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BW 350 PSRU Failure, please share the word!

Andreas K

I'm New Here
Fellow Pilots,

I?d like to share the gearbox failure that I recently experienced in my Vans RV-10.

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My setup:

Airframe: Vans RV-10
Engine: GM LS 7 (approx. 400 hp, 800 ft/lbs of torque @ 2700 prop RPM, WOT @ SL)
PSRU: BW 350 (Bud Warren, Geared Drives), Cast Aluminum Case, 1:1.75 reduction ratio
Propeller: Whirlwind Aviation RV-10 composite prop, 80 inch, 46 lbs

I was in phase one at around 33 hrs. when I experienced a total thrust failure shortly after takeoff. As it turns out the prop shaft of my PSRU broke and caused a disconnect between the engine and the prop.
In my third flight a couple month back I made a 3 G turn for 20 seconds as part of my flight testing.

Several conversations with the manufacturer of the BW350a and BW350b at AUTOPSRUS revealed that this turn caused the gearbox case to flex and misalign the supporting bearings. The prop shaft bent slightly and possibly cracked at this time.
I could fly another 30 hrs. before the shaft finally broke. The shaft is splined at the area of breakage and the core of the shaft is 1.25 inches thick (compare that to a Lycoming main bearing).

I believe a combination of the 3 G turn and torsional vibrations caused fatigue and subsequently shaft failure.
I never used more than 75% of the available power and flew the airplane like an airliner (minimum G?s) except for that one turn.

I am reaching out to you in order to spread the word. Maybe you know somebody who is flying or building an airplane with this PSRU. I destroyed my gearbox because I was unaware that a 3 G turn could do such harm.

Please remember, that I am not an engineer and every setup is different. I CANNOT provide a G-limit that is safe.

I can only suggest to STAY AWAY FROM ANY G?S and STAY OUT OF TURBULENCE

Many Happy Landings
 
Fellow Pilots,

I?d like to share the gearbox failure that I recently experienced in my Vans RV-10.

open


open



My setup:

Airframe: Vans RV-10
Engine: GM LS 7 (approx. 400 hp, 800 ft/lbs of torque @ 2700 prop RPM, WOT @ SL)
PSRU: BW 350 (Bud Warren, Geared Drives), Cast Aluminum Case, 1:1.75 reduction ratio
Propeller: Whirlwind Aviation RV-10 composite prop, 80 inch, 46 lbs

I was in phase one at around 33 hrs. when I experienced a total thrust failure shortly after takeoff. As it turns out the prop shaft of my PSRU broke and caused a disconnect between the engine and the prop.
In my third flight a couple month back I made a 3 G turn for 20 seconds as part of my flight testing.

Several conversations with the manufacturer of the BW350a and BW350b at AUTOPSRUS revealed that this turn caused the gearbox case to flex and misalign the supporting bearings. The prop shaft bent slightly and possibly cracked at this time.
I could fly another 30 hrs. before the shaft finally broke. The shaft is splined at the area of breakage and the core of the shaft is 1.25 inches thick (compare that to a Lycoming main bearing).

I believe a combination of the 3 G turn and torsional vibrations caused fatigue and subsequently shaft failure.
I never used more than 75% of the available power and flew the airplane like an airliner (minimum G?s) except for that one turn.

I am reaching out to you in order to spread the word. Maybe you know somebody who is flying or building an airplane with this PSRU. I destroyed my gearbox because I was unaware that a 3 G turn could do such harm.

Please remember, that I am not an engineer and every setup is different. I CANNOT provide a G-limit that is safe.

I can only suggest to STAY AWAY FROM ANY G?S and STAY OUT OF TURBULENCE

Many Happy Landings
You can get 3g in some good turbulence, if you are going fast. If it can't handle 3g, it doesn't belong in a light aircraft.
 
. Several conversations with the manufacturer of the BW350a and BW350b at AUTOPSRUS revealed that this turn caused the gearbox case to flex and misalign the supporting bearings. The prop shaft bent slightly and possibly cracked at this time.

I don't see how conversation can possibly reveal that a 3 G TURN caused this failure. Nor do I see how a conversation can determine the shaft bent and POSSIBLY cracked.:confused:

While this may in fact be true it may also be possible that the shaft was defective. I would think a more detailed investigation is in order before coming to a conclusion as to cause.

Has the manufacturer seen this before? Have they flexed the case/engine in testing and seen this failure? If so have they published a warning or cautionary note to customers.

Also if they have determined, without doubt, that this was, in fact, the cause are they issuing warnings to past, present, and future customers?

As Kevin said if 3Gs caused this it probably doesn't belong in an aircraft, at least not in this configuration.
 
Early Bud Warren shaft design, photo courtesy Contact magazine. Judging from the photos Andreas posted, it does not appear the shaft has been redesigned, at least not at the point of interest here.

As a general rule of shaft design, a section change intersecting a spline is a bad idea. In this case, the shaft failed right where this rule-o-thumb predicts.



