Torsional Vibration on a long shaft?

Discussion in 'Mazda Rotary' started by RSD, Jun 1, 2019.

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  1. Jun 4, 2019 #41

    RSD

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    I don't know how back then, but the speed I'm planning on running it at is 800 fps tip speed.
     
  2. Jun 4, 2019 #42

    RSD

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    Essentially it works out at 162 hp per rotor at max power, and 125 hp per rotor in cruise - going with the all aluminium engine will help with the cooling, down here we have a company called PWR who make some really excellent cooling products including a water cooled oil cooler that I think are going to help with the cooling quite a bit.

    That's pretty much why the project suddenly went in the Allison direction - at the direction of the German Air Force - they didn't want the logistics of dealing with a different fuel.

    Will do.
     
  3. Jun 4, 2019 #43

    RSD

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    If I spin the fan faster than 3800 rpm the noise will get brain shattering due to the tip speed. 3800 rpm on a 37.5 inch fan is 0.7 mach - over that and the noise levels get deafening
     
  4. Jun 4, 2019 #44

    RSD

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    That sounds about right - a tip speed of 800 fps seems to be the best compromise between thrust and noise.
     
  5. Jun 4, 2019 #45

    RSD

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    Tracy Crook's "Aviator's Guide to Mazda Rotary Conversion" and "The Mazda Papers" just landed in my mailbox:)
     
  6. Jun 5, 2019 #46

    wsimpso1

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    It seems to me that the OP is also intending to push to high power and rpm levels relative to present experience.

    I have no doubt that the Crook design PSRU with a Mazda 13B works fine. The fact that the planetary used works fine in really big hp drag cars and diesel trucks should give you some idea of how overbuilt it had to be to operate as a stiff system in front of a 13B.

    Ev Hatch saw no market room for another 160 hp rotary - he saw market room in figuring out how to get to a bigger increment of power. Once he had his low lash gear set worked out, he and Alan Tolle flew his engines at that hp as a means of testing his equipment to make sure it would be fine with a more typical airplane dutycycle for that max power. Even the nitrous experiment that ran in the Sun-n-Fun time to climb event was aimed that way.

    SInce the original post is about getting way more power per rotor than Tracy Crook's scheme is intended for, I figured that the guys who were getting comparable HP per rotor and no gear/shaft/housing damage might be of interest...

    Ev Hatch was experienced at building and maintaining many engines of many types in hydroplanes and race cars. He built race engines that both ran all season and won races, then freshened them up over the winter. His rotary powerplant for airplanes came along only after building durable race engines based on various go-fast parts for Mazda's. He was also a champion road racer in Camaro's. He knew his way around making power and not breaking.

    When I first chatted with him, I was prepared to discuss firing frequencies and resonance with Ev as the student. I quickly found out that he understood resonance and isolation theory, having also designed and built a multiple order torsional pendulum that worked great for these engines. That device was just too pricey to make a marketable powerplant. So, Ev was familiar with his vibe problem.

    Ev Hatch was intent upon opening up his product envelope. He talked with me about how his experiments with relatively high inertia props were the means for checking if resonance was close at hand with his stiff system or if he had put resonance out of range. His thoughts were to establish if he had good margins on prop MMOI and rpm for his customers. He did not like the idea of saying "oh I never tried that" to his customers.

    No V-8 crank in what has been successfully made into a race car is a "wet noodle" - production crankshafts are typically designed to give torsional resonance above 2.5x firing order at intended redline. The designers know about this issue and it really does come into journal diameters and the like. Here is why: The two biggest torsional forcing functions in even fire engines come at firing order and at twice firing order. 4x and 8x rotation in V-8's, with 4x going with peak combustion chamber pressures, while 8x is simple from the pistons accelerating back and forth in the bores and thus independant of throttle position. If your primary frequency of the rotating parts is lower than 2x firing, it will fully resonate before you get to redline, and no crank will last doing that. So, production engines are designed to put resonance 2.5x or more above firing frequency at redline. That puts practical limits on how high some engines can be turned, and can drive changes from iron to steel cranks and lighter reciprocating parts - both drive k/m higher which is the root of higher resonant frequencies. Crankcase stiffness can also come into this, and is why high rev versions of some engines use more than two bolts on main bearings and have more serious webs, structural oil pans, etc. The crankshaft still has to be stiff enough to put resonance out of range high. So, no crank capable of serious rpm for the engine size is a "wet noodle".

    Furthermore, as a powertrain guy charged with confirming isolation of different engines from the rest of the powertrain at two major automakers, I can tell you that the crank is WAY too stiff to play much of a role in protecting anything else. It is just too stiff. Wankel eccentric shafts may look inherently more stiff, but the diameter and width of the eccentrics is set by rotor geometry. After that, the rest of the size and shape follow directly. I suppose someone could skinny down the eccentric shafts and flirt with resonance, but that inertia and weight save is tiny indeed compared to things like iron rotors and iron housings and the counterweights.

