# Airframe from non-aircraft grade material.

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#### WonderousMountain

##### Well-Known Member
Still waiting on someone to "invent" brazed brass plumbing pipe fuses.

#### BJC

##### Well-Known Member
Supporting Member
Still waiting on someone to "invent" brazed brass plumbing pipe fuses.
Back when I was working for a living, it was common for electric utility transmission line protective relaying circuits to use class CC fuse holders in PT circuits, with copper tube replacing the fuse. That was an inexpensive disconnect for the rare occasion when the circuit needed to be open. (Usually for setting / maintaining / testing the relaying.)

BJC

#### AeroER

##### Well-Known Member
Could you explain it a little deeper ? According to my knowledge, most of our trusses are limited by members buckling, not by tension.

And modulus of ss steel is similar to 4130 at 200 GPa / 27000 ksi.

Buckling in this truss context is defined by three subcategories, crippling, short column buckling, and Euler buckling.

All three are functions of material gauges and Young's Modulus, and the first two are functions of compressive yield and ultimate strength. The trick with Euler buckling is to be certain of the mode, and that it's not in the transition region where mixed Euler and short column behavior occurs.

Crippling is a local buckling phenomenon independent of the length. Crippling plus short column behavior also must be considered.

Short columns are also called Johnson Columns.

#### stanislavz

##### Well-Known Member
Supporting Member
Buckling in this truss context is defined by three subcategories, crippling, short column buckling, and Euler buckling.

All three are functions of material gauges and Young's Modulus, and the first two are functions of compressive yield and ultimate strength. The trick with Euler buckling is to be certain of the mode, and that it's not in the transition region where mixed Euler and short column behavior occurs.

Crippling is a local buckling phenomenon independent of the length. Crippling plus short column behavior also must be considered.

Short columns are also called Johnson Columns.

Thank you.

So this one shell also be considered, for situations below critical slenderness ratio

#### stanislavz

##### Well-Known Member
Supporting Member
Found this one calculator for buckling loads :

Beware - it looks like their material have wrong Yield stress'es ? (For 2024 their show 76, when other source show 345 Mpa), and critical slenderness ratio for 2024 ~ 66.

But, back to our numbers.

This one for 304 Ys at 210 Mpa:

This one for good old 1020 Ys at 340 Mpa

But it is with a a few mm for 4130 with Ys at 435 Mpa..

Thank you all for good discussion. So yes, it looks that we are facing not only Modulus for material, but we may face Yield stress too if our colums are on shorter side, than Scrit for given/new material.

#### wsimpso1

##### Super Moderator
Staff member
Could you explain it a little deeper ? According to my knowledge, most of our trusses are limited by members buckling, not by tension.

And modulus of ss steel is similar to 4130 at 200 GPa / 27000 ksi.

Usually fuselage truss tubes in existing 4130 tube sizes are buckling limited. When all other loads are checked, some airplanes down grade to mild steel, Piper made a lot of Cubs that way. But they still had to pass not only Euler, but the Johnson criteria and crippling criteria.

Issues with most thinwall stainless are consistency of wall thickness - crippling load and buckling load are reduced with increased variation in wall thickness as you go around the tube. Euler buckling is reduced by the variation in wall thickness, and Euler buckling is 10% lower in stainless than in 4130 by E. Standard tubes are seamless tubes for a reason - welded tubes produce lower strength to weight ratios…

There are reasons the standard tubes are so favored….

Billski

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#### stanislavz

##### Well-Known Member
Supporting Member
When all other loads are checked, some airplanes down grade to mild steel, Piper made a lot of Cubs that way. But they still had to pass not only Euler, but the Johnson criteria and crippling criteria.

I can see it now... But now, especially on this point of globe - thin walled steel tubes are "unobtanium". And that ss steel is the only way to get thin walled tube.

#### BJC

##### Well-Known Member
Supporting Member
Does anyone know what became of the short-time HBAer who was determined to build a fuselage with welded EMT?

BJC

#### stanislavz

##### Well-Known Member
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And one more - we may have even better ss steel compared to 304. It is 444, which is used here, in eu for drinking water application. And available in ok size for us in 18x1 and 15x1mm. Ands its Yield strength is at 340 Mpa. And it is cheaper, than 4130..

#### TFF

##### Well-Known Member
At least EMT is galvanized. No paint. Can’t be worse than the Gere Sport with the tube clusters held together with a bent over nails. You have to think, aviation grade is nail in the box; non aviation is picking out of the dirt. Or maybe I got it backwards.

For Cubs the longerons were 4130; the verticals are 1018. Cost savings. I think it was Taylorcraft that had the first full 4130 fuselage to be a jab at Piper.

#### AeroER

##### Well-Known Member
Thank you.

So this one shell also be considered, for situations below critical slenderness ratio

View attachment 125768

Crippling lives on the left hand side of this curve in the transition from short columns to block compression.

Crippling stress is not constant at compressive yield, the mode starts at stresses less than yield then continues to a cutoff at ultimate allowable stress.

Whether you should use common materials will probably be dictated by the variety of outer diameters and wall thicknesses available.

It is critical to understand how the tubes are formed. Rolled with welded seams is usable with adequate quality assurance of the joints, but there is no question that seamless is preferred. Weld seams add a fabrication complication where tubes are telescoped, and grinding an interior seam might not be the correct solution.

#### BBerson

##### Light Plane Philosopher
Supporting Member
Welding galvanized will kill you before it is completed.

#### stanislavz

##### Well-Known Member
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Welding galvanized will kill you before it is completed.
Never ever. Just aisi 444.

#### wsimpso1

##### Super Moderator
Staff member
And one more - we may have even better ss steel compared to 304. It is 444, which is used here, in eu for drinking water application. And available in ok size for us in 18x1 and 15x1mm. Ands its Yield strength is at 340 Mpa. And it is cheaper, than 4130..
340MPa for 444, 435MPa for 4130. 22% less strength. Where the Johnson or crippling criteria are in play, that 22% could drive you to the next larger tube diameter... None of this is to say it won't work, you just have to do your homework and may have to accept some weight gain in the tube parts.

#### sming

##### Well-Known Member
But if it's stainless steel, you won't need paint ? (Always a win in my book!)

#### stanislavz

##### Well-Known Member
Supporting Member
But if it's stainless steel, you won't need paint ? (Always a win in my book!)
Yes. You do not need to paint it. And if you are above critical slenderness ratio - you are in same position as with 4130..

Below - you are in 10-12% lower in allowable load level or you are forced to redraw you truss for shorter sections or you need to take next size of tube.

#### stanislavz

##### Well-Known Member
Supporting Member
And all this is down to stresses and geometry. Mine example with truss for high wing is on the edge of failure criteria between Euler and Johnson. Remove some load - truss will be same for 4130 and 444. Add more load or make it lower one for low wing bird - it will be lighter in 4130..

But this lighter is in 500-700 grams region max. Or in 0.1% of mtow.

Need/want some live examples of tail load. For slow flying stol, and for faster cruiser and gentelman acrobat.

Going Orion way of 1.5mtow is an overkill for most uses. Othervise 150-180 kg from ul regulations looks and feel too small.

#### BBerson

##### Light Plane Philosopher
Supporting Member
At Airventure 2021 I stopped at an auto engine conversion booth (won’t say who).
My eye caught the mount tubing first. It was welded stainless.
The owner (engineer?) came up and I asked a bit about the stainless mount. Finally, I asked about “toughness” or ductility. He didn’t understand.

#### stanislavz

##### Well-Known Member
Supporting Member