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fuel9m

Prop fell off! HELP!

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Hi chaps

So there I was, paramotoring along quite happily with my friend over the east coast of scotlands beaches when 'whirring' noise started. Looked around to see no prop! Fortunately there was no-one around for miles and it was over grassy fields.

All 6 bolts which hold it on had sheared completely. There was no abnormal vibration before I lost it and I've never crashed the paramotor. All I can think of is that some of the bolts may have been loose - this perhaps means the metal plate that the prop attaches to was out accelerating the prop putting a shear force on the other bolts which eventually gave up. If anyone else can shine some light on the matter I'd be interested as I don't want this happening again - obviously.

The motor is a PAP 1100 AS with inverted engine and prop spins anticlockwise. Does anyone know where I could get a replacement 2 blade prop? Are the bolts special or just standard items?

Like I say, if anyone has any other thoughts on why this might have happened I'm all ears. All my mate and I could come up with is that a few of the bolts must have been loose. The prop was virtually new and fitted by the previous owner. I checked them before my first flight on it and they were fine. I've put 4 hours on it since but havent checked those bolts on my last 2 flights :oops:

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Hi Fuel,

I'm only a newbie and can't really comment on your exact problem although your diagnosis seems to fit in my head. But, I would have thought checking your prop nuts would be a definite in your preflight check?

I certainly wouldn't want to be going up there without checking first. I did have a preflight checklist somewhere but can't out my hands on it...but i'd imagine that prop nuts rates up there with checking your throttle linkages etc.

Ben

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You're totally right mate, I shouldve checked them before every single flight, I checked everything else last night but forgot about the prop bolts. Maybe this wouldnt have happened if I'd done that but I'm not 100% convinced yet that it was loose bolts that caused this, its just my theory that a few were loose. I was so so lucky that it happened where it did. An expensive mistake to make but its only gonna cost me to get the 6 sheared bolts removed (dont have the tools or time) and a new prop which is a small price to pay compared to what wouldve happened if the prop had hit a building, vehicle or worse.

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Did you have high tensile steel nuts and bolts fitted? They are more susceptible to fatigue and fracture than mild or medium steel. It might be worth going for a quiet walk over the route you flew, to pick up the debris. There may be clues to the cause there. It's a relief that it didn't end disastrously either for you or anyone else.

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Yep, I guess that simple check is for both pilot safety, but probably more for the innocent bystanders down below. Ouch, serious ramifications there. If you lose the prop mid flight (and assuming it doesn't hit you or the lines) then worse case you just glide to safety, but if a 1 pence peice does that damage from top of Eifle tower...

I would suggest, and am ready to be argued with (long time since I played around getting dirty), that you invest in some grade 8 (or whatever is stronger these days) bolts and a torque wrench. Not sure how tight you'll need to go, i'm sure someone on here will say or it'll be in your engine manual.

I think it would eitehr have been caused by over tightening, or not tightening enough.

A lesson learned and luckily nobody hit - other than your wallet :)

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The prop bolts are the only fasteners on my motor that I tighten with a torque wrench.

With the new prop being fitted, it has possibly dried out slightly, and thus shrunk. It wouldn't take much shrinkage to loosen the bolts.

Because the bolts rely on friction between the assembly components to minimise torsional loads and thus the risk of shear, any loosening at all is potentially disastrous. Although the normal tensile load on the bolts would all but disappear, the sheer force mentioned above, and secondly, a small but significant bending moment on each of the bolts would increase. With each acceleration and deceleration of the engine, this bending moment would reverse direction, increasing massively the risk of fatigue fracture.

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If you have bolts with nuts you should use nylocks (new ones each time you undo them)

Just bolts should have spring washers on only. (new ones each time you undo them)

Normal bolts will do, you do not need high tensile as the torque on the bolts is very low and the prop is held by friction between the plates.

Pete b

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Some good feedback so far, thanks much appreciated. I did think that the metal plate and prop mating face would have to be super tight together to minimise any minor twisting motion during acceleration and deceleration. Never thought about a brand new prop shrinking but it makes sense now I think about it, that really could have created slightly loose bolts which just rattled looser with time. Damn, I checked them and they were tight 2 flying hours ago but, lesson learnt, they need checked every time.

The bolts did not go right through the plate so nyloc nuts weren't an option - however spring washers could've been fitted but weren't - they will be next time! Anyone reckon its worth using loctite thread lock as well? Any tips to know when fitting a new prop assuming that if its brand new it will be balalnced properly?