    As I said above, I am sure that the existing planetary drives work fine with normally aspirated 13B's. The OP is going WAY beyond that level of power both per rotor and for the whole engine as well as up-scale on rev's too. I think that the experience of folks who both had trouble and solved those troubles would serve as a marker for the potential issues he might see. I thought he should be so warned that it might become a challenge...

    Billski
     
  7. Jun 5, 2019 #47

    rv7charlie

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    Thanks Bill; I'm constantly learning from your posts. I hope you know that my 'wet noodle' comment was hyperbole.

    When I mentioned that the latest version of the RWS reduction didn't use a soft coupler, I didn't mean to imply that it became a stiff system; it's not. It does have a somewhat higher mass flywheel, but it still behaves as a 'soft' system, with resonance below idle. Lash in the planetary handles keeping resonance below idle. The 2.85-1 ratio drives, with 6 planetaries, have been run successfully on both supercharged (on an RV-10) and turbo'd 2 rotors (on an RV-6), and on at least one P-ported 20B 3 rotor on a Lancair ES (obviously should help a bit with TV issues). An electric MT 3 blade prop (around 30 lbs) on both the Lancair and the RV-10. The Lancair guy found some creative ways to break stuff, but the planetary gearset itself never gave any issues. Unfortunately, the Lancair is no longer flying with the 20B. Cancer took its builder (a friend), and the plane was parted out. I currently own both the gearbox and the MT prop.

    Charlie
     
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  8. Jun 5, 2019 #48

    RSD

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    Billski,

    In one of the earlier posts someone mentioned that a four rotor engine would create less problems than a two rotor engine - is that your view too? At the moment I'm waiting for measurements for a six rotor engine to see if we can fit that in which might mean the engine stays normally aspirated.

    Another question - this is a drawing of the original arrangement in the Fantrainer, do your think it best to connect the engine output shaft to the flexible coupling that is closest to the fan or directly connect it to the fan?
    upload_2019-6-5_19-21-12.png
     
  9. Jun 5, 2019 #49

    RSD

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    A quick update - we believe that an all-aluminium six rotor engine will fit.
     
  10. Jun 5, 2019 #50

    wsimpso1

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    I wish that I had been able to know Ev Hatch better. I did not agree with his design path, but it is hard to argue with success, and his scheme did work.

    The drive systems I have seen had several "pin in doughnut" connections between two discs, one attached to the E-shaft flange, one on the gearset input. This produces a rising spring rate and can be tuned as to preload and initial rate. The scheme can make for a compact system and can also make for a resonant frequency that is always safely below various vibe orders through the rpm range. Is this what Tracey Crook was doing? With a rising spring rate, this could be very compact, but still be spring isolated... Hhow did he change it more recently?

    The C6/E4OD/4R100/5R100/5R110 planetaries were large and intended for big torque, but they were always pretty well isolated from various engine vibe orders, first by open torque converters and then in later iterations by substantial spring isolators integrated into the converter clutch. I have a bunch of time on dynos with 5R110's, measuring and then analyzing torsional vibe. The planetaries had helical gears significant radial and axial lash. In automatic transmission use, the lash is only gone through during transitions between positive and negative torque. The lash accumulation through the various joint in the transmission is several degrees and so these transitions are usually heavily damped in the ECU to prevent hearing and feeling the crash as the lash comes out. Lash impact is almost religiously avoided in the business.

    Are you telling us that the isolation in Tracey's PSRU is achieved by opening and closing the lash of the gearset? At idle level power, we are a little over 1 degree of firing throw and I can follow the lash opening and closing and impact being low enough for survival of a heavily overbuilt unit. As rpm rises to flight speeds, the firing throw goes to more like 0.1 degrees while the torque reaction (from the prop) grows with the square of rpm. This will load up the planetary and all other torque carrying joints, reducing and probably eliminating lash opening while driving up the apparent spring rate.

    It would seem to this engineer that the system would still need compliant elements. The more recent coupling that Tracey developed, is it still an elastomeric doughnut but of higher stiffness? Perhaps with a small amount of lash at its joints to allow isolation at starting and idle? Did Tracey include anything that could be construed as a quill shaft in the system? This could add to the total compliance and help keep the vibe from amplifying.

    As for running three and four rotor systems, the firing orders go up and firing vibe amplitudes drop, making isolation easier, maybe even possible with quill shafts or other compliant elements but requiring more torque capacity. Are the 20B systems running the same PSRU's? Hmmm.

