Torsional Vibration on a long shaft?

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

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  1. Jun 2, 2019 #21

    RSD

    RSD

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    At times I get the feeling that I have taken on far more than I can chew given that I'm neither an aerospace or mechanical engineer...

    Who is that? Very few people have even heard of them so anyone who knows of them on this site would be a useful contact.

    At the moment I've done a lot of guestimates of what the change will mean, for the rotary engine I'm planning on using the aftermarket alloy end and centre plates which will bring the engine weight down very similar to the Allison's 114 kg, but then I have to add in radiators, coolant etc. I will get back some weight with the removal of the ejection seats and replacing them with carbon fibre motorsport seats.

    I'm certainly hoping to succeed with the Wankel to fan connection as I haven't really considered any alternatives. The performance of the planes that have the Allison + fan & duct is very satisfactory so hopefully I can make it work as it will be a sweet plane to fly if I do.
     
  2. Jun 2, 2019 #22

    rv7charlie

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    Would you mind some thoughts, from someone who's been around the rotary a/c community in the USA for a couple of decades?

    1st, most would say that Perry Mick's efforts were not really a ducted fan; they were ducted *propellers*. None of his props looked anything like what you'd typically see called a 'fan' in successful commercial a/c. (Some believe that was a factor in his relative poor performance using the ducted system.)

    2nd, you haven't mentioned what rpm the fan needs to run at. Assuming you're keeping the original duct/shroud, that should be be constrained by the existing design. I didn't see a name/model of the a/c you're working with, so no idea from that video how big the a/c is or the duct dia.

    3rd, will the 2.17/1 reduction get your fan at that rpm?

    Now, we start getting serious.

    4th, the 2.17 sounds like an early Tracy Crook/RWS drive, or something similar like the early Ross. Is it a RWS drive? If so, be aware that the input shaft thrust bearing is limited to around 6,000 engine rpm. So, is that fan designed to run at under 2800 rpm?

    Now, to get more serious.

    5th, Tracy (designer of the RWS drive and likely the best living authority on making a rotary fly), has said on multiple occasions (both in his writings and in person, at rotary gatherings I've hosted at my home airport) that the Mazda rotary is good for a max of around 90 *continuous* HP per rotor in an aviation application. I consider that a sensible 'real world' (pardon the pun) number. Even though the metal bits might be strong enough to handle the power, you can't cool the engine beyond that continuous power level. Even Mazda themselves have extracted a max of ~165 HP/rotor, in their 650 HP 4-rotor Lemans winner, and you're talking about the same HP from only 2 rotors (325 HP per rotor). And you must remember, a road racer's duty cycle is closer to 50% than an a/c's 100%. A 7-second dragster motor is not going to survive in a 7-hour a/c.

    6th, assuming your gearbox is a RWS or similar, at 650 HP, its life will be much shorter than the engine's (likely measured in minutes, or even seconds).

    I think rotaries are great engines for a/c; I'm installing a Renesis in my RV-7. But I'm realistic in my expectations, and I think you should be, as well. I don't like being a 'downer', but I'd hate to see more nonsense show up on the forums about a rotary 'causing another death', and more importantly, I'd hate for you to be that statistic.

    Charlie
     
    Last edited: Jun 2, 2019
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  3. Jun 2, 2019 #23

    BBerson

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    The turbine doesn't have radiator drag.
    I don't know any commercially successful fans powered by other than gas turbine, which have a huge power to weight ratio advantage.
    Electric might have a power advantage, but only for about 6 minutes.
     
  4. Jun 2, 2019 #24

    TFF

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    Fantrainer? My experience with Allisons is small helicopters. What the apples to oranges comparison is this. I work with the c20w mostly. 420 hp derated to 308 hp. 420 is only sea level. Realistically we only have a 308 hp motor that can be blown up easy at low altitudes. Lycoming or Continental at 300 hp is sea level unless you add a turbo. Does this thing need 650 hp or is that the non derated hp? If you need 650 hp, you better find a 800 hp engine. A turbine will scream it’s little heart out making power blow up if it has too. Internal combustion is falling on its face the minute it starts to climb without a turbo. Cooling a turbine is a non event unlike regular engines too. 650 but derated to 400, you may have a chance. An Allison output shaft is turning about 6000 rpm. That is a pretty fast drive shaft. My Alfa GTV6 has the driveshaft connected directly to engine; transaxle in back. You can feel the driveshaft spin. The Jetranger helicopters drive the tailrotor directly from the engine output shaft. It has a coupling at every hangar bearing to keep alignment good all the way to the tailrotor gearbox. It is only absorbing 20% of the horsepower. A driveshaft to support 400-650 hp load will be big. Did the fan have a gearbox or was it spinning 6000 rpm?
     
