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rtfm

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Good design shouldn't be a lot of afterthoughts cobbled together. There are several basic things a powered aircraft needs, a fuel tank is one of them. Surely you considered the shape, placement, material and construction of this before you got this far. Duncan, before you waste any more effort, materials and money, Stop and finish a workable design and consider how you are going to build it and from what materials.
I had always planned to use a 20 litre plastic gas tank from one of the marine supply shops. I have one in the workshop, which as it turns out is a tad too tall. That was a miscalculation on my part. But, hey, this is my first design. It's all very well to say "finish a workable design" but there are gotchas at every turn - things I hadn't even considered. Every time I think I have EVERYTHING designed, I discover that either it doesn't work, is unbuildable or something else. I'm TRYING to do as you say - but it's not easy.
 

rtfm

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A plywood fuel tank is not an award winning idea, the only use I could see is to use light ply as a permanent in-situ mould. I would make the ply bottom and sides out of say 3mm ply, since you have a CNC you could finger joint it to make it easier to assemble. Fillet the inside corners with micros, lay up glass/vinylester/epoxy inside to a thickness that provides the necessary strength with any baffles sumps etc, form inward flanges at the top, lay up a glass/resin top, fit the filler/fuel gauge, glue and rivet it to the flanges. Looks like a coffin.View attachment 113236
See - this is what I mean. Your sketch is better by far than my plan. The finger joints, the flange at the top... I like your approach much better. Back to the drawing board...
 

rtfm

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Question regarding the load testing of the fuselage.
Hi. Any suggestions on how to test the fuselage? It seems to me that the obvious ones would be to load up the seat area to simulate 4G (or at least, to see how many G's it will withstand before collapsing). The other is to test the torsional rigidity of the aft section (although there is little torsional stress other than the effect of the rudder).

Anything else? And any wise comments about how to actually perform these tests?

Duncan
 

rtfm

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Question:
After wasting about an hour (and two cups of coffee) on the laptop searching for suitable fuel drain plugs, not a single manufacturer/distributor cares to specify the diameter of the thread going into the fuel tank. It seems I am looking for a 1/8" NPT, but I need to know what size hole I need to cut into my tank to accommodate this. Any ideas?

Duncan
 

TiPi

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Question:
After wasting about an hour (and two cups of coffee) on the laptop searching for suitable fuel drain plugs, not a single manufacturer/distributor cares to specify the diameter of the thread going into the fuel tank. It seems I am looking for a 1/8" NPT, but I need to know what size hole I need to cut into my tank to accommodate this. Any ideas?

Duncan
search for "1/8' NPT bung" and select the best option for your tank material and thickness.
 

rtfm

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Next question:
Strengths of materials is not my long suite. So a sanity check would be helpful.

Instead of bankrupting myself by buying streamlined tubing, I thought I might try carving a streamlined strut out of plywood. I checked the shear strength of "plywood" online, and it is listed as 250 to 300 PSI, which is 0.1757 kg/mm^2 This number is suspect, since it is a generic figure. Aircraft grade plywood is (presumably) better than this?

My first attempt was to carve a short test piece of Gaboon ply, with a 2mm recess cut out of one side to allow for a 4mm steel attachment to be inserted. I cut two pieces, and backed them, like this: Chord = 50mm, thickness = 15mm (once bonded together)

1627120892601.png

Cross section: 229.4mm^2 x 2 = 458.8mm^2
So shear strength of this strut would be .1757 x 458.8 = 80.6kg

But that's rubbish. I weigh about 80 kg, and there is no way this "strut" will break in shear with just my weight hanging from it.

What have I done wrong?

A second test piece had the following dimensions:
Chord: 100mm
Depth: 30mm
Cross section: 929mm^2 x 2 = 1858mm^2
So, shear strength would be: 1858 x .1757 = 326.45kg

This seems a more comfortable number, but being based on the same figures, it just looks better than the smaller strut, which is definitely not accurate.

Duncan
 
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Vigilant1

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Instead of bankrupting myself by buying streamlined tubing, I thought I might try carving a streamlined strut out of plywood. I checked the shear strength of "plywood" online, and it is listed as 250 to 300 PSI, which is 0.1757 kg/mm^2
You've mentioned shear strength. If this strut is to be (always??) in tension, then isn't tensile strength the attribute you want to find? The example tensile strength specs of plywood that I've seen are much higher than the specs for shear strength.

If the struts will also be in compression (on the ground, drag loads in flight, side loads on the wing, etc), then other calculations (esp buckling) will be required.

With plywood, won't strength be dependent on grain direction? This is a case where plywood wouldn't be expected to be as weight efficient as other choices.

I think the most common approach to this design issue is to use a metal (AL or steel) structural strut wrapped with a light streamlined metal fairing. I would go that way.

Otherwise, wrapping your plywood core with FG or CF might provide some valuable attributes in a part that has high loads of a critically nature..

Obviously, computing the stresses and doing accurate tests will be required.
 
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rtfm

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You're right. I would probably go with Hoop Pine, anyway - but I don't have any HP stock at the moment. A far better choice.

And you're right - tensile strength is what I'm after. Let me see what tensile strength numbers I can find for Hoop Pine.

Thanks for your help.
 

TFF

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So you are not going to turn and create G force or not ever going to fly in gusty winds or bounce a landing? You need to make your struts multiplied by the biggest force you will encounter plus some. If they are at an angle when mounted, they loose a little bit in being offset. You need to add that up and make your struts that strong.
 

rtfm

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For struts why not just go with round (or square?) steel tubing and make a fairing for it?
Hi,
That would certainly take the guesswork out of it. I could do either of these. A 20mm OD tube up the centre. The left hand one looks more complicated, but not really. The router doesn't care.

And before anyone suggests this is to much work, and that I should just uses some plastic wrapped around the tube to make a streamlined fairing: First, this isn't much additional work for me - the router does all the work while I do something else. Second, while this would be heavier than a plain tube with plastic fairing, it doesn't look HALF as cool...

I'm just trying to get out of buying steel tube. Besides, I like working on the router.

1627165993796.png

Duncan
 
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rtfm

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Ah - sorry...
MAUW: 300kg
Wing masts: 650mm
Struts: 975mm

I can't find the tensile strength of hoop pine. From what I've read, however, it falls somewhere between Spruce and Douglas fir.

Regards,
Duncan
 
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