BW 350 PSRU Failure, please share the word!

Discussion in 'Chevy' started by Andreas K, Aug 19, 2016.

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  1. Aug 20, 2016 #21

    Vigilant1

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    That makes sense, but then the mfgr should make that clear. Maneuvers that produce very mild G's but a rapid change in heading or pitch (spin, a power-on stall, etc), especially under power, can produce high gyroscopic loads and would need to be prohibited.

    Who would sell (or purchase) a PSRU that breaks and causes a loss of power as the result of a departure stall?
     
  2. Aug 20, 2016 #22

    don january

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    I don't, and never have ran a PRSU on an engine in flight but I think it should handle at the least +/- 3 g's and not break. Nothing worse then spinning up your prop and have the rubber band break.:depressed
     
  3. Aug 30, 2016 #23

    wsimpso1

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    I don't normally lurk on the Chevy forum, but somehow spotted this. Bummer about having a failure, glad to hear everyone is OK.

    I have seen a lot of gearboxes where stuff is misaligned (manufacturing errors). Gears and bearings get torn up, but shafts, well, it could happen, but I have not seen it. Was there any pitting on the gear faces or the bearings? How about evidence of tooth tip or root contact? What evidence was present that made AutoPSRU's conclude that case deflection resulted in fatiguing the prop shaft?

    Either the telephone game messed up the message or the message has some big errors. I do not know which.

    I do know a few things about gearboxes, and g's causing failures sounds thin to me. Now yaw and/or pitch rates can bring up the loads in the prop shaft. I did some engineering wild-ass guesses to get some order of magnitude numbers on this guy. Gear loads on the prop shaft at max torque will result in bending moments in the shaft of somewhere around 5000 in-lb. Bending moments in the prop shaft due to the prop being pulled by a 3 G turn are around 500 in-lb. Bending moments in the prop shaft due to P-factor I did not estimate, someone else can do that. Precession at 15 degrees/second (slower than a spin entry) can get on the order of 5000 in-lb on the prop shaft. Moments due to alignment of vibration modes with forcing functions do not have an upper bound, but there are usually damaged parts (plural) when this happens.

    Conclusions? G's can only increase loads in the prop shaft a little, but even short periods of decent pitch or yaw rates can double the loads in the shaft. G's distorting the housing and causing shaft damage? My bet is something other G's did the damage. Maybe rudder and/or elevator input. It does not sound like torsional resonance, but without a review of the condition of ALL of the parts, it is hard to say.

    I do have a view on the bigger issue, having been in un-forecast severe turbulence. If I can move the controls somewhere while at Va, the engine and PSRU should be able to stand it. You should not have to keep an airplane below 2g's to have it be durable, because nature won't cooperate with that plan.

    Billski
     
    Last edited: Aug 31, 2016
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  4. Aug 31, 2016 #24

    Marc Bourget

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    Billski said: "You should not have to keep an airplane below 2g's to have it be durable, because nature won't cooperate with that plan."

    And Hubris keeps us from recalling (as to Nature, that we're just a mote in God's eye!:)
     
  5. Aug 31, 2016 #25

    wsimpso1

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    With my admittedly ballpark estimates of stuff like shaft lengths and prop inertia, I doubled the bending moment in the shaft with 14 degrees per second Yaw/Pitch rotation. That could happen on a ham handed rotation for takeoff (5 degrees in a third of a second). Anybody ever seen what happens when you push on the wrong rudder first in a stall recovery? I know that you all have done it some time... More like 45 degrees per second for a brief time. Ugh. so the moment in the shaft would quadruple during that foreseeable event. Let's not even get into snap rolls and spins. I sure hope that folks building PSRU's have thought about precession forces in design of their prop shafts. Might be a good question to ask...

    Billski
     
  6. Aug 31, 2016 #26

    billyvray

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    This thread makes me wonder why more airboat gearboxes aren't used. They seem to see some pretty serious duty. I'm sure they may be a bit heavy, but can't be too bad.





