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Tube construction: Alternatives to conventional welding

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BBerson

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4) I've read (unsubstantiated?) opinions that adhesive will cause the rivet effectiveness to decline significantly, since it provides a soft mushy layer that reduces the grip strength of the rivet and hastens the day when they rattle loose. Any thoughts on that?
That was using polysulphide rubber sealant which is not a true structural adhesive and can cause problems with the rivet grip. A hard adhesive should have good results if preparation is correct. Preparation in a home shop may be an issue.
 

cheapracer

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Oblong, read Rectangle, 1/2" X 3/4" x 0.035 thick wall, 6061-T6
While technically true, fact is most reconise 'oblong' as having either curved ends (flat sides) or an elongated in one axis circle with continuious curvature..

.. and a rectagle as having 4 flat sides.
 

flywheel1935

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Thanks for the offer, it looks like a nice build.
1) Can you comment a bit on the recommended AL prep and HYSOL application/bonding process? According to what I've read, preparing the aluminum is key to any success with goo.
2) Can you comment on any analysis of joint strength with vs without the adhesive? Obviously, the designer thought it important to use HYSOL, but there are a lot of flying aircraft with good service histories that just use pull rivets.
3) Are there other approved adhesives?
4) I've read (unsubstantiated?) opinions that adhesive will cause the rivet effectiveness to decline significantly, since it provides a soft mushy layer that reduces the grip strength of the rivet and hastens the day when they rattle loose. Any thoughts on that?
Thanks!
VIGILANT,
1/ De-grease alloy with Acetone, scuff with oxide paper or flap wheel, both tube and gusset, degrease again, apply HYSOL, then rivet
2/ Going by build manual, to be fair 'hysol' not as strong in peel as it is in shear ??? hence having the rivets.
3/ looking at 3M products, with cure temps from +15C
4/ Nothing of concern at present using this system, but the LMA kits came from Florida, and I think that they believed all builders had access to high cure temps.
Main issue being poorly designed airframe, so were doing a few mods then redraw MK2 to make it "Fast Build" using more common parts, ie 2-3 gusset designs for all the airframe IMG_1852.JPG IMG_1853.JPG
 

Winginitt

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Could you rig up a "booth" with a plastic drop cloth from the board behind the workbench and then hook an electric heater and a thermometer to it for curing the Hysol or ???

Edit: After looking at the set up again, I have to wonder how you place the "glue" on so many joints at once and then get the rivets installed ? Is the gluing done as one operation and then the riveting done after the glue dries, or do you try to rivet as the glue dries?
 
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SVSUSteve

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Welded steel cabins have a good reputation for crash safety.
For something designed and built by a layperson, they should be considered the best option. They are the F1/IndyCar of homebuilt cockpit structural integrity. Why do you think I not using aluminum on composite in my design’s cockpit frame?

Several posts in this thread have noted that AL can offer strengths higher than 4130 at the same weight.
In theory or with the use of advanced alloys by extremely thorough engineers, yes.

In real world homebuilt crashes, it’s more like (to paraphrase a military adage) “no aluminum cockpit survives first contact with the enemy (IOW the ground) much above about 10 G longitudinal (often less)”.

Plus those aluminum fuselages tend to do this nasty little thing where they collapse under vertical or “bending” (think nose down impact) loads and then spring back open so they look like they stayed intact or “are strong enough” until you realize everyone is dead from having their skulls crushed and/or their cervical spine damaged.

perhaps there's no need to go with 4130 at all, just build to the same strength (which may be well above flight loads) using aluminum
Show me an example of someone doing that.

You can have strong, light, cheap or easy to work with. Pick any three.

yes, some composite planes fly apart into a jumble of foam and glass on impact, but perhaps that's due to a design that accounted for flight loads and not impact loads.
Pretty much every one of them except for a handful of gliders (where it’s not that hard to make a cockpit stand to the low speed of your average glider crash).



There are some well engineered high performance composite sailplanes with good crashworthiness, and some Formula 1 cars with composite crash protection that is tremendous.
On the F1 end also because they’re using resins and manufacturing technology that would make the pearl clutchers who complain about the cost of even the most basic things start bleeding intracranially due to their blood pressure spiking. More importantly they aren’t designed by amateurs using shortcut methodology.
 

flywheel1935

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Could you rig up a "booth" with a plastic drop cloth from the board behind the workbench and then hook an electric heater and a thermometer to it for curing the Hysol or ???

