Cracked fuselage

[skywagon8a]

An update for everyone: fuselage wasnt just cracked in one spot. We have now had several instances of "this is worse than we thought." First, all lower longerons are bad. Several compression members on the sides are bad. Lots of tubes with paper-thin walls.

Now we are faced with the time-money problem of either repairing or replacing with new. Fun times!

No surprise at all with a 70 year old steel tube salt water seaplane.
A thought, contact http://bbiaviation.com Ask them about a new -12 fuselage. You can remove the weld on serial number data tag on your old junk and weld it on the new one. The exchange rate is great. Each of your dollars is worth $1.30 in Canada.
I didn't say this

[mvivion]

I find it interesting that nobody has mentioned the different metallurgy of the old Piper fuselages vs a new, all 4130 fuselage. That's got to be worth something in my book.

MTV

[dgapilot]

I find it interesting that nobody has mentioned the different metallurgy of the old Piper fuselages vs a new, all 4130 fuselage. That's got to be worth something in my book.

MTV

For what the majority of the tubes in a fuselage do, there is no real advantage of 4130 over 1020 or 1025. Most are in compression, and column buckling is more a function of diameter, wall thickness and length than tensile strength. Remember that every tube is sized for a positive margin with 150% safety factor. The only reason to use 4130 in places that weren’t originally 4130 is availability. Most of the tube sizes we use are no longer available in the lower alloys unless you want to buy a mill run.


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[Crash, Jr.]

4130 does have some pretty marked advantages, namely better strength for equal wall thickness versus mild steel and slightly better corrosion resistance. I don't think simple availability is the reason why every frame maker, aircraft maker, and everyone down the line went to 4130. It's just a better alloy.

[mvivion]

For what the majority of the tubes in a fuselage do, there is no real advantage of 4130 over 1020 or 1025. Most are in compression, and column buckling is more a function of diameter, wall thickness and length than tensile strength. Remember that every tube is sized for a positive margin with 150% safety factor. The only reason to use 4130 in places that weren’t originally 4130 is availability. Most of the tube sizes we use are no longer available in the lower alloys unless you want to buy a mill run.


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Read the discussions above on this thread and look at the pictures.....there's a heck of a lot more than just compression loads going on in this tubing.

MTV

[dgapilot]

Read the discussions above on this thread and look at the pictures.....there's a heck of a lot more than just compression loads going on in this tubing.

MTV

Yeah, Jeromy’s fuselage has some corrosion issues, and likely some low cycle fatigue. That said, 4130 is a harder material. Not sure how it compares to 1020 or 1025 from a fatigue standpoint.


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[jadebons]

I'll try to post more pictures later this weekend. It's rough. I put a rubber mallet through one longeron. Others are caving to a wire wheel. It seems that 73 years is a bit too much for this mild steel.

[dgapilot]

Jeromy, are you at Stan and Sandi’s place? I heard someone was there with a Supercruiser doing instruction.


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[jadebons]

Yes I am. Are you nearby?

[mike mcs repair]

. That said, 4130 is a harder material. Not sure how it compares to 1020 or 1025 from a fatigue standpoint.


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Not good. You can bend and then straiten the old fuselages. The 4130 ones crack adjacent to the welds when bent and straitened


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[dgapilot]

I work on base, but live in northern MD by Frederick. I’ve only been down there twice since February. I think you showed up once at our QB meeting before all this Covid stuff. Wish I was closer so I could give you a hand getting the Cruiser back in the air. I used to do a lot of seaplane flying when I lived in NY. Did a fair amount of instructing in Lakes, PA-22 on floats, 172 on straight floats, 180, 185, and 206 on amphibious floats as well.


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[wireweinie]

Not good. You can bend and then straiten the old fuselages. The 4130 ones crack adjacent to the welds when bent and straitened


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You engineer types on here, what are the technical differences between 1025 and 4130 when it comes to welding and shaping (as in bending or re bending)?

Web

[dgapilot]

Low carbon steel (1020 or 1025)is much more malleable. 4130 is harder, can be heat treated, and will work harden when bent or formed. If you don’t take care to cool slowly after welding, you can increase the strength, but at the same time increase the brittleness with 4130. If you use 4130 rod, the weld metal will be very hard and brittle. Always use mild steel rod, with TIG, I use ER70S2 or S6. With gas welding I use RG45 wire. If welding over about .120 (not often with airplanes), it is recommended to pre heat the base metal before welding.

SAE 1020 has about .18 to .23% carbon. 4130 has about .28 to .33% carbon 4130 has about 90k psi tensile in Cond N, but can be heat treated to over 125 k psi(ultimate) 1020/1025 has about 57k psi tensile (ultimate). Both 4130 and 1020/1025 have a modules of elasticity of around 29k psi.


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[Gordon Misch]

1025 CD
Yield 54 KSI
Ultimate 64 KSI
Elongation 15%
Brinell hardness 126
http://matweb.com/search/DataSheet.a...5f3e1c7&ckck=1

4130N
Yield 63 KSI
Ultimate 97 KSI
Elongation 25%
Brinell Hardness 197
http://asm.matweb.com/search/Specifi...bassnum=m4130r

Note the greater elongation of 4130. It will stretch more between yield and failure.

As pointed out above, elastic modulus (stiffness) is the same for both, and that's what matters for slender column buckling.

[Aeronut]

Perhaps I misread it the other day but FAA AC 43.13-1B Table 4-15 selects 4130 filling rod for 4130 metal. This is the only place I‘ve ever seen this and everywhere specifically states to avoid 4130 rod on 4130 metal. If I‘m not misreading this AC it really makes me skeptical of anything the FAA prescribes.