    Bill
     
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  7. Aug 31, 2016 #27

    rv6ejguy

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    With the success so far of Jeff Ackland's P85 with the Ballistic airboat drive and the price delta, I frankly don't see how Auto PSRUs will sell too many units. The only big minus on the airboat drives is that they have no easy provision for a hydraulic C/S prop which is a big deal on RV10s. You could go electric which would be fine on a -10. Certainly the airboat drives have hundreds of times more hours under often more extreme loads than the Auto PSRU units.
     
  8. Aug 31, 2016 #28

    Monty

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    Since I am working on these problems currently I guess I can add a little something.

    As far as housing deflection from precession loads goes. Not so much a problem for the gearbox itself usually. The problem is the bellhousing. For my situation the problem was not strength, it was deflection. Under the worst case loading the bellhousing was deflecting about 15 thou. That is a lot of misalignment for an input shaft. Automotive flywheels are large diameter to accommodate a clutch or torque converter. The bell housing must span this large flywheel. For many engines, there is no good load path for the bottom of the bell housing. Fortunately in my case the stock oil pan has structural gussets I could tie into. It took quite a bit of design iteration to get the deflection within reasonable limits.

    I believe the gearbox in question was using a CS prop, which aggravates the problem.

    RE air boat gearboxes. I would urge caution. The drag racers don't do that for hours at a time. The boats see bursts of full power and then they limp along at 3000 rpm while cruising. High cycle fatigue issues are less likely to show up in this instance. Put it in an airplane, operate at 75% power continuously and you may have problems sooner than you expect. Obviously they are testing for Low cycle fatigue, but the stress level in the material can easily be below plastic deformation, but significantly above the fatigue limit. An input shaft spinning at 4000 rpm sees 240,000 cycles per hour. It only takes about 4 hrs to get to 10^6 cycles, and whether you operate at 30% power or 75% can have a huge effect on life. But if one assumes a 10 second full power drag run, that number of cycles is roughly 1400 drag passes...It is relatively easy to put something together that will last for a while, much harder to make something that will last for 3000 hours at 75%. The loading on the components is complex and not easy to determine exactly. Especially when surface finish, press fits, key ways, splines, snap rings, section changes, and other stress concentration factors are taken into account. Not saying there aren't gearboxes that will work, you just need to do your homework. Thoroughly.
     
  9. Aug 31, 2016 #29

    rv6ejguy

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    Many of the airboat guys are putting 500 to 1000 hp through these drives though in comparison to about 300-350 for about 5 minutes and maybe 200 continuously in cruise for aircraft. A gearbox which will withstand 700hp+ for a few hours total will likely last hundreds or thousands at 200 hp.
     
  10. Sep 15, 2016 #30

    TXFlyGuy

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    Mr. Kronk was advised by friends not to fly that engine/PSRU combination. The V8 was highly modified putting out huge horsepower numbers. The failure occurred just after takeoff as Terry was on his way to an airshow. The plane was immediately engulfed in flames preventing anyone from offering aid to extract the pilot from the cockpit. That 80% scale P-51 was perhaps the most beautiful replica flying.
     
