# Tube construction: Alternatives to conventional welding

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#### FritzW

##### Well-Known Member
Log Member
This has probably been discussed but... square tube isn't cheap (neither is round tube for that matter). I could see something like a square tube Himax fuselage costing $800-$1,000. I suspect using proper aircraft wood wouldn't be a whole lot cheaper.

As much as I love working with wood, the "easy factor" of square tube is hard to ignore.

EDIT: it needs a mental "cost to hassle" analysis...

#### Vigilant1

##### Well-Known Member
Lifetime Supporter
3/4" square 6061 extruded is $1.88/ft. https://www.onlinemetals.com/en/buy/aluminum-square-tube-6061 Just heavy. I would prefer .032" thick or even thinner. They have it about that thin in round tube, but 1" dia and .035"wall thickness drawn tube is over$5 per foot.
You'd probably be better off to have somebody bend up some hat sections from sheet, and you could get just the profile you need.

#### FritzW

##### Well-Known Member
Log Member
I was looking at the 3/4" x .049 stuff at Aircraft Spruce at $4.47 a foot. The AS tube has rounded corners. Rounded corners would be *better but not$2.60 a foot better.

*better = less stress concentrations in the corners? I don't know if that's a big deal or not.

EDIT: the weight difference between .062 and .049 for 100' of tube would be ~4.5 lbs. ...significant, but maybe not worth the extra $200-$300

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#### Vigilant1

##### Well-Known Member
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Any concern about the buckling strength of the corner (in blue)?
I should have mentioned there's corner gussets on the other side of the blue sheet at every brace point..

View attachment 89881
Oh. Never mind.
And forget those nice things I said about the looks and aerodynamics--it'll be as boxy and draggy as a milk crate .

Compared to tubing, the long "legs" of the corners should make them plenty stiff.

Out of curiosity, any idea what it would cost to have the corners made with a rolled, 2 cm radius rather than a sharp, simple 90deg bend made with a brake?

#### litespeed

##### Well-Known Member
We are looking at alternatives to welded steel, which are traditionally the entire airframe.

I would contend that yes, ideally the best would be a very stiff cage for the cabin and everything else is far weaker and takes up the crash impulse.That is exactly what we see in some advanced metal aircraft that are built for a sudden stop. Examples are Crop dusters and AAK Hornet- a very tough farmers aircraft from here in Aust, which has a substantial frame covered by traditional skinned alloy construction.
That construction method also involves extra weight.

These designs are outliers as most have very limited steel and are primarily just alloy sheet.

A pure steel cage I contend is good at gross pilot protection but often very poor in reducing the loads felt, as the rest is relatively too strong.

A square tube truss that is sandwiched can be extremely strong as a cage and easily taper the strength as it gets away from the cockpit as compressable areas. We can used less material where desired. Any additional crush zones can be very light. Crush zones Do Not have to be heavy just clever.

Riveting- always a fun subject but most criticism fails to look at the actual sandwich truss, just, at simply tubes (often round- which have issues with loads and limited rivets).

A sandwich truss, does not have a few rivets at point loads using a a often single sided gusset. It is a world away and the rivets and alloy the only common denominator.

A riveted sandwich truss has the sandwich skins riveted along their length for all contact surfaces with the tube, including all the cross beams. We are talking hundreds of rivets not a handful holding it together on flimsy gussets on only one plane. But it encapsulates the truss on both sides. The sandwich stops the tubes flexing and bending out of shape. The mission is not to crumple and stay intact for cockpit safety, and do it without too much weight. The shear strength of the riveted sandwich truss is far beyond a normal design, it acts more like a monolithic part.
A equivalent would be a steel truss that has full length steel skins welded to the truss at every contact point. It is about comparing like for like as far as strength and load paths go.

I contend we are stuck in the old school thinking of steel tubes been the best when that is just a function of what has been available, which is dictated primarily by manufacturers choice, builders skills, knowledge and legacy designs. The average sheet alloy design is also a big compromise safety wise and is prevalent for the same reasons.

Some will contend that hat sections can suffice- I disagree, that is a false economy to save a bit of weight at far lower strength.

The airframe is one of the aircraft components that is not super heavy as a fraction of AUW if designed well. We have lots of examples of much lighter aircraft replacing a 152 with the same useable load and substantially lower weight overall.
A 152 is not the gold standard of crash survival.

I contend that any additional weight of a safety design can be small enough to make it practical, keep landing speeds down and still meet our expectations and done at a reasonable price.

If we are so worried about any extra weight, we would strip all designs of anything that does not help it stay in the air. So goodbye seatbelts, a quality seat,fuel systems that don't barbeque the pilot, instruments bar a mark one eyeball and certainly no ballistic cutes or pilot parachute. Why- well that is stuff you need to be safe when stuff goes wrong. That is a fantasy we would never consider. So why be so shy of a little extra weight to make a sudden stop more survivable?

People get too fixated on what has been done before and what the others guys might think, and adding dodads with shiny bits and flashing screens. These attitudes do nothing to help a safety culture and lead to a lot of dead or mangled pilots and foster a poor record in the eyes of the public.