    Billski
     
  11. Jun 6, 2019 #51

    rv7charlie

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    I'm on the road for a few days. I'll post some pics when I get home showing the original coupler, and if I can find them, the updated rigid coupler.
    The original uses 4 ea 'rubber'~2" dia discs (donuts) on ~6" radius. They seem to start getting tired at around 500 hrs on the 13B/Renesis.

    The rigid switched from an auto trans flex plate to an aluminum racing flywheel (+~8 lbs), with roughly the same size input shaft.

    Same 6 pinion drive on 20B, but the P-port 20B guy had to beef up a lot of the hardware 'around' the planetary gearset. Never had any issues with the gearset itself, though.

    My layman's understanding is that (on the 13B rigid) lash handles keeping resonance below idle; iirc, it will start to 'rattle' below ~1200 engine rpm. Of course, there's minimal impulse strength at that rpm/prop load. Remember, torque never goes negative with 2 or more rotors, so the gears never see reversed torque at normal operating rpms.
    Edit: RE, quill shaft action: IIRC, at one point during discussion on the Flyrotary list, Tracy ok'd modifying the original box to using the original input shaft direct coupled to our choice of an aluminum or lightweight steel racing flywheel. So quill shaft style decoupling seems unlikely.
     
    Last edited: Jun 6, 2019
  12. Jun 6, 2019 #52

    wsimpso1

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    RSD and traded notes on this privately. I will share my responses as they may help the larger community understand the topic. Yes, I edited it a bit for clarity

    Let's define what I think will be your possible issues:

    Reliable power - The normally aspirated 13B rotor system can make roughly the same peak torque, depending upon intake and exhaust design, at anywhere from 6000 to about 11000 rpm. The higher the revs where it makes peak torque, the higher the horsepower. Peripheral ports and fuel injection will help. Given the 215HP that Ev Hatch got at 6500 rpm, you can probably get 550 HP at 8000 rpm from a normally aspirated four rotor. Is it going to be long lived cruising at 7250 rpm and 385 hp? I do not know. I know that road racers go over that all the time, but their average power on the course is more like 60% power - they have braking at closed throttle and cornering at modulated power, etc. An airplane engine needs to be reliable at continuous power. Boost to get 600-650 hp from a four rotor and maintain cruise power to high altitudes seems modest enough to be doable. One hint - Stainless exhaust works in piston engines, even turbocharged ones, but Ev Hatch found that he needed Inconel at 110 hp per rotor;

    Coolable power - Airplane duty cycle is a few minutes at 5% power, 5 minutes at 100% power, hours at 60-75% power. All of the parts go all the way to steady state temps and the amount of waste heat going into the rotor and housings and exhaust system per unit of time go with POWER being produced. We know from Tracy Crook that factory 13B housings are thought to be reliable and coolable at 80 hp per rotor. We also know from Ev Hatch that 110 hp per rotor with aluminum housings works too. I do not if 150 hp per rotor continuous can be adequately cooled or not. If there is more heat-in than the system can move out on a continuous basis at any given coolant temp, temps will go up until equilibrium is reached, the power side of the housings and rotors expand with the temps, and coolant temps will go up, with a limit around 250 F. I do not know what continuous power the aluminum housings will cool effectively;

    Torsional vibration issues - Each rotor makes 1 power pulse per turn of the eccentric shaft, plus an unknown (to me) number and timing of secondary accels and decels. That is the same as an even fire twin. 2 rotors is 2 per rev, and can be dealt with by either stiff systems (low lash, very sturdy everywhere, potentially heavy) or by soft systems (adequately low springiness between the engine and gearbox with adequately high flywheel and fan/prop inertia). Go to four rotors and you have four pulses per rev, with a lot of overlap between pulses. A four rotor will be similar to a V-8 and should be much easier to deal with than a 2 rotor. Torque fluctuations will be much smaller (due to pulse overlap) and half the time between them than in the two-rotor, making the pulses more easily dealt with. Going three or four rotor will make you much less sensitive to misfire issues too;

    Wankel engines have had more severe vibe issues than similar firing frequency and similar power piston engines. I do not know if the issues arise from the size of the firing pulse or if there are large secondary vibrations. I have looked in the literature and not found info on secondaries like we have with pistons. Ev Hatch designed a torsional pendulum for his two rotors, and if memory serves, he had pairs of rollers running at what looked like three or four different radii - he had tuned weights for these different orders of rotation, so he must have had some info on which orders to absorb with pendulums;

    Now a six rotor? Has anyone built one? Will the eccentric shaft tolerate the torque and firing frequencies? A normally aspirated six rotor could be a pretty mild build and make 650 hp at 8000 rpm, but turbo-normalizing may still be desired. You still have to cool it, but you are in the known range on reliable continuous power and cooling. Vibrationally you are at 6 per rev firing, which is V-12 territory for smooth, and you may be able to run with just the shaft designed as a quill shaft. Seems like a lot of weight to get there, but it should be on your list of possible solutions. Oh, and if six rotor is possible, any count of rotors less than that is doable too.

