Author Topic: Stress cracks in carbon fiber props. (Read 2925 times)

Will Hinton · « **on:** September 24, 2007, 08:46:15 PM »

Hi y'all,
I recently had a bad news - good news experience. I ticked the tips of my favorite carbon fiber three blade at the FCM weekend this year. Darn. I pulled it off and examined it, intending to sand it down and balance it. I soon gave that idea up as impossible for I found tiny stress cracks in the hub!!

This prop had been carefully checked by me quite often, BUT-I failed to remove the spinner and examine the hub area, never dreaming that this might happen.
There were cracks both paralell to the one blade and cracks ACROSS EACH BLADE at the point where the blades and hub meet. I am glad to have ticked that rascal, it taught me a lesson - always remove the prop and examine the WHOLE bloody thing for flaws.

I won't go into the make of the prop because I think this can happen to any of them , so pilots, please, please start a good PM schedule on all your props, and include the hubs. These things aren't as indestructible as I thought!
Blessings,
Will

Ron King · « **Reply #1 on:** September 25, 2007, 05:54:41 AM »

Will,

I am very glad to hear you are okay and there was no other damage.

Now a question: Do you hand flip your engine to start it?

Hand flipping a carbon prop puts a lot of stress at the root of the blade. I know it's still the "macho" thing to do, but I shattered one while hand flipping and now use an electric starter.

We should also be careful when we are flipping the prop to draw fuel, too. These props are fragile - and expensive.

Take care,

Ron

Alan Hahn · « **Reply #2 on:** September 25, 2007, 07:52:13 AM »

Ron,
I don't have any carbon fiber props, but I am surprised that hand flipping could stress them more than the engine firing pulses. Yes you can put a lot of torque on the prop by hand flipping, but when it is running, I have to believe the torque as delivered to the hub must be larger than I could give by flipping.

Not really meaning to be argumentative, and maybe you are right, but it just doesn't sound right.

Jim Oliver · « **Reply #3 on:** September 25, 2007, 08:09:32 AM »

Will,
Thanks for the "heads up".

None of the guys I fly with use the carbon fiber props (we're sorta low techie types) but this type info needs wide distribution. Maybe a note to Len for the Safety article?

Thanks,
Jim

Ron King · « **Reply #4 on:** September 25, 2007, 10:25:11 AM »

Quote from: Alan Hahn on September 25, 2007, 07:52:13 AM

I don't have any carbon fiber props, but I am surprised that hand flipping could stress them more than the engine firing pulses. Yes you can put a lot of torque on the prop by hand flipping, but when it is running, I have to believe the torque as delivered to the hub must be larger than I could give by flipping.

Not really meaning to be argumentative, and maybe you are right, but it just doesn't sound right.

Alan,

I don't consider your question to be argumentative.

There is a big difference in the location of the force acting on the prop. When the engine is running, the forces are coming from the center hub and acting outward along the prop. These are the forces it was designed to handle.

When you hand flip a prop, you are exerting force out on the blade - essentially using it as a lever to move the crankshaft - and the force travels inward to the hub. If we could hand flip the props right against the hub, we might be able to minimize the stress, but we usually apply the stress out on the weaker outer section of the blade.

There is a world of difference in the load a prop (full scale or model) can handle if the force is applied in different places. This problem is not limited to carbon blades. I've also seen wooden props break when flipped.

Hope this helps explain my first post.

Ron

Bill Little · « **Reply #5 on:** September 25, 2007, 01:09:47 PM »

Hi Ron, Alan,

I can definitely see where the flipping can cause stress on the base of the blade. I have personally broken more than one wood prop while flipping!

As to the CF props, we always try to crank them by the "spinner snap". With the PAs, it is the way to go! Sunday we got in a bunch of flights with a PA 40 (not UL) and an Aero Tiger 36. Both (especially the PA 40) were cranking with one "snap".

Ron King · « **Reply #6 on:** September 25, 2007, 02:14:59 PM »

Quote from: Bill Little on September 25, 2007, 01:09:47 PM

As to the CF props, we always try to crank them by the "spinner snap". With the PAs, it is the way to go! Sunday we got in a bunch of flights with a PA 40 (not UL) and an Aero Tiger 36. Both (especially the PA 40) were cranking with one "snap".

Allen Goff is the best I've ever seen with the spinner snap. He would probably do well in West Side Story.

My arthritis just doesn't allow me to give the spinner much "snap".

I'll stick with my wimpy electric starter.

Ron

Bill Little · « **Reply #7 on:** September 25, 2007, 02:30:20 PM »

It seems to just take getting it as far off compression as you can and then whipping it back onto compression. One GREAT BIG NO-NO that Randy informed me about, not too long ago, was DO NOT FEEL FOR THE BUMP. If you do, you just wasted your "start"! And once the engine has run that day, I have found out he is right. (DUH!! like he's ever wrong! LOL!!)