[mike mcs repair]

Perhaps I misread it the other day but FAA AC 43.13-1B Table 4-15 selects 4130 filling rod for 4130 metal. This is the only place I‘ve ever seen this and everywhere specifically states to avoid 4130 rod on 4130 metal. If I‘m not misreading this AC it really makes me skeptical of anything the FAA prescribes.

Just another WRONG thing in 43.13. Never will be changed.

Look at the bend radius chart twords the bigger end. Someone was whisky-ing and playing with their slide rulers when that table was made....

Don’t believe everything that’s written.. just because it’s written in a book we are supposed to live by....


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[Steve Pierce]

Lots of fuselages out there with mild steel not falling apart. I am sorrybut that one was used and abused, probably some not so good repairs.

[Gordon Misch]

Just another WRONG thing in 43.13. Never will be changed.

Look at the bend radius chart twords the bigger end. Someone was whisky-ing and playing with their slide rulers when that table was made....

Don’t believe everything that’s written.. just because it’s written in a book we are supposed to live by....

Not the only factor. A large weld section can create an adjacent stress concentration notch. And an asymmetric weld can be worse yet. Also, the rapid rate of heating / cooling with tig or mig can cause self-quenching. Lots of variables - - - So a yielding weld filler can help compensate.

[TurboBeaver]

Well first of all this corrosion condition on 73 year old 12 Airframes isn't going away.
Trying to decide weather to dive into a massive welding project or simply buy a new 4130 airframe is not really the point
here. What should be is: the simple fact that from a resale perspective; regardless
of who did the work......... Generally speaking, A Cruiser with a brand new 4130
fuselage, is a different animal altogether than a patched up wanta be, when it comes time to sell it.......
Anyone that thinks 1025 tubing is just as good as 4130; has obviously never whiped a Cub around in the tundra, without the tail boxed in, with a load of moosemeat onboard, only to find the fabric saged and tail twisted.
So if you are a welder yourself and wanta
"Fix er up cheap" get an old fuselage ( or bed frame ) and have at it. Or if your not
this is a "no brainier" as when you are done
with "patches" say it's worth 60 grand, now
Compare that to same 12 with a brand spanking new 4130 airframe that will easily
fetch $80K. Where is the extra $20k, you had to put up front, right on the back end!!
It isn't really going to cost jack, you saved a year in time (at least) your back in the air,
And the word around is " that thing has a brand new fuselage.
Good Luck
E

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[Kid Durango]

Gordon, does a post-heat have as much effect as a pre-heat and then weld?? I’ve had mixed teachings over the years, but mostly with pipe.

[DENNY]

I spent 1 1/2 years looking at cubs before I got mine. Having a new fuselage on a 1951 cub was a huge selling point. Nothing wrong with a properly repaired/rebuilt cub but that "properly" part may be a bit hard to find.
DENNY

[Gordon Misch]

Gordon, does a post-heat have as much effect as a pre-heat and then weld?? I’ve had mixed teachings over the years, but mostly with pipe.

I don't actually "know" the answer to that, and I think the correct answer is "it depends" but here's my take on it.

Post heat can do two things. It can relieve residual weld shrinkage stresses, and it can re-normalize after welding induced self quenching. For re-normalizing a temp of 1600 - 1650 is required, and I don't know how long that temperature has to be maintained.

Pre heat can reduce or eliminate self quenching, eliminating the need for re-normalizing. Then post heat can be applied for stress relief if needed.

As a practical matter, I do know that 4130 is not highly susceptible to quenching, and is commonly TIG welded to aerospace criteria without either pre or post heat. A high end shop near me does that and destructive testing at a nearby metallurgy lab has shown it to be suitable, in the thin sections found in tubing. Heavy sections would be more susceptible to self quenching and pre/post heat could be required. But we don't have those heavy sections in our Cubs.

I'd say if I were in doubt about thickness and quenching risk, I'd preheat to around 400+ (amber color). That's merely opinion.

Google "steel temperature color" for some charts.

[CharlieN]

The amount of Pre and post heating a weld has more to do with the duration it took to perform the weld and the mass, in this case the thickness of material. The goal is to achieve evenness in the heating and cooling to reduce the chance of uneven stress. If you are gas welding the process is slower and the heat spreads more evenly. A slow withdraw of the torch while spreading the heat is most you need to do. This also maintains the shielded envelope of the flame till the material cools below the critical level, generally considered the color change.
With tig, starting the heating away from where the weld will begin is a good thing. This might be doing a tack near the endpoint, then move the flame to where you intend to start the weld and proceed back through the tack. Long welds or thick material one would stitch the weld and then close out with making sure the whole heat affected zone tends to cool evenly. Again you want to maintain the shield gas till the initial cooling has taken place. I find I commonly need to tap the pedal to keep the post flow going till the material is below critical level.
Larger, thicker weldments take more care during cooling such as burying the part in dry sand or using a Nomex wrap.

[wireweinie]

Question: In TIG, if the gas shield is important to protect the weld from oxygen, why don't welders pipe argon into the tube being welded? Wouldn't the shield be needed on both sides of the weld?

Web

[CharlieN]

Question: In TIG, if the gas shield is important to protect the weld from oxygen, why don't welders pipe argon into the tube being welded? Wouldn't the shield be needed on both sides of the weld?

Web

They do, most common with stainless and Ti. Seen with aluminum, not as often with the simpler alloys in steel. I have capped the ends of steel tubes just to reduce the air exchange and with flat sheet in all material used flux on the backside.

[Gordon Misch]

Question: In TIG, if the gas shield is important to protect the weld from oxygen, why don't welders pipe argon into the tube being welded? Wouldn't the shield be needed on both sides of the weld?

Web

Yup - purging is used on critical welds.
Edit: Oops - I see CharlieN already answered.