor, the entire eccentric mass of the shaft and rotor has to be balanced with counter weights. With two or more rotors, the primary (up and down) imbalance is taken care of by having the eccentrics evenly distributed, and the counterweights only have to balance the pitching moments. The more rotors, the smaller this whole effect is and less counterweight size needed.

The firing frequency on a single rotor is one per rev and really strong. Torsional pendulums would be useful, but they would have to be really big to tame that low a firing order. A soft system would require pretty low spring rates, which means a lot of travel and volume occupied by the spring system. You might need both...

In the end, Wankel single rotor engines will be heavier and more complex than two rotor engines of the same power.

Billski
I think some clarification might be in order, here.

My airplane, a Q200, originally had a single-rotor Wankel that weighed, firewall-forward, right at 200 lbs with a Ross PSRU and aluminum end housings, wet, with a wooden prop. The builder wrote extensively about this airplane in Kitplanes during its development and did a nice job on all of it, though not without a learning curve. He put the output, naturally-aspirated, at around 130 HP. He has stated to me that he plans to use another Mazda engine in his next airplane, which will be his seventh build, having been very impressed with the end results on airplane number six.

Unfortunately, someone else made an offer on the engine/PSRU separately and he sold it a few hours before I contacted him about the plane. I'd like to duplicate the previous engine in rough numbers, with an aluminum single-rotor and a similar PSRU.

Single-rotor 13Bs don't have as good a HP:weight ratio as dual-rotors, but they're still very good, and excellent for aviation. Using aluminum end housings saves a lot of weight, as well, but they're expensive at around $2000 each. #### imacfii ##### Member I think some clarification might be in order, here. My airplane, a Q200, originally had a single-rotor Wankel that weighed, firewall-forward, right at 200 lbs with a Ross PSRU and aluminum end housings, wet, with a wooden prop. The builder wrote extensively about this airplane in Kitplanes during its development and did a nice job on all of it, though not without a learning curve. He put the output, naturally-aspirated, at around 130 HP. He has stated to me that he plans to use another Mazda engine in his next airplane, which will be his seventh build, having been very impressed with the end results on airplane number six. Unfortunately, someone else made an offer on the engine/PSRU separately and he sold it a few hours before I contacted him about the plane. I'd like to duplicate the previous engine in rough numbers, with an aluminum single-rotor and a similar PSRU. Single-rotor 13Bs don't have as good a HP:weight ratio as dual-rotors, but they're still very good, and excellent for aviation. Using aluminum end housings saves a lot of weight, as well, but they're expensive at around$2000 each.
you might try Paul Lamar's website http://www.rotaryeng.net/
also
there are a few planetary reduction drives being developed down in Australia and NZ,

#### aeromomentum

##### Well-Known Member
While I have no direct experience with PSRUs on Wankel engines I would guess the intrinsically low moment of inertia is the main issue. Add a high moment of inertia flywheel and low moment of inertia propeller and most gearbox issues will decrease. Since the rotors only turn at 1/3 the shaft speed they have less effective moment of inertia on the output shaft turning at 3 times that speed. The actual output shaft has very little moment of inertia.

Currently we are very busy with a few projects so we are not planing on entering the Wankel market directly. We have our 2 gear PSRU that reverses the direction, 2.588 ratio and has a 3" offset. We also have a new large offset 3 gear PSRU that has a 9.8" offset that keeps the prop rotation the same direction. This PSRU has a 2.435 ratio and is designed to be about 58% stronger.

#### rv7charlie

##### Well-Known Member
It's good to hear that you have a 3-gear drive. Any pics, etc on the website? Weight? Price? Is the rubber donut 'damper' built in? Any way to roughly estimate the 2-per-rev engine rpm resonant point, with such limited info?

With the offset, it's not going to help the guys currently flying (most are flying planetaries, like the RWS box), but new builders who haven't built their motor mounts could compensate. I would consider the ratio almost perfect for for a rotary, putting engine max rpm at around 6500. Most of the guys actually flying 13B & Renesis engines aren't under any illusion that they can be run reliably at more than ~90 HP/rotor, continuous, so 6500 is plenty of rpm.

To the earlier post about rotaryeng.net: there is some useful info there, but bring your salt shaker (or maybe a case of it). If you don't have the background to sort the wheat from the chaff, tread cautiously.

Dave,
While I'm committed to a Renesis for my RV-7 project (already hung on the firewall), I'd never even attempt to cut a 13B down to one rotor. It's just too easy to get 100 HP out of many small watercraft/snowmobile/etc engines these days, and save at least 50 lbs, even if the rotary has aluminum housings. A 1-rotor must have a balance weight that weighs as much as the rotor, so there's very little weight savings. If you're irrevocably committed to a 13B based single rotor, have you contacted Richard Sohn? He's almost certainly the guy with the most knowledge (and raw intellect) that you're likely to find.

