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Steel Wing Straps and other Steel parts


By petehdgs - Posted on 04 May 2013

I started by laying out an ordinate table for the GA35A415 Long Coot (LC) wing at 75 inches long.  This information gave me a maximum wing thicness of 11.244" at station 26.25", or 35% of wing chord length.  Subtracting 1/8 for each wing skin gives me a maximum main spar height of 10.994 inches.  This is the outside dimension the spar is to be made. 

I then looked at the wing plans and fitting hardware.  Molt called for the wing attachement straps and disconnect pins to be made of 4130 steel and heat treated to 120,000 PSI strength.  I worked out some stress math for the Long Coot wing and determined that these straps are not strong enough for the added load of the LC wing.  So I know they have to be re-designed based on the actual wing dimensions and loads.  They have to be larger, or stronger, or both.  This led me to an investigation into the strength of 4130 steel and it's fabrication processes, including heat treatment.  I discussed this with some weldors and machinists in the area and was given some copied pages of a steel selection catalog from Earle M Jorgensen company.  I was initially looking for a steel that might be more forgiving than 4130 steel as Molt's notes had discussed the dangers of creating embrittlement through welding and poor control of heat while processing the steel.  I was also looking for something that might be stronger than 4130 after processing. 

Discussions with my machinist about various steels in the catalog, heat treatment and looking at his heat treatment oven in his shop led me to investigate heat treatment techniques and what the various terminology means.  I spent much of the following week immersed in this research.  The best discussion so far has been ACF180B Steel Fundamentals http://www.asminternational.org/content/ASM/StoreFiles/ACF180B.pdf and and the EMJ Metals Blue Book page on 4340 steel, a stronger version of steel than 4130, with finished properties that I find particularly attractive.   

"This Chromium nickel molybdenum alloy is widely used deep-hardening constructional steel. It is used at a variety of strength levels and at each level possesses remarkable ductility and toughness. With its high alloy content uniform hardness is developed by heat treatment in relatively heavy sections. High fatigue strength makes E-4340 ideal for highly stressed parts. It maintains its strength and hardness at elevated temperatures."

It is clear to me that I must design new wing straps, build them, heat treat them, and then test them to destruction to be sure the wing attachment points will not fail in service.  To this end I am investigating heat treament literature and equipment for my shop.  4340 steel can be heat treated to an ultimate strength of 260,000 psi, so I am hoping that 200-210,000 might be about right to suit my needs with only moderately larger straps to hold the intended loads. 

I have to make room in the shop for the heat treatment oven, so I am selling a Shopsmith Mark V if anyone is interested. 

One miracle at a time. 

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