Tube construction: Alternatives to conventional welding

[litespeed]

As far as volume loss goes- not a big deal if a trussed skinned cabin is only 30mm thick. Not enough to be a issue and helps with mounting items.

As a practical example I provide the infamous caught from clean air, the spectacular amazing death defying, ferris wheel bait........The Morgan series of aircraft.

Uses a trussed skinned design of tube and sheet. It has the basic design idea baked in but could be improved. Gary thought outside the box and made a lovely aircraft. Not only strong but very easy to build accurately in a home shop with minimal tools. No special jigs, no welding skills and gear. But it is no a spam can or RV copy- lessons we can all learn from.

The build time is fast and not a thousands of rivets.

 

[BBerson]

Few designers consider or test crash criteria.
What you want is a material with high elongation in yield to absorb the crash progressively. Progressive yield makes the steel stonger as it yields (work hardens). That's why cars are steel but not fully hard steel that would shatter like a drill bit.
Steel has the most elongation. Welding doesn't induce the failure points of holes.

 

[cheapracer]

At the risk of mischaracterising, existing homebuilt (and factory built?) aluminum and composite cockpits aren't designed/built to perform as well in crashes as 4130 tube cockpits are.

I think I already said it, consider that most 4130 tube aircraft are slower moving, high wing, they crash slower hence a statistical misrepresentation.

A similar aircraft in terms of usage is a Jabiru, slow, high wing, made from near paper thin fiberglass, but they have an excellent reputation for crash survival, considering how many of their engines have failed, it's been well proven, sadly.

 

[litespeed]

I would not say paper thin at all. The strength is where it needs to be.

But it is very hard without really trying to kill yourself in a Jabiru.

Us Aussies have tried and tried. If you die in a Jabiru it is Cummulus granite or some other feat of Darwinsim. If your at landing speeds and have a sudden stop the are remarkably survivable

 

[Vigilant1]

Two things:
1) That's my point in asking "show me a practical example".
2) There's a reason why they aren't. Either you end up with a heavier structure to account for the downsides of the material in question (or the limits of the designer's knowledge) or you have to have so much volume (or awkward geometry) to account for load paths, that it's just simpler to go with a steel rollcage.

Thanks for the points, I appreciate it. I think our reasoning is not very different, but it is taking us to different conclusions.
An example: (All round tubes)
.....What..............................................................Weight............Compressive..........Tensile....... Max column
....................................................................................................Strength.............Strength..... buckling stress
A) 4130 tube, 27"L, Dia: 0.5", Wall: 0.035"............0.39lbs.............545 lbs...............4857 lbs........10,664 lbs
B) 2024 T-3 tube, 27"L, Dia: 0.85", wall: .058"........0.39lbs...........1617 lbs...............8947 lbs........11,206 lbs
Both parts have the same weight, the aluminum part is stronger in all of the ways shown.

C) 2024 T-3 tube, 27"L, Dia: 0.6", wall: 0.06"..........0.27lbs............534 lbs................6311 lbs.........5246 lbs

A designer working in welded 4130 has a minimum wall thickness--it's probably too much to expect an amateur builder to reliably produce good welds with very thin wall tube. And the density of 4130 means that larger diameter tubes (that would give higher stiffness) get prohibitively heavy (again, since we have a min wall thickness driven by the welding requirement).

A designer in AL who needs 525 lbs of ultimate compressive strength for flight loads can choose Tube B or Tube C--they are the same to him. Tube C does the job well enough and it weighs 30% less than Tube B, so he chooses tube C. Now, Tube B would have been much stronger in buckling and tensile strength than Tube C (in fact, stronger than the steel tube), and this could be important in a crash (where loadings can be high and in directions different from flight loadings). A designer concerned with crash loadings could pick Tube B. And, again, that more crashworthy AL part would weigh no more than the steel one.

Obviously, we need joints, and designs, that are up to the job, too.

I can see how a 4130 steel cabin, designed for flight loads only, could wind up providing better crash protection, as an unintended by-product of the wall thickness requirement, than a lighter AL structure. Similarly, I can see how this minimum conveniently weldable gauge for 4130 provides very strong welded joints without benefit of extensive engineering ("experience shows that . . ."). In this way, 4130 could give good crashworthiness without that being a deliberate design goal. That's great, but how much better could things be if we >did< design for crashworthiness using a lighter, stronger material (aluminum or composites)?

As far as "show me a practical example": I'm all for looking at what we can learn from existing designs. We should always appreciate the (usually good) logic behind the design of things that already exist (and ask why some things don't exist). But, if the lack of readily available existing examples was sufficient evidence that an idea was not worthy of consideration, then why design the Praetorian? I mean, if it were possible to design a plane with more crashworthy fuel systems, a strong occupant cage, good energy management to protect occupants in a crash etc--wouldn't all planes already be that way? I don't think it's impossible to improve on all the things we already have, but "improvement" is in the eye of the beholder--sometimes it is an "improvement" only if our priorities are different from previous priorities.