  5. Jun 2, 2019 #25

    wsimpso1

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    PSRU will drive a bigger fan. The bigger the fan, the larger the low speed and mid range thrust, and it can not really hurt you much until tips approach sonic speeds - look at how big fans are on jetliners. Direct drive means smaller fans, which use more runway to get off...

    Direct drive or psru - either one will take some serious engineering and test to do right and know it is right. You have, flex joints at each end, shaft critical speed and all of the torsional vibe orders from engine and fan to worry over either way.

    I understand the appeal of the Mazda, compact and all that. Also loud and reputed to be poor on fuel. Hot ticket could just be the big LS engine with a PSRU. Not sure if the reducer should be at the engine or the fan - it would require some work with hard numbers and careful analysis. Probably lighter at the fan, but the PSRU may choke the duct unless you really get serious about the flow around the PSRU while it is also going through the duct.

    But then some folks think there really is nothing to this stuff...

    Billski
     
  6. Jun 2, 2019 #26

    rv7charlie

    rv7charlie

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    I thought that this is for an existing a/c. The fan size is fixed by the a/c, unless a new duct, new (different length) gear legs, structure to support the duct *and* the tail, etc etc are planned.

    The rotary can be loud, but not all installations are. I can introduce you to a couple of Renesis installations that are quieter than Lycs on the same airframe. Real world (sorry) fuel consumption is about 10% higher than a Lyc at the same power levels, *assuming a properly leaned Lyc*. The way most people fly Lycs (rich of peak in cruise), the spread is going to be closer to 5%; possibly a wash. That 10% number is from me flying my properly leaned 160 HP RV-4 with Tracy Crook's Renesis powered RV-4 on a couple of different cross countries covering around 400 miles each time. I was burning *E-free premium* mogas; he was burning whatever the cheapest, lowest octane, E-laced mogas was available at the time. Dollars per mile were pretty close to equal. If I'd been burning avgas, his fuel cost per mile would have been significantly lower.

    RE "Your choice of a Mazda rotary though may get interesting. They are known to be more trouble to adequately isolate from powertrains than piston engines. The same fundamentals apply as for a four banger, but the Wankel can get interesting because it is more severe." : Do you have evidence if this being factual, save the now-infamous Powersport dynamometer coupling shaft incident? It's a 2 per rev motor, like a 4 cyl a/c engine, but because the power 'stroke' duration lasts more than 180 degrees, there's never a torque reversal on the E-shaft, unlike a 4 cyl engine. Tracy Crook has been running his Renesis directly coupled (no 'damping' mechanism) to his 2.85/1 reduction drive on his RV-4 for several years and several hundred hours. It's dependent solely on the lash in the planetary gearset to keep resonance below idle. Before his retirement, he had flown it enough to start marketing the new 'hard' coupler, with the stipulation that propeller MMI be limited to what's typically seen with wood props.

    Interesting note from the link below: "The first prototype (registration D-EATJ), powered by two EA871 [2]110 kW (150 hp) NSU Wankel engines, made its maiden flight on October 27, 1977. This engine installation proved troublesome, and the second prototype (D-EATI), fitted with a single 310 kW (420 shp) Allison 250-C20B turboshaft engine, flew on May 31, 1978."

    I found the a/c design of interest here:
    https://en.wikipedia.org/wiki/RFB_Fantrainer
    According to the Wik, the fan is 3' 11" diameter, 5 blade. HP ranged from 420 on the original up to 650. Should be relatively straightforward to estimate rpm required for a 5 blade, 4' dia prop to absorb that HP. Based on dia, an LS motor would probably be able to drive the fan directly while producing power in the 450 HP range, and be more reliable than a rotary *at that continuous power level*. I don't know, but would suspect that a 1970s era Allison 250 wasn't that much lighter than a 21st century LS motor.

