And in the process turns the wooden Bolly 28x10 into matchsticks and sprays the debris up to 30m away!

I've read a whole bunch on several forums where prop bolt failure is often the root cause of throwing props. The thing here is I don't see how throwing the prop would make the engine stop dead still from 6k+ rpm instantly. I think the sudden stop is what caused the prop to shatter and depart, tearing off all the bolts in the process. But that would point to detonation or maybe pre-ignition.
Ofcourse the crank was bent after that but was not obvious until the crank case bolts were released and the engine turned over showing a wobble between the front / back case interface.

I'd love to hear any thoughts on this one. Especially any experience with motors stopping dead still. Scared the crap out of me I can tell you!

Norm.

Sounds like either the engine seized (does it still turn freely?) or one blade failed, and the resulting imbalance caused shaft bending and rapid stop. But to me it sounds like it seized, the abrupt stop destroying the prop and bending the shaft....

I don't see your inspection results on the bearings and crankshaft surfaces. What were they like? And the connecting rod and wrist pin?

An engine doesn't just stop, and a crankshaft doesn't just bend. From your description, I would be inclined to look for a flawed fillet in the crankshaft (subsurface inclusion? Machining flaw?), and a resulting crack, with distortion looking like a bent crankshaft. If that's not the case, there should be a seized bearing or wristpin bushing, or something that caused the abrupt stop.

Let us know what you find.
Dave Olson

Maybe a piece of metal debris getting sucked in. But that is a stretch. I think bearing failure is more likely.

Yes, to clarify, the engine is brand new and was on its second tank from bench running @ around 2500rpm with a short smooth acceleration to 3/4 for about 3 secs at the end of the run, and a number of shutdowns and restarts to see how hot starts go etc. I do this to get familiar with the new engine before committing to an airframe. Just like to make sure all works normally with full access to check it over between runs. Anyway, I found the motor to be a pleasure to operate (just like my 85twin but a monster mamojam amount more power :0. OMG, talk about getting the hairs on your neck standing up and something else tightening! This engine is a freak!)

Back to the post mortem.
The engine's crank was thrown out of alignment due to slipping on the crank pins. That's to be expected from such a sudden stoppage with a high resisting force on the shaft via the prop inertia. The engine turned freely (normally) and no defects found anywhere and the motor was clean. I sent the motor to the 3W distributer here in Oz and he realigned the crank segments and replaced the bearings by default (which were still fine). The motor was deemed in perfect condition and he put it down to prop bolt failure.

I guess I'm wondering if anyone has seen an engine stop cold 'due to' the prop leaving or is it usual for the motor to keep running. I'm convinced that something happened from the other end of the motor and the prop thing is a symptom rather than the cause but I just need more clues from guys who have millions of hours experience with these motors. eg. Could the ignition be dodgy and have fired once miles too early to cause this?

I've since run the motor for two more tanks with no problems but am apprehensive and need to feel that it's not gonna throw a stink again.

Any more thoughts would be greatly received, thanks.

Now you see the need to check those prop bolts between every flight with a new prop till you are sure the have seated. And then they will need to be checked occasionaly still.
What stopped the engine was, when the first 4 or 5 bolts let go, the prop got off balance on the hub, the pendulum effect of throwing the prop the rest of the way off (kind of like throwing a giroscope, they don't want to move) stopped the engine.

Have you actually checked the crank runout? Have you checked the bearing alignment inside the case? Have you checked the fit of the bearings in the case? You will need to use some good machine tools to do this. Two of the three noted above have the ability to stop an engine in it's tracks and shatter the prop in the process, although I tend to think the prop bolts may not have been torqued correctly to begin with.

There is a general assumption that our engines are manufactured with a high level of precision while using excellent QA methods. If only that was true....

Look at everything before making the conclusion.

....I'll usually torque the prop bolts the first time, fly a short flight, re-torque and repeat three times. Usually after the third round I find the bolts and/or hub haven't loosened/compressed, respectively. Some props are softer than others, and I find I sometimes have to repeat more than three times, and with carbon, usually less will do. I just make a habit of repeating three times regardless of the material, to be sure.

