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JonathanMilbank
Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area
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 Posted: Wed May 02, 2018 12:39 pm Post subject: Metallurgy and undercarriage legs |
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Would you heat and straighten a Europa tri-gear undercarriage leg if it had become slightly bent as a consequence of a heavy landing? I thought not, and neither would I for the simple reason that I know nothing about how steel responds to very high temperatures. It might become softer, or it might become brittle.
However the heavy landing in question didn't involve me, nor any Europa. A friend landing a recent version of the Rans S6 with supposedly unbendable legs in gusty conditions, unfortunately got caught by a gust and departed the runway. His nose-wheel leg snapped, the propeller got smashed with a little engine power applied and sideways drifting to the right caused the right wheel to dig into the earth (mud on tyre sidewall) causing the right undercarriage leg to bow up slightly (or the end bowed down). It looks slightly "convex" while the left leg looks slightly "concave".
The right wheel now has a few degrees of positive camber (top outwards) and the right leg lower end is now closer to the aircraft centreline, causing the aircraft to lean a little to the left. After speaking to the Rans agent, who says that he's never heard of the latest legs bending and suspects that there will be distortion in the airframe, we've invited our inspector to come and take a close look. Several experienced sets of eyes have already looked very closely at the airframe and careful measurements have been made, thus far not revealing the slightest evidence of airframe distortion or cracking anywhere in the vicinity of main leg or nose leg attachments.
There is a strong temptation to get the local blacksmith to come and apply some intense heat while we pull the wheel sideways, or push down on the bow, until it stands vertical and the aircraft stands level again. I'm not going to make encouraging noises about doing this, but my friend wants to give it a try. If the aircraft were mine and knowing how light it is being a micro(ultra)light, I would give it a try before spending over £800 on another leg. The bow really is only a few degrees. But can you tell me, would it be SIGNIFICANTLY weaker?
Please answer if you have knowledge and/or experience in these matters. Rather not reply thanks, if you're only expressing your gut feelings. And another thought has just come to me; it would be simpler to do heating and bending in situ, where the outcome is readily seen. Removing the leg and going back and forth between workshop and hangar would likely give a less accurate result.
Of course we're already preparing to remove the Jabiru engine in readiness for sending it to the agent for shock load testing. If you have other advice, all help gratefully received thanks.
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tennant
Joined: 19 Apr 2011
Posts: 121
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| Posted: Wed May 02, 2018 11:45 pm Post subject: Re: Metallurgy and undercarriage legs |
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Hi Jonathan,
Without knowing the material and the heat treatment condition it is difficult to give any advice.
The important property for a spring leg is the yield strength, as stresses above this Level will bend the leg permanently.
Assuming that the legs are quenched and tempered to increase the yield strength (probably SAE4130 as they are made in the US), any extreme heating of the metal will reduce it again so that the next "not so hard" landing will bend the legs again.
I remember some years ago that the nose wheel legs of the RANS were regularly bent here in Germany, and we did an Analysis on one and changed the material to something slightly stronger, but not so that the structure would be damaged instead of bending on a hard landing.
Sorry I cannot help any more, but I would suggest that a set of new legs is probably the way to go.
Best regards
Barry
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Barry Tennant
D-EHBT
At EDLM - Germany |
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clivesutton
Joined: 23 Jan 2013
Posts: 187
Location: KENILWORTH
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| Posted: Thu May 03, 2018 12:29 am Post subject: Re: Metallurgy and undercarriage legs |
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Hi Jonathan,
My two-pennarth is that there really may only be a handful of experts in the whole World who could give a fully informed answer to your question. Phase diagrams, microalloy additions, stress intensity factors - it's another world . . .
However, general steel industry knowledge tells me that as per many spring steel grades, susceptibility to fatigue-cracking is made worse by the introduction of surface blemishes induced by e.g. corrosion, dents/local impacts and even factory surface hardening treatments like carburisation. So the risk of creating surface stress-cracks during a local bend repair for the typical grades used (AISI 6150 et al), is high.
But your S6 friend may find the following EAA article more informative and authoritative and save themselves a lot of grief . . .
https://www.eaa.org/en/eaa/aviation-communities-and-interests/homebuilt-aircraft-and-homebuilt-aircraft-kits/resources/building-articles/landing-gear-wheels-and-brakes/spring-steel-landing-gears
Quoting here from a paragraph within it:-
"You may as well face it, rebending and heat treating a spring steel gear leg in your own work shop, with a single welding torch, is not practical. You simply cannot get enough uniform heat on it, over a wide enough area, to allow you to bend the metal without inducing stress cracks. Countless builders have tried to anneal and straighten, or rebend, the gear legs themselves but only a few have succeeded while many have thoroughly botched the job"
I hope useful
regards,
clive.
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area
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| Posted: Thu May 03, 2018 1:14 am Post subject: Re: Metallurgy and undercarriage legs |
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Barry and Clive,
Thanks very much for your quick responses. That gives me more than enough information, making it simpler for me to encourage my good friend towards obtaining a replacement leg.
Best wishes,
Jonathan
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area
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| Posted: Thu May 03, 2018 1:15 am Post subject: Re: Metallurgy and undercarriage legs |
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Barry and Clive,
Thanks very much for your quick responses. That gives me more than enough information, making it simpler for me to encourage my good friend towards obtaining a replacement leg.
Best wishes,
Jonathan
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