Suitable steel for all brackets etc.

[misterpeter]

Spruce would be the choice, as available - very expensive, when you can get it. Wood is hard to get at the moment, so it will probably be steel after all…

 

[misterpeter]

Spruce would be number one, but is expensive - if you can get it. At the moment wood is hard to get here and is generally in short supply, the good stuff, anyway. So, I’ll probably end up making it out of steel in the end… Whatever, it’s good to kick ideas around and there are bound to be lots of questions more from me as I go along

 

[wsimpso1]

Hi everyone out there! I have a question is 1.4301 austenitic stainless steel usable for the hundreds of brackets and connections in a wooden aircraft construction - in this case a cub? The reason I ask is that I can find specs for the chrome-moly tubing everywhere, but not a mention of the ‘requirements’ for these parts. My job involves this material almost exclusively and since I’m running the 4kw laser-cutter at the moment and can use a 70-ton brake for the bending; I’m otherwise actually employed as a TIG-welder, so it would be very ‘convenient’ for me if this material is allowed in aircraft construction… just a thought!
Look forward to your expert opinions! Peter

Two issues with substituting 304 stainless for 4130:

• Young's Modulus (tensile stiffness of the material) is 28 Mpsi vs 30Mpsi - this is most likely a minor issue;
• Tensile yield is 27 kpsi vs 63 kpsi for 4130 - This means that a fitting made of 304 will stand 42% as much load as one made of 4130.

Given that Factor of Safety (FOS) of most metal parts in airplanes can go as low as 1.5, you can go from a perfectly safe fitting to one that will yield and fail before reaching limit load. Moreover, it will fatigue and grow cracks at approximately one half the loads that the 4130 fitting could. While it is true that many fittings are overbuilt, you do not know which ones are near limits and so can not know which ones must either remain in 4130 or be beefed up, much less know which way to beef them up.

This is a classic case of substitution not being a wise idea. If you start with an airplane that suits your mission, has a fleet of successfully built and flown airplanes, and folks speak well of its handling etc, why would you want to start changing stuff? If you want what the other folks have gotten, many of us will say "please build it per the plans". The exception is when there is a known deficiency, and a known successful fix for the deficiency, then we will append with "and build the known good fixes per their plans".

Now if you just can not help yourself, and you feel you must substitute a stainless steel for 4130, atleast pick one with strengths equal to or greater than the strengths of 4130. The alternative is to go back through the entire design, and calculate all of the loads on all of the fittings and redesign in stainless.

Are you qualified to estimate the flight, landing, and handling loads on the whole structure and then design it anew in some other material than what the plans specify? If YES, well, knock yourself out, but you will be flying far sooner by building to plans and know what it will fly like too. If NO, let's just build the plane that is known.

Billski

 

[WonderousMountain]

Although a lot more Dense, Birch is stiffer &
stronger, and Aluminum is a lot less dense:
These are my alternate material suggestions.

 

[Laromin]

How expensive is 1.7734.4 sheet right now?

It is one of the best steels for tig welding.The Extra aerobatic planes use it in all their fuselages.It's about 15% stronger than 4130.
Certainly you won't be using that much of it,even though it is expensive.

 

[misterpeter]

I wasn’t out to ‘change a winning team’ by substituting 4130 with 304 anywhere along the line. I don’t intend to take any risks and the local BFA Inspector would absolutely not allow its use if it was not suitable. It was a question of the convenience of being able to cut, weld and bend it at work, where we are only allowed to work stainless. I will be taking the trouble to CAD the necessary drawings, which is more than half the cost of getting them cut, wherever I get it done! That is my intention, anyway. First off, I have to find a space upwards of approximately 8m x 4m for the build! Without that, this is all just so much pie in the sky! thanks for all the input so far, all very interesting…

 

[Martti Mattila]

When I come to metal industry there was a lot 304 tubing around, but my new colleagues whom were professionals, told me it is fatigue prone. Then I went TIG welding class and learned a lot more about metallurgy. Dont they care to educate welders in Germany. Mannesman made al the 4130 tubing to U.S. earlier, now its made in China.

 

[misterpeter]

There seems to be a slight misunderstanding here: I never spoke of using 304 tubing instead of 4130! Hell no! I was talking about brackets and such stuff…

 

[TFF]

What brackets then ?

 

[Dan Thomas]

What brackets then ?

I think he's referring to the usual amount of metal stuff in a wooden airplane. Wing, strut, and tail attach parts, control surface hinges and bellcranks, control stick and rudder bars, and so on. There's an amazing amount of welded steel stuff in a wooden airplane.

 

[proppastie]

My boss would kill me if I brought 4130 in to cut or work on!...

Different cultures but you might ask...it is not like the two look alike
One is black the other is silver so unless there are quality issues as a policy it shouldn't be a problem. Here it is so hard to find motivated skilled help little accommodations such as you need usually are not a problem as long as you do it on your own time.