Charlie

#### wsimpso1

##### Super Moderator
Staff member
Log Member
While I have no direct experience with PSRUs on Wankel engines I would guess the intrinsically low moment of inertia is the main issue. Add a high moment of inertia flywheel and low moment of inertia propeller and most gearbox issues will decrease. Since the rotors only turn at 1/3 the shaft speed they have less effective moment of inertia on the output shaft turning at 3 times that speed. The actual output shaft has very little moment of inertia.
Listen to Mark. Inertia of the prop and crankshaft (eccentric shaft) along with the spring rate of the isolator are the primary elements in determining resonance rpm in all PSRU. Ross at sdsefi.com wrote about how much he changed the character of his EJ22 powered RV6 by adding a little engine side inertia.

Reflected inertia is an interesting thing as it depends on the gearing of the element and which rotating shaft is your frame of reference. Since most of us watch engine rpm, the crank flange (eccentric flange) is usually the reference point and the point where the soft element is inserted to bring resonance below idle.

If something seems heavy, but has low gearing, its inertia gets a LOT lower as the reflected inertia goes with inertia times speed ratio squared. The rotors on a Wankel are kind of heavy looking, and between their weight, "diameter", and the eccentricity, you might expect a lot of inertia at the crank flange. But their inertia as measured at the output flange is MMOI*(1/3)^2 = MMOI/9. That takes some of the sting out of their weight.

Even a fixed pitch prop for a 200 hp Wankel turning the prop at 2700 rpm is a pretty big inertia, but its inertia is divided by something like 2.4^2, so it does not seem quite so huge here.

Building a "stiff" PSRU will most likely benefit from low inertia at the engine and the prop. Building a "soft" PSRU might need those cast iron rotors and a nice thick flywheel. Once you run it find your resonance rpm, you might be able figure out how much of that flywheel can be removed on a lathe and still have enough isolation to min operation speed.

This is one of the reasons why I feel we should be asking PSRU sellers what combinations of engine side and prop side inertia are OK with their boxes and isolators...

Billski

#### GESchwarz

##### Well-Known Member
I’m looking to buy a PSRU right now.

#### micah.powell

##### New Member
While I have no direct experience with PSRUs on Wankel engines I would guess the intrinsically low moment of inertia is the main issue. Add a high moment of inertia flywheel and low moment of inertia propeller and most gearbox issues will decrease. Since the rotors only turn at 1/3 the shaft speed they have less effective moment of inertia on the output shaft turning at 3 times that speed. The actual output shaft has very little moment of inertia.

Currently we are very busy with a few projects so we are not planing on entering the Wankel market directly. We have our 2 gear PSRU that reverses the direction, 2.588 ratio and has a 3" offset. We also have a new large offset 3 gear PSRU that has a 9.8" offset that keeps the prop rotation the same direction. This PSRU has a 2.435 ratio and is designed to be about 58% stronger.
Hey Mark, I'm not an engineer at all so this may be a dumb question. What are your thoughts on using a dual mass flywheel with centrifugal pendulum vibration absorbers to 1) add mass moment of inertia on the engine side as you said and 2) dampen/absorb torsional vibration?
I am unsure about the effectiveness of a DMF/CPVA at higher RPMs since they have mostly been used to help isolate the torsional vibrations of a 3rd order torque impulse signature of 3 cylinder engines at lower revs.
But do you think a combination of a DMF with one of your gearboxes with a low MMOI propellor could be a reliable unit?

I like the sound of your 3 gear gearbox with the 9.8" offset. From what I have seen from other rotary conversions, most builders are forced to custom build intake manifolds to be able to fit under cowling due to the zero or low offsets of their redrive. Therefore they give up the opportunity to utilize the many well tested manifold options out there for rotaries in the automotive world because of the height issue.

And while the lower ratio of that gearbox may not work well for a peripheral port rotary, if you eliminate the need to custom build an intake, then the builder has other porting options than just peripheral ports. I think a semi-peripheral port might be a great option as it reduces the intake/exhaust overlap and therefore brings your power curve to a slightly lower rpm while still providing an ample amount of airflow.

- Micah

#### rv7charlie

##### Well-Known Member
Hi Micah,

I don't want to 'drift' the thread too much, but there are good reasons to *not* use a stock Mazda intake manifold in an a/c. Happy to discuss further; here or in another thread.

Charlie

#### Jimb0

##### New Member
New member here hunting for the business end of a rotary mazda powerplant. As per the title of this thread I want to say "I am interested in a gearbox for the mazda 13b REW."

I will go back and read this entire thread but I wanted to bump the thread to the top, say hello, and ask for an update on a gearbox.

