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philr

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In post 120, you seemed to show diagonals on your "keel", here, not so much. Without them, or a skin of some sort, your torsional strength and stiffness will be poor. And I know from personal experience that getting long bends in thin aluminum with the tooling available to amateurs is TOUGH, and in a beam like this, if they are not straight, they will fail in compression (buckling) quite readily.

New Kolb sells boom tubes for $500 plus shipping, just sayin'

The fuselage of a conventional light plane is loaded in fight with the bottom of the fuselage is in compression, yet your keel beam has more material on top, where the loads, especially on a tri-gear airplane (no landing loads on the tail in a NORMAL landing), are in tension.

Look at how spars are built, particularly box spars, for some ideas on how to improve this.
The plan is to use .016 2024 T3 (stated in my post above) for the skin which will be stressed skin like the Zenith 701 uses in the rear fuse although the angles they use at the corners are heavier.
 

challenger_II

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Before you jump out and build a brake, hunt around and find your local Air Conditioning and Heating companies. They often have very large shears, and brakes, for making duct work.


For ease of fabrication you cant beat working with aluminum sheet. To cut out bulkhead use a olfa knife and straight edge I can cut out this part in under 10 minutes with the 13$ tool below from harbor freight the flanges are folded in under 5 minutes. If .016 is too light to hold rivits well in a bulkhead step up to .020. It is true longer folds are harder to do well but we can build a 8' brake for pretty cheap and make that the longest length just join parts for the longer ones.
View attachment 114523
View attachment 114524
 
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philr

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Before you jump out and build a brake, hunt around and find your local Air Conditioning and Heating companies. They often have very large shears, and brakes, for making duct work.
Oh thats a good idea I'm sure most areas there would be a contractor that would bend your stuff for you.
 

rotax618

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I have built an ICP Savannah (CH701 clone), which used 0.016” skins, as someone said you will probably need to sandwich the skin between two 0.025” angles to prevent the rivets simply tearing out. The Savannah used approx 40mm rivet spacings. The angles don’t have to be as wide as you have suggested in your drawings as long as they equal the required cross-sectional area.
 

philr

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BJC

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I’ve bent 12’ pieces on a brake, but I’ve never used a brake like the one in the link. I think that it would be difficult to get straight bends with a constant bend radius with a brake like that, especially a long piece, as for a spar or longitudinal stiffener, of and significant thickness.


BJC
 

Victor Bravo

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The most cost-efficient, time-efficient, and safety-efficient way to achieve this, for a one-off E-AB "garage builder", has already been posted.

Sandwich the actual piece of metal between two sacrificial "spacer" sheets of low-cost/non-aircraft quality aluminum flashing. Figure out how thick these sacrificial spacers have to be, to achieve the actual correct bend radius on the "real" piece in the middle.

Then you can go to Bubba and Billy-Bob at the local air conditioning sheet metal shop (I can't say "Jose' and Hose B" any more, the PC Police will s**t themselves) and tell them to put a 90 degree bend on that line right there, and never mind why there are a couple extra pieces of metal.
 

Victor Bravo

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Another thought, which can save a lot of time and material in many cases, is that you can make the "blank" piece of metal half an inch wider than it actually needs to be, then put your bend in it, and THEN you can trim it so the dimensions of the finished part are exactly what you want to achieve.

In a factory environment, with highly trained WW2-era sheet metal workers - guys who do this every day - you can figure out the exact bend allowances, "setbacks", offsets (adjusting for Jupiter's gravitational field and Coriolis effect).... and cut the exactly correct width piece of metal within .005", and put the bend exactly where it needs to go, and have the finished part come out perfect.

As for the rest of us.... I can show you a Volkswagen-size pile of scrap parts that should have had the bend 1/16" further this way, and parts where the flange has 1/16" too long on one side and too short on the other, etc. That pile of scrap parts is not cheap in terms of time, money, or emotional exhaustion.
 

challenger_II

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VB: for, say, a 60 degree bend, in .025" 2024 T4, you would allow 5/32" additional material, for a corner radius of 1/8". Will not be exact "missile-quality dimensions", but will work well, in this application.
 

philr

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The aluminum angles for carrying the main loads around the cabin see pic below. 2x2x.125 5.3 ft 3.08 lbs, 1x1x .125 20 ft 5.64 lbs, 2x1/4 bar 2.5 ft 1.5 lbs. All 2024 t3 joints bolted like the Texas parasol. Weight 10.22 lbs and around 25 lbs for skin and .025 corners but still working on weights.
1629823767163.png
 

philr

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I looked into options for bending the fuselage stringers with a brake and I think it may be an easier option to use aluminum angle. The 2 sizes needed are 1/2x1/2x1/16 and 5/8x5/8x1/16. see pic below and tell me what you think. Staying with .016 skin sandwiched between the corner angles and .020 formers with 4" lightening holes flanged. Weight for these angle 7.92 lbs cost from Aircraft spruce $681 hmmmm what do you think?1630151593125.png
 
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