Hi guys (and gals)
I am restoring an Enstrom 280C helicopter that crashed earlier this year, and I
am seeking advice on how to straighten some of the bits.
Firstly, it is being restored for static display only, so none of the airworthiness
caveats apply. It will be fine so long as no bits fall off on to someone’s head!
The tail rotors and the main rotors were bent. I have straightened out the tail rotors which was relatively easy because they were each only about 2 feet long, but the main rotors (3 off) are 10 feet long, thus I am looking for a simpler method than the one I used on the tail.
The main rotors are 10 feet by 6 inches wide and are made from stressed aluminium skinned over a leading edge aluminium longitudinal spar and stuck together at the trailing edge.
When fixing the tail rotor, I had to resort to cutting right through the leading edge spar before I could bend the rotor straight. I then replaced the cut section with various bits of aluminium rod and lots of body filler. The wrinked skin would not flatten, as being aircraft spec., it was brittle and springy. I had to resort to cutting sections out and replacing them with body filler.
This worked well, and could be applied to the main rotors, but would be a bit tedious.
Hence my questions:
Is it feasible to heat soften the aluminium main spar- and if so, what sort of heating gear would I need? (gas torch?)
If the thin aluminium skins were clamped flat for a period of time, would they cold flow back into shape?
I suspect that neither of the above would work, but thought I would ask the experienced members of the Forum for their advice.
Cheers,
Ian.
By: QldSpitty - 1st December 2006 at 08:38
Have an emergency number???
Do we have to precall the fire brigade?Good luck mate and have fun.But be bl00dy careful. 😉
By: iws - 1st December 2006 at 08:09
Many thanks to those who replied with advice.
I have just bought a gas torch and will have a go this weekend.
I am away now to count my fingers!!
Cheers,
Ian.
By: QldSpitty - 30th November 2006 at 07:21
To find the correct temp go down to your local welding supplies or engineering workshop and look for temperature crayons.These are different coloured crayons that change colours at different temps.Also it all depends on what grade ally you are working with.2024 is used for skins and general parts where 7075 which is different is used in high strength parts.Thing with alli is that it is a lot more brittle than steel.It can be worked but it will work harden quickly.Good luck and don,t hit your fingers…Ouch…
By: MarkG - 29th November 2006 at 21:03
The wrinked skin would not flatten, as being aircraft spec., it was brittle and springy.
Ian, as you say aircraft spec. aluminium alloy is brittle and springy and so will crack if bent ‘as is’. In order to work it you will need to anneal it first. That will return it to a soft and malleable state.
You’ll need a gas torch or similar but you must be very careful as the melting point is quite low and there is a danger of melting it if you get a bit handy with the flame! You should hold the torch a little way off the surface of the metal and ensure that you heat the piece as evenly as possible to avoid it distorting further. One the alloy has reached the correct temperature let it cool slowly. It will then be annealed and easy to bend and work.
A top tip is to rub soap onto the surface of the alloy before heating it. When the soap turns black the alloy is at the correct temperature. Remove the heat and let it cool. Be aware though that some alloys annealing temperature varies so the soap thing is not always perfect. It’s generally a good guide though.
It will ‘work harden’ however as you are bending, bashing etc. so you may need to periodically re-anneal it. Once you’ve finished the alloy will re-harden itself over time.
Edit: Ah! I see that Gowans fellow nipped in with sound advice while I was typing my reply!
By: stuart gowans - 29th November 2006 at 20:49
It is possible to anneal dural, but it is harder to judge the temperature at which it is annealed; the standard guide of soaping it and waiting until it turns black, is unreliable because soap turns black way before dural is annealed , (I am reliably informed that soap turns black at about 275 deg wheras dural anneals at 475deg or thereabouts), and once the soap has turned black ,it can’t really get any darker!
One way, is to fix said piece of metal at one end and apply pressure with one hand whilst heating it with a torch(not a battery torch!) with t’other ; as it starts to become maleable you can take the flame away ,and feel the metal tighten up and pull the other direction, works best when the bit you are holding is at least a foot away from the naked flame!
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November 29, 2006 at 8:12 pm