Frame tubing welding question

[cgalutera1]

It's something I am not clear about in the fuselage section of the bearhawk 4 place plans. For example if I was viewing the plane from the front and looking at STA. B-P, do I weld T10 to the T11 as option 1 or 2. T10 is 1 inch diameter while T11 is 3/4 inch. If I weld to centerline to centerline, there would be a "bump" at the bottom of the fuselage (see option 2). My guess is to make sure the bottom of the fuselage is flushed as in option 1.


OPTION1.jpgOPTION2.jpg


Carlo

[alaskabearhawk]

Carlo, are you building a 4 place Bearhawk? If so, double check your tubing sizes. T10 is .750 x .035. T11 is .750 x .049. Same diameter. There are other places where larger diameter tubing is welded to smaller diameter tubing. In these cases you always weld centerline to centerline, as in your option 2, regardless where they are located.

[cgalutera1]

Oh, I meant T18 instead of T10. Anyway, you answered my question as option 2. In this case, the bottom surface of the fuselage will have "humps" in places where the tubing diameter is larger the than T11 which 0.750 diameter, right?????

Carlo

[alaskabearhawk]

That is correct. The load path has to be direct. But, the bottom of the fuselage has formers and stringers, so the fabric and cowling will be smooth.

http://www.mykitlog.com/users/display_log.php?user=yeeeha&project=527&category=3583&log=53251&row=248

[cgalutera1]

Alaskabearhawk: Is drilling for oiling the fuselage necessary? What is it for?

[alaskabearhawk]

Hi Carlo...

There is a wealth of info on oiling/sealing aircraft tubing structures on the internet and has been done for a long time. Google "oiling fabric fuselage tubing" and see what comes up! This is out of AC43.13-1B:

6-42. TUBE INTERIORS. Protect the interiors
of structural steel and aluminum tubing
against corrosion. A small amount of water
entrapped in a tube can corrode entirely
through the tube thickness in a short period.
Coat the tube interior by flushing with hot linseed
oil, paralketone, or other approved corrosion
inhibitor. The flushing liquid is usually
introduced through small holes drilled in the
tubing. Allow the flushing liquid to drain and
plug the holes with a screw or by other means
to prevent entry of moisture. Air and watertight
sealing of the tubing will also give adequate
protection against corrosion if the tubing
is internally dry before being sealed.

Russ Erb did his entire fuselage. I just did just the bottom part. It really isn’t hard to do, just make sure there is a hole drilled so the oil can pass through at each cluster. The other thing to keep in mind is to make sure you have all your welding done before you oil the inside, that is if you decide to do so.

Is it necessary? Well, I would recommend at least doing the bottom longerons. As a conventional gear aircraft, it will be sitting tail-low for most of it's life. Any moisture that finds it's way into the fuselage will corrode the tubing from the inside out near the tail, making early visual detection almost impossible.

Here is a link to the Polyfiber product: https://www.aircraftspruce.com/catal...s/tubeseal.php

Hope this helps...

[way_up_north]

Hi Carlo...

There is a wealth of info on oiling/sealing aircraft tubing structures on the internet and has been done for a long time. Google "oiling fabric fuselage tubing" and see what comes up! This is out of AC43.13-1B:

6-42. TUBE INTERIORS. Protect the interiors
of structural steel and aluminum tubing
against corrosion. A small amount of water
entrapped in a tube can corrode entirely
through the tube thickness in a short period.
Coat the tube interior by flushing with hot linseed
oil, paralketone, or other approved corrosion
inhibitor. The flushing liquid is usually
introduced through small holes drilled in the
tubing. Allow the flushing liquid to drain and
plug the holes with a screw or by other means
to prevent entry of moisture. Air and watertight
sealing of the tubing will also give adequate
protection against corrosion if the tubing
is internally dry before being sealed.

Russ Erb did his entire fuselage. I just did just the bottom part. It really isn’t hard to do, just make sure there is a hole drilled so the oil can pass through at each cluster. The other thing to keep in mind is to make sure you have all your welding done before you oil the inside, that is if you decide to do so.

Is it necessary? Well, I would recommend at least doing the bottom longerons. As a conventional gear aircraft, it will be sitting tail-low for most of it's life. Any moisture that finds it's way into the fuselage will corrode the tubing from the inside out near the tail, making early visual detection almost impossible.

Here is a link to the Polyfiber product: https://www.aircraftspruce.com/catal...s/tubeseal.php

Hope this helps...

Years ago I heard something about pressurizing the tubing to see if you have any leaks, I can't remember the source. Is that sort of thing something you've come across?

i sorta think of rust as a very slow burning fire....if you remove all the fuel from the system...let's say purge the tubes of air and fill with nitrogen or carbon dioxide would that help....or does moisture burn through..no mater what you do?