Thanks
James

#### Lendo

##### Well-Known Member
Jimb0
Speak to BillRSV4, he's the expert. Internal Spur gear up to 200hp +.
Is it Peripheral Ported? He's also the expert there as well.
Hope that helps.
George

micah.p
Pendulum Dampers are very good but expensive to manufacture, Powersport did one for the Rotary, but felt it too expensive for production.
If your equipped to make your own, might be the way to go.
George

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Hey Mark, I'm not an engineer at all so this may be a dumb question. What are your thoughts on using a dual mass flywheel with centrifugal pendulum vibration absorbers to 1) add mass moment of inertia on the engine side as you said and 2) dampen/absorb torsional vibration?
I am unsure about the effectiveness of a DMF/CPVA at higher RPMs since they have mostly been used to help isolate the torsional vibrations of a 3rd order torque impulse signature of 3 cylinder engines at lower revs.
But do you think a combination of a DMF with one of your gearboxes with a low MMOI propellor could be a reliable unit?
I understand that you asked for Mark's opinion, but if I may, I have professional experience in these very areas

Dual Mass Flywheels were invented to help with isolation of engine from transmission. The issue in automotive applications is that the soft spring in such a system is normally in the clutch hub, and thus downstream of the large inertia turning and oscillating with the crankshaft. Big mass - spring - little mass. Achieving good isolation requires very low spring rates and then a lot of spring to both have low rates and enough torque capacity. This can get expensive and take a lot of volume that simply is not there. An alternative is to split the engine side inertia and put the soft isolation spring between the inertia, effectively moving a bunch of inertia downstream without adding much mass or total inertia. It also makes a lot more volume available at the OD of the flywheel for packaging the springs. The large operating radius of the arc springs used in DMF also become subject to large hysteresis - lots of friction from centrifugal forces - and so their spring rates rise rapidly with rpm. This has two undesirable tails: Transmitting much of the firing pulses at high rpm to the downstream components, and; raising natural frequency of the system as rpm goes up. Also, the springs for these devices are typically designed for the engine and transmission combination, and they are arc springs for minimum drag in their tracks.

So, does this rather highly optimized for a car gadget work for an engine-PSRU? Depends upon what you are trying to do. The original post is about a "stiff" system. You keep the stiffness of everything way up and keep the inertia way down. You can not have any soft elements in this system. If instead, you are trying to run a "soft" system, your spring rate will rise quickly with engine speed, making your natural frequency go up with engine speed, and maybe faster than the firing frequency does. If the approach each other, the system will try to tear itself apart. It would have to be carefully checked out to make sure that you do not bring resonance in as rpm comes to flight levels, and then checked again to make sure that the transmitted firing pulses are within the long term capacity of the downstream parts to survive reliably. If your needs do not match the available pieces from the dealer's part counter, you then have to design new springs and see if you can get them made. I do not even want to think about how many thousands of dollars your set up cost will be for each spring you scheme out.

They might be made to work, but I would sure not want to try to make any money doing it for a little airplane PSRU.

Now to centrifugal pendulums. These gadgets have been around since the 1930's in big piston engines. Generically, they are order tuned absorbers. They were introduced in cars over a decade ago, with high end diesels being the first applications. They are tuned to specific number of oscillations per engine turn and for the fluid they are operating in. Air is different from automatic transmission oil. If you find a two-per-rev gadget for a manual transmission and your engine has two firing pulses per rev it could work. But if you swipe the two-per-rev pendulum damper from a torque converter, well, you will have to run it in ATF if you want it to pull off firing pulses. Everitt Hatch built a pendulum damper for the Powersport conversions, and it was expensive to make, so he backed out and came up with the stiff system being revived by the OP of this thread. Ev's pendulum damper appeared to have more than one tuned order (looked like three to me). Besides their being expensive, in airplanes they have had a tendency to be sensitive to rapid movements of the prop lever.

The OP is not working in either of these areas. And Mark Kettering has figured out where the sweet spots are for his engines, PSRU, and likely props are with his elastomeric spring isolators.

I like the sound of your 3 gear gearbox with the 9.8" offset. From what I have seen from other rotary conversions, most builders are forced to custom build intake manifolds to be able to fit under cowling due to the zero or low offsets of their redrive. Therefore they give up the opportunity to utilize the many well tested manifold options out there for rotaries in the automotive world because of the height issue.
True. Wankels have been sometimes dicey to make work for turning props. If I were Mark, I would stay away until I ran out of other engines to develop. He has some very good packages in the works. As to making room for appropriate intake manifolding, Ev Hatch laid his Mazda on its side for pacjaging of engine, manifolds, etc, and it worked fine. Too bad we lost that fine gentleman so early.

