# What PSRU Can we Really Trust???

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#### plncraze

##### Well-Known Member
HBA Supporter
I would like to thank Billski for sticking his toe in the waters of reduction drive design for us. These waters are hot and frothy with lots of contradictory fish. His explanations help to understand why there are not more drives around. The more I read about designing one of these beasties the more reasonable in cost an aircraft engine (used) is and if I remember correctly an aircraft engine is what Billski is using in his own project!
I remember talking to a transmission engineer from one of The Big Three with a masters degree and lots of experience in design and management. He was an EAA member with a Bonanza. I told him he should design, build and sell plans for a reduction drive. His reply was not printable. The short version was "NO!!" He said something about possible failure modes and started getting very excited. And then he mantioned liability.
I work in a shop that builds "one-off" machinery for manufacturers and when they send out P.O.'s sometimes they don't like what they get back as the "completed" product. So there is another issue to deal with and it happens concurrently with design. Last week I was assembling a some parts and I mis-assembled two parts that could not be taken apart and the designer called out the wrong thread for another part. Two hours of assembly turned into a week of waiting and then we will try again.
If I remember correctly the reduction drive that Jim Stewert sold for his S-51 was $20,000 (approx) ten years ago and others that I have seen recently are at least$10,000. Some of these guys are charging $20,000 with the GM crate engine and the crate engine is probably$3,000 to $5,000. I would be curious to see what it would cost to get a timed out aircraft engine and overhaul it yourself. The cost might be the same and in less time. #### plncraze ##### Well-Known Member HBA Supporter The issue of using a reduction drive ,I believe, are compounded when you try to figure out how much horsepower you are really getting at the prop. You can dyno the engine but then you have to know the friction of the drive when it is at operating temp. I doubt many folks have actually tested their reduction drive to how horsepower the drive alone needs. #### PTAirco ##### Well-Known Member Assuming the "soft" were done with the metal parts, wouldn't fatigue become a problem? I was thinking more of using one of the many types of rubber/polysomething types of couplings out there. #### Topaz ##### Super Moderator Staff member Log Member I was thinking more of using one of the many types of rubber/polysomething types of couplings out there. The question has been bubbling around in my mind since we were discussing thin, low-frequency driveshafts. Popped up again here. Seems like if you were to use a long slender 'soft' metal shaft or other component to push the resonant frequency off-range low, it would be getting a lot of deflection in torsion with each start, stop, and throttle change. Maybe I'm missing something, but that seems like a recipe for fatigue failure. #### plncraze ##### Well-Known Member HBA Supporter The people who did the P-39/63 drive train put in many joints in their drivetrain that allowed them to control the "whipping" of the shafts. These joints were rigidly mounted to the airframe. #### wsimpso1 ##### Super Moderator Staff member Log Member I did mention building a soft unit by including spring package. It has the neat advantages of being tunable and you can put it where it does you the most good too. Change inertias or number of cylinders, and change springs. The single biggest problem with deliberately driving the resonance frequency down, is that you tend to also drive some of the higher order vibration modes down into your operating range... Doing this on a computer, adjusting inertias and spring rates until everything is safe is a task. Imagine doing this by build and test... Long slender shafts are a soft element. Quill shafts are used for accessory drives on many big piston engines. With any shaft, you must check its max speed against its critical speed. If your operating speed gets close to its critical speed, it can go unstable and flail. Then you have to break it into more than one shaft. You see it in truck drive shafts - with a bearing and a couple Hooke joints midway between trany and axle. And that sounds exactly like the P39 and P63. Billski #### MKIV ##### Active Member Long slender shafts are a soft element. Quill shafts are used for accessory drives on many big piston engines. With any shaft, you must check its max speed against its critical speed. If your operating speed gets close to its critical speed, it can go unstable and flail. Then you have to break it into more than one shaft. You see it in truck drive shafts - with a bearing and a couple Hooke joints midway between trany and axle. And that sounds exactly like the P39 and P63. Billski[/quote] Think I actually watched something along these lines recently with TV coverage of Aussie V8 Touring Cars, they had 'under car' camera focussed on rear of driveshaft & Differential, as car proceeded up thru gear changes & driveshaft RPM approached engine RPM ( 5th/6th gears ) there was a point where the driveshaft tube'''went crazy''' & Im sure the tube appeared to 'snake'. Have also been involved in some discussions re transaxles ( MKIV) recently & one manufacturer was advocating splitting the input shaft into two pieces for similar reasons. Sorry for the sidestep toward car info, but its my background & a lot simpler for me to relate what I have learnt from that field & try to cross reference it to an A/C scenario. #### Topaz ##### Super Moderator Staff member Log Member ...Quill shafts are used for accessory drives on many big piston engines. With any shaft, you must check its max speed against its critical speed. If your operating speed gets close to its critical speed, it can go unstable and flail. ... Aside from the 'whipping' aspect, is fatigue a factor in slender shafts such as this? Seems like they'd see a lot of torsional deflection in use, between power pulses, throttle changes, and starting and stopping. Or do you design so that the deflections are small enough that the stresses are below the "infinite fatigue life" limit? #### wsimpso1 ##### Super Moderator Staff member Log Member MKIV: You just described a shaft going through critical speed. Nothing will last very long with that going on. It sounds like that car is a candidate for a larger diameter driveshaft tube or a two part driveshaft. Topaz: In this context, soft pertains to spring rate not material condition. Designing it to live and to give the right spring rate may conflict, but they are not mutually exclusive. Deflection angle of a torsion bar element is T*L/(G*J) where T is torque, L is length, G is torsional modulus of material, and J is torsional section modulus. Spring rate is thus G*J/L. Make it long (big L) and slender (small J) and it will have a low spring rate. Build it with low G and it will have a lower spring rate too. Oh, and J goes with diameter to 4th power... Now shear stress in a torsion bar element is T*r/J. So the design task is to obtain the right spring rate at a low enough stress. It can be and is done. One thing to remember is that the light way to make a stiff shaft is large diameter and thin wall, while the way to make a soft torsion element is to make it small diameter and long. A lot of attention has to be paid to ends and stress concentrations and heat treat. And then you have one springy shaft. Worked great for isolating pumps and generators and the like from crankshafts. The problems with using flexible shafts for isolation is several fold. First, if you need a different spring rate (another engine comes along or your design needs some tuning), and you are putting most of your flexibility in that shaft, you need to design and prove out a new shaft. You might not have enough room if you need to