Manish Chandrayan Posted January 30, 2023 Share Posted January 30, 2023 Yes, repair. Before saying no! no!, do please read on .  Have a carbon 22x8 prop that has probably glass fiber filled hub (evident during drilling). The four bolt holes I drilled were drilled wrong 😌. I intend to fill up the holes and re-drill the hub correctly.  What would be better option? Fill with birch dowel epoxied in or fill with solid carbon rod epoxied in?  I am assuming wood will be a better option since it will be end grain and have better crush resistance.  Suggestions please  Quote Link to comment Share on other sites More sharing options...
Jon H Posted January 30, 2023 Share Posted January 30, 2023 Does the prop have a wooden core in the hub? If so then use wood.  Basically you want to make sure the plugs have the same density as the rest of the hub or the drill will wander.  Quote Link to comment Share on other sites More sharing options...
Manish Chandrayan Posted January 30, 2023 Author Share Posted January 30, 2023 Jon The prop hub has glass filling as was evident during drilling, where instead of wood boring or carbon powder what came out was whitish material. The intention to not re-drill holes at the same location but move off the plugged holes. It may make hand starting position difficult but should not be an issue using an electric starter.  Quote Link to comment Share on other sites More sharing options...
Solly Posted January 30, 2023 Share Posted January 30, 2023 Hmmm. If the repaired prop decided to throw a blade and there was damage to property or injuries to a person, I suspect your insurance might be affected. Personally I wouldn’t chance it. Quote Link to comment Share on other sites More sharing options...
Paul De Tourtoulon Posted January 30, 2023 Share Posted January 30, 2023 Maybe a photo would help, and is it on a 2 or 4 stroke ?. Quote Link to comment Share on other sites More sharing options...
Jon H Posted January 30, 2023 Share Posted January 30, 2023 I thought you were going to plug the old holes and then drill new ones in almost the same position. If you plan to move off the existing holes and drill new ones say 90- degrees off then then i agree with solly. you risk weakening the hub too much as it will be like a swiss cheese by the time you are done. Chuck it out and buy a new one.   3 Quote Link to comment Share on other sites More sharing options...
Manish Chandrayan Posted January 30, 2023 Author Share Posted January 30, 2023 1 hour ago, Paul De Tourtoulon said: Maybe a photo would help, and is it on a 2 or 4 stroke ?. Paul, if it works, it's going to go on a DA 50. Quote Link to comment Share on other sites More sharing options...
Rich Griff Posted January 30, 2023 Share Posted January 30, 2023 Safety says get a new prop. Â Make a shield or display piece with the ruined prop, even a raffle prize object. Quote Link to comment Share on other sites More sharing options...
Manish Chandrayan Posted January 30, 2023 Author Share Posted January 30, 2023 33 minutes ago, Jon - Laser Engines said: I thought you were going to plug the old holes and then drill new ones in almost the same position. If you plan to move off the existing holes and drill new ones say 90- degrees off then then i agree with solly. you risk weakening the hub too much as it will be like a swiss cheese by the time you are done. Chuck it out and buy a new one.    Would that mean it's better to drill at the same spot?  As for swiss cheese, the old holes are filled with plugs. And then there are larger engines with 6 holes (albeit with proportionally larger hubs).  The other option is that I plug the holes with wood /carbon rods and use the prop exclusively on single bolt engines. That way no need to drill new holes  My original question was, what would have a better chance in terms of crush resistance? End grain hard wood or solid carbon rod? Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted January 30, 2023 Share Posted January 30, 2023 What have you got Manish ? 3 correct holes and one off ? Do you have to redrill or is another method gonna work, could the holes be enlarged slighty and brass bushes fitted, or some other work around used. Â Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted January 30, 2023 Share Posted January 30, 2023 Don't you want the same crush resistance as the rest of the prop hub or all be it less? If you use carbon won't it act like pillars which will set up unusual stress between the back of the hub and washer at the front, probably resulting the prop slipping and coming loose.  If the old holes are filled with anything and then re-drilled surely the hub will not be as strong as a hub with only one set of holes?  Having experienced a spinner cone disintegrate at bench WOT testing and seeing how far the debris travelled (about 30 ft) I would really not want to be around if the prop hub failed and a blade departed. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted January 30, 2023 Share Posted January 30, 2023 Even drilling a prop correctly weakens the hub - correctly done is acceptable but even then there are preferred locations.  Once the grain structure across the hub is weakened by drilling then plugging restores virtually none of the lost strength - perhaps contact the manufacturer for advice but I would be reluctant to make such a repair. 4 Quote Link to comment Share on other sites More sharing options...
Don Fry Posted January 30, 2023 Share Posted January 30, 2023 You have messed a prop up. Swear words spoken. Start again. Not the first setback if you are using that size prop. Quote Link to comment Share on other sites More sharing options...
Brian Cooper Posted January 30, 2023 Share Posted January 30, 2023 From experience, the DA50 will happily turn a Mejzlik 23x8 propeller.  This would be an ideal time to change the prop. Quote Link to comment Share on other sites More sharing options...
kc Posted January 31, 2023 Share Posted January 31, 2023 The disadvantage to putting your own name on the forum is that if there were any accident then it would only take a few seconds for someone to Google your name and find that you have altered/damaged the prop and had many people advise against using the prop!   Don't use it! Quote Link to comment Share on other sites More sharing options...