Jim Treace · « **Reply #8 on:** September 26, 2007, 04:10:14 PM »

Will:
Excellent advice to all. I recently checked my cf prop and sure enough, there was the crack. Part of the hub actually had broken off. NOT GOOD!
Jim

Will Hinton · « **Reply #9 on:** September 27, 2007, 03:13:49 PM »

I think eberyone has made good points here, especially the stress of hand cranking. Actually, I've not started with anything but my electric starter since I got bit really hard with my old VF while trying the spinner filp thing. Ron, maybe my starter is wimpy, but I'm even wimpier!!! I just can't get into pain.
I guess I thought maybe these CF props were bullet proof at the hub and never bothered to check them. I had run this one THREE seasons!

I'm carrying this one with me all next year to spread the word, be safe guys.
Blessings,
Will
Ron, email me with an update on your situation.

Howard Rush · « **Reply #10 on:** September 27, 2007, 07:49:25 PM »

That's interesting. I wouldn't have expected a crack across the fibers at the hub. Did it actually break any carbon fibers, or is the crack just in the epoxy?

Hand flipping can sho' 'nuff be a problem. I hand flipped (with a piece of heater hose) my hefty Bollys, but I would probably refrain from clubbing anything thin or flexible or having visible gaps between fibers. I was holding a model once that had a three-blade carbon prop that was being hand flipped. The blade being flipped had a gap between fiber bundles. You could see through the resin. That's where it broke, and when it did, the model disintegrated. Big holes appeared in the covering, a flap exploded, and parts were flying off everywhere. Attempts to hold the plane nose-down and pinch the end of the (detached) pipe came to naught. It was cool to watch, because it wasn't my plane.

Steve Fitton · « **Reply #11 on:** September 27, 2007, 07:57:09 PM »

Carbon fibre/epoxy cannot fatigue, right? So the theory is that only the flipping is damaging them?

Jim Treace · « **Reply #12 on:** September 28, 2007, 09:06:50 AM »

I really don't know what caused the "cracks" in my cf hub, but they sure are there. Probably would take some micro analysis. On very close examination I don't see any loose fibers. Could be just epoxy void that broke off. More than likely, the basis was an original inclusion (void) or artifact (defect) in the molding (forming) of the prop. Put a little stress on a molded defect and that caused my problem. Don't know how you screen for this since it would just have to be done visually. Can't destructive test them. Kind of like testing flash bulbs!
Jim

Andrew Borgogna · « **Reply #13 on:** September 28, 2007, 09:40:40 AM »

Is there anybody here on the board that actually knows what the process is for making a carbon fiber prop. The question I would have is how the resin is poured/injected into the mold. Is it a simple gravity feed or is it presure injected and for sure I would like to know way precausions are being take to make sure there are no voids internally. Do the makes perform any kind of quality assurance inspection to make sure the prop is safe. I would like them to be xrayed for internal flaws, but the cost of that would make an already expensive prop way too expensive.

There are very few people at my flying site that use carbon fiber props, for myself I will never use them. I help a friend clean up after he crashed his plane with a carbon fiber prop. I was picking out fibers from my fingers for quite a while. Just tiny little strands of the fiber had penatrated into my fingers. Not fun! I would never touch a broken carbon fiber prop again without serious gloves.
Andy

Bill Little · « **Reply #14 on:** September 28, 2007, 09:57:17 AM »

HI Andy,

I have seen the process described years ago. The prop was all hand layered up from CF tow, the top of the mold clamped on, and then put in a vacuum pot to cure. Not a great description, but I hope you can visualize it.

Andrew Borgogna · « **Reply #15 on:** September 28, 2007, 11:08:46 AM »

Bill
The vacumn pot you mentioned is that for the purpose of pulling bubbles out of the resin?
Andy

Will Hinton · « **Reply #16 on:** September 29, 2007, 09:04:34 AM »

Howard,
I love your description of the incident in which you were involved! I had to laugh even though it was obviously a serious matter.
As to whether or not the fibers are broken on my prop, I really have no way of checking that out that would tell for sure. Any destructive process to check it would be false info because it would require damaging the prop and then who is to say it didn't happen during the testing?The only other way I can think of would be to magniflux and since these are not metal that won't work. Therefore, my only recourse in my case was to saw off part of a blade to insure I'm never tempted to use it again.
As to concerns about the layup process and general safety concerns with these, I watched Dan Winship lay up a prop and can attest to the procedure being sound. The bubble where someone could see through the prop blade is something that should never have been allowed out of the shop that built it. I examine every prop I buy before I buy it since seeing something similar once.
I've used CF 's since '98 with this being the first negative incident. I will continue to use them. As to the danger involved, I've had wooden props where the hub compacted and then flew apart while running - not as spectacular as Howard's experience, but not good either. Fortunately, no one was in line with the prop arc when it happened. Another reason for checking and setting the needle from BEHIND the airplane!!!

Blessings,
Will

peabody · « **Reply #17 on:** September 29, 2007, 12:25:50 PM »

I've watched Jim Casale "rebuild" early carbon props with super glue....

I realized that I was ruining these things while flipping with a piece of heater hose...

I ordered up a battery and a will use it from now on.........

But what about some stratigic CA

? Safe?