Chad

I watch a 3W 160 four cylinder throw a carbon prop during flight. No damage to the engine. Never could find the prop.

I think you're saying the crankshaft is assembled with press-together segments. Would that be correct? I was not aware.

Best wishes,
Dave Olson

I've never seen a thrown prop actually "stop" an engine dead. I have seen one rip the firewall from the plane due to the resulting imballance. And I've seen a shaft run, where a racing glow engine shed it's prop and revved out no load. Never a dead "Bang!" stop.

A single cylinder pre-igniting would be hard pressed to stop the engine. "Ping" to "CRACK" would be about the range of noises, but not instant stoppage. To transmit that much energy thru the piston would break something before the prop.

Still voting for some kind of seizure... But no idea how or why.

jburry's reply is precisely the the reason for suggesting checking component alignment inside the engine. Even if the prop torque was incorrect the engine would usually spin up to higher rpm when shedding the prop, not instantly stop.

I'll assume he was bench running the engine. And stick to my theory of gyroscopic momentum stopping the engine after the prop bolts failed. Otherwise we'd be seeing a post about a engine ripping a firewall off the plane from a sudden stop. Crank case bearrings don't have enough leverage to stop a prop. The crankcase would fail first. The only thing that could stop a prop that suddenly would be a catostrophic piston failure. But that hasn't been reported. Just the opposite, the engine turns freely. Also a catostrophic piston failure would have ripped the engine mounts off before the prop went poof. So the prop bolts failed and offset momentum stopped the engine.

Hate to tell ya, but it wasn't that long ago a certain engine manufacturer had bearing alignment so bad that from time to time the vibration and friction would cause the crank to tear the front of the engine off. I still see similar issues from time to time. One engine manufactuer has such poor quality control they have a slip fit sizing process between pistons and cylinders. They make four different cylinders and pistons for the same engine since they can't seem to maintain a reasonable tolerance spread...but only one ring to fit the four Some engines have front to back bearing alignment so bad the crank won't spin freely in the crankcase with the plug removed. In others the bearing races themselves can have tolerances so loose they require a bucket full of sleeve lock to retain the bearing. Too much sleeve lock and the stuff loosens up when the engine gets hot causing the bearing to float in the race, permitting the crank to run out of true with the crankcase. One of the reasons some cranks develop a twist over time. Another problem that periodically crops up is inconsistent cylinder deck heights, requiring a stack of head gaskets to provide an acceptable squish band.

Lot's of things that can be done wrong that by themselves won't make a lot of difference. Gang a few of them together and catastrophic failures can occur. It's not very often anymore, but it has and still can happen.

Thanks guys for your great input!

When I first got the motor I did infact check the crank runout against the prop hub face and found it 2 thou out. Not perfect but not bad either. The prop tracked within about 2mm from memory. There was no slop detected in the bearings in any direction but that was only from basic physical/visual checks and not disassembled and measured properly.

It's true that I didn't check the bolts as often as suggested above so this may end up being the cause. Ok, I need to order a new set of bolts as I borrowed the replacements from my 85. Is there a tensile strength I should look for? The M5 bolts I have here look like they have an 8.8 stamped on them.

Many thanks, Norm.

The only thing that will stop an engine dead in its tracks is a physical interuption of movement, either mechanical or hydraulic, even a partial siezue would only slow it down gradually a full siezue would show by not being able to turn the engine over afterwards and would show on the piston and cylinder liner, the racing Kart engines I used to work on would Ping very loudly when the siezed, but would stop dead, which stopped the wheels and hence lots of gyrations of the Kart, these where usually caused by lack of oil and to much heat, minimum oil for best performance, after they coolded down the engine would turn over again, quick lap back in with wet and dry and out again on the track.

So question how much oil did you have in the petrol mix?

Mike

Good question.

Back to the run out. Industry standards seem to be that a max of .0055 is acceptable but anything between .002 and .0045 is considered very good.

Oil was a good 2 stroke mineral oil @ 32:1 for break in with 96 octane unleaded. Mixture was on the rich side with 4 stroking happening at about 1/4 down.