    Charlie
     
  7. Jun 2, 2019 #27

    TFF

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    An Allison is about 200 lbs less than a IO-540. Essentially free passenger if you can afford the fuel.
     
  8. Jun 2, 2019 #28

    wsimpso1

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    The basis for my concern over torsional resonance with high output Mazda rotaries is several:

    There were all those dead stick landings Alan Tolle did with Ev Hatch's various iterations of the early Powersport reduction drives, all way over built and all beat to living heck before leaving Alan with the prop not connected to power. Ev Hatch was a smart and very experienced guy, and the fact that he had trouble, well, it says something.

    The guys at Dyken Clutch never did reveal any details when I asked them if they knew about Mazda rotaries, but their eyes did widen significantly, and they did indicate that they took way more work to get right than piston engine clutch dampers...

    Billski
     
    Last edited: Jun 2, 2019
  9. Jun 3, 2019 #29

    rv7charlie

    rv7charlie

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    I've read a lot about Powersport's issues, as well. But I've also read that they were pushing everything to the limit, including hanging heavy metal props on them, running nitrous, etc. It might be worth pointing out that they were hard core hot-rodders at heart. When you push the limits, you're likely to break stuff.

    Any chance that PS & the other the guys that had serious issues with the rotary, had been getting help from 4-per-rev, and the relative wet noodle of a crank when playing with V-8s, & got no help from the E-shaft when they switched to the rotary?

    Weigh the PS experiences against the dozens (hundreds?) of RWS & modified Ross drives that are flying successfully with hundreds to over a thousand of hours on them. There is absolutely nothing 'special' about the RWS damper/coupler. (It doesn't even exist in the most recent version of the drive.) If the problem is the engine, then the same problems should show up in other iterations that have the same configuration and in some cases, the same gearsets. Even the old Ross drives (made by Ross Sr.) seem to have held up just fine except for the fact that Ross failed to include a thrust bearing on the drive's input shaft, and the sun gear/input shaft would axially load the E-shaft, and destroy the rotary's internal thrust bearing in short order.

    Apologies to the OP for the massive thread drift...
     
  10. Jun 3, 2019 #30

    RSD

    RSD

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    I've got no problems with people's thoughts if they are well constructed and based on facts or first hand experience etc.

    I just took a look at the fan pictures on his website - he was only running three blades whereas I wouldn't call anything with less than five blades a Fan.

    Fan needs to run at 3800 rpm for maximum thrust. The aircraft is a Fantrainer 600, designed in Germany in the late 70's early 80's and probably a long way ahead of its time - https://en.wikipedia.org/wiki/RFB_Fantrainer Fan diameter is 1200mm / 37.5 inches

    Yes = 8250 RPM which is in the ballpark for a Mazda Rotary

    No, planetary gear set made by Dowty Rotol in the U.K. - http://dowty.com/

    Was never going to be a two rotor engine - looking at a turbo four rotor with the aftermarket aluminium end and center plates which are both lighter and aid heat transfer / cooling

    Should be OK as the Dowdy Rotol gearbox was designed specifically for this aircraft with the 650 hp Allison.

    Thanks - the only statistic I'm aiming to be is the first person to get one of these flying again.
     
  11. Jun 3, 2019 #31

    RSD

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    Yes - ex-Royal Thai Air Force Fantrainer 600 minus engine

    I'm not sure exactly what it needs, but they were sold with a 650 hp Allison C30B

    Has a 2.17:1 gearbox. The original design etc was for the fan to run at 3800 rpm, but then the various air forces who were interested in it wanted to fit the Allisons so the fan was running at a lower speed with the Allisons but the performance was still OK.
     
    Last edited: Jun 3, 2019
  12. Jun 3, 2019 #32

    RSD

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    Most of the modern ducted fan aircraft that have flown (Dreamer, UL-39 Albi, Airbus's e-thing) have a maximum fan tip speed of 0.7 mach regardless of fan size - that seems to be the sweet spot that everyone is finding for maximum thrust

    Not going to be quick and simple, but hopefully achievable...