Thanks

Ward Van Duzer · « **Reply #18 on:** September 29, 2007, 02:40:58 PM »

Yup, I've repaired (and seen many others) many CF props with CA. Guess I learned that from Jimmy! BUT, only the tips where you may get small splits in the fibers, maybe from nicking the pavement, or...

But while on the subject of hub problems, beware of this one! Beautiful Bill Lee Prop from my TT .36. I guess I didn't tighten the aluminum spinner down enough and the cone vibrated against the prop enough to cut a circle on the hub of the blades. This didn't photo real well but look close and you can see what are really serious slices in the prop hub.

Oh well...

Ward-O

Howard Rush · « **Reply #19 on:** September 30, 2007, 12:03:32 AM »

Those nicks may not be a problem. There should be plenty of continuous fibers remaining, and I wouldn't expect cracks to propagate across the grain, although you should ask an actual structures guy. I got confidence in carbon prop hubs after laying up some Y&O copies. It took way more fiber than I expected. After stuffing all that yarn into the mold, I figured that it had to be pretty strong.

Ward Van Duzer · « **Reply #20 on:** September 30, 2007, 01:16:42 PM »

Thanks, I thought the same BUT...

I'm not willing to risk a thousand(?) dollar airframe over a 30 dollar prop!

W.

Patrick Rowan · « **Reply #21 on:** September 30, 2007, 07:33:56 PM »

I saw Will's prop today at the Columbus Stunt Contest. The cracks looked pretty bad.

I would not want to use it on my planes.

Bill Little · « **Reply #22 on:** September 30, 2007, 09:26:50 PM »

Quote from: Andrew Borgogna on September 28, 2007, 11:08:46 AM

Bill
The vacumn pot you mentioned is that for the purpose of pulling bubbles out of the resin?
Andy

That's my understanding, Andy. I had considered buying out a guy who was making props once (couldn't scare up the needed cash! LOL!!) so I haven't made any of my own..... yet.

Everyone that I talk to that mass produces anything *molded* always refers to a vacuum pot.

Andrew Borgogna · « **Reply #23 on:** October 01, 2007, 11:24:55 AM »

Thanks Bill, it makes sense that there would be something in the process to eleminate bubbles in the resin matrix. Now if they just had an xray machine for final examination. Not going to happen.

Will Hinton · « **Reply #24 on:** October 02, 2007, 04:46:32 PM »

Okay All,
Kenny Stevens took the prop that started this thread home with him from the Columbus contest this weekend. His wife is an xray specialist and they are going to try and xray it to determine the depth and extent of damage of the cracks. Should be interesting! I hope they can provide some definite information. I'll keep ya posted!
Blessings,
Will

Greg Hart · « **Reply #25 on:** October 03, 2007, 02:14:54 AM »

Ron,
I have only thought using a elec. starter was a added safty issue and a more sure way to start your engine. I guess I'm wrong. I'm glad I read this post! I use a leather glove to flip all my props. Wood CF and apc. Apc I think would still be OK to hand flip with my glove, but as long as my spinners are not to wet I will start using my electric starter. I did chip my 3 blade cf and now I'm going to inspect the hub area. I was able to repair the end that was chiped. I will only use it for a back up if needed.

Alan Hahn · « **Reply #26 on:** October 03, 2007, 02:29:50 AM »

I am guessing the cracks probably are between lines of fibers. As I "understand" the process---and those quotation marks mean I don't really know a lot!--the fibers are mostly laid out lengthwise, and there is very little cross connection between the fibers but the epoxy itself, which I think doesn't have a lot of intrinsic strength. If this is true, there wouldn't be a lot of strength in the transverse direction, an a prop could split pretty easily.

Andrew Borgogna · « **Reply #27 on:** October 03, 2007, 11:34:24 AM »

I agree that the xray could be interesting. Regarding the epoxy matrix, its purpose is just to keep the fibers in place. All the real strength comes from the carbon fiber. The same holds true with fiberglass, that's why you really need so little epoxy to attach the fiberglass to whatever surface you use it on.

As far as starters go, I am a believer in them and I use them. In the R/C world (yes I came back from the dark side) there is no shame in using a starter, in fact on my giant scale planes (read 22x10 props 63cc gas burner) I have a belt reduction drive 24v starter. I wouldn't think of putting my hands anywhere near that prop.

This has been a good thread, I am thinking we should get the word out to other people who use carbon fiber props that they may want to start using a starter or at least perform regular inspection of the hub.
Andy

Alan Hahn · « **Reply #28 on:** October 03, 2007, 12:41:32 PM »

Andrew,
What I was getting at is that you need some transverse fibers to bind the prop against splitting along the prop diameter. I know some of the RC carbon props I have seen in ads have a woven cloth visible on the surface. On the few CL props I have seen, I don't recall seeing this feature, so I wonder if they are adding some fiber perpendicular to the long lengthwise fibers. It is the transverse fibers that give you strength against the power pulses and prop flipping abuse. The long ones hold it together under the strong "centrifugal" forces, but do nothing in the direction of rotation. As you say (and I said too) the epoxy by itself gives you very little.

So here is the question. If your prop has split, can you peel off lengths in the longwise direction. If so I am guessing that the props need some fibers laid down at 90 degrees to the diameter direction.