Egg Gearbox Failure

<<I sometimes get a quite noticeable vibration on the ground with a gusting tailwind or in flight in turbulence which I attribute to the composite prop blades flexing.>>

Naaa. Come on now, why not fundamentals? Assume a wind from any quarter during runup. The result is a cyclical variation in AOA for each blade as the prop rotates, a well known issue and the reason we are taught to face directly into the wind for runups. I think the frequency of the cycle is (RPM x #blades)/60 = hz, but I reserve the right to think about it some more <g>

Anyway (and similar to the previous prop tip-fuselage example), the prop would resonate if it had a natural frequency matched by the frequency of AOA variation. It is a victim of the variation, not the cause. If not matched, it would not itself resonate but would pass the cyclical variation in load onward to the powertrain. If a powertrain natural frequency is matched, the shaft system resonates. And so on.

Dampers on the free end of the crank are often replaced with lightweight, solid aluminum pulleys when used in race cars and aircraft. Bad idea in most cases and I've seen a number of crankshaft breakages over the years shortly after these were installed.

My advice is don't. What kind of damper was Subob using? This could be an alternative cause for failure. Again, we usually can't feel these high frequencies at the rpms these failures usually occur at. More things to worry about. That Lycoming is starting to look pretty good right now.

Ressonance is not needed for destructive vibrations. Ressonance in a loaded structure, like a shaft, will in most cases lead to destruction very fast, usually within seconds. When designing machines involving rotation, you allways set stiffness and inertia so that the RPM during normal operation is far away from ressonance.

There are two methods doing this, running overcritical or subcritical. This means running above the ressonance frequency or below. My experience is in measuremens and analysis of hydropower turbines. They run sub-critical, meaning they never experience ressonance - ever (at least not mechanical torsional vibrations, but sometimes experience electrical torsional vibration caused by the synchronous generator). A washing machine typically operates over-critical when centrifuging, meaning it accelerates fast to an RPM far above the ressonance RPM. I also believe the rotor on a helicopter operates far above ressonance RPM.

The general rule is (not to be confused with cause-effect):
high RPM and/or large inertia and low torque = over-critical.
Low RPM and/or low inertia and high torque = sub-critical

Anyway, bad destructive vibrations are usually vibrations that occur outside ressonance, since problems with ressonance are solved during design.

With aero engines I am not sure. I would think that a lycoming runs sub-critical? I also think that a Rotax 912 operates sub-critical, and that is one of the reasons for very limited propeller weight (torsional vibrations). But I really don't know. How about the subaru with complex gear involving several shafts and a heavy propeller?

SvingenB said:

Anyway, bad destructive vibrations are usually vibrations that occur outside ressonance, since problems with ressonance are solved during design.

With aero engines I am not sure. I would think that a lycoming runs sub-critical? I also think that a Rotax 912 operates sub-critical, and that is one of the reasons for very limited propeller weight (torsional vibrations). But I really don't know. How about the subaru with complex gear involving several shafts and a heavy propeller?

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The problem with many PSRU/ engine / prop combos is that no "design" work in the engineering sense was ever done.

Many rotating systems like car drive trains or our propeller drive trains have a TV period somewhere in the possible operating rpm range. We try (or hope!) that this rpm is somewhere below the idle range and above the maximum rpm range.

The three systems we have mainly discussed all have some bad periods around the 1200 rpm range perhaps coincidentally. By idling above this rpm, we avoid long term operation here and passage through here is only transient and hopefully not destructive.

Dan's earlier calcs showed that prop MOIs had only a small affect.

rv6ejguy said:

The three systems we have mainly discussed all have some bad periods around the 1200 rpm range perhaps coincidentally. By idling above this rpm, we avoid long term operation here and passage through here is only transient and hopefully not destructive.

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Yes, but are you sure this is ressonance problems? It could equally well be too low rotational momentum in the propeller at RPM below 1200-1400 so the end result is torque reversal due to the (relatively more pronounced) pulses from the engine. It could also be engine mounts being calibrated for 5000 rpm causing the engine to oscillate around the propellor axis below 1200 rpm or a combination of both.

