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Prop failure

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Jon - Laser Engines05/08/2019 11:40:55
4765 forum posts
179 photos

I had a pretty rough time of it on saturday with various issues with my 3 models which limited my flying. The worst of which was my trusty La7 which was doing its usual thing when just as i was pulling up for a loop at full power there was a massive bang, followed by a thrubbing sound and a shower of debris from the front of the model.

I killed the engine and was able to glide down and land safely.

After i landed it was apparent that the prop was missing a blade and once we found the spinner that was missing a large chunk of itself too.

Looking at the damage its hard to tell if the prop went first or the spinner did. The most damaged blade of the prop has red paint on the tip so its clearly whacked the spinner at some point. This could have been as the spinner flew off after the prop failed, or the spinner could have failed and the prop was shattered as it departed. I cant see any signs of fatigue in the metal spinner or discolouration in the wood prop suggesting a long term crank that grew. I guess i will never know as it would all have happened so fast even a video of it would not help.

In any case, i will be doing even closer inspections of props in the future to hopefully avoid another issue.

20190805_110433.jpg

Chris Freeman 305/08/2019 11:51:44
233 forum posts
301 photos

I have 2 prop failures and it is not fun. the first was also a partial failure and the vibration caused huge damage to the airframe. The second failure resulted in the prop spitting through the hub and one blade embedding itself in the ground about 20 meters from were I was standing, it was also a 20 inch prop.

Please check the airframe well for damage

Cuban805/08/2019 11:59:54
2639 forum posts
13 photos

Wow! Quite a close shave, you did well to recover the model safely. Interesting that the failure occured at the point of pulling up in a full power loop - a small fault in the prop material + high gyroscopic forces perhaps led to a failure? Having less mass and being smaller - lower stress on the spinner - unlikely to be the cause of failure?

Edited By Cuban8 on 05/08/2019 12:02:13

Shaun Walsh05/08/2019 12:07:38
174 forum posts
12 photos

How was the spinner attached to the backplate and crankshaft? Perhaps the frequent use of an electric starter was placing a lot of stress on the spinner cone leading to stress cracks spreading through the material, then, when you open the throttle centripetal force is too much for the cracked spinner and away it goes.

Bob Cotsford05/08/2019 12:18:43
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7923 forum posts
436 photos

Is it possible that the spinner had slipped putting loads on the prop leading edge and the spinner cutout? I don't think it needs an electric starter to cause slippage, just the gradual compression of the wooden prop letting it slacken off or the spinner itself creeping during compression/power stroke pulses?

Robert Welford05/08/2019 12:53:53
160 forum posts
4 photos

Jon, what engine is the propeller/spinner off? 20x10 wood - I assume not a Laser 4 stroke.

flight105/08/2019 14:15:35
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609 forum posts
33 photos

Going by the fact that the split has traveled throught the hub to the other side rules out the spinner as it will have sheared of at the rub point. More likely to be a split or fault in the wood near the tip which opened up under the high loading flex as you pulled up and this has split apart folloing the wood grain.

as allways with wood props they require extra vigalnce each flight due to this vunrability (laminated one are less prone) and i always loosen the prop nut up when in storage/ after the days flying as the wood hub will keep compressing if not.

Jon - Laser Engines05/08/2019 14:18:07
4765 forum posts
179 photos

The engine is a laser 310v prototype engine which has been in the model for about 2 years. The prop is a many year old but little flown menz 20x10 that i have used for testing on and off. I have used menz props for years and not had issues before. The spinner is the original for the model and is about 9 years old. It was retained in the conventional way with a bolt down the middle.

This was only the 3rd flight for this prop on this engine as i damaged the last prop when at wings and wheels.

The engine revs relatively slowly at 7000 with this prop on the ground so over revving was not an issue. On the circuit before i did a full throttle descending turn and flat out pass. I do these from time to time just to get the hooligan out of my system. it didnt show any distress. I then did a half cuban into another circuit. at the end of that i pulled up for a loop and just as i pulled past the vertical it went. 