 

[Vigilant1]

What you want is a material with high elongation in yield to absorb the crash progressively.

Sometimes we might want this plastic deformation (e.g. in a crush zone), sometimes it is not acceptable (in the cage that is designed to keep the rocks, trees, engine, etc away from the occupants). It would be better to design for each attribute specifically.

 

[TFF]

Any material can be engineered to work. Although crash survival is always thought about, I doubt real design thought, past the average, has only been pushed for maybe the last twenty years. Airbags in cars changed auto safety a lot. That and ABS. It changed the way we accepted safety. People started asking questions like can they retrofit it to their 65 Mustang.

In the homebuilt airplane world, 4130 probably wins for safety, but not because of maternal but engineering. Not very many new 4130 airplanes. Those that are are from legacy designers. Engineering on these is old school with a high factor of fudge built in. It’s built in for success. It’s designed to survive average welds, average material, average alignment, average pilots. Average math designed.

Aluminum in sheet or tube form has to trade. For someone who has minimum skill, it’s easier to convince them riveting is easier. The engineering has to step up. The fudge built into 4130 is there for a reason, success of build. New aluminum design sees “free” performance gains because they are not adding the same type of fudge in. Much more conscious engineering for the performance. If the same engineering was done to aluminum that was done in the steel tube day, the parts would be thicker, more rivets. You will end up with the between the wars construction like Hawker. These things were not designed cost per pound like launching a rocket. They were designed so the average idiot could get it together.

Back to the crash. Probably most 4130 planes are overbuilt for today’s engineering. I doubt any material is actually better if crash was designed in as a primary issue. Manufacturing is where the issue comes in. What does it take to build it given some set of circumstances. In homebuilding simplicity wins out over extreme dreaming for the 99% of the 10% of the starters who actually finish a plane.

 

[Winginitt]

I think the key to survivability should be to put more work into designing for slower landing speed and control during that slow speed. Knocking 10-20 mph off ones landing speed does wonders for surviving an engine out.
How much more survivable would most airplanes be if they took lessons from Helio Courier designs. 3600 lb gross 170 mph max slow speed control and lands appx 30/35 mph in a third of the space a Cessna 172 lands in.
Now if you build a plane that has a 2000 lb gross you should be able to achieve 150 mph and land at similar or even slower speeds. I think thats the biggest "bang for the buck" as far as improving chances of survivability.

 

[Vigilant1]

I think the key to survivability should be to put more work into designing for slower landing speed and control during that slow speed. Knocking 10-20 mph off ones landing speed does wonders for surviving an engine out.

Yes, slower speed (less energy) helps a lot. But, since not all crashes are controlled off airport landings, and because there are other design constraints that limit how slow we can land, I think this is a case of "and" vs "instead of."

 

[flywheel1935]

Ok, I asked because if the bond breaks down over time then you are relying on the rivets only. Thanks.

I understand Grumman/AA Yankee aircraft are/were bonded airframes, and no doubt many others too, such as ARV, even Airbus and Boeing bond airframes, so happy with assembly choice,

 

[wsimpso1]

I can see where Dr Krieger is coming from and agree, that often the design work and manufacturing do not take best advantage of alloy over steel.

If we compare a steel and cloth fuselage to a similar alloy with gusset fuselage also covered in cloth- the steel should win. The cloth offers no strength at all, just keeps the wind out. However we look at a truss type fuselage in alloy that is skinned and gusseted where appropiate- the scenario changes a lot. The tubes of the truss are square, with high bonding/ riveting area. The skin on the outside is needed anyway of some type, the interior one is handy for mounting things.

We are now looking at a truss that is sandwiched. The combined strength of the sandwich is considerable. It will not flex, kill the occupants and spring back. It is very vault like, add in insulation and quiet as well. That is for a cockpit, the cage you wish to survive in. A normal tube and gusset rear and tail can still be used to maintain low weight.

The point is we can beef up a fuselage to meet the demands we expect of a steel frame and still build in alloy and have deformable crash attenuation in our design. In theory, we could make it of the best material for each load and crash survival but that often, ends in many different materials. A example would be a steel cabin and alloy remainder or even a steel cabin and glass or a lot of combinations we see in design and that is before we get serious about some safety.

I think the idea of designing for the particular attributes alloy presents and its potential is the key. The spam can, can and will happily give the impression of strength until it kills and acts like nothing happened. The steel frame will bend a bit but is quite solid- it does not takeup the impulse of collision well for the occupants either. We need to design so anything bar the cabin is treated as crush zone, then we can give ourselves the ability to dump the big g loads in crushing the front, the wings breaking off and the tail compressing a bit. But the cabin stays in tact- any additional loads are then transmitted around the cabin perimeter and minimal g loads reach the poor mushy bit inside.