Thanks, Joe

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Dont use thread lock as this will not let you take out any slack after it has set..

You should only really have to retighten the bolts once after a few hours on a new prop.

I did 250+ hours on my pap and only tightened them once after approx 2 hours from new

Pete b

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Once the prop has settled down environmentally, unlike Pete, I probably would use loctite. I'd torque down the nbolts without it initially, then after a few hours running, remove each bolt in turn, loctite it, then replace and re-torque to spec.

The alternative would be to replace the bolts with studs (threaded externally both ends). Then loctite the end that screws into the hub, and put nylock (or similar) nuts on the outboard end.

Adventure actually state in their manual that you can remove the nylock nuts up to five times before you need to replace them, and they also supply a spare set of four in the toolkit.

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Adventure actually state in their manual that you can remove the nylock nuts up to five times before you need to replace them, and they also supply a spare set of four in the toolkit.

Standard nylock drills for any use. Simply put its because the rubberised threadlock moulded into the nut is F****d after 5 goes.

On my 320bhp motorbike, the nylocks were binned EVERY time.

SW :D

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Just inspected all the damage and its worse than I thought. The 6 threaded holes where the bolts fit into are pretty much knackered. The bolts must of been moving around inside the holes just before they sheared.

I was thinking of taking the whole cog (is that what its called) to a machine shop and asking if they could remove the 5 remaining bolts, drill open the holes and re-thread them for a bigger bolt. Then I'd have to open up the holes on the plate that fits over the prop and buy a prop with bigger holes to.....aaaargh, nothing is ever easy. I could always just try and get hold of a new cog but no idea how or the cost involved, not cheap I'm sure.

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I personally would have them drilled out to the next size and get them to do the plate at the same time.

Its no problem to drill the prop.

Pete b

But at the end of the day do what you will be happy with.

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Your suggestion will work, as you have the bits you may as well do it as suggested above.

If you have a bit of cash burning a hole, get a new one and keep the other as a spare.

If skint, I would go for the above.

SW :D

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Yeah money doesnt grow on trees....at least not in my garden. Thought I might have been able to drill right through and fit a nut on the protuding thread of a longer bolt but the geometry won't allow it.

Hopefully the cheaper option will be to get the M6 holes opened up to M8, along with the plate. I need to buy a new prop as this one is trashed, I will try to find one with same hole pattern but larger diameter holes.

Cheers

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There will almost certainly be no issues in going up from a M6 to M8 thread in your prop pulley. Removing the remains of the old bolts may be tricky. If you can get a sharp centre punch on the centre of the bolt remains, then drill out, preferably on a good pillar drill or lathe, that would be my choice. Whatever you do DON'T use e-z out stud extractors, cos if the remains are even slightly seized, the extractor WILL snap and leave you well and truly kyboshed. The other alternative would be to go half way between the existing knackered holes, and have some new ones drilled, either for M6 or M8.

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There will almost certainly be no issues in going up from a M6 to M8 thread in your prop pulley. Removing the remains of the old bolts may be tricky. If you can get a sharp centre punch on the centre of the bolt remains, then drill out, preferably on a good pillar drill or lathe, that would be my choice. Whatever you do DON'T use e-z out stud extractors, cos if the remains are even slightly seized, the extractor WILL snap and leave you well and truly kyboshed. The other alternative would be to go half way between the existing knackered holes, and have some new ones drilled, either for M6 or M8.

Good idea about going halfway between the holes (as long as it doesnt affect structural integrity of the pulley). My machine shop friend with all his tools will try and remove the broken bolts and open the existing holes up to M8. If it proves impossible I'll ask him to drill the new M6 holes. Cheers Phil!

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On trying out my new(2nd hand) paramotor in the garden twice the prop bolts came loose.

My solution has been to drill each bolt head through and tie all four together with tying

wire.It wasn't very easy but I have done it-have yet to test this in flight.

Some cars and bikes also use this system.

Andrew (Luton).

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A quick way to check for movement of prop bolts and nuts is to apply a tiny dab of nail varnish on the nut or bolt head, and on the prop plate next to it. If there is any movement, the spots will not be aligned with each other. Nail varnish sticks to most surfaces and does not get wiped off easily. A bit of blue loctite on the bolt in addition to this along with new nylock nuts would prevent / spot loosening.

On the hollow headed shackle bolts on PAP swing arms, I pass a small cable tie through the head and around the shackle bit to prevent it from loosening.

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