Prop cap crushed Prop.

[fuel9m]

Hi, I'm after some more opinions on this coz I'm not sure!

I just fitted a new propellor and hope I've not damaged it seriously in the process. The old one fell off because the bolts sheared, the broken bolts were removed from the pulley and the holes opened up to M8 from M6. The torque loadings for the original M6 bolts were 10 nm but as I was using new M8 bolts I torqued them up to 14 nm (from an online chart detailing appropriate torque figures for certain bolts).

So all 6 bolts were torqued to 14 nm and when I stepped back to check it I noticed that the wooden prop had been crushed ever so slightly at the edge of the cap. There is also one section where a tiny hairline crack about 5mm long goes from the cap outwards. Im hoping this is not a major issue and will monitor it regularly, looks like surface damage only. I guess the recommended torque of 10 nm for the M6 bolts was also assigned because they knew that wouldnt crush wooden props, I've backed the M8 bolts off and torqued them to 10 nm to ease my mind.

Does anyone think its dangerous to leave it like this? Cheers.

[pete_b]

Keep a VERY close eye on the crack/mark

and retorque the bolts after running for a while.

Pete b

[fuel9m]

Yeah I've marked it with a permanent marker so I can see if it grows and will check it before every take-off in addition to checking the torque. Thanks.

[morgy]

Any crack in a prop sounds like bad news to me. I fly model planes and i wouldnt fly one of my models with a chipped, cracked split or damaged prop of any kind. I would get it checked out by a pro before even starting it. As i said i wouldnt fly one of my planes costing a few £100's how much is your life worth.

A photo on here would be good for those in the know to have a look at.

[fuel9m]

Any crack in a prop sounds like bad news to me. I fly model planes and i wouldnt fly one of my models with a chipped, cracked split or damaged prop of any kind. I would get it checked out by a pro before even starting it. As i said i wouldnt fly one of my planes costing a few £100's how much is your life worth.

A photo on here would be good for those in the know to have a look at.

How do you post photos on here? I will take one tomorrow in daylight and stick it on.

It looks like the pressure from the cap has just distorted and crushed the wood slightly, the crack is hairline on the surface - could even just be in the varnish. When I looked at my old prop it had an identation mark where the cap had sunk into the wood prop about 1 or 2mm. I've been running it on the ground to test it through the rev range and there hasn't been any changes so far. Will run it a few more times on the ground and just monitor it for a while.

[gordon_dunn]

Crack anywhere near the prop hub is bad news- don't even consider running the prop on the ground never mind flying it. Theres no point monitoring the crack, it will potentially snap at any time- and the consequences could be serious. Discard it immediately, and replace with a genuine spare part.

(Not advice- instruction!)

[Guest]

i think you can get resin to repair it which is very strong put some in it if you can and if you aint gonna change it just to be safer

[morgy]

the only thing i can think of is that as you have used a larger diameter bolt and used the correct torque setting for that bolt. BUT the prop was not ment to withstand that torque setting. i would have used the old setting with a bit of lock tight. the only down side to this would be removing the bolts is you need to transport it to your field or back home. personaly i would have got a new part and kept it all standard..

As for the photos put then on a photobucket acount or simular and then use the like and the IMG box above the smileys and add it between the two IMG'S

[Phil_P]

I think that it is very hard for folks on here to give a definitive answer to this problem. Without seeing the crack, it is impossible to form a sensible opinion. That is why you will get so many folks saying 'don't do it', because they are erring on the side of safety. However, it's only you that can make this choice based on what you can see. If it truly is just a crack in the varnish, then I suspect it is a non issue (I personally would continue using a prop if there was a crack only in the varnish), but only you can make that call. I have glued a few chips and small cracks in prop tips, and I know that there are experts who produce very effective repairs on really quite badly damaged props.

As for the crushing issue, I think that 10Nm is probably a sensible sort of value for the torque, and I think that a lot of people (myself included) will have experienced some degree of crushing of the prop hub. My wooden prop quite clearly carries an indentation of the drive flange on the motor, from where it has been pressed in.

As others have said, if you decide to go ahead and use the prop, recheck the torque after a fairly short period of running. Also, remember that atmospheric conditions will affect the tightness. If the air was moist when you first tightened it, the wood would be slightly swollen, and if it dries out then it is likely that there will be a tiny amount of shrinkage and hence loosening of the prop.

[norman]

Wooden props aren't that expensive, one of the great qualities of wood is its resilience, but how do you know where a crack, shake or split goes within the prop structure? Would you really want to find out how sound your prop is as you cross the boundary fence of your flying field at 200 feet? Or worse still have the thing come apart when you are ground running it next to people.... If it holds together for a couple of flying hours you will begin to trust its integrity and stop worrying about the crack... that is when it will come onto the stage. Sod's Law.

Sounds to me as though Gordon has sound advice and someone else has a new wall adornment for his study.