The poor bugger in the pilot seat should have a proper seat that allows for some movement to take up the big loads of a sudden stop. Additionally the landing gear is crush area/ g load shedding. If we a add a small and relatively light alloy structure bellow the structural cabin we can have extra crush zone with minimal weight. This can help protect the pilot for big vertical loads. A steel frame still has relatively unyielding structure and any extra added pieces are still quite stiff to be useful in sucking up G loads.


I see that a Alloy structure can not only be strong, cheap and not too heavy if designed well- the safety for bad days is in the design. If we compare steel and cloth vs a traditional monocoupe we are making the wrong comparison. Neither is ideal if traditional thinking is used. We need to think more of a blend including sandwiched trusses-then we can really take advantage of the material and the ease of manufacture for a safer aircraft.

No matter what materials we use, builders are often stuck with whats available and is it economic?- a very big deal outside the USA where tube is cheap, alloy abounds, epoxy flows from gutters and Spruce just falls next to you. I would expect the most economic material for the vast majority is alloy tube and sheet. So that is what I would choose- if the aim is the most builders getting a aircraft in the air, cheaply and quickly, that if all goes to shite, may not kill them. Use the right grades and follow simple instructions and no specialised skills required. But the design is paramount- no amount of skills will make a Affordaplane a awesome design. But the materials can be used properly, its all in the minds eye.

What the aircraft looks like after the crash is irrelevant, as long as the skin bags of flesh get to walk away. I know that sounds ghoulish but the man who lives laughs the loudest.

We have a lot of ways to make the cockpit sturdy enough to protect the occupants in a forced landing. The question is not "can it be done?" but "how much does each method weigh?" These are airplanes and above all else, WEIGHT IS THE ENEMY. We have to decide how much we will protect the occupants and then do that level of protection at the lowest weight we can manage.

As SVSU Steve points out, we already know from looking at crash results, conventional welded steel tube fuselages per the old school designs do really well as is. Designing a safety cage in welded steel is about the simplest design scheme we have - truss design and monkey-see monkey-do engineering are as low in difficulty to design and execute as we have and work well in welded steel tube structures.

You can do as well in composites but they are at the other end of being home design difficulty to do well. You had better have an A-level game to do the design and execute it well. Aluminum tube-and-gusset or tube reinforced monocoque are between welded steel tube and composite for design difficulty for this topic.

Part of what makes welded steel and well designed composites work well in these cases is that they distribute the stresses pretty well, and welded steel tube does it at relatively low stresses relative to weld strength. Composite joints properly designed will hold together beyond what the rest of the structure will carry, but in riveted or bolted tube-and-gusset work, the fasteners and how they penetrate the other members are frequently the weakest spot in the scheme.

Traditional aluminum structures are not designed to stand the several loading modes of forced landings, are designed to modest flight loads, materials will buckle readily and fasteners fail when the design loads are exceeded. There is no doubt that the cockpit of such birds can be designed around the likely crash pulses with increased FOS in joints, and we are remiss in design if we do not make efforts to do so.

When we have someone decide on crash pulses, then design cockpits in welded steel tube and reinforced monocoque aluminum to those loads, then tell us what they each weigh, we can actually say what will be suitable of not.

In the meanwhile, I do suspect that traditional welded steel tube will be your crashworthy scheme of choice for all but the most capable designer/crash analysts.

Billski

 

[Vigilant1]

Keep in mind that properly designed riveted and bolted joints don't see shear through the fastener. If they do, their fatigue life is drastically smaller. The residual compression forces imparted during installation should be large enough to allow the joined parts to take up all the in-plane and normal forces without ANY additional static or alternating stresses on the fasteners.

So few people recognize that friction caused by preloading fasteners is what holds joints from moving and keeps the fasteners from fatiguing. If the joint shifts at all, the fasteners see fatigue and joint is doomed. You could Loctite nuts or not fully set rivets and then the assembly will either break the fasteners or the holes will open up and the joint will fly apart.

If the friction between the parts (say, for example, the faces of the tubes against the gussets) which results from the grip pressure of the rivets/bolts is what determines the joint strength in normal use, then:
1) Round tubes would seem to be at a significant disadvantage to square/rectangular tubes with regard to "in service" joint strength
2) The right adhesives would appear to be attractive as a means to increase the resistance to shifting of the respective parts. The rivets/bolts would provide clamping force during the cure and provide additional peel "insurance" and in-plane strength for extraordinary (destructive, one-time) loading. The negatives: Another (preparation-required, time-sensitive, temperature sensitive) construction step and difficulty in disassembling parts in the case of builder mistakes/needed repairs, etc.