    If I were to be running the powerplant program for this bird (and I am NOT!) I would look closely at your gross weight, wing loading, field capabilities, and then evaluate just how much power you really ought to have in the bird. Maybe 400 hp will still leap from the runway, climb like a bat, and easily cruise near max structural cruise airspeeds. Then I would start with known engine territory for durable power and continuous power cooling on a per rotor basis and use as many as are needed, then check weights, fuel burns, sizing of the cooling system, etc. That can be made to work, but be aware you have the entrepreneur's risk here - there are payoffs but they come with extra work and risks. This will be your minimum fuss path to flight.

    Only after doing the whole analysis with a known secure power/cooling level (in aircraft duty cycle), I would look closer at increased power per rotor, evaluate it carefully for reliability and ability to cool it, and maybe bump down in rotor count, knowing that you are experimenting, knowing that you are likely to have additional problems to solve, and knowing how much weight benefit it gives you to do so.

    Now to layout in the airplane... The Allison had an offset gearbox to reduce output shaft speed from turbine rpms to PSRU input rpm, and offset the drive.

    The long shaft and flex joints can augment your vibration isolator. To use a long shaft arrangement within the current engine spaces, you would face the RFOB (rear face of block, where the bell housing and output shaft flange are) forward and would need a gearbox to offset the output shaft much as the Allison had.

    I use gearbox euphemistically - it could be gears, cogbelts, silent chain, etc. Making it all work at these power/rpm levels is a task all by itself. You then have the soft system or stiff system question to deal with here.

    If you have no isolator between engine and this offset gearbox, the gearbox and all of its internals would have to be capable of standing more than 100% of the firing pulses of the engine - there is always some amplification in stiff systems. Lash must be kept very small or eliminated (cog belts are nice for this).

    Alternatively, if you put in an isolator spring set between engine and offset gearbox, the box only has to carry engine torque plus whatever fluctuations that are passed through the isolator. And the shaft can augment the isolator for making a capable soft system.

    If you run the RFOB facing aft, you skip the offset gearbox and your shaft will be much shorter. You will still have the soft or stiff question.

    Soft will require a fully capable isolation spring set on the eccentric shaft output, and perhaps some additional inertia on one or both sides of the spring set. The shaft and flex joints will be short and may be of little use for isolation - lots of rotors does make shaft as isolator more possible, but you have to design the shaft to carry the torques and have adequately low spring rate to really isolate the PSRU from the engine. V-12 quill shafts between crank shaft and PSRU were less than a foot long and they had massive gears too, so maybe this scheme can work. Analysis will show;

    Stiff, well, with higher numbers of rotors, the fundamental frequency needed may be way higher than makes any sense to build to...

    Billski
     
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  13. Jun 6, 2019 #53

    D Hillberg

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    Check out the surplus turbine dealers out in the world, had a movie production Co. buy a ducted fan ground unit [Boeing T 50] why go piston with its power pulse issues most are about $20,000 price range (airliner APUs & GPUs)
     
  14. Jun 6, 2019 #54

    BBerson

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    Probably some surplus T53 or something from an old grounded Bell Huey.
     
  15. Jun 7, 2019 #55

    RSD

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    I want to avoid turbine due to the extra training tickets required, the plan is pretty much locked in as being an all aluminium six rotor developing 650 SHP. It probably won't be the quietest plane that ever flew...
     
  16. Jun 7, 2019 #56

    pictsidhe

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    If the power limit per rotor is a thermal one, coud it be turbo'ed to run direct drive rpms? That would also help with the godawful noise of a NA engine.
     
  17. Jun 7, 2019 #57

    BBerson

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  18. Jun 7, 2019 #58

    RSD

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    I think that bit got lost in translation somewhere - what actually happened was that Audi (which owned NSU - the makers of the Wankel engines) made a decision to stop manufactuing the Wankel motors so supply became a problem, and at the same time the Luftwaffe (who were funding the development) requested that they switch to Allison turbines as they had an oversupply of them and they didn't want the logistical hassle of needing mogas as well as avgas.
     
  19. Jun 7, 2019 #59

    RSD

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    Going direct drive would require a rework of the fan hub assembly as the PRSU is built into it and I don't want to go there as that is just more work, and I'm not sure that a turbo rotary would produce enough power at those sort of much lower revs - I don't have the knowledge of rotaries to know
     
  20. Jun 7, 2019 #60

    BBerson

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    Can you strip out a significant amount of military weight?
    What is the primary mission if not training?
     

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