<<Dampers on the free end of the crank are often replaced with lightweight, solid aluminum pulleys when used in race cars and aircraft. Bad idea in most cases and I've seen a number of crankshaft breakages over the years shortly after these were installed.>>

The wrong damper can be pretty useless too. Take the classic Lanchester or Houde dampers for example. If the damping rate is too high, all you have is a flywheel. Too low and it does nothing useful.

FWIW, you can also place damping in parallel with a torsional soft element. System damping doesn't have to be at the end of the system.

<<Anyway, bad destructive vibrations are usually vibrations that occur outside ressonance, since problems with ressonance are solved during design.>>

.......among professionals. Love your washing machine example. Is anyone shocked to hear washing machine engineers pay more attention to torsional issues than the average PSRU vendor?

<<I would think that a lycoming runs sub-critical?>>

Great question next time I pass the Lycoming booth at Sun'nFun.

<<I also think that a Rotax 912 operates sub-critical>>

Our typical system (Rotax, Subaru, etc) is mid-critical. For a well-designed system we pass through a resonant RPM or two during startup (and idle above them if we're smart), with the next critical above redline. The M14 radial (short, stif crank) is sub-critical for the 4.5 order, but mildly critical for the 9th.

<<Ressonance in a loaded structure, like a shaft, will in most cases lead to destruction very fast, usually within seconds.>>

Disagree, more or less. It will lead to rapid destruction if the designer was really dumb, but there are lots of systems operating with resonant periods in the operating range. You just need to limit amplitude by design. Heck, even I designed one that worked fine, and I am very near dumb <g>.

Our alt-engine systems operate over a wide speed range and must be lightweight; we don't have the luxury of designing around a narrow speed range, or using huge shafts (high torsional stiffness) to push the criticals up above the speed range.

<<Yes, but are you sure this is ressonance problems? It could equally well be too low rotational momentum in the propeller at RPM below 1200-1400 so the end result is torque reversal due to the (relatively more pronounced) pulses from the engine. >>

I think the 1200 RPM range is a resonant period. The 912 is a 4-cyl 4-stroke, so 180 degree firing intervals (meaning it has crankshaft torque reversal), plus it has no flywheel on the crank. The lack of flywheel inertia and the crankshaft torque reversal just means the forcing frequency is quite powerful. And I judge the shafts to be too small (admittedly without hard numbers) to push the 2nd order critical above the operating range.

Here's a story from my own experience. It involves an example worse than the 912, a 3-cyl (270 firing interval) 4-stroke with pumped-up compression. The application (a WW1 replica) required a very low idle speed because it had little tiny brakes and high pressure tires on 21" wheels. It was impossible to drop the first critical below a 500 RPM engine idle speed, so the drive system had to be designed to pass through the first major critical without harm. After much engineering, I had a system that only reached about 90 ft-lbs of resonant torque running steady state at the worst-case 1-1/2 order critical around 800 engine RPM.

Here's why I think low rotational momentum and torque reversal mean little by themselves. The above system would cheerfully idle like a clock at 500 engine RPM, meaning less than 250 prop RPM. It did so without significant noise or complaint, at relatively low oscillating shaft torque, even though you could darn near see the prop stop and start. It also sounded like a Model A Ford, much to the delight of onlookers <g>

The same system with Rotax dogs would be noisy, but I don't think it would shake around more than the 3-cyl.

BTW, I'm not guessing about the low oscillating shaft torque at idle, or the 90 ft-lbs when resonant. I had telemetry and a strain gauge on the propshaft, and I can look up the actual values if anybody cares.

Grounding of pre P3 units?

I read on the Glastar site a note that indicates Eggenfellner is grounding all the non P3 PSRU units. I can't find anything to verify this, so I consider it rumor now, but does anyone know if there is any truth to it?

Grandy said:

I read on the Glastar site a note that indicates Eggenfellner is grounding all the non P3 PSRU units. I can't find anything to verify this, so I consider it rumor now, but does anyone know if there is any truth to it?

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It is a fact.

If you bought a Gen II (now grounded), do you pay for the new Gen III gearbox? How much do they cost?

DanH said:

If you bought a Gen II (now grounded), do you pay for the new Gen III gearbox? How much do they cost?

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Yes on the first question.

I do not know the answer to the second. Mine cost $2495 but it was below cost to manufacture for sure. The company in CT doing the machine work to create the case is not cheap.