 

[cluttonfred]

As I said in this thread two years ago, it would be very instructive to talk to someone who has restored and maintained the old New Standard D-25 biplanes for lessons learned and best practices with a bolted and riveted aluminum angle structure. I can't think of an easier truss-type fabrication method for amateurs or small-scale production.

 

[cheapracer]

I can see where Dr Krieger is coming from and agree, that often the design work and manufacturing do not take best advantage of alloy over steel.

You have pretty much stated my case for a weight increase for LSA


Here's one I'm playing with at the moment, and will test out next week.

Rather than an actual tube, the blue gussett forms a longeron tube using the corner of folded sheet, and then whatever bracing/gusseting you need for the application...

 

[FritzW]

Here's one I'm playing with at the moment,

Nice but looks like lots of bending. Not a problem if your equipped for it.

 

[Vigilant1]

Here's one I'm playing with at the moment, and will test out next week.

Rather than an actual tube, the blue gussett forms a longeron tube using the corner of folded sheet, and then whatever bracing/gusseting you need for the application...

View attachment 89880

It looks easy to build and will give an attractive (and maybe less draggy) corner than a square tube would.
Any concern about the buckling strength of the corner (in blue)? The shape, esp the "caps" at less than 90 degrees, would give a fairly low moment of inertia and might require a substantial thickness to equal a tube (or even an angle).

 

[cheapracer]

Nice but looks like lots of bending. Not a problem if your equipped for it.

Correct and correct.

Comes back to that old argument of what material is best, it's the one you know and have the tools for.

 

[cheapracer]

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..

 

[wsimpso1]

If the friction between the parts (say, for example, the faces of the tubes against the gussets) which results from the grip pressure of the rivets/bolts is what determines the joint strength in normal use, then:
1) Round tubes would seem to be at a significant disadvantage to square/rectangular tubes with regard to "in service" joint strength

Not as big a deal as you might think. There are several types of joints. The first is sealing surfaces with a lot of stiffness to distribute loads, like in a cylinder block to cylinder head joint, the loads in the fasteners approach or equal yield loads, and the compressive loads in the members are well distributed by design of the mating parts and by design of the gasket between them. Next is lighter systems where sealing is not needed - almost all of the compressive load from the fasteners is carried within a cylinder 2 times the fastener shaft diameter, and again the fasteners can approach or even equal yield loads. Then there are lightly built parts where fastener loads are low.

Your tube and gusset design is in the second bunch, big loads in the fasteners, local compression close to the fastener, little in the way of distributed load. If the tube did not change shape at all under the bolting or riveting forces, the contact would primarily be taken up in the areas along the long axis of the tube. Usually, the round tube deforms slightly and you get high contact stresses around bolt head and nut or rivet head that looks quite similar to what you see with square tube. If you doubt what I say, build a sample joint or two , torque the bolts or set the rivets, then spritz a little spray paint along the joint, wait for it to dry, then open the joint back up and see how close the paint gets to the bolt or rivet hole...

2) The right adhesives would appear to be attractive as a means to increase the resistance to shifting of the respective parts. The rivets/bolts would provide clamping force during the cure and provide additional peel "insurance" and in-plane strength for extraordinary (destructive, one-time) loading. The negatives: Another (preparation-required, time-sensitive, temperature sensitive) construction step and difficulty in disassembling parts in the case of builder mistakes/needed repairs, etc.

If you are talking bolted aluminum members with adhesives on the joint, yeah, adhesives seem like a way to make a good joint into a bulletproof one. Some things to remember. First, there is ZERO natural metallic aluminum on this planet. All metallic aluminum on this rock is man-made, and it is all trying to react with oxygen and other chemicals. So, when we try to bond aluminum parts into a joint, please recognize that if we can not 100% prevent entrance of water, it will make an oxide layer that will grow in thickness and wedge the mating parts away from each other. From this mechanism, we must recognize that bonding to aluminum is not a long term process. There are some processes that can be applied in specific conditions and be almost permanent, certainly up to the life of airplanes. Most of our home craftsman stuff does not rise to that level. I do prime my aluminum parts and assemble them with the plan that we can keep corrosion at bay for the lifetime of the airplane. This will help with the permanence of joints, but I would design the joints to be slip free with just the rivets and/or bolts, and then take adhesives as corrosion protection. This is at its maximum where dissimilar metals are painted with epoxy primers, riveted together with rivets dipped in paint (at one time) and epoxy (more recently).

Billski

 

[BBerson]

Here's one I'm playing with at the moment, and will test out next week.

That's a full skinned structure.
This thread is about tube/angle and fabric covered trusses.