  11. Sep 16, 2016 #31

    Winginit

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    Seems to me that anyone reading this thread might come to the conclusion that direct drive is the best way to go. Everyone has heard the arguments both for and against the use of alternative engines. The old song and dance about auto engines not being as reliable as aircraft engines has pretty well been neutralized. My opinion is that failures in the prop adaption methods is where the real problem lies, but the engines took the brunt of criticism because of generalizing by a less than knowledgeable public. It seems that over the years there have been many people/companies that have created reduction drives. People believed that auto engines produced far more hp at higher rpms (true) and that they must somehow make use of that available HP. The NEED for a reduction drive to accomplish that purpose became generally accepted. Many times builders just wanted bragging rights for their airplanes HP even though in most cases the HP would be unmanageable in many airplanes.
    The way I see it is that reduction drives used on smaller HP engines often have proven successful. There has also been a lot of failed redrives on smaller engines, but all in all there have been some successful redrives for smaller engines. I think in many cases a redrive is the way to go with some of the engines. Certainly the VWs and Subarus have proven successful although more than a few companies failed before the successful ones became known. Moving on up to the larger displacement engines above 320 cu inches. There is where the failure rate seems to be uncommonly high. Almost every company producing redrives for larger engines has either failed eventually or is still struggling to survive. People building high end airplanes spend quite a lot of money on their airframe, and then use a high dollar reduction drive. This thread is yet another instance of reduction drive failure. Personally I would like to see someone come up with a reduction drive that proves to be reliable for larger engines.
    Having said all of that, I think that most of the people attempting to utilize an automotive engine will be better served by building for direct drive. In the case of the RV10, a builder may want a constant speed metal prop, and attendant higher HP . Many others want to build more along the lines of Steve Wittman by using a direct drive wooden or composite prop. I think the average builder (reasonably low buck) will fare better if he accepts the premise that a reduction drive is not a required feature of an auto conversion, and that utilizing a reduction drive often increases chances of failure. Depending on the airplane being built and what the builder wants to do with it, direct drive may limit choices somewhat, but a creative person can usually find a reasonable solution to adapt. The thing about a direct drive is that it is much simpler to design and build than a reduction drive and chances of success are better. Just the fact that there are fewer components involved lessens the chances of harmonic issues. Usually engines with more cylinders create smaller individual pulses. Couple that with a lighter and less harmonic wooden or composite propellor and you have a better chance at success. If someone has need for an engine that produces 200/250 hp (and more), then look at increasing displacement of an existing engine or even turbo charging.
     
    Last edited: Sep 16, 2016
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  12. Sep 16, 2016 #32

    TXFlyGuy

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    Gear reduction drives have been used successfully since WWII. The Titan / Autoflight PSRU, made in New Zealand, has had great success with no failures that I know about. The engines used range in horsepower from 180 to 300+. The company has hundreds (thousands?) of trouble free hours on this design.

    The few instances of "off airport" landings by Titan Mustangs has been the result of propeller failure, engine failure, fuel exhaustion, etc. None that were directly attributable to the PSRU.

    There was one pilot in Australia that suffered a failure that looked like it was the PSRU, however the problem was the propeller being heavier than recommended, plus lubrication issues that resulted. The plane nosedived in from 50' AGL. The pilot knew he was dead. But he walked away from the impact. That speaks highly for the "roll cage" like construction of the Titan Mustang wing center section and cockpit area.
     
  13. Sep 16, 2016 #33

    rv6ejguy

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    Direct drive auto engines generally cannot compete in the power to weight department, hence the widespread use of PSRUs. Like the engine itself, the PSRU just needs to be properly engineered and tested if you expect it to be reliable.

    The vast majority of Experimental aircraft PSRUs were neither professionally engineered nor tested for TV issues. You have to accept therefore that a good number will have reliability issues.

    I was up flying yesterday evening enjoying the Fall colors and the sun reflecting gold off the cut hay and wheat. Lovely flight. I wasn't worried about my Marcotte PSRU blowing up. The gearbox has never been touched internally in 13 years on my RV6.
     
    Last edited: Sep 16, 2016
  14. Sep 16, 2016 #34

    BJC

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    Are you eki?


    BJC
     
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  15. Sep 16, 2016 #35

    Winginit

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    Yes, I agree with that statement. The vast majority were not properly tested and as a result failures happened. The bottom line is that available reduction drives often prove to be unreliable and therefore may not be a good choice when considering what to use. My SWAG on the success/failure rate of reduction drives for large engines would probably be at least a 10 to 1 failure rate. By that I mean that for every 1 company producing proven reliable reduction drives for larger engines, 10 others proved unreliable and or went out of business.
    A builder has to first decide just how much HP they want and how much they can really use. In the case of the failed reduction drive on the RV 10 at the start of this thread, you are looking at an engine in the 230/260 HP range. Using an LS3 block, a builder could easily assemble a 408/416 cu in version and make 260 HP (or more) at 3400 rpms. He would have an installation that probably would be a minimum of 30 lbs lighter on the nose of the airplane. So, for the purposes of this particular example, max usuable hp would be available. As you say, there are other installations where a higher HP might be desirable/usable and unattainable by direct drive. I just feel that in the majority of installations, direct drive could furnish the max usuable HP. Its really more of a function of rpm range and propellor selection to attain the desired HP.