A lot are saying it is too expensive and would rather save a few hundred dollars and a few pounds. Think of the next owner or even your family. " we are gathered here to remember John, a pilot and loving father. We note he saved \$500 building his aircraft and made a investment for his kids education, he was such a caring father- he bought them a new phone".
That will not be considered a investment at your funeral. "Bravado till death, But I saved Money, Was always a old school man, or even, Hey hold my beer" are not epitaphs you want on your head stone.

Any concerns about corrosion been used as a reason not to follow this path are straw man arguments. No matter what we use- its real life is always limited by many factors and corrosion is not just a alloy problem. Composites hate UV and temperatures, they can delaminate. Wood can rot, have glue problems, steel can rust esp when mated to alloys.

No aircraft is a island but a real world machine with real world issues and real world use and abuse.

Perfection may be the enemy of good enough but good enough is often the enemy of life- we need a middle ground.

#### Vigilant1

##### Well-Known Member
Lifetime Supporter
A riveted sandwich truss has the sandwich skins riveted along their length for all contact surfaces with the tube, including all the cross beams. We are talking hundreds of rivets not a handful holding it together on flimsy gussets on only one plane. But it encapsulates the truss on both sides. The sandwich stops the tubes flexing and bending out of shape. The mission is not to crumple and stay intact for cockpit safety, and do it without too much weight. The shear strength of the riveted sandwich truss is far beyond a normal design, it acts more like a monolithic part.
Adding 50 sq ft of .020" skin on the inside of the cabin area would add about 13 lbs to our plane, plus the weight of the rivets. That would be acceptable in some cases, crazy talk to other folks. Adhesive might also be considered--at 4000 psi shear strength and able to smoothly spread the load over a large area, it might make a contribution to keeping everything together when loads exceed flight loads. Applying adhesive to only one skin would allow rivets to be drilled out and the structure more easily accessed for repair or inspection, if required.
Some specific questions:
- Noise: One piece of flexing AL is noisy, I can only imagine what these resonating chambers would be like. Noise canceling headphones only go so far. Include foam inside some of these for noise attenuation, insulation?
- Access/moisture: There's no getting into those cavities once they are fully skinned on both sides, so the structure is hard to inspect. And these enclosed bays are the kind of spot water condenses and stays in once either surface gets below the dew point, and it roosts there for a long time (sometimes increasing over time). Drip holes or larger cutouts for inspection/ventilation would seen to be important.

Anyway, I can see why it would be attractive. Only a thorough analysis (or building some test cabins and crushing 'em!) can tell us if it is lighter/better than other approaches.

P.S. I think we need to note that the Morgan Sierra 200 that hit the Ferris wheel in NSW has a welded steel tube frame in the cabin area. From the Morgan web site:
The Cheetah Sierra 200 is an all metal 2 seat high performance aircraft that uses a simple aluminium tube, aluminium sheet and pop rivet construction method. The main support frame that holds the undercarriage, pilot and wings to the fuselage is a welded steel tube frame

#### cheapracer

##### Well-Known Member
Log Member
That's a full skinned structure.
This thread is about tube/angle and fabric covered trusses.
It's a fabricated tube, you can fabricate or extrude a tube to that or other shape, or use that technique, modified accordingly, for existing tube shapes. I threw it out there for ideas for others to expand on.

BTW, nowhere in this thread title or OP is there any mention of angle or fabric. All my tube and gusset builds and experiments are skinned with aluminium sheet.

Oh. Never mind.
And forget those nice things I said about the looks and aerodynamics--it'll be as boxy and draggy as a milk crate .
Actually planes like the BD-4 and Whitman Tailwind have proven to be quite slippery, and the long list of other "boxy" planes from Austers to the thousands of Zeniths out there, just don't care.

Better a cheap, stupidly easy to build plane that gets people in the air rather than fantasies of how to build a plane that will never happen or ever be finished.

Out of curiosity, any idea what it would cost to have the corners made with a rolled, 2 cm radius rather than a sharp, simple 90deg bend made with a brake?
If you mean the folded outside skin corner the blue gusset adds to to make the longeron, no need to roll it, it will curve around that radius by hand, but I don't want it to for a number of reasons.

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#### Vigilant1

##### Well-Known Member
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Actually planes like the BD-4 and Whitman Tailwind have proven to be quite slippery, and the long list of other "boxy" planes from Austers to the thousands of Zeniths out there, just don't care.

Better a cheap, stupidly easy to build plane that gets people in the air rather than fantasies of how to build a plane that will never happen or ever be finished.
Oh, I agree. Just joshing you a bit. My Sonex has square sides all along the bottom and it doesn't bother me at all. The piano hinged-on-one-side rudder is dirt simple but drives some folks crazy. It gets an honest 130 MPH on 75 HP with two aboard and is fun to fly, just not everyone's cup of tea.

#### FritzW

##### Well-Known Member
Log Member
Better a cheap, stupidly easy to build plane that gets people in the air rather than fantasies of how to build a plane that will never happen or ever be finished.
That's it in a nutshell. Theorizing is great but at some point you have to build something.