make it longer. And the shaft might not be in the right place to do the job, or will require a three piece shaft with two supports... If instead, you have torsional spring packs, you just design a new coil spring set and have a spring shop make you a few. You can even start out with several in mind and have all of them available for trial with instrumentation... Billski Last edited: #### Jman ##### Site Developer I'm curious as to why these geared re-drives are not making use of chip detectors. There are 6 in the helicopter I fly. The main transmission has two, the engine accessory gear box has two, the Free Wheeling Unit (OWC) has one, and the tail rotor gearbox has 1. The gearbox at the T/R also has a temp gauge to detect impending failures. Even with the money and resources bell helicopter has to design and test these components, they still feel it's necessary to have adequate early warning sensors. I feel good about having them there. Occasionally a mission is aborted when a sensor is tripped by shavings that are later determined to be fair wear and tear by-products. Every once in awhile we do get some serious problems caught in advance though. Just the other day a friend had some good size tooth chunks stuck to his lower transmission chip detector. Are these practical for PSRUs used in this application? #### Tom Kay ##### Well-Known Member Jake; I think I have read about at least one geared PSRU that had a chip detector, but even with this equipment, isn't it a case of a shaft giving up the ghost without apparent pre-damage? I mean, in the case of a crankshaft, or input shaft, that's twisted back and forth to death, do you get chips? Or just sudden breakage? Yes, Billski has put some commendable effort into helping us understand the complexities of the full "system" as has that EPI website (which I suggest as reading material for anyone). Tom. #### wsimpso1 ##### Super Moderator Staff member Log Member Jake, Chip detectors are COTS, and should be more widely used in light airplanes. Remember that they detect metal pieces in the oil. If a bearing or a gear has had enough sub-surface damage (occurs as one surface rolls across another many many times), you begin to free pieces, and that is what the chip detector is able to see. Hopefully, it spots the chips soon enough to be able to abort the mission and safely conclude the flight. For the topic of PSRU's, they can be useful - We have bearings and gears that could tear up, and the warning could prevent a dead stick landing, but will only give us warnings on the failure mode mentioned. They give virtually no warning on others. Those have to be precluded by design. If a shaft or case is being fatigued, generally single cracks will emanate from a tiny flaw in the surface of a part, and it will grow with successive load cycles until the remaining area is reduced enough to cause complete failure of the element. Generally no chips are released by this process until the end. You could detect the cracks by disassembly and any number of non destructive methods, but you generally will not see them with the Mk I eyeball or the chip detector. Magnaflux, dye penetrant, etc are very useful for catching cracks in parts at rebuild time if the parts in question have long but not infinite life. When we are talking about PSRU's managing torsional vibration inadequately, we are talking something that should easily last way over 2000 hours that instead is lasting minutes because torsional vibration and feedback is increasing the loading by double or triple or even more over what was expected. So, useful? Yes. The careful operator, could use them for the same issues that they are in your Scout, but only after the vibration management issue is properly addressed. The chip detector would not be very useful for detecting flawed vibration management, as the detector warning will not sound until just about the time the powerplant goes into death throes... Bill #### Topaz ##### Super Moderator Staff member Log Member ...Topaz: In this context, soft pertains to spring rate not material condition. Designing it to live and to give the right spring rate may conflict, but they are not mutually exclusive. Deflection angle of a torsion bar element is T*L/(G*J) where T is torque, L is length, G is torsional modulus of material, and J is torsional section modulus. Spring rate is thus G*J/L. Make it long (big L) and slender (small J) and it will have a low spring rate. Build it with low G and it will have a lower spring rate too. Oh, and J goes with diameter to 4th power... Now shear stress in a torsion bar element is T*r/J. So the design task is to obtain the right spring rate at a low enough stress. It can be and is done. One thing to remember is that the light way to make a stiff shaft is large diameter and thin wall, while the way to make a soft torsion element is to make it small diameter and long. A lot of attention has to be paid to ends and stress concentrations and heat treat. And then you have one springy shaft. Worked great for isolating pumps and generators and the like from crankshafts.... Ah, okay. I see what you're getting at. I'd seen successful applications with tail-rotor drives and such, so I figured there must be a flaw in my thinking somewhere. I was missing that the ability exists to tailor the spring rate and still keep a low stress level. Thanks. #### Tom Kay ##### Well-Known Member Bill; For those of us who aren't up on our acronyms, what's a "COTS?" Tom. #### bmcj ##### Well-Known Member HBA Supporter Bill; For those of us who aren't up on our acronyms, what's a "COTS?" Tom. I think he means "Commercial Off The Shelf". #### Midniteoyl ##### Well-Known Member Sounds as if the only sensible way to go is to sell engine/PSRU combinations as a unit - no individual sales of the PSRU. Limits your market on one hand, but your liability on the other. This is why Jason at Vista will only sell combos... #### MalcolmW ##### Well-Known Member Bill; I've read your comments a couple of times which gives me reason to believe that you can offer some good advice. The problem is I'm not a mechanical engineer (adhesives and bonding aluminum, now I can do that stuff all day long until your eyes glaze over), and I'm not sure of all the terms you're using (one of my educational shortcomings). For example, you refer to an 'OWC,' I must apologize but I don't know what that is, nor, what a 'Rattler' is, which both seem to be useful in curbing torsional resonance problems (if sized properly). Should I want to use a Subaru engine as an alternative power plant and the same PSRU that Ram Engines uses (also available directly from New Zealand), what would you recommend as a 'belts and suspenders' approach to the technically challenged (specifically me)? This thread has been most informative, and your comments lead the pack. Thanks! Fly safe, MalcolmW #### wsimpso1 ##### Super Moderator Staff member Log Member Yeah, COTS is Commercial Off The Shelf, meaning you can buy it now and plug it in... OWC is one-way-clutch. There are two common types, sprag and roller, and they allow torque to be carried in one sense, but overrun if you try to make them carry torque of the opposite sense. Used in a lot of machines, including at least two in every automatic transmission I know of... Hit Wikipedia for way more... Rattler is a commercially available tuned pendulum vibration damper. Right now, they are available for most Detroit V8 and V6 engines. They work pretty well, but they put out a short but distinct rattle at engine shut down or start up. The company decided to just make a point of it by naming it that... Wiki on "pendulum damper" for lots more. Read my writing close. I don't know what WILL work for any application. I have been warning of the pitfalls that will give you no warning, but could suddenly give you a powerless airplane... Without actually putting torsional vibration equipment (a$60-100k set of equipment that I no longer have access too) on a system, I can not tell you if there are issues with any particular setup.