Billski

#### rv7charlie

##### Well-Known Member
Hi Billski,

Can you elaborate on "Wankels have been sometimes dicey to make work for turning props", when placed in the larger universe of making *any* engine work for turning props (meaning, when using a reduction drive, of course)? What do you see, in its 2-per-rev impulse, with ~270 degree duration and no negative-going excursion in the torque curve, makes it more dicey than, say, a 4 cyl 4 stroke engine?

Charlie

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Can you elaborate on "Wankels have been sometimes dicey to make work for turning props", when placed in the larger universe of making *any* engine work for turning props (meaning, when using a reduction drive, of course)? What do you see, in its 2-per-rev impulse, with ~270 degree duration and no negative-going excursion in the torque curve, makes it more dicey than, say, a 4 cyl 4 stroke engine?
When Mazda rotaries were first run on a water brake dyno by Ev Hatch, he blew out his U-Joints that worked fine on big dragster engines. They figured out that they had to do stuff differently for rotaries. Some folks had gearboxes that would only live a modest power settings. You will notice that neither Ross nor Tracey conversions run more than about 160 hp and spend most of their time down below 120 hp. These were isolator equipped systems. Ev Hatch built a series of engines in the 200-220 hp range with a series of isolators and gearing. When they worked on the stand for a few hours and were clean on inspection, Alan Tolle flew them in an RV3. The man has a lot of dead stick landings - they broke several in flight. Ev Hatch came up with a torsional pendulum damper for the engine. Light, worked great, costly to build. Before committing to that, Ev Hatch tried out a zero lash high stiffness scheme and it worked. Since it was reasonable on weight and cost, Ev Hatch was working with this when we lost him. Looked like a heart attack... This very box is what the OP of this thread is attempting to revive. Very low gear lash, high stiffness, and it ran well. I hope to see it return to the market.

The following thread discusses the big picture theory of forcing functions and vibration isolation as applied to airplanes:

Next, all this talk about the long firing pulses and no negative torque - it is just so much marketing. All engines make firing pulses. When turning things with inertia no higher than our props, they ALL speed up and slow down, with acceleration occurring around the highest torque and deceleration around the lowest torque. Even V-12's and those multiple row radials have torque and rotary speed that oscillates at firing rate and another string of pulses at twice that rate. It just is not something that disappears, although the amplitudes get smaller and the frequencies get higher.

So, with any engine speeding up and slowing down, we can either:
Accept it by running a stiff system with resonance taken at least half an octave above 2x max firing rate (conventional airplane engines do this) or;
Isolate it by running a soft system with the soft element between the engine and the rest of the downstream stuff, and resonance at least half an octave below min firing rate.

Actually, this is not the whole story on Wankels. I do not know anything about its harmonics. Piston engines have firing pulses, then they have another significant forcing function at 2x that frequency and about 1/3-1/4 the WOT firing pulses. Then there are bank-to-bank and cylinder-to-cylinder orders too. Wankels almost have to have something, but I know nothing about that part of the situation. Ev Hatch's pendulum damper appeared to have three different orders it was picking off. Four and six cylinder Lycomings and Continentals (models that have pendulum dampers) usually have pendulums tuned for two different orders as did the big radial engines.

Billski

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#### rv7charlie

##### Well-Known Member
I've heard the Hatch dyno story. (I've heard additional details, too, that kinda question the validity of their assumptions as to cause.)

As to the RWS drives: there are numerous copies that have run for years on 3-rotor 20Bs, one of which was P-ported (likely north of 300 HP) and flying on a Lancair ES. That drive went through some incredible abuse during the install's 'teething' stage, and the only real damage was to the aluminum adapter plate that carries the 'damper' rubbers, and, IIRC, the screws that secured the stationary part of the planetary set to the housing.

I do get it that all engines make pulses. But is it just 'marketing' that a V-8's pulses are easier to tame than a 4 cyl engine? Isn't it easier to tame even if rpm is adjusted to the same number of pulses per minute? Does the peak-to-average torque ratio matter at all?

You're certainly much more knowledgeable than most of us here (certainly than me) on this subject. So I'm hoping you can help me understand why the rotary's 2-per-rev is worse than a flat 4's 2-per-rev.

Thanks for helping me understand what's happening.

Charlie

#### wsimpso1

##### Super Moderator
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Log Member
I've heard the Hatch dyno story. (I've heard additional details, too, that kinda question the validity of their assumptions as to cause.)

As to the RWS drives: there are numerous copies that have run for years on 3-rotor 20Bs, one of which was P-ported (likely north of 300 HP) and flying on a Lancair ES. That drive went through some incredible abuse during the install's 'teething' stage, and the only real damage was to the aluminum adapter plate that carries the 'damper' rubbers, and, IIRC, the screws that secured the stationary part of the planetary set to the housing.

