Welded Aluminium Structure

[drawoh]

I am looking into the possibility of a welded, sheet metal aluminium chassis to be mounted in an aircraft, possibly outside. Probably, the walls will be around 1/8" (3mm) thick. Probably, I would use 5052-H32 or 5086-H32, which is bendable, and has reasonable annealed strength. This would be an optical device so it will be designed for rigidity. It will be very, very strong.

How difficult will it be to get this certified safe for flight? Has anyone any experience with this?

Another option is a casting, which I understand can be another can of worms.

I have not started modelling yet. I am staring at an arrangement in 3D[ ]CAD.

--
JHG

 

[rb1957]

welding isn't uncertifiable ... the difficulty is in the design, referencing the welding specs, detailing certificated welders, ... details.

I'd consider 6061 as a "good" weldable Al.

Not for helicopter ?

Design to a low stress level, there's potential that you may have to test (to ultimate) the flyable specimen. another possibility is welding specimens (details) to test.

another day in paradise, or is paradise one day closer ?

 

[dgapilot]

"How difficult will it be to get this certified safe for flight? Has anyone any experience with this?"

Look at the certification regulations related to the type aircraft you are proposing to install it on. If in the US, Part 23 for normal, utility, aerobatic or commuter airplanes, Part 25 for transport airplanes, Part 27 for small helicopters or Part 29 for large helicopters.

 

[drawoh]

welding isn't uncertifiable ... the difficulty is in the design, referencing the welding specs, detailing certificated welders, ... details.

I'd consider 6061 as a "good" weldable Al.

Not for helicopter ?

Why 6061 as opposed to 5052? I want to bend it.

It might go on a helicopter.

--
JHG

 

[berkshire]

Drawoh,
If you use 6061 condition O, you will find that it is reasonably easy to bend, not quite as malleable as 5052 but close enough to cold bend without too much trouble. Then you can heat treat it to T6, If you even want to.
For some reason Aircraft people associate strain hardened alloys like 3003 and 5052 with things like fuel tanks and formed cowlings.
B.E.

You are judged not by what you know, but by what you can do.

 

[tbuelna]

As part of obtaining FAA approval for your rotorcraft component you will need to submit analysis documentation showing it meets FAR part 29 requirements. You can use welding or casting processes to manufacture your optical platform. But there are specific requirements in FAR part 29 for using each one, including analysis factors (FAR 29.621) and manufacturing QA procedures (FAR 29.605).

 

[rb1957]

You can bend and weld 6061T6, but use 50xx if you prefer.

My caution about helicopters was that they are nature's fatigue machines, so more caution is required.

Why not a machined structure ? (finding qualified welders, who do good work, is going to be at least as hard as finding a good machine shop)

another day in paradise, or is paradise one day closer ?

 

[drawoh]

rb1957,

Machining out of billet is my fall-back strategy. I suspect though that by the time I find a qualified welder and set up the paper trail, the welding may still be cheaper. As noted above, this is an optical device with a laser. By the time I make it rigid enough to stay aligned under all the shock and vibration the helicopter imposes, it will be very strong.

My managers want this thing to look cool. I will find out how determined they really are when I propose casting it.

--
JHG

 

[Sparweb]

There are many welding shops capable of welding 6061 aluminum according to practices acceptable to your DER / FAA authority.
Any bending of 6061 should be done in the O condition, especially if it is 1/8" thick as you say.
Welding of 6061-T6 aluminum tends to cause local annealing around the welded area, plus residual stresses that can deform the part.

Hence...

Complete all bending operations in O condition, complete all welding in O condition, leave the structure in the fixture you used for aligning the parts for welding, and put it all in a heat-treatment bath. I would dispense with the heat-treatment if I could structurally test the finished welded assembly and show that it doesn't fail under ultimate load. Local deformations aren't important at ultimate load, provided nothing breaks, but at working loads, of course, any deformation is unacceptable, and then you would be required to use heat-treated material. Annealed 6061 is very soft.

STF

 

[berkshire]

Sparweb,
It would appear that you and I are on the same page, If Drawoh makes it in condition O and it passes all of Drawohs tests, he is ahead of the game.
If it does distort, it only needs enough heat treat to make it strong enough. Using heat treated material defeats the purpose unless you can accept the large bend rads. Taking T6 quenching, then forming in the Aq condition circumvents this, But this presumes that Drawohs contractor has heat treat facilities.
B.E.

You are judged not by what you know, but by what you can do.

 

[verymadmac]

Unless there is some unusual geometry or strict weight requirements, for the typical helicopter loads associated with role equipment, I would be looking at making it out of 6061-T4 and welded as is, especially given that your design criteria is stiffness. As for welding, in New Zealand for this type of kit we wouldn't control the weld procedures and would just call out for each weld assembly to proof loaded (not sure if the FAA DER's can use that approach), it just depends on the expected number of units.

The other point is this what type of use on the helicopter will it get, most role equipment tends to be on and off the helicopter frequently, so addressing fatigue (given that strength of these units they easily grow a visually inspectible crack before being critical length) is often picked up by requiring visual inspections for damage to the kit on installation &/or 100 hourly as finding the damage relating to handling / stowage of the kit off the frame is likely be a bigger problem.