If other folks are using a particular setup successfully (several powerplants with a thousand hours each) I would say it is a good bet. But changes that seem insignificant cause vibe problems in cars, trucks, trains, motorcycles, airplanes, ships, machine tools, commerical powerplants, etc. It is real...

Billski

#### Mac790

##### Well-Known Member
Since we have PSRU discussion already, I'm wondering, does anyone have any info about Thunder Engine TE495-TC700 (attachment1) and Bonner Sapphire V6 (attachment2). Unfortunately I have only those two pictures, no more info.
Seb

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#### Tom Kay

##### Well-Known Member
So what have we learned? Would it be fair to say that the average aircraft enthusiast doesn't really understand torsional vibration, or am I the only one? Taking this further, could we say that the average PSRU maker doesn't either? I mean, are claims about cogged belts, hy-vo chains and one-way-clutches absorbing/attenuating/defeating this type of vibration fair or nonsense?

If the goal still remains to have a power plant based on a V auto engine, how do we go about this? And I know you could ask for a range of engines, the 150 HP class, the 250+ class, the 400 HP class, it depends on what you're building.

I think I can accept that the entire system must be matched to really be reliable, but again, who on the open market offers this, and has the years of successful service, plus the vibration survey to back it up? I can accept the concept of not scrapping a PSRU program just because of a few "failures" as was mentioned earlier, but I guess I did mean fwf failures of any kind, either prop breakage due to vibration, crankshaft breakage or PSRU failure. I guess it's not fair to simply blame the reduction unit.

All of the replies are indeed appreciated, but at this point, I'm still not sure where to turn for a 150 HP fwf package to power a 3/4 entry-level performance fighter.

Still eager to see further discussion, plus ultimately, a magic solution, of course.

Cheers all, Tom.