I dont think the spinner came loose as there were no fret marks on the backplate or locating lip. The remaining prop also shows no sign of touching the spinner. I have also had the cone come loose before and there is an obvious rattle that tells me to land. 

The start of the damage on one side of the spinner lines up perfectly with the damage on the broken blade and it seems the spinner then peeled itself like a satsuma round to the other prop cutout.

I looked at the failed surface of the spinner (cast material finish turned inside and out) and can see no sign of fatigue failure but i also cannot see any discolouration of the wood suggesting an old crack in the prop that spread.

My leading suspicion is that the sleeve nut was too tight in the hole and it split the hub as this was one of my old props that i reamed with a 13mm reamer. On wood this sometimes left the hole tight due to expansion of the wood so i now ream them 13.04mm and it has cured the problem. part of me doubts that .04mm can make enough difference to split the wood but its the only thing i can thing of.

As for the model, it survived well as i stopped the engine right away. The crankshaft is slightly bent, but its only about half a thou of runout and i dont have any spare cranks as it was a prototype engine so it will stay as is. Firewall integrity is good and there seems to be no other damage.

 

Edited By Jon - Laser Engines on 05/08/2019 14:41:27

Nigel R05/08/2019 14:51:39
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2970 forum posts
471 photos

"310v prototype"

...he says, quite casually, as if it was old news.

Jon - Laser Engines05/08/2019 16:05:21
4765 forum posts
179 photos
Posted by Nigel R on 05/08/2019 14:51:39:

"310v prototype"

...he says, quite casually, as if it was old news.

it is old news to me! i built it in about 2012 when we brought out the 155 so perhaps its been in the model longer than i thought.

Anyway on test when i first built one it wasnt as easy to use as the 300v so it never went into production. Since then i have made some mods and it would be fine to sell now but the twins are currently out of production. I also dont know if it would be worth replacing the 300v as the 310 is just a smidge taller.

Not sure if the 310 will ever see the light of day but its running well now and pulls the la7 like it was made of thin air.

Paul Marsh05/08/2019 16:24:14
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3666 forum posts
1039 photos

As mentioned before, I don't use wooden props on four stroke engines - for this reason. They are too light and probably had a backfire, due to insufficient mass, which destroyed the prop.

I had a OS 40FS with a wooden prop on a while back, and it split length-wise and flew off - now only use them on petrol or electric models.

Jon - Laser Engines05/08/2019 16:54:06
4765 forum posts
179 photos
Posted by Paul Marsh on 05/08/2019 16:24:14:

As mentioned before, I don't use wooden props on four stroke engines - for this reason. They are too light and probably had a backfire, due to insufficient mass, which destroyed the prop.

I had a OS 40FS with a wooden prop on a while back, and it split length-wise and flew off - now only use them on petrol or electric models.

Despite this incident this statement is incorrect. I fly all of my large 4 strokes on wooden props and i will continue to do so. Our twins run smoothly vs a single and the 4.5inch ali spinner provides plenty of flywheel effect. A backfire will not effect the prop in any way as the shaft will slip and the nut will come off. The exception to this would be a multi bolt hub and its a reason i hate multi bolt hubs.

In my case the prop nut was still tight and holding the prop on the crank when i landed so it certainly didnt backfire. this model has flown with wooden props for the last 9 years using 3 different engines without issue and the 20x8 that was on it before ran much faster so it wasnt an overloading issue.

Even in the case of a single cylinder engine wooden props are not a big issue. My saito 45 is happy as a clam on its wooden prop and i have used 13 and 15x5 props with great success.

The OS fs40 is a vicious little so n so. Mine will throw nylon props without any trouble so i dont think the material matters much. My enya 53 is a bit vicious too as the carb was too big. Putting a sleeve in the intake stopped it throwing props immediately.