Good practice places the spline roots outside the shaft diameter. Illustration credit Carroll Smith's Engineer To Win:



In this case there is plenty of material available in the center of propshaft gear, so a design change should be relatively painless.



Possible contributing factors...

Andreas, are there two large support bearings on the large shaft section in the nose case, and third bearing in the main case for the shaft tail?

Does AutoPSRUs specify a prop? Were you using an aluminum blade?
 
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It's my (possibly erroneous) recollection that a lot of these drives are used on airboats.

I'd think typical turbulence loads are much higher in boats than airplanes.
 
Andreas,
You and I have discussed this, since I have the identical cast aluminum gear box. You've left out some important details.
First, you don't mention the Gyroscopic Torque. Also, your gear ratio yields over 200 more rpms on the engine than my 1.667 ratio. Also, my LS1 only develops 350 ft lbs of torque at WOT, while your LS7 develops almost double that.
Last, I was aware of the high "g" loads issue. It's not the flight load that causes the problem, it's the Gyroscopic Torque. Our engines develop a tremendous amount of torque at these higher rpms. If a Lycoming was running at 4500 rpm, it would trash the engine, and the prop - I reference the prop governor failures in the RV-10s using the MT governor.
Plus, the RV-10 is not designed for high g loads, or acrobatics. The limit is 3.8 positive, and 1.5 negative. I hope you don't feel that I'm trying to discredit you, but I felt that your statement:
"I can only suggest to STAY AWAY FROM ANY G?S and STAY OUT OF TURBULENCE"
is a little uncalled for in this case.

Respectfully,
John Goodman
 
Looks like a classic progressive torsional fatigue failure to me.... So torsional vibration would be high on the suspect list, but I doubt the 3 G load had anything to do with it.

You can see in the photo at the upper link what looks like a somewhat flat area across ~ the left 1/2 of the diameter. This area was progressively cracking for a while before the complete failure occurred when the remaining cross section could no long handle the applied load.

This was definitely not a one time overload event.
 
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It's my (possibly erroneous) recollection that a lot of these drives are used on airboats.

I'd think typical turbulence loads are much higher in boats than airplanes.

I doubt if there are too many of these particular drives used on air boats due to the high cost of them but drives are in widespread use there on most auto conversions.

Jeff Ackland is using a Ballistic air boat drive on his LS powered P85 with good success so far. No provision as yet for a Hydraulic C/S prop but you could go electric on an RV10. This gearbox cost is a fraction of the box we're talking about here and they have tens of thousands of hard use hours on them collectively. Food for thought.

The LS7 outputs quite a bit more torque than the LS1 so you've immediately moved into uncharted territory.
 
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My initial reaction is that the G loading is less relevant than the pitch and yaw rate combined with the moment of inertia of the propeller.

The classic case is the hammerhead turn where G loading is small but prop speed and precession rate is high, putting large loads on the front bearing and shaft

The precession moment is being reacted at the prop flange and putting a large bending moment into the shaft.
 
Early Bud Warren shaft design, photo courtesy Contact magazine. Judging from the photos Andreas posted, it does not appear the shaft has been redesigned, at least not at the point of interest here.

As a general rule of shaft design, a section change intersecting a spline is a bad idea. In this case, the shaft failed right where this rule-o-thumb predicts.



Good practice places the spline roots outside the shaft diameter. Illustration credit Carroll Smith's Engineer To Win:



In this case there is plenty of material available in the center of propshaft gear, so a design change should be relatively painless.



Possible contributing factors...

Andreas, are there two large support bearings on the large shaft section in the nose case, and third bearing in the main case for the shaft tail?

Does AutoPSRUs specify a prop? Were you using an aluminum blade?

And substantially aggravated with the stiffness added with the gear spines coming right up to that transition zone.

I think he said whirlwind, 46#, which would not be aluminum blades.

At least the OP got the plane on the ground in one (unbent) piece.
 
A couple of comments FWIW:

1) Carrol Smith consistently cracks me up, and is also usually technically very accurate. (see post #4)

2) I have spent a good bit of time looking at and dealing with mechanical failures. Not necessarily shaft failures, but mechanical machinery and structure.

3) I agree that this looks like a failure caused by a fatigue crack propagation that finally had a catastrophic failure. Yes, it could have been some single event that started the crack, then propagated from there, but it is more likely from a manufacturing defect like bad heat treating. If this is true, it could effect a much broader population of drives. In any case, it warrants a close inspection and, in my opinion, the manufacturer should be clamoring to get his hands on the part for inspection and analysis.

4) If the failure was initiated by a 3g turn (which I don't believe it was) then the airplane should not even be considered normal category (3.8g), it should be considered... maybe... an air boat.

Tim
 
The rest of the story

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
 
Thanks for coming on here and providing the link to the rest of the story. A blocked vent won't just take out a seal on these units, it will do the same thing to a Lycoming.
 
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

Thanks Stuart,

Always good to hear the rest of the story.
 
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