# 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