I do get it that all engines make pulses. But is it just 'marketing' that a V-8's pulses are easier to tame than a 4 cyl engine? Isn't it easier to tame even if rpm is adjusted to the same number of pulses per minute? Does the peak-to-average torque ratio matter at all?

You're certainly much more knowledgeable than most of us here (certainly than me) on this subject. So I'm hoping you can help me understand why the rotary's 2-per-rev is worse than a flat 4's 2-per-rev.

Thanks for helping me understand what's happening.

Charlie
I knew Ev Hatch. The "whys" of the issues were just out of his comprehension for a while. He may have mis-interpreted the sources of his problems, but in the end he cam up with two very successful solutions. He understood it all well enough to both scheme out a pendulum damper that worked and a stiff system that worked.

Pendulum dampers are one of those things that has to be pretty much right on the money for tuned order and be able to store enough energy so that it can work to settle down the engine after transients. Miss on order by 4% or short on energy storage, and your data looks the same as if it were not there. Want to make it suck up fiing pulses really well? Gotta hit it within about 1-2%. And a stiff system? All the vibrating torque of the output goes through the gears and is reacted in the bearings. Shortfalls here bite too. He got solutions.

It IS just marketing to tell people "And this engine never goes negative torque". They ALL have torsional vibe at firing order.

Is it easier to tame a V-8 over a I-4? Define easier:

To an engineer practiced in the art, they require the same amount of work. You must either be given transmitted vibe targets or come up with them yourself, come up with estimates of the rpm range, estimate the spring rate to isolate to required level of transmitted vibration, estimate the spring travel to absorb the firing accel and re-emit it during the compression decel, find the right package of how many and how big springs with all the rest of the hardware, then package it all in the space you have. If you come up short of space or high on cost, you have to go back to the bosses and get them to free some space or say we can compromise on the transmitted vibe somewhere;

To a person guessing at what they need to make it all work, one may succeed right away and the other may fail, requiring many cycles of changing design features before a solution is achieved.

A three rotor has 50% higher pulse train frequency at any given rpm, and less ripple. Is that easier? I can calculate the changes required of the springs and shafts and gear systems. The damper has two big design criteria. First is high enough torque capacity and second is low enough spring rate. Shafts and gear also must have high enough torque capacity. Assuming a successful 2 rotor, the three rotor has 1.5 times as much mean torque but a little less ripple, and the three rotor makes that ripple 1.5 times faster. So roughly 1.5 times as much torque and only allow 1.22 times the two rotor's spring rate. That is a substantial increase in total stored energy for the soft element - go short and the soft element will be overloaded/overheated/bottomed or otherwise have either short damper life or hammer the gears to death. Then the gearset has to have plenty of capacity to go 1.5 times the torque of the two rotor. If the existing system had that much headroom, terrific. If it did not, they might have to find some room and build stouter parts that store more energy.

Let's go through an example:

I have four of these rubber donuts in my two rotor (or I-4) and I know it is OK at 150 ft-lb but not at 180 ft-lb. My new three-rotor (or V-6) is 225 ft-lb, so I need six of the same donuts to handle the torque without turning them to powder. If this is tried and the spring rate of the donuts was just low enough for the two rotor, the new spring rate is 1.5 times as high and it can only afford to be 1.22 times as high - it will now not isolate well, and is likely to also overheat and turn them to dust, if the gears stay together long enough and the prop does not fly apart due to all the hammering. All three are possible and in a race to see which one dies first. So you need 150% more total capacity with a spring at less than 122% more spring rate, which means more energy storage in the springs. I hope you have space for bigger donuts? Same with coil springs or giubos...

Now maybe your only problem area is at the low rpm end of things. Could you solve it by speccing the idle speed up 25%? Well, let's see what the tails are: If you have the same prop, you will have 56% more idle thrust that makes taxi and approach more challenging, but that bigger engine is likely to need 50% more prop blade area, so that will actually mean 134% more idle thrust - Oh, nobody's gonna buy that... But that larger prop will have more inertia, so that will help a few percent on low rpm isolation. Well it is something,but not much. Then there is the fact that once you get to firing speed (while cranking), the engine does have to accelerate itself and that prop through a now higher resonant rpm before it can get to idle. I got news for you, if resonance rpm is higher, it takes longer to go through the resonance, giving the system time to be captured at resonance rpm where its vibration is growing without an upper bound and can break or fatigue things before it finally goes on up to normal idle speed. Yeah, you really do have to design the damper for your inertia at both ends and to give resonance neatly between firing rpm (during start) and nominal idle speed, so it can accelerate through it quickly.

