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The illusion of safety?
« on: Nov 04, 2019, 5:42 pm »
Fibreglass versus some high speed plastic blades - n competition!
https://youtu.be/YGlL9rAC7So?t=624

Re: The illusion of safety?
« Reply #1 on: Nov 04, 2019, 8:04 pm »
Thats a near miss if ever i saw one!

Anybodies guess what happened, but overstressed bladed almost certainly played a part. If i was investigating it I'd be looking at tip clearance, hub failure, or belt failure as likely contributors.

My advice is keep blade tip speeds within the manufacturers limit and they will taje all sorts of punishment without failing.

Ian
Ian Brooks
Gloucester, UK

Re: The illusion of safety?
« Reply #2 on: Nov 04, 2019, 8:53 pm »
It looks as if the blades pulled out of the hub and the securing flanges failed. Last time I had a fan let go ( thankfully in the mid 1980's ) the roots failed due to over speeding and a pressure surge back thro the duct - they must have been absolutely on the limit already .


Any idea on blade life Ian ?   It can't be UV as the part that failed is inside the hub. 


Very lucky the failure happened where it did and not on the water or mud flats.  B good reason for a routine blade change at service time!


That said, the containment worked.  Some bits came out,  but not at significant speed by the look of it.



Re: The illusion of safety?
« Reply #3 on: Nov 04, 2019, 10:08 pm »

Any idea on blade life Ian ?   It can't be UV as the part that failed is inside the hub. 


I did this for the MCA back in 2013. At the historically accepted 168m/s a 5z blade has a very short life. A 127m/s the blades should be OK, so this should represent the absolute limit. However, bringing the speed down to 110 m/s will result in infinite life with safety margin for unexpected events - like hats!

Ian
Ian Brooks
Gloucester, UK

Re: The illusion of safety?
« Reply #4 on: Nov 04, 2019, 10:16 pm »
If i was investigating it I'd be looking at tip clearance, hub failure, or belt failure as likely contributors.


That would be my list too Ian! Of the three though I wonder if the most likely is the belt (or something else) going through the fan? Seems like the sort of blade failure that a sudden mechanical load on a blade would create. I also notice they have gone back to BHC to purchase a replacement craft...

Cheers

Paul
« Last Edit: Nov 04, 2019, 10:24 pm by Paul H »
Paul H
Herefordshire (i.e. a loooong way from the coast...) 🤔

Re: The illusion of safety?
« Reply #5 on: Nov 04, 2019, 10:21 pm »

That would be my list too Ian! Of the three though I wonder if the most likely is the belt (or something else) going through the fan? Seems like the sort off blade failure that a sudden mechanical load on a blade would create. I also notice they have gone back to BHC to purchase a replacement craft...

Cheers

Paul

That would be a reasonable hypothesis given that they had just arrived back at the slipway - max revs to get up the beach followed by a rapid shutdown on a non-too-stiff fanframe could easily cause the belt to track off.
Ian Brooks
Gloucester, UK

Re: The illusion of safety?
« Reply #6 on: Nov 06, 2019, 7:29 am »
This seems a reasonable place to ask the following question...
What is the criteria for a fan/prop guard ?
Is it to stop people walking into the prop, in which case a fairly
light structure, or
Is it to contain a catastrophic prop failure ?
Obviously the later would be better but is likely to be rather heavy.
All pointers appreciated !!

Re: The illusion of safety?
« Reply #7 on: Nov 06, 2019, 8:29 am »
In my book it's definitely to contain a catastrophic prop failure, stopping stuff going in is secondary but necessary.
National Sarcasm Society - like we need your support
http://www.patsure.com

Re: The illusion of safety?
« Reply #8 on: Nov 06, 2019, 10:14 am »
There are two issues here - 1st is that the prop/fan should never catastrophically fail.  It's design and application should allow for sufficient safety margin for normal operation AND include a margin for "unforeseen" events (by that I mean a wave strike or minor debris impact (gravel, etc. that are KNOWN possibilities).  The 2nd issues is that, once the safety limit has been established by design, the guarding should protect the prop/fan from ingesting objects above the design limit (that could cause catastrophic failure).  Get those things right and the risk is negligble

If you want to reduce the energy of a blade then the single most effective way to do it is by reducing the speed it rotates at!

In the real world, it is impossible to protect from catastrophic blade failure using guarding.  You would need to guard the intake making the device useless!  A high speed/energy plastic fan/prop will cut through almost anything - including steel mesh.  A stiff duct wall and mesh make very poor guards - ideally the energy in the blade should be gradually (over distance) absorbed by something "soft".  On ducted fan craft, the addition of a few layers of loose (NO resin)  glass or Kevlar cloth installed inside the duct cavity would probably be very effective at containment in that area (archery netting demonstrates the principle!).

A proper risk analysis should be carried out - question being what can cause catastrophic blade failure.  First thing I'd look at is what is inside the guard?  In this case, the hub, belt, frame and any other bits and pieces - should/can any of those go through the blade will it fail?  If the answer is yes AND they can't be moved !outside the guarded volume, then they should all have secondary retention (belt retainers, etc.).

Re: The illusion of safety?
« Reply #9 on: Nov 06, 2019, 6:34 pm »
https://youtu.be/RxqX2P4cTz4?t=99
Marlin owners would be advised to do some checks on the electrical system?  Specifically that a fuse of the correct rating to protect the wire used, is installed and functional.

Re: The illusion of safety?
« Reply #10 on: Nov 06, 2019, 10:14 pm »
Hi

As John suggests, its practically impossible to contain a broken fan at high speed with anything of reasonable weight. The guard is there to protect the fan!

Ian
Ian Brooks
Gloucester, UK