 

[MikeHalloran]

If you are indeed designing for stiffness, aluminum has no weight advantage over steel, which may be easier to fabricate.



Mike Halloran
Pembroke Pines, FL, USA

 

[Sparweb]

Steel is suitable indeed, but aluminum should have a greater resistance to corrosion in the "typical" external environment of a rotorcraft.
We don't know what facilities are at Drawoh's disposal for finishing & painting the parts when they are complete, but many kinds of durable finish can be used.
Some people swear by the commercially-available phosphate-bath plus powder coating, though to my knowledge there is no Mil-spec process for this. Such a finish would be ideal for low-stress high-stiffness steel components - much less suitable for heat-treated aluminum parts.


STF

 

[drawoh]

MikeHalloran,

I need to keep weight down. If this were a tubular space frame, there would be no advantage for aluminium over 4130 steel. This is an enclosed chassis. The wall of an aluminium chassis will be thicker and stiffer.

--
JHG

 

[rb1957]

I am not a fan of working with O condition sheet and heat treating it, because of the potential of warping.

Are we talking about bending sheet or welding it ... there seem to be two discussions going on !?

I'm alittle confused about the material choice ... if you're comparing 0.04" Al with Steel, then the Al will be lighter but the Steel will be stiffer, if you're comparing 0.12" Al with 0.04" Steel then the two should be much the same (weight and stiffness).

Could we see something of what this looks like ?

IMO welded Al doesn't look "cool" ... "cool" might be a composite, with all the extra difficulties that has.

another day in paradise, or is paradise one day closer ?

 

[drawoh]

rb1957,

Steel has approximately three times the mass and three times the stiffness of aluminium. In a truss frame loaded in pure tension and compression, there is no difference in stiffness between the two structures. If the structure is loaded in bending, the aluminium piece will have a higher second moment of area. The aluminium structure will have a higher stiffness to weight ratio.

The aluminium structure would be a sealed sheet metal box. The walls would be at least .10" (2.5mm) to control warping, and to assure that there is room to grind off edges. There appears to be a requirement for this thing to look cool. I don't have a good gut feeling for warping. I understand that steel is better, but I don't think I can take advantage of its superior strength and stiffness.

--
JHG

 

[rb1957]

we can debate the relative merits of the different materials ... it'll come down to how you make it ... ie you won't have an optimal stress everywhere in the structure, you'll have a structure as complicated (or simply) as fits the manufacture of it.

0.1" thk Al is really thick.

a "box" is rarely an efficient way to react pressure ... so already the design has compromised weight for other reasons.

a thick walled box is rarely an efficient structure.

give us a clue as to what we're looking at ... how big? how many (1 off?) ?

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

oh...

Rb1957's last post poses a very important question to YOU: The design/configuration complexity is an all-important aspect.

In several cases I have seen weld assembly designs that could be far-more-easily reproduced as machined components [3D model required, simplified for machining purposes]. These assemblies could easily be simplified to a single machined part... and/or with some odd details fastened to the machined base component.

In a few rare cases I have seen where weld assemblies... such as for complex pneumatic system components... could only be made using multiple sheet-metal weld details and perhaps some castings.

In some cases, perhaps, a lost-wax casting could also be employed for the base part and/or complex details. The base/detail component is[are] made from one-or-more stereolithography wax/plastic detail [including risers, sprues, vents, etc]; which are coated with multiple layers of ceramic compound to build a thick shell for containing the casting metal; which is then heated to a temperature which both hardens the ceramic and 'burns-out' the core wax/plastic. The ceramic mold is then filled with a typical casting alloy [A356, etc] at a foundry. Finishing the part becomes a simple matter of trimming off casting elements/flash, heat treatment [if necessary]; and then pressure testing the casting for general soundness.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[Sparweb]

At the risk of sounding like a broken record (or worse), I'll quote myself "... leave the structure in the fixture you used for aligning the parts for welding, and put it all in a heat-treatment bath..."

There's no other way to guarantee that all the holes line up if you release the assembly from the fixture too soon, if the assembly was welded. Now that I re-read this I'm sure I have a specific configuration in mind - but maybe not at all what the OP has in mind. I'm rethinking my assumptions, but still believe the "camera box" in question doesn't need to contain air pressure. I expect the OP would have mentioned it, given how important this would be if it was. I think it needs to be sealed to to "keep the rain out", but there's no reason I couldn't be wrong.

Hmm, I wonder if there aren't features in the OP's concept that are driving higher and higher thickness of certain parts... seeking higher stiffness... sometimes these pursuits go in the direction of the Tacoma Narrows bridge, or the Fokker D.7 monoplane. I have seen this happen, many times, with 1-off external mods on rotorcraft.

Going back to the first posting: is this going inside or outside? I would want to know where it's mounted, before figuring out HOW it's mounted, and then determine how it must be protected.

STF

 

[tbuelna]

It would be helpful to see a sketch of the optical platform with a few dimensions. Other details such as production rate/quantities would also be helpful for providing guidance on what manufacturing approach might be best. If weight and structural stiffness are important, a composite material might be best. There are many low cost methods for prototyping composite parts. If the hollow box platform is not too large, it can be rapid prototyped in a single piece with thin walls (~.06") using laser sintered aluminum.