And we have not even gotten into failure mode management yet. Any cylinder or rotor or maybe even bank of cylinders can stop firing, giving you additional large torsional inputs, but the system has to be secure against tearing itself apart for at least some time while you find an airport and put down, right?

Is one easier than another? Only if you are very lucky. To be assured of having a salable product at the end, each product has to be engineered. If one were willing to guess and play and set a cancel date and be willing to just cut losses, yeah, it might be easy. Most businesses do not like those kinds of gambles, but some might.

Billski

#### rv7charlie

##### Well-Known Member
I believe (hope, anyway...) that I've got a reasonable grasp of what you're saying, and none of it conflicts with my prior "layman's" grasp of the forces at work.

Where I kinda hit a wall is the never-ending assertion that the rotary is harder on the power transmission path than a piston engine; apparently driven almost entirely by the original Powersport's published (and 2nd/3rd hand) info. In conflict with that are the dozens of rotaries that have been flying for decades, using 'loose' (as in resonance below operating rpm) planetary gearboxes, and even a few cogbelt drives. Beyond *that* is the fact that the latest version of the RWS planetary that was marketed before the owner's retirement, used a slightly heavier flywheel (~9 lb aluminum racing wheel instead of the auto trans flex plate) and no isolation soft parts at all. The planetary input shaft bolts directly to the flywheel. RWS did limit prop inertia to wood props on the hard coupled drive, and as far as I know, it was never even tried with heavier props (or on a 3 rotor). The RWS 'test mule' RV4 is still flying with that drive configuration. The P-ported 3 rotor I mentioned earlier was running the same coupler plate & bushings as the 2 rotor engines, with an electric 3 blade MT prop that weighs a bit north of 40 lbs. The bushings were due for replacement by the time the engine came off the plane (owner died due to health issues & widow sold off the assets), but even the 2 rotor guys eventually have to replace them, as do many in the piston power world.
[edit: For clarity, the 3 rotor did eventually need to have the aluminum rivets (used to assemble the plate to the steel spline assy) replaced with A/N bolts.]

I wholeheartedly agree about losing a rotor; that's one thing that I consider a significant negative about the rotary. It actually happened to one of the early rotary guys, who for years and hundreds of hours had been using a stock RX-7 transmission running in 2nd gear, on an E-Racer. One day when he cranked the engine, for some reason that I can no longer recall, it started on only one rotor. At idle, it stripped the teeth off the gearset in the manual transmission.

No doubt the 'tight' gearbox they developed worked well. But as is hinted at above, development/production costs were/are a bit excessive.

I just wish that I could see some real, engineering-based, actually-tested, evidence that a properly running rotary is harder to make work (from a reduction drive standpoint) than typical 4 & 6 cyl engines. Now, cooling, intake design, *exhaust* design...Those really can be a bit tougher, because it really is a different animal in those areas. One or more of those three things are what causes most rotaries to be removed. Well, actually, installers' (potentially including me) inability to sort them out. But that's true for any alternative engine.

Here's another 'old hangar tale'. 'Rotaries are horribly inefficient'. I've flown two different flights from my home airport in MS to central TX with the RWS test mule. Both RV4s; mine with a 160 HP Lyc O-320 burning premium E-free mogas, and the test mule running a Renesis burning 87 octane E-mogas. I'm not shy about leaning a Lyc; at cruise power I'm not afraid to run as lean as I can get it and still run smooth. Both trips (same flight profile; basically a loose formation), the rotary burned about 10% more fuel, but it was about 30% cheaper to buy (and a lot more convenient to get).

I really appreciate you taking the time to show us the real world engineering drivers for problems with alt engines; please keep it up.

Charlie

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
I believe (hope, anyway...) that I've got a reasonable grasp of what you're saying, and none of it conflicts with my prior "layman's" grasp of the forces at work.

Where I kinda hit a wall is the never-ending assertion that the rotary is harder on the power transmission path than a piston engine; apparently driven almost entirely by the original Powersport's published (and 2nd/3rd hand) info. In conflict with that are the dozens of rotaries that have been flying for decades, using 'loose' (as in resonance below operating rpm) planetary gearboxes, and even a few cogbelt drives. Beyond *that* is the fact that the latest version of the RWS planetary that was marketed before the owner's retirement, used a slightly heavier flywheel (~9 lb aluminum racing wheel instead of the auto trans flex plate) and no isolation soft parts at all. The planetary input shaft bolts directly to the flywheel. RWS did limit prop inertia to wood props on the hard coupled drive, and as far as I know, it was never even tried with heavier props (or on a 3 rotor). The RWS 'test mule' RV4 is still flying with that drive configuration. The P-ported 3 rotor I mentioned earlier was running the same coupler plate & bushings as the 2 rotor engines, with an electric 3 blade MT prop that weighs a bit north of 40 lbs. The bushings were due for replacement by the time the engine came off the plane (owner died due to health issues & widow sold off the assets), but even the 2 rotor guys eventually have to replace them, as do many in the piston power world.
[edit: For clarity, the 3 rotor did eventually need to have the aluminum rivets (used to assemble the plate to the steel spline assy) replaced with A/N bolts.]