 

[proppastie]

First off, I have to find a space upwards of approximately 8m x 4m for the build! Without that, this is all just so much pie in the sky! thanks for all the input so far, all very interesting…

No....you only need that when you start to assemble, all your ribs and fittings probably could fit under your bed. You could spend a long time building before you needed to rent the larger space which would save enough money to buy the the 4130 and or the spruce.

 

[TFF]

The only stainless I think I have ever seen were tabs on tail wires. Everything else I have seen was 4130. The plans for the Pietenpol have I think 1018/1020 but it costs as much as 4130 and usually not available in small quantities. Legacy of the times it was designed. Legacy now is 4130. From what I get, you must work somewhere that must remain clean like a medical instrument manufacturer and can use their stainless. Lotta of pretty CNC tools. I get the want. I wouldn’t use stainless for anything structural. I would use it on hinges and handles and cosmetic details.

 

[BBerson]

I saw a stainless tube engine mount in a display area at Airventure 2021. Of course, stainless flying wires and cables are common. Some stainless is very high strength, just depends on the alloy. I don't have much experience with the various alloys other than offered at Aircraft Spruce.

 

[Aviacs]

For stainless, I love machining 17-4PH / condition A. It is even marginally machinable with HSS at H(900) which is aprox 45Rc. Heat treatment is low temp, so it is relatively easy to make parts and HT them in house, the tricky issue being how closely the temperatures need to be controlled (25deg ranges, more or less) for each condition. It is stronger than 4130 at any condition. (Tensile ult and yield) I don't know enough to guess whether 16% elongation at break in condition H1150/Rc28-"ish" is adequately tough. (Compared to normalized 4130 at around 25%) Since it is steel, Youngs mod. is in the typical range 28 - 30 Mpsi

smt

edited: Per follow-on comments below: 17-4PH is martensitic, as opposed to austenitic.

 

[misterpeter]

Great bandwidth of ideas! And yes, I work in an environment where we can’t afford contamination due to the possibility of rust - all medical and food applications, kitchens, breweries, hospitals etc.
fitting under the bed is probably right, but try cutting and welding at home with a wife and one-year old baby boy getting under your feet… not to mention what my dear wife might have to say about it… and the landlord!

 

[PMD]

As a general rule of thumb: more austenitic the structure, the more it is prone to work hardening and intergranular / stress corrosion cracking (thus the fatigue issues). I know just enough phys-met & NACE stuff to know when I need to seek professional advice before challenging the status quo. The thought of watching my wing fluttering down while looking up through a spinning canopy powers these thoughts. And YES: these are subjects directly related to aluminum alloys as well (ref: deHaviland Comet).

 

[Gregory Perkins]

As a general rule of thumb: more austenitic the structure, the more it is prone to work hardening and intergranular / stress corrosion cracking (thus the fatigue issues). I know just enough phys-met & NACE stuff to know when I need to seek professional advice before challenging the status quo. The thought of watching my wing fluttering down while looking up through a spinning canopy powers these thoughts. And YES: these are subjects directly related to aluminum alloys as well (ref: deHaviland Comet).

I knew a guy that took the plans for a J3 Cub copy and substituted titanium tubing for 4130 tubing but I have no idea about the diameters or wall thickness changes or what exact titanium
alloy was selected. He did say the result was lighter and stronger. He was a welder for an aerospace company that fabricated a lot using titanium and he knew how to do it.

 

[PMD]

I knew a guy that took the plans for a J3 Cub copy and substituted titanium tubing for 4130 tubing but I have no idea about the diameters or wall thickness changes or what exact titanium
alloy was selected. He did say the result was lighter and stronger. He was a welder for an aerospace company that fabricated a lot using titanium and he knew how to do it.

I will try to be careful writing this:...there was once a motorcycle race shop that sold snowmotwitty machines to keep the doors open in the winter. A certain DC3 captain had a kid who was a pretty decent snow rider, so some "friends and relatives" who worked in a nearby...uh...BUSINESS that did a LOT in unobtanium fabricating took apart a relatively common small displacement snowmo chassis and duplicated most of the parts in said exotic alloy. These parts and sub assemblies, well, uh...SOMEHOW made it to the DakJock's home and turned into what looked like a fairly pedestrian production machine - with a VERY high performance motor. His kid won a lot of cross country races (under sponsorship of afforementioned shop) but let's just say very few people ever knew just why that darned thing was so bloody fast and strong.

I would dearly LOVE to build a titanium cub (when you realize how much backgas or full welding atmosphere is required to make good welds, it is a bit of a daunting prospect). I would probably call mine instead of J3 a J-917 or something.

 

[misterpeter]

Back in the 90s I was in the bike business and we had a lot of dealings with exotica, beryllium as a frame material, for instance… complete Mountainbike frame and forks set 1.2 kilos without the bearings. Cost? Around $10,000 a pop! There were a lot of titanium frames around, too, also in the Moto-Cross field and nearly all of them broke - or if you were lucky, just cracked - around the headstock after a short while. Not a material I would want to use as a home-builder, better left to the professionals. If they can’t get it right, at least your family can sue the hell out of them after the funeral…