[fuel9m]

Thanks for the input and advice gents. I took everything off again last night and closely examined the prop. It really does appear to be minor surface damage. I could see by looking down the centre hole that the 'crack' doesnt have any depth to it - leading me to believe that its just the surface finish that is damaged.

I spoke to a few prop suppliers and they said if I was really concerned I could sand out the tiny split and fill the gap with superglue and baking powder which apparently sets rock solid.

I am very safety conscious but also interested to see if I can effectively repair this so I'm 100% confident with it before I shell out for another prop. If I had bought a new pulley it would of cost £180 from PAP and PAP also quoted £180 for a new prop. In the end a machine shop very professionally drilled out the M6 holes to M8 and re-threaded for £35 and I got an identical prop from Manchester Paragliders for £120. So, all in I saved over £200. Buying like for like is not always the best solution.

The only thing that has gone wrong was my doing - torqueing the M8 bolts to an appropriate value for themselves but not stopping to think that the torque would crush the prop. If Id just torqued them to the M6 value of 10nm then I prob wouldnt have crushed the prop! - oh well, you learn something everyday.

[Guest]

There are two very distinct groups who paramotor. Those who apply all the rigours of years of aviation theory and practise to these funny little machines and those who do not.

You can make frames out of aircraft grade materials with certified welders or you can bend some tube and mig it together.

You can strip down and inspect after any stike however minimal or you can use a bit of glue.

You can report incidents to a central database or you can keep it quiet.

You can modify your machine with different bolts and torque settings or you can refer to its manufacturer for approval, specifications and acceptable servicing.

You can comply with the law or you can be a maverick.

Or you can wander between these extremes..................................

Edited November 26, 2008 by Guest

[huffhuff]

I am very safety conscious

...so you'll be doing your preflight checks a bit more thorough from now on

[norman]

But then if it IS just a scratch.....

At last you can crack test metal with dye and powder, carbon fibre and wood are a little more difficult aren't they?

[fuel9m]

I am very safety conscious

...so you'll be doing your preflight checks a bit more thorough from now on

Ha ha , yeah you got that right mate, maybe I was being a bit hypocritical then but, seriously, I felt like an idiot when it happened and kept playing over in my mind what could've happened. If it was loose bolts then it wont be happening again as I have a shiny new torque wrench which will be getting used before every take off.

[gordon_dunn]

There are two very distinct groups ....

You can comply with the law or you can be a maverick.

Or you can wander between these extremes..................................

That's right Francis.... but the guy with the cracked prop won't get hurt.... he's probably in the safest position.... it will be someone else that will get clipped with the prop fragments... We all have a responsibilty for safety, for ourselves and others.... and should encourage others to do things right.

I've always been following the guidance- anything other than minor dressing, repair to the blade tips or trailing edge should be avoided.

I wouldn't be flying in the same field as someone if that had a potentially unsafe item such as this.

Another useful quote my instructor taught me- "If there's doubt, then there's no doubt"-- if you are doubtful at all that there is a crack in that hub, then discard it immediately- the risk is not worth taking. You must have 100% confidence in your kit before you decide to fly.

[pete_b]

There are two very distinct groups ....

You can comply with the law or you can be a maverick.

Or you can wander between these extremes..................................

That's right Francis.... but the guy with the cracked prop won't get hurt.... he's probably in the safest position.... it will be someone else that will get clipped with the prop fragments... We all have a responsibilty for safety, for ourselves and others.... and should encourage others to do things right.

I've always been following the guidance- anything other than minor dressing, repair to the blade tips or trailing edge should be avoided.

I wouldn't be flying in the same field as someone if that had a potentially unsafe item such as this.

Another useful quote my instructor taught me- "If there's doubt, then there's no doubt"-- if you are doubtful at all that there is a crack in that hub, then discard it immediately- the risk is not worth taking. You must have 100% confidence in your kit before you decide to fly.

As I understand it is not the hub but the prop and is probably just the varnish cracked, but as you say it should be checked and not used unless you are 100% sure it is just the varnish.

Pete b

[Guest]

Seems to me Fuel_9m you are a very concerned and conscientious pilot. Having had one prop fall off in flight and having reported same you now invite peer review of your options. "Power to your elbow my friend", you will come to the responsible course of action that is right in your individual case through your researches.

[fuel9m]

Seems to me Fuel_9m you are a very concerned and conscientious pilot. Having had one prop fall off in flight and having reported same you now invite peer review of your options. "Power to your elbow my friend", you will come to the responsible course of action that is right in your individual case through your researches.

Thanks. The 'crack' appears to be only on the surface and not extending deep into the wood - it looks more like a crushing force has made a slight split on the surface. To confirm this I am going to sand out the damage to create a small hollow. If no further evidence of damage is visible then I will fill the hollow either with resin or other appropriate filler. Money isn't really an issue if a new prop is required but I find it more rewarding to salvage/repair where possible.