Edit: After looking at the set up again, I have to wonder how you place the "glue" on so many joints at once and then get the rivets installed ? Is the gluing done as one operation and then the riveting done after the glue dries, or do you try to rivet as the glue dries?
We set up about 10 gussets at each bond session, thats about the most a glue dispenser will last (50ml), My build partner glues, I rivet. If temps are likely to drop we use an electric blanket at about 30C to post cure for a couple of days . We also beefed up the airframe at high stress points, and doubled the surface are to give a larger bond area. All holes are pre-drilled 1/8", excess glue that oozes out can then be smoothed with a acetone soaked cloth.



20190824_131832.jpg
 

Aerowerx

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A thought about the crash safety discussions...

Are you talking about a crash or a forced landing?

I would think in a crash (for example caused by a close encounter with another aircraft) all bets would be off. You have no idea what the forces will be, and the occupants may very well be dead before they hit the ground.

Now, in a forced landing gone bad the pilot will still have some control, but is just running out of air to fly in, is a different situation. Either he has a nice flat area to land in or there are obstacles. Flat area, no problem. But if there are obstacles then coming in at minimum controllable airspeed would be what you want to do. Then the plane should be designed to protect the occupants with crush zones as much as possible.
 

flywheel1935

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Have you performed the wedge test to see if its even worth using adhesive?
The whole process works in tandem, ie bonding only or riveting only, would leave the trusses compromised at major stress points, if its good enough for
Lotus and Aston Martin Chassis, I'm happy to use it. Its not my design, I'm just improving on the original kit suppliers build method.
 

Vigilant1

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In theory or with the use of advanced alloys by extremely thorough engineers, yes.

In real world homebuilt crashes, it’s more like (to paraphrase a military adage) “no aluminum cockpit survives first contact with the enemy (IOW the ground) much above about 10 G longitudinal (often less)”.

Plus those aluminum fuselages tend to do this nasty little thing where they collapse under vertical or “bending” (think nose down impact) loads and then spring back open so they look like they stayed intact or “are strong enough” until you realize everyone is dead from having their skulls crushed and/or their cervical spine damaged.

You can have strong, light, cheap or easy to work with. Pick any three.
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. But the AL and composite materials themselves are stronger (on a pound for pound basis). So, it sounds like either a difference in priorities or a deficiency
in engineering.
 

trimtab

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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. The practice of designing for shear flows or shear stresses based on fastener shear cross sections is so far wide of the mark of what is actually going on, it is a wonder why it is used as the default structural minimum criterion for joints at all.

Bolted joints can be considerably stronger than welded joints where material condition plays a role. Aluminum goes to a T0 condition in the HAZ, for example- a huge strength reduction. 4130 can be gang drilled and then treated to be twice as strong, then bolted or riveted with a properly conceived joint design to be far stronger than a welded joint.

Manufacturing is a compromise of skills, costs, weight, performance, etc. There are downsides to using hard 4130, T6 aluminum, etc., just as there are upsides.
 

SVSUSteve

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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.
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.

But the AL and composite materials themselves are stronger (on a pound for pound basis). So, it sounds like either a difference in priorities or a deficiency
in engineering.
The "pound for pound" basis argument is misleading if not outright fallacious.If it doesn't hold up as well in a given scenario then it does not really matter if a cubic foot of the stuff is lighter now does it?

If it's stronger FOR A GIVEN APPLICATION then yes, the argument is valid. But just saying X material is stronger per pound (with an asterisk "....but it doesn't work as well for the application you have in mind because you wind up needing more of it-- either in terms of weight or volume (reducing the interior volume of the aircraft or making the wetted area bigger)-- to achieve the same result") is misleading. It's the same excuses we hear from the composite crowd about how their materials are superior BUT when it comes to crash survivability only if you have the high end materials or methods that most of us cannot afford AND if you or a colleague working with you have an advanced degree in designing those sorts of structures.
 

wsimpso1

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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. The practice of designing for shear flows or shear stresses based on fastener shear cross sections is so far wide of the mark of what is actually going on, it is a wonder why it is used as the default structural minimum criterion for joints at all.
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. Just ask your tire shop guys what happens if the nuts are not tightened...

Billski
 

litespeed

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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.
 

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