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Old Oct 08, 2012, 04:08 PM
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Your probably right and that has caused the trouble. But it's also true that the molded dummy radial engine could have been ripped apart by the engine wobble after the props parted company. Isn't it?
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Old Oct 08, 2012, 04:37 PM
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Here's a close up of the hub. Same exact thing that happened to my 1700 P-47.
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Old Oct 08, 2012, 04:45 PM
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Aros -

With the mechanical advantage of the spinning mass, those parts contacting the dummy motor would have simply torn it up and not suffered breakage of the prop itself if the strength were there in the prop assembly in the first place.

Like Kaikokid suggests, there's possibly enough flex in the motor mounting that could allow the spinning disintegration and the motor can to hit the parts behind.

When you look at the edges of the hub breaks, that is ALL of the area of the material that holds the unit together at that point, a really small fraction of the entire hub around the blade. Whether the blades broke first or the hub it would be hard to tell but it's the hub's 'job' to hold this all together and there is so pitifully little hub material and integrity at these spots.

Just another of my $.02 worth.

Jack
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Old Oct 08, 2012, 05:00 PM
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Originally Posted by Aros View Post
Here's a close up of the hub. Same exact thing that happened to my 1700 P-47.
Curiously no issues and same setup as my FMS T-28 using 3 blades I know .... are you turning the same rpms as the 500kv installed in the t-28... 6s make it turn less or more? This seems really odd I could only think that the blades are larger mass stressing the hubs..... hmmm
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Old Oct 08, 2012, 05:06 PM
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Quote:
Originally Posted by gnats82 View Post
Aros -

With the mechanical advantage of the spinning mass, those parts contacting the dummy motor would have simply torn it up and not suffered breakage of the prop itself if the strength were there in the first place.

Like Kaikokid suggests, it's flex in the motor mounting that allowed the spinning disintegration and the motor can to hit the parts behind.
I slightly disagree...haveing done many full size FAA crash investigations and also with over 30 years at this and the hobby. Not to mention having the similiar P-47 situation here my self...

When the back of the screws and slightly exposed lock nuts that are behind the prop hub hit the protuding part of the dummy engine radial due to any slight. Wobble, out if ballance, or motor shift...at that ammount if revelolution and ammount of spinning mass...would cause what we all saw in Aaros's video
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Old Oct 08, 2012, 05:24 PM
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I slightly disagree...haveing done many full size FAA crash investigations and also with over 30 years at this and the hobby. Not to mention having the similiar P-47 situation here my self...

When the back of the screws and slightly exposed lock nuts that are behind the prop hub hit the protuding part of the dummy engine radial due to any slight. Wobble, out if ballance, or motor shift...at that ammount if revelolution and ammount of spinning mass...would cause what we all saw in Aaros's video
That's fine, good discussions bring up the points to ultimately bring more light to this kind of situation and I edited that post to reflect clearer thought.

I'd suppose, from my understanding, that if you were to assemble the prop blades and hub with, say, a silicone glue that is inert to the plastic parts but has good shatter resistance, the assembly would have the stresses spread out over a large area and forestall an event like this.

And, again, look at the area of the hub break. The outer screw doesn't even include the front half of the hub in the assembly for strength.

Just points to ponder.
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Old Oct 08, 2012, 05:27 PM
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One thing is for certain. FMS needs to redesign these hubs. The most vital element to these planes is the part that allows the propellers to spin thousands upon thousands of times per minute. That part (the hub) should be destruction-proof. This cheap plastic is simply not cutting it and it's a matter of time before somebody gets hurt.
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Old Oct 08, 2012, 05:34 PM
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Originally Posted by Aros View Post
One thing is for certain. FMS needs to redesign these hubs. The most vital element to these planes is the part that allows the propellers to spin thousands upon thousands of times per minute. That part (the hub) should be destruction-proof. This cheap plastic is simply not cutting it and it's a matter of time before somebody gets hurt.
Aros,

Did you recover the blades? It would be interesting to see what kind of shear caused this?
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Old Oct 08, 2012, 05:40 PM
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Originally Posted by Aros View Post
One thing is for certain. FMS needs to redesign these hubs. The most vital element to these planes is the part that allows the propellers to spin thousands upon thousands of times per minute. That part (the hub) should be destruction-proof. This cheap plastic is simply not cutting it and it's a matter of time before somebody gets hurt.
Well said and that was an incredible image capture.
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Old Oct 08, 2012, 05:41 PM
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Originally Posted by Aros View Post
One thing is for certain. FMS needs to redesign these hubs. The most vital element to these planes is the part that allows the propellers to spin thousands upon thousands of times per minute. That part (the hub) should be destruction-proof. This cheap plastic is simply not cutting it and it's a matter of time before somebody gets hurt.
Absolutly....I agree 100%
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Old Oct 08, 2012, 05:54 PM
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Aros,

Did you recover the blades? It would be interesting to see what kind of shear caused this?
No we didn't find the props. However, the props from the P-47 crash showed break points at the base where they are screwed in. This is a photo of the P-47 hub. Same type of damage. You can see a piece of the prop still inside.
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Old Oct 08, 2012, 05:59 PM
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AROs
Are there parts of the props still in the hub of the F4U like your P47's hub?
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Old Oct 08, 2012, 06:00 PM
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Yes indeed.
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Old Oct 08, 2012, 06:01 PM
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"incredible image capture" - thats for sure. I showed the video to my dad. He didn't know what to expect and he sure got a shock. Thought someone must have hit it with a missile!!
It's a brilliant video, at least something good has come out of it. Also, because of it FMS will have no choice but to do something about it. I hope HK are watching in regard to their 1700 P40 also.
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Old Oct 08, 2012, 06:03 PM
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Another Theory

Another question is - how is the cowl attached...screws or magnets. If magnets I see a potential problem. Our 1400 cowl was modified to look like this particular version with the chin scoop. The first flight was fine, but the second was interesting. I used pink insulation to replace the leading edge of the cowl and use this a foundation for the chin scoop. Sometime during the second flight, I believe the cowl volume got overpressured and pushed the cowl outward toward the propeller. Since this was foam, albiet a denser foam, it made this horible noise - very similar to the video. We were able to recover since the props merely shaved away some of the dense foam - but this ruined the prop. Since the cowl on this boy is plastic and if the cowl released because of internal overpressure - I could see this kind result.

You can see by the video, two blades ejected immediately and at the same time - the third a mili second afterward. Without the benefit of the connection between the cowl and airframe this is all guess work based on what happened to us once.

I posted some of the still frames with all due respect. These are some of the most incredible in RC in flight sequences I have ever seen. Hopefully we make make lemonade out this little lemon episode.

Hink
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