    . Yes, I have not heard of any failures from Autoflight and they appear to be developing an excellent reputation. I wish you all the best/success with your project.
     
    Last edited: Sep 16, 2016
  16. Sep 16, 2016 #36

    Winginit

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    What is eki ?
     
  17. Sep 16, 2016 #37

    rv6ejguy

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    The problem with larger planes and engines is you can't turn 72ish inch props at 3400rpm so direct drive auto engines are out of the equation right there if you need over 250hp, unless you turbocharged them of course.

    I look to Jeff Ackland running the Ballistic airboat drive on the P85. He's having good success so far at a fraction of the cost of EPI or BW350 drives. If they can fit a pump and governor for a hydraulic prop, it would fit the bill for many RV10/ Glasair, Lancair class aircraft.
     
  18. Sep 16, 2016 #38

    BJC

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    Said in jest.

    eki is a poster here, not heard from lately, who regularily lectures us about engines. I PM'ed him a few weeks ago to see if he is OK, but got no answer. I did recently see a post from him on another farum, so he may be taking a break from HBA.


    BJC
     
  19. Sep 28, 2016 #39

    Wheeler90

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    I am Stuart Davis, the owner of Auto PSRU’s. I have always had a policy of staying off of group sites to avoid postings that degrade too easily into ugly mudslinging disputes. If someone had not sent me an email about these postings I would not have known about it.

    As ‘cheapracer’ pointed out early in these postings, this amounts to another abuse of social media. What is even worse are the postings from “experts” in other fields that chime in with their miss-informed opinions. Survival and success in this industry is hard enough without this sort of fanning the fire.

    I am not going to make any direct response to any of the postings made here. It would only degrade too easily into ugly mudslinging disputes and spin off even more miss-informed opinions.
    Anyone that reads website postings has to keep three things in mind.
    1 - They should question the history, facts, and motivation behind anyone’s complaints.
    2 - They should keep a neutral perspective until more information comes out or checkout the facts themselves.
    3 - They should realize that research shows the motivation to post complaints outweighs posting positive reviews by a ratio of over 20 to 1.

    I have always been happy to explain the details of our gears, shafts, materials, clutch mechanism, and harmonics (TV) to anyone that asks. Most of what was posted here is wrong.

    If you want to know the facts instead of someone’s miss-informed opinion just go to my website at www.autopsrus.com. To read detailed responses to these postings and learn more facts behind Andreas’ problems with his used gearbox that was 9 years old when he bought it back in 2014 look under the ‘Articles’ link for ‘Andreas Fiasco’. On that page is a button that will open a PDF file.

    Thank you,
    Stuart Davis
    Auto PSRU’s
     
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  20. Sep 29, 2016 #40

    pictsidhe

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    After reading this thread and Stuart's PDF riposte, I'm inclined to take Stuart's view of misapplication.
    I was wondering about how TV problems were averted and the clutch is the key. The springs in the clutch will put the first TV frequency below the operating range. Ever driven a manual gearbox car and taken the revs too low and had that horrible shudder? Hello resonance! With a prop and no clutch, no way to avoid that at some point, some designs try and put it above the operating range, some below. The big boys use harmonic absorbers to deal with troublesome engine harmonics. You still need to get the 1st resonance out of the operating range though. Add centrifugal clutching, you can disconnect the engine until it's safely above a critical speed. Change the crank, rods, prop, gears or clutch springs and the frequency changes. Too much torque through a clutch can bottom the springs. Personally, I suspect Stuart's theory of excessive g loads with a heavy prop is the culprit here. But there are still an awful lot of ways to step outside the limitations of a tuned system like a well engineered psru. How many engineer hours and patents did it take to fix the TV problems on the Wright cyclone?
    This thread is a good example of why I'm going direct drive. It may be slightly inferior to a psru system, but the peace of mind of not having to play with the potential fire of a psru is worth it especially on a one off.
     
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