# 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.