#### cheapracer

##### Well-Known Member
Log Member
P.S. I think we need to note that the Morgan Sierra 200 that hit the Ferris wheel in NSW has a welded steel tube frame in the cabin area. From the Morgan web site:
Having built a Morgan Sierra 200, I can tell you that is news to me ..... The only welded steel frame is the wing spar support frame, not really any great difference in a crash.

The dark coloured frame ... though note the aluminium longerons are 3mm thick (1/8"), quite substantial.

#### cheapracer

##### Well-Known Member
Log Member
.. and while on the subject of square tube frames, where straight or slight curved tube runs are made, plan on 'fold and double overlap' skins.

This increases strength greatly with very little weight penalty.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Post 1 was about steel tube end joints other than welding. Now it's morphed into a Sonex....
I gave an alternative steel tube end connection in post 4. The assumption was a traditional steel tube structure normally covered with fabric. If the "skin" is other than fabric, then the steel tube end connection probably doesn't matter much. But a steel tube frame covered in metal or some composite is likely to be heavy.

#### litespeed

##### Well-Known Member
The OP post might have been a alternative for steel joins- but the topic just means alternative to a welded steel frame for a tube fuselage.

Hence a tube and gusset sandwich- no welding and far more usable for a builder than replacing a welded joint for a bolted joint or riveted in steel. Why bother with steel if you are not going to weld it. Just more weight and no benefits when done in steel.

The Morgan steel component is minimal and has no bearing on the cockpit safety, it was just a way of providing a support for the gear/legs/fuselage join that the builder does not have to do. Replacing it with alloy would have no bearing on impact strength of the cabin.

#### litespeed

##### Well-Known Member
What about oilcanning and other nasty noises?

Pretty simple really but depends on taste.

You can insulate the sections, makes it quieter- should be gently bonded to both internal sides when assembled. That is the new school method.

Or you can bead form it to have lots of structural strength and low oil canning with no weight added. Bead forming is very common in motor racing for strength and sound reduction. Easy to do, cheap and can be done with style for those worried about that. A well done bead job makes a very flimsy panel very very stiff. The same can be applied to add extra strength anywhere you desire rather than increase the thickness and weight.

Even a thing coat of Rhino liner will also reduce noise substantially.

As far as inspection goes- add a inspection panel. No deal breaker.

A bead rolled sheet firewall, looks lovely and much stiffer.

This is a basic job but can get much more strength if needed or even write the aircraft name in the sheet.

Bead rollers are very useful when doing sheet metal even for just making fast accurate toggles. A bead rolled gusset is easy to do and very stiff, which resists buckling.

A car interior- could be internal panels for a cockpit.

#### cheapracer

##### Well-Known Member
Log Member
Post 1 was about steel tube end joints other than welding.
What about if we fabricated a chrome moly tube of various shapes and made suitable gussets, maybe even cover it with fabric ...

12 pages and 2 years, and there's a minor thread drift with lots of people offering altermate builds ideas to welding, as the Pyschologist said to me when I told him I couldn't stop thinking about Tom Jones all the time, "It's not unusual".

#### BBerson

##### Light Plane Philosopher
HBA Supporter
So then it's alternatives to steel tubes now?
Just asking.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Nothing wrong with seeking alternatives to steel tubes. Actually that's what I have been doing for years.
I don't think much interest exists for welding anymore and steel tube is expensive for an economy airplane. And I have been looking at all alternative ideas in my aircraft test lab. I tried all sorts of built up tubes like Cheapracer proposed.
Something IKEA simple is needed, I think.

#### Vigilant1

##### Well-Known Member
Lifetime Supporter
Having built a Morgan Sierra 200, I can tell you that is news to me ..... The only welded steel frame is the wing spar support frame, not really any great difference in a crash.

The dark coloured frame ... though note the aluminium longerons are 3mm thick (1/8"), quite substantial.

View attachment 89892
The Morgan steel component is minimal and has no bearing on the cockpit safety, it was just a way of providing a support for the gear/legs/fuselage join that the builder does not have to do. Replacing it with alloy would have no bearing on impact strength of the cabin.
Thanks, I understand now about the Morgan design and the steel. I do think it is a good use of welded steel (taking/tansferring the heavy and repetitive point loading of the wing spars and landing gear, and firmly lashing the seats to that cage). It also makes good business sense, allowing Morgan to offer a critical part that would be difficult for the average homebuilder to reverse engineer and fabricate. He's got the jigs, templates, proper welding equipment, and a trained welder, so he can probably offer the part at a price that would be a bargain to someone without the tools and expertise to do the work himself.

#### cheapracer

##### Well-Known Member
Log Member
I do think it is a good use of welded steel (taking/tansferring the heavy and repetitive point loading of the wing spars and landing gear, and firmly lashing the seats to that cage). It also makes good business sense, allowing Morgan to offer a critical part that would be difficult for the average homebuilder to reverse engineer and fabricate.
LOL, it's a simple arrangement of mild steel square tubing, nothing fancy and terribly easy to replicate.

The reason it's there in steel is because of the risk of damage removing and installing the wings is mitigated.

The wings are tankless, and they only take less than 10 mins to remove, 2 bolts, the aileron and flap cables, then they are slid out over the steel.