Actually that's good

To Eggs credit he is taking action, tough action. I went on the Egg web site and read all the service bulletins and at least he addresses it and is proactive. It's still ugly, but it has to be done. Some companies deny or do nothing in hope the issue will go away and save face (say for example Ford, Chevy....). I find hope in that he has upgraded the box and grounded the Gen II. However it sucks for those that have to shell out the cash.

Dumb question

For Ross, Dan, and others who understand this better than I do...

I've been following this thread from a distance with some interest.

It seems to me that one of the problems is identifying the areas where you might have destructive resonance/vibration, and "seat of the pants" us an unreliable method.

Given that solid state accelerometers and their supporting electronics have become so cheap, would it be possible to instrument the drivetrain and just watch it in real time?

Maybe make it another engine limit--keep the CHT below a certain number, and keep drivetrain vibration below a conservative limit.

flyeyes said:

For Ross, Dan, and others who understand this better than I do...

I've been following this thread from a distance with some interest.

It seems to me that one of the problems is identifying the areas where you might have destructive resonance/vibration, and "seat of the pants" us an unreliable method.

Given that solid state accelerometers and their supporting electronics have become so cheap, would it be possible to instrument the drivetrain and just watch it in real time?

Maybe make it another engine limit--keep the CHT below a certain number, and keep drivetrain vibration below a conservative limit.

Click to expand...

The EGG H6 GEN3 with a MT7 prop was recently flight tested with considerable vibration detection equipment. This was accomplished on a private airplane with MT equipment and a MT engineer recording the numbers.

I e-mailed the pilot to ask if MT had released the data, to date they have not. But the verbal conclusion after the flight was "it is very smooth".

<<I e-mailed the pilot to ask if MT had released the data, to date they have not. >>

Has MT ever released raw prop telemetry data? I got $5 here for you David. You collect by sending me the data. I collect if all they release is "We approve our prop on this engine". Is it a bet? <g>

DanH said:

<<I e-mailed the pilot to ask if MT had released the data, to date they have not. >>

Has MT ever released raw prop telemetry data? I got $5 here for you David. You collect by sending me the data. I collect if all they release is "We approve our prop on this engine". Is it a bet? <g>

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I think not. But a certificate indicating the combination is OK by them is better than no certificate.

flyeyes said:

For Ross, Dan, and others who understand this better than I do...

I've been following this thread from a distance with some interest.

It seems to me that one of the problems is identifying the areas where you might have destructive resonance/vibration, and "seat of the pants" us an unreliable method.

Given that solid state accelerometers and their supporting electronics have become so cheap, would it be possible to instrument the drivetrain and just watch it in real time?

Maybe make it another engine limit--keep the CHT below a certain number, and keep drivetrain vibration below a conservative limit.

Click to expand...

I've looked at doing TV testing with accelerometers and have some components and an idea picked out. Once the -10 is done, I might have time to pursue this so I can use it when I'm ready to run the engine. Not sure if it will work but looks feasible in theory.

<<Given that solid state accelerometers and their supporting electronics have become so cheap, would it be possible to instrument the drivetrain and just watch it in real time?>>

I'm on record as saying "no", for two reasons. One, I'd like to encourage the conventional method (measuring oscillating shaft torque directly with a strain gauge set). You net a vital piece of info, actual shaft stress, and for the most part the signal contains nothing but shaft torque. The drawback is the requirement to somehow get the signal off the rotating shaft. My own fantasy in this regard involves an electronics wizard lurking here, reading this post, and responding with "Ah, heck, I can make a single channel digital telemetry radio with a hatpin and some junk PC parts" <g>

The other reason is practical; It might be difficult to sort out the indirect indication of torsional oscillation (accelerometer signals due to block rotation around the crank axis) from all the other block motions. The signal from any particular accelerometer will be the sum of all motions. Best I can tell (no serious study here) the signal can be broken down into discrete frequency components with FFT software and other vibration analysis software tools. That's fine, but then you have to identify the source of each discrete frequency. When you get done, you still don't know what you really want to know; amplitude, or "How bad is it?"

Having said all that, I would be delighted to see someone try the accelerometer method, and will cheerfully help if I can. I'd love to be wrong, if the result encourages others toward real test and measurement.

All my best Christmas wishes to you and your families.