Electric propellor fatigue resulting in possible failure

[Owdlad]

Today I was loading up the car with my models when I noticed that the propellor on my model looked like it had developed white lines in the material around the hub.

The propellor is by a well know manufacturer of the electric thin type. Power plant is E-flight power 25 turning a 13x6.5 electric prop using 4s lipo.

On closer inspection of the prop the white lines around the hub looked like stretch marks. When I checked the blade for stiffness it didn't feel as it should, it felt rubbery then blade just broke away from the centre of the hub. Likewise the other blade did the same.

I believe that I was lucky as the prop was ready to fail under load either on the ground or in the air with the resulting carnage that follows.

My advice is to always check your props before you fly or run up the power plant.

Owdlad

[Jon H]

This can happen if props dry out. It sounds mad, but if you boil it in a pan of water this can sometimes help!

[onetenor]

JUST DON'T TAKE A CHANCE. NOT EVEN BOILED just not worth the risk for the price of a new prop. I have seen even quite thin prop blades driven into pine planking. Once a break occurs there is absolutely no way of knowing where the blade might end up.

Please discard any props with any sign of damage of any sort . PLEASE PLEASE

[Trevor Crook]

I've never had that happen. Does the model belly land, or has it experienced nose overs or prop strikes? I can't think of other stresses that would cause this, as there should be no vibration if the prop is balanced.

As said above, very important not to take any chances with props. I still use some of my old glow props if they are the size I need, they seem to perform pretty well and are more robust than dedicated leccy ones.

[Nigel Dell]

Posted by onetenor on 30/06/2016 22:19:55:

JUST DON'T TAKE A CHANCE. NOT EVEN BOILED just not worth the risk for the price of a new prop. I have seen even quite thin prop blades driven into pine planking. Once a break occurs there is absolutely no way of knowing where the blade might end up.

Please discard any props with any sign of damage of any sort . PLEASE PLEASE

I echo your sentiments onetenor, I think Jon was meaning to boil before using as was the practise many moons ago with Nylon props to relieve the manufacturing stresses, obviously white stress lines = new prop.

Edited By Nigel Dell on 01/07/2016 08:11:33

[ken anderson.]

i had a folder throw a blade in the air...I never knew they were rated up to a certain RPM ...until the lad in the LMS told me.....

ken Anderson...ne...1 tech/not dept.

[Trevor]

I agree with all the cautionary advice. Assuming we are talking about APCe props here, I have used them almost exclusively for many years and never seen this issue. The 13 x 6.5 on a 4s LiPo happens to be my favourite too!

Like Trevor C., I wonder whether this prop been subject to any external stresses? A nose over which would break a wooden prop can leave a nylon one apparently intact so this experience is a useful reminder to check them regularly, particular after any untidy landings.

Trevor H.

[Owdlad]

I always ballance my props and regally re-check if I detect any vibration, I also ballance the spinner.

The prop in question has probably had 3hrs run time since it was a replacement for one I broke earlier.

Yes the prop has had the ocational nose over when wind milling on landing on the grass strip and I accept that this is probably the root cause

Last week under my mentors guidance I have been practicing violent manoeuvres with the associated rapid throttling response....... coincidence? Possibly

My mentor who is a retired F3a / display pilot says he remembers encountering this issue many years ago with this manufacturer of electric prop when he was pioneering Electric poweed systems for F3a. He says that he reported his findings to the manufacturer who subsequently issued a safety concern on their Web page.

I have posted this issue as a matter of safety just to highlight that it is important to check your prop before powering up.

I believe that the Bmfa guidelines also state this.

Safe flying

Owdlad

[Jon H]

Posted by Nigel Dell on 01/07/2016 08:10:59:

Posted by onetenor on 30/06/2016 22:19:55:

JUST DON'T TAKE A CHANCE. NOT EVEN BOILED just not worth the risk for the price of a new prop. I have seen even quite thin prop blades driven into pine planking. Once a break occurs there is absolutely no way of knowing where the blade might end up.

Please discard any props with any sign of damage of any sort . PLEASE PLEASE

I echo your sentiments onetenor, I think Jon was meaning to boil before using as was the practise many moons ago with Nylon props to relieve the manufacturing stresses, obviously white stress lines = new prop.

Edited By Nigel Dell on 01/07/2016 08:11:33

Pretty much Nigel. Props that got white lines in them like that used to get a boiling and as you say new props sometimes got it as well. These days with props only being a few quid its not worth it.

As for the cause of the failure, the flexing from normal use could be all it is. Especially if the prop had dried out a touch. In any event, i would just get another one and be done with it !