Given that this is in fact my first prop failure in 30 years i think i will just chalk it up and move on. As i said before my feeling is that it was a hub failure caused by the hole for the sleeve nut being a little on the tight side. Its also possible the prop had some weakness in it and the two factors combined to cause the failure but to be honest i think it was probably the nut which makes it no one's fault but my own.

cymaz05/08/2019 18:22:16
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8579 forum posts
1160 photos

Jon, how long has the prop been tight against the thrust washer with the nut?

I only ask as I loosen all my wooden props after a days flying. The theory behind this that I’ve been told is that with the wood drying out and swelling due to differences in humidity and air temperatures, the props can be come loosened naturally over time.

Don Fry05/08/2019 19:30:58
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3816 forum posts
42 photos

I am a little suspicious of wooden props. I accept Jon's failure rate. But, when you look at an old glass plastic prop, with its pitted surface, you have to accept, that that's a mixture of grit, and more likely insect damage.

I've hit a hawk moth on a motorbike. I reckon that's a possibility. My recollection was this thing, hitting the visor (accepted at over the speed limit), was much the same as taking a knee to the face, in rugby.

Tom Sharp 205/08/2019 19:59:48
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3458 forum posts
17 photos

If you've got the other half it should mend with super glue.

Jon - Laser Engines05/08/2019 20:46:50
4765 forum posts
179 photos
Posted by cymaz on 05/08/2019 18:22:16:

Jon, how long has the prop been tight against the thrust washer with the nut?

I only ask as I loosen all my wooden props after a days flying. The theory behind this that I’ve been told is that with the wood drying out and swelling due to differences in humidity and air temperatures, the props can be come loosened naturally over time.

The prop has only been on there a few weeks. I busted the previous prop at wings and wheels. I fitted a spare 20x8 that i had in the car for the rest of the show but it was too noisy for my club field so i fitted the 20x10. I didnt fit this at WW as i didnt have a file and needed to modify the spinner cutout to get it to fit. Incidentally, i did consider this as a potential cause but have disregarded it as i didnt remove much and all the edges were nice and smooth.

When it comes to wood props i fit them, fly, then retighten once the engine is cool. Over the first 4 or 5 flights i can usually get a bit more torque on it but this gradually reduces. Once its stabilised i leave it well alone and they never come loose. The prop on my saito 45 powered nieuport must have been on there 3 years and i check it periodically. The stampe is a similar story.

In general i find wooden props pretty stable after they settle down. Its the first 4 or 5 flights when they tend to need tightening

Chris Freeman 306/08/2019 05:25:31
233 forum posts
301 photos

The prop failure I had was with a Master Airscrew 20x8 on a ST 3250, it failed right in the hub area. I had been given the prop by a well respected modeller and he had done some balancing of the prop, I thought it was ok but it failed. This is how it looked after the failure and was no more than 20 meters from where I was standing.

ma1.jpg

Jon - Laser Engines06/08/2019 08:21:42
4765 forum posts
179 photos

ooo ouch.

If memory serves, didnt those large ST engines have two spikes in the prop driver to prevent it slipping? If so i wonder if they caused a crack in the material just as i suspect my over tight sleeve nut hole did.

Frank Skilbeck06/08/2019 08:24:58
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4430 forum posts
101 photos

Just to add to to the wood vs composite props, friend had a 3 blade carbon fibre prop on his DLE 35cc powered zero start to de-laminate after a few flights, fortunately without a failure. The only wood props I have had fail have been as a result of a coming together with terra firma.

Peter Miller06/08/2019 08:49:12
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10064 forum posts
1192 photos
10 articles

I have had a couple of plastic props fail over the years but they were small ones on two strokes. One was when a spinner front came off on a control line model. It remove the entire front end of the model. The other was on an ST 25 and was due to a bubble in the hub of a Master airscrew

I do remember Neil Tidy saying that one should use wooden props on four strokes because there was a variation in acceleration as the engine ran because it only fired once every two revolutions which could cause flexing of the prop on plastic ones with eventual failure

This was back in the 80s when I was writing "The Engine Bay"

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