I wholeheartedly agree about losing a rotor; that's one thing that I consider a significant negative about the rotary. It actually happened to one of the early rotary guys, who for years and hundreds of hours had been using a stock RX-7 transmission running in 2nd gear, on an E-Racer. One day when he cranked the engine, for some reason that I can no longer recall, it started on only one rotor. At idle, it stripped the teeth off the gearset in the manual transmission.

No doubt the 'tight' gearbox they developed worked well. But as is hinted at above, development/production costs were/are a bit excessive.

I just wish that I could see some real, engineering-based, actually-tested, evidence that a properly running rotary is harder to make work (from a reduction drive standpoint) than typical 4 & 6 cyl engines. Now, cooling, intake design, *exhaust* design...Those really can be a bit tougher, because it really is a different animal in those areas. One or more of those three things are what causes most rotaries to be removed. Well, actually, installers' (potentially including me) inability to sort them out. But that's true for any alternative engine.

Here's another 'old hangar tale'. 'Rotaries are horribly inefficient'. I've flown two different flights from my home airport in MS to central TX with the RWS test mule. Both RV4s; mine with a 160 HP Lyc O-320 burning premium E-free mogas, and the test mule running a Renesis burning 87 octane E-mogas. I'm not shy about leaning a Lyc; at cruise power I'm not afraid to run as lean as I can get it and still run smooth. Both trips (same flight profile; basically a loose formation), the rotary burned about 10% more fuel, but it was about 30% cheaper to buy (and a lot more convenient to get).

I really appreciate you taking the time to show us the real world engineering drivers for problems with alt engines; please keep it up.

Charlie
Ev Hatch was a very experienced race engine guy, and was famous for building race winning engines that would last all season. He was B Production champion in Camaros, and had also been building Mazda drag cars with some awesome ET's. He was no stranger to making engines that made power and lasted.

The way I got it directly from Ev was that they started with a Ross gearbox - he did not set out to build gearboxes. He intended to sell engines in area of 200-220 hp because all of the other Mazda's were 160 and less. He talked about getting into the bigger hp niche. He started by installing the Ross box per instructions and promptly trashed it on his stand. Ev told me that was when he started visiting folks running other Mazdas. He found that they were usually running low power - engines that could make 160 hp, but running props pitched flat enough that they could make full rpm at modest throttle and thus running 100-120 hp. His engine ran fine on the Ross box at 120 hp, but he felt that niche was already filled, and he decided to go for 200+ hp and really use it there.

This info all says to me that he had trouble because he ran high prop pitch and high torque. When he started doing his own gearboxes, he also saw these high torque C-6 racing planetaries tearing up with his 180 ft-b and figured it had to be resonance.

The fact that folks are finding the RWS rubber donuts need regular replacement indicates something is seriously off. They are either seeing more torque than they should or they are seeing a lot more temperature than they should. Maybe both, as high torque cycling means heat build up in the donuts. Too much stroke for the piece and rubber chosen comes from either design or resonance, which is also design. Going back to how these things work, they are either undersize for the torque or see too much stroke or they are too close to resonance. Ends up destroying the rubber through fatigue and being cooked.

Along comes a guy with a three rotor - and it ran fine. Is that with regular donut replacement too or did he get past that? If that three-rotor ran the same isolators as the two rotors, do we think that they had that much excess torque capacity that the three rotor did not exceed it either? I do know this, if the three rotor with RWS damper was OK for peak torque of the three-rotor, it was running 50% higher firing frequency while also having higher engine inertia, which puts it nearly another octave above the resonance rpm for better isolation while it has less ripple to absorb in the donuts. It well could have been OK IFF the whole thing was overbuilt and the donuts would stand the direct torque ...

Ev told stories of efficiency too. Once they had the stiff system, they put one in an RV-6, and flew formation with a per plans Lycoming engined RV6 on flights, filling with fuel when they got back. Ev said that they used less fuel in the Powersport version on several flights where they took off together, climbed together, cruised together, descended together and landed together. At the time, he attributed it to a slick cowl and cooling system - looked like he had a turbine in the cowl with a single inlet below the prop and a modest outlet too - as he knew from dyno work the base engine was less fuel efficient on a BSFC basis than the Lycoming.