    Loud if normally aspirated, quieter if turbo'd is my understanding. Fuel consumption similar to the Allison is what I'm expecting

    Possibly, but I'm not finding any power curves that have the power in the right range for any of the common PSRU's to spin the fan at 3800rpm, and direct drive isn't an option either as again the power is all wrong at 3800rpm

    The Fantrainer has the 2.17:1 PSRU at the fan.

    You are slowly educating me that it isn't that easy:)
     
  13. Jun 3, 2019 #33

    RSD

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    Definitely not going to that much trouble - will be keeping the existing duct, fan etc - only change might be to make some carbon fibre fan blades at some stage.

    I'm probably going to have to run Avgas as Mogas isn't readily available at a lot of airports here.

    I think that this will be a "time will tell thing"

    I think that something got lost in the translation with this article - the trouble with the NSU Wankel engines in this case was that NSU stopped making them which meant that they had to choose another engine. The Allisons weren't the engine of choice as far as the plane manufacturers were concerned, but the German Air Force wanted to use them and as they were funding the development...

    LS engine direct drive at 450 hp is spinning the fan too fast - 3800 rpm is optimal fan speed for maximum thrust.
     
  14. Jun 3, 2019 #34

    RSD

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    I suspect that Dowty Rotol will have over-engineered it knowing that it was air force applications, but only time will tell if it is going to work with a Mazda rotary.
     
  15. Jun 3, 2019 #35

    BBerson

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    I don't see how such a small fan can absorb 650hp at such low tip speed. ( 621feet per second)
    Normal props are 1000 feet per second.
     
  16. Jun 3, 2019 #36

    rv7charlie

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    RSD,

    Thanks for the update/reply; good to see you're somewhat more realistic in expectations, including choice of redrive at that HP level. And four rotors should make any TV issues easier to deal with. Given the somewhat limited details in the original post, I was fearful that you might be like some of the folks that participate on what I'd call the 'other' rotary a/c forum here in the USA. I've seen guys expecting to fly a 2 rotor 13B making that kind of HP (in some cases, on something like a Van's RV-x). Really scary to think about trying to do that. Even with 4 rotors, that's a lot of HP per rotor, with the understanding that you'll need at least 75% of that, continuously. I wonder if cooling might be a bigger hill to climb than the mechanical issues.

    On the NSU vs Allison issue, my bet is that there was never a chance that any well funded military would have ever bought the a/c with anything other than a turbine in it. Here in the USA, even Beech T-34s, that originally flew with avgas piston engines, got turbines decades ago in the military. It's just too painful to support two fuels, plus they have a real aversion to gasoline due to its higher volatility. You can bet that the only reason the early Predator style drones were purchased with Rotax gas burners was because there was no kerosene burner that could meet both the weight and fuel burn limits imposed by the a/c.

    Good luck with the project, and please keep us updated on your progress. (And thanks for not taking offense; I was just concerned for safety reasons.)

    Charlie
     
  17. Jun 3, 2019 #37

    pictsidhe

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    Turbocharged LS may work DD.
    GM make a supercharged LS that should be ripe for conversion.
    Keep to GM torque +~10% and it should be reliable. A turbo engine won't have the supercharger drive power loss.
     
  18. Jun 3, 2019 #38

    BBerson

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  19. Jun 3, 2019 #39

    Toobuilder

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    650 HP @ 3800 is 900 pound feet torque.

    Even a 500 inch LS will need a BMEP of 271 PSI to get there. That's not happening as a NA configuration.

    Since the plan is to turbo it anyway the DD LS is worth at least a trade study look.

    Back off on the 650 HP requirement or spin the fan a bit faster and things get easier on the engine side
     
    Last edited: Jun 3, 2019
  20. Jun 3, 2019 #40

    BBerson

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    I found the Q-Fan paper. The Q stands for "Quiet" so they were after 18dba quieter than GA aircraft (with high tip speed props). They proposed 3' feet diameter, 387hp reciprocating or rotary, 640fps tip speed/4060rpm.

    But 4500rpm and 750-800fps or more is optimal if not concerned with extreme quiet.
     

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