[Daniel]

One of the first Porsche to run the 24 hours of Lemans had a nitrogen filled tube chasis. It was engineered right at the limit so they filled it with nitrogen and monitored guges to ensure the chasis was staying together during the race. It's a good idea and would certainly alert you to a cracked weld and avoid corrosion. But alaskabearhawk is right. It would need a Schrader valve to top off.

[alaskabearhawk]

I think you would be fighting a losing battle. The ability to contain an inert gas for the life of the airframe would be almost impossible. From another site:

"After drilling holes for all crossmembers on my frame and tack welding,
I took the entire frame to a professional aircraft welding company. This
company does certifcated welding for Aerospatiale Helicopters. They
create some of the most beautiful TIG welded clusters I've seen. Even
with this level of expertise, it took two weeks of soapy water spray and
rewelding to get the frame to hold 10PSI for 4 days. At this point you
had to spray a cluster and wait about 60 seconds for a bubble to show
up. After sand blasting and epoxy primer the frame would hold 10PSI for
5 days. I think I'll live with refilling it with nitrogen until the
linseed oil has a chance to coagulate and seal the remaining pin holes.
Lot's of work but that gauge in the cockpit showing positive pressure
sure does inspire confidence when you're pulling and pushing 9G's."

Remember, there are thousands of airframes that are decades old and are still ok without the monkey business of gauges and inert gas. KISS principle certainly applies!

[way_up_north]

I think you would be fighting a losing battle. The ability to contain an inert gas for the life of the airframe would be almost impossible. From another site:

"After drilling holes for all crossmembers on my frame and tack welding,
I took the entire frame to a professional aircraft welding company. This
company does certifcated welding for Aerospatiale Helicopters. They
create some of the most beautiful TIG welded clusters I've seen. Even
with this level of expertise, it took two weeks of soapy water spray and
rewelding to get the frame to hold 10PSI for 4 days. At this point you
had to spray a cluster and wait about 60 seconds for a bubble to show
up. After sand blasting and epoxy primer the frame would hold 10PSI for
5 days. I think I'll live with refilling it with nitrogen until the
linseed oil has a chance to coagulate and seal the remaining pin holes.
Lot's of work but that gauge in the cockpit showing positive pressure
sure does inspire confidence when you're pulling and pushing 9G's."

Remember, there are thousands of airframes that are decades old and are still ok without the monkey business of gauges and inert gas. KISS principle certainly applies!

My post is from someone who knows squat about welding.....your posts get me wondering....why gas welding has endured so long....those old tube frames running around...

maybe the process of tig welding creates micro pathways...not weak..but welds that are pourous on the micro level ....I'm wondering how a gas welded airframe would have fared under the soap test....maybe no diffrent...but it's something I'd like to test out when I get to the airframe stage

[Morto]

Hi folks. Really good stuff. My question is about storing the tubing prior to working on it. Alaskabearhawk, you mention to have all you welding done prior to oiling the inside - I’m guessing this is to avoid contamination and get a clean weld? We battle high humidity here in Dubai, should the tubes be covered with a weld through primer or similar for storage?

[alaskabearhawk]

That is an excellent question. For the first part, the oil typically used to protect the interior of the tubing is linseed oil. It creeps and eventually gets hard and forms an effective barrier against water and helps prevent corrosion. Any welding after application of any oil will burn the oil and render it useless for preventing corrosion.

For the second part I really don't know. Alaska is pretty dry and it's not an issue where I live. Someone that is in the southeastern US might have some suggestions. You can also contact some dealers like https://www.stockcarsteel.com/contact-us and ask what they recommend for storing 4130. If you know anyone in the oil industry in Dubai that would also be a potential resource. Sorry I'm not much of a help. Good luck and let us know what you find out!

https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/basic-construction/rust-protection

[Morto]

No that’s great, many thanks. We do have oil industry contacts.

[Morto]

I guess a dose of linseed before storage and again after finish welding to restore the protective layer?

[fairchild1934]

My intuitive feeling are that ANY oil inside a tube -- hardened or liquid--- if you are going to Tig weld - it would cause a nightmare. Any type of contaminate--- especially ones which are volatile under heat--- would contaminate the puddle and more so- contaminate the tungsten. When that happens-- welding stops..... until you clean the electrode. If the whole inner surface had oil---- you would be screwed.

I think it would be less of an issue for oxy----- but it would still badly contaminate the puddle. And it would generate so much smoke you couldnt see or breathe.

Now there is a type of primer they spray on structural steel to protect it from rust until can be installed and welded. But I suspect stick welding is the most tolerant of that compared to Tig, mig or Oxy. I suspect Tig would not like it.