There are so many anecdotes and so little real data. Is it possible that Ev fooled himself into building his own gearboxes? I really doubt it. He told me he only got into gearboxes because the ones he could buy failed on his engines. He and Alan Tolle beat the S-51 in the time to climb drags and they placed high in the Sun 60 races a couple times when it was dominated by Lancairs and Glasairs. This after Alan flew the airplane all the way across the country diagonally. I gotta believe he knew what he was doing.

Billski

#### Billrsv4

##### Well-Known Member
Ev Hatch was a very experienced race engine guy, and was famous for building race winning engines that would last all season. He was B Production champion in Camaros, and had also been building Mazda drag cars with some awesome ET's. He was no stranger to making engines that made power and lasted.

The way I got it directly from Ev was that they started with a Ross gearbox - he did not set out to build gearboxes. He intended to sell engines in area of 200-220 hp because all of the other Mazda's were 160 and less. He talked about getting into the bigger hp niche. He started by installing the Ross box per instructions and promptly trashed it on his stand. Ev told me that was when he started visiting folks running other Mazdas. He found that they were usually running low power - engines that could make 160 hp, but running props pitched flat enough that they could make full rpm at modest throttle and thus running 100-120 hp. His engine ran fine on the Ross box at 120 hp, but he felt that niche was already filled, and he decided to go for 200+ hp and really use it there.

This info all says to me that he had trouble because he ran high prop pitch and high torque. When he started doing his own gearboxes, he also saw these high torque C-6 racing planetaries tearing up with his 180 ft-b and figured it had to be resonance.

The fact that folks are finding the RWS rubber donuts need regular replacement indicates something is seriously off. They are either seeing more torque than they should or they are seeing a lot more temperature than they should. Maybe both, as high torque cycling means heat build up in the donuts. Too much stroke for the piece and rubber chosen comes from either design or resonance, which is also design. Going back to how these things work, they are either undersize for the torque or see too much stroke or they are too close to resonance. Ends up destroying the rubber through fatigue and being cooked.

Along comes a guy with a three rotor - and it ran fine. Is that with regular donut replacement too or did he get past that? If that three-rotor ran the same isolators as the two rotors, do we think that they had that much excess torque capacity that the three rotor did not exceed it either? I do know this, if the three rotor with RWS damper was OK for peak torque of the three-rotor, it was running 50% higher firing frequency while also having higher engine inertia, which puts it nearly another octave above the resonance rpm for better isolation while it has less ripple to absorb in the donuts. It well could have been OK IFF the whole thing was overbuilt and the donuts would stand the direct torque ...

Ev told stories of efficiency too. Once they had the stiff system, they put one in an RV-6, and flew formation with a per plans Lycoming engined RV6 on flights, filling with fuel when they got back. Ev said that they used less fuel in the Powersport version on several flights where they took off together, climbed together, cruised together, descended together and landed together. At the time, he attributed it to a slick cowl and cooling system - looked like he had a turbine in the cowl with a single inlet below the prop and a modest outlet too - as he knew from dyno work the base engine was less fuel efficient on a BSFC basis than the Lycoming.

There are so many anecdotes and so little real data. Is it possible that Ev fooled himself into building his own gearboxes? I really doubt it. He told me he only got into gearboxes because the ones he could buy failed on his engines. He and Alan Tolle beat the S-51 in the time to climb drags and they placed high in the Sun 60 races a couple times when it was dominated by Lancairs and Glasairs. This after Alan flew the airplane all the way across the country diagonally. I gotta believe he knew what he was doing.

Billski
Billski,
These arguments are the biggest reason I haven't spent a bunch of money to get the gearbox into production. The unknowing anecdote repeater. Everything I mentioned in starting this thread that the original Powersport had TRULY TESTED the rotary and debugged it. Raytech had torsional vibration tests done by Steve winziril of E.P.S. As his master thesis. What I have to ask these guys is if Ev and Steve had a custom built planetary that would run at their power levels trouble free, why wouldn’t they have just started selling it? It takes a real gut check to start over and test a different system entirely when your focus is building engines not gearboxes.
T.O. Bill (OP)

##### Well-Known Member
A side note on soft dampers: they are dissipating power as heat.
They need airflow to survive above low power.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
A side note on soft dampers: they are dissipating power as heat.
They need airflow to survive above low power.
While it is true that rubber spring elements waste a fraction of the spring energy stored and released on each cycle, they can be designed to last thousands of hours with decent airflow over them. Evidence is engine mounts on things like, cars, trucks, and airplanes and giubos used as universal joints in many automotive powertrains. These gadgets are doing the same thing - deflecting to new equilibrium position under mean torque and then vibrating while isolating the engine vibration.

Rubber springs are poor thermal conductors, which can make the task vexing. One approach that greatly extends part life is simply to use elastomers compounded to stand higher temperatures while maintaining modest temperatures with decent airflow over them.

Billski