GrumpyGnome Posted January 31, 2023 Share Posted January 31, 2023 I know the initial post said don't tell you to not repair it but personally, I would never, ever repair a propeller, unless it's very slow turning - e.g F1D.  A 'shed' blade made of carbon could travel very quickly, a surprising distance, and with a not inconsiderable energy.  No idea what the ensuing vibration would do to your engine - maybe it'd stop straight away...... Quote Link to comment Share on other sites More sharing options...
Wookman Posted January 31, 2023 Share Posted January 31, 2023 On 30/01/2023 at 13:20, Manish Chandrayan said: I intend to fill up the holes and re-drill the hub correctly. Â NO, just NO. Quote Link to comment Share on other sites More sharing options...
jeff2wings Posted January 31, 2023 Share Posted January 31, 2023 Unfortunately Manish hasn't stated what the error is, as in PCD or radial ,so you're all just guessing at solutions for the problem ,no doubt the reason for his question is that this prop would cost about £40 , so the only safe solution would be to have a prop driver and washer with the holes drilled to mach the prop,otherwise it's scrap. Quote Link to comment Share on other sites More sharing options...
Richard Wills 2 Posted January 31, 2023 Share Posted January 31, 2023 (edited) 3 hours ago, GrumpyGnome said: I know the initial post said don't tell you to not repair it but personally, I would never, ever repair a propeller, unless it's very slow turning - e.g F1D.  A 'shed' blade made of carbon could travel very quickly, a surprising distance, and with a not inconsiderable energy.  No idea what the ensuing vibration would do to your engine - maybe it'd stop straight away...... I have had a prop hub fail on a 20x8 prop at full throttle on a Moki 180, resulting in one blade coming off. The vibration caused the solid aluminium engine mount to shear off both beams , engine and half a prop were found 2 weeks later 200yds away at the bottom of the field, not a nice experience. Edited January 31, 2023 by Richard Wills 2 Quote Link to comment Share on other sites More sharing options...
GrumpyGnome Posted January 31, 2023 Share Posted January 31, 2023 (edited) 39 minutes ago, jeff2wings said: Unfortunately Manish hasn't stated what the error is, as in PCD or radial ,so you're all just guessing at solutions for the problem ,no doubt the reason for his question is that this prop would cost about £40 , so the only safe solution would be to have a prop driver and washer with the holes drilled to mach the prop,otherwise it's scrap.  Sorry, no. The only safe solution is to throw it in the bin and accept the wasted cost of around £40.... irrespective of PCD or radial. Or anything else. Edited January 31, 2023 by GrumpyGnome Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted February 2, 2023 Share Posted February 2, 2023 Punching some numbers into a centrifugal force calculator, it gives a 4737 Newton centrifugal force at 6000rpm, assuming a single blade mass of 120g and an effective radius of 100mm (rough guess of centre of gravity of one blade from the centre). Centrifugal force calculator Given how few glass or carbon fibres will actually cross all the way from one blade to the other after drilling two sets of holes, plus the cyclical acceleration /deceleration forces experienced at lower RPMs as the engine slows coming up against compression, then accelerates after firing. It sounds like a case of 'when' the blade will come off, not 'if'. 1 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 2, 2023 Share Posted February 2, 2023 On 30/01/2023 at 13:20, Manish Chandrayan said: Yes, repair. Before saying no! no!, do please read on . Â I read on... Â On 30/01/2023 at 13:20, Manish Chandrayan said: What would be better option? Â The better option is a new prop. Â Hang this one up. Or throw it away. Â A rotating knife rapidly transitioning to a ballistic state is not an outcome to tempt. Quote Link to comment Share on other sites More sharing options...
MattyB Posted February 2, 2023 Share Posted February 2, 2023 (edited) Guys, whilst I agree with you all there is no value in continuing to flog a dead horse. The OP seems to have got the message and left the thread, but on the off chance that he hasn't he certainly won't be able to say he wasn't warned if things do go wrong. Time to close this out methinks. Â Edited February 2, 2023 by MattyB 1 1 Quote Link to comment Share on other sites More sharing options...
Geoff S Posted February 2, 2023 Share Posted February 2, 2023 5 hours ago, Robin Colbourne said: Punching some numbers into a centrifugal force calculator, it gives a 4737 Newton centrifugal force at 6000rpm, assuming a single blade mass of 120g and an effective radius of 100mm (rough guess of centre of gravity of one blade from the centre). Centrifugal force calculator Given how few glass or carbon fibres will actually cross all the way from one blade to the other after drilling two sets of holes, plus the cyclical acceleration /deceleration forces experienced at lower RPMs as the engine slows coming up against compression, then accelerates after firing. It sounds like a case of 'when' the blade will come off, not 'if'.  When I was gainfully employed I was involved in the design of a rotating radio telemetry system to make measurements of strain and temperature on a gas turbine running on a test bed. The components were embedded in approx 40mm square modules in epoxy in an annular structure about 450mm in diameter (all this from memory). The g forces at about 7000 rpm were something like 40,000! So the forces acting on quite modestly sized rotating objects get very high very quickly. Amazingly we got very few electronic failures considering the temperature and g forces but the wiring to the thermocouples and strain gauges tended to fail progressively during a run.  In other words don't risk anything. A plastic spinner disintegrating on the ground was bad enough. Quote Link to comment Share on other sites More sharing options...
Paul De Tourtoulon Posted February 2, 2023 Share Posted February 2, 2023 I was waiting for a photo maybe it was just a leg puller,,,, Quote Link to comment Share on other sites More sharing options...
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