[IanN]

Posted by Jon Harper - Laser Engines on 30/06/2016 21:57:40:

This can happen if props dry out. It sounds mad, but if you boil it in a pan of water this can sometimes help!

That takes me back. Used to boil all our plastic / nylon props for our control liners back in the day

[john stones 1 - Moderator]

Posted by Jon Harper - Laser Engines on 30/06/2016 21:57:40:

This can happen if props dry out. It sounds mad, but if you boil it in a pan of water this can sometimes help!

Always remove prop from model before boiling

John

[Phil Green]

Split prop? Easy fix, just follow the advert:

[Stuphedd]

I do use wooden and Air Master ic props on my leccy models , not all the time , but I find them in many cases just as good and sometimes better than the spindly leccy ones .

cheers

[The Wright Stuff]

Posted by Phil Green on 01/07/2016 15:17:48:

Split prop? Easy fix, just follow the advert:

I hope the tape is applied to both sides of the prop blade, otherwise this could be dangerous!

[Nigel Dell]

Glad to see even now Phil you are practising safe propping!

[Erfolg]

Of passing interest is that it was common practise when i worked in the plastic moulding industry some +40 years back to drop some plastic mouldings into a vat of water. The reason being that straight out of the injection moulding machine the plastic would be brittle. Left in the vat for some time, they would become flexible. Polypropylene from memory.

In the case of nylon (66 from memory) once the bag of chips was opened, there was only a limited time to get the stuff into the machines hopper. If left out all day, the material would absorb water, which then turned to steam in the moulding process.

I am not sure that in the case of a propeller that the white lines are what used to be called fibril alignment, where the Mers had become aligned due to forces such as stretching. If I could remember there was a whole lot more about the phenomena. Most frequently seen on the handles of supermarket polythene bags, when heavily loaded, Although it would be a surprise if this was the same thing on a model propeller. Although it could be if a cheap low strength polymer was the material.

Just had another thought, the viable temperature range for the majority of polymers is very narrow , compared to metals. In cold weather the properties are very different to the data sheet, the same is true at higher temperatures.

Edited By Erfolg on 01/07/2016 22:25:33

[jonryan]

Be aware. Be very aware

http://www.dailymail.co.uk/sciencetech/article-3537631/Bizarre-slow-motion-footage-captures-drones-catapulted-slabs-pork-study-blades-tear-human-flesh.html
 

Edited By jonryan on 06/07/2016 14:25:50

Edited By jonryan on 06/07/2016 14:26:24

[IanN]

Yep, just because a prop is small, or isn't attached to a noisy i.c. engine, doesn't mean it's not every bit as dangerous

[The Wright Stuff]

Posted by IanN on 06/07/2016 15:08:10:

Yep, just because a prop is small, or isn't attached to a noisy i.c. engine, doesn't mean it's not every bit as dangerous

Agree with the sentiment, but I don't agree literally. A smaller prop is (a) likely to be attached to a lower power source of energy than a big prop, (b) it weighs less and so has less angular momentum, and (c) for any given RPM, the tips will be travelling more slowly. So still dangerous, but not every bit as dangerous!

I have a gut instinct that props intended for electric use are thinner than ones intended for IC use, and therefore possibly sharper.

[IanN]

Posted by The Wright Stuff on 06/07/2016 15:32:30:

Posted by IanN on 06/07/2016 15:08:10:

Yep, just because a prop is small, or isn't attached to a noisy i.c. engine, doesn't mean it's not every bit as dangerous

Agree with the sentiment, but I don't agree literally. A smaller prop is (a) likely to be attached to a lower power source of energy than a big prop, (b) it weighs less and so has less angular momentum, and (c) for any given RPM, the tips will be travelling more slowly. So still dangerous, but not every bit as dangerous!

I have a gut instinct that props intended for electric use are thinner than ones intended for IC use, and therefore possibly sharper.

In the interests of good practise, and preserving my digits intact, I'll stick with treating them as "every bit as dangerous"

[Peter Christy]

The prop on my little 0.8cc PAW diesel bites a lot more than the ones on my bigger engines - and I have the scars to prove it!

--

Pete

[The Wright Stuff]

In the interests of good practise, and preserving my digits intact, I'll stick with treating them as "every bit as dangerous"

Oh, absolutely. My comment was in the context of scientific characterisation (as in the pork study link) - no suggestion of changing good practice.

[Bill Wood]

Owdlad - you mentioned that you also balance the spinner. I have asked this question on the forum before - How do you balance a spinner? With the risk of going off topic, can anyone tell me?