thinning down from 6061 to 2024

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Norm Langlois

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hull modification 002_00001.jpg Does any one know if for example something constructed of .050 6061 t4 would be as strong or better if thinning down to .040 2024 t3
The KSI numbers show a much superior strength for 2024
The mono spar I Made a 60 degree triangle for my UL is 6061 T4 .050 it did not have a full third diaphragm it was not even the same thickness for what was ,the 3rd side
I plan to make a new spar . this new spar will be enhanced by a full 3rd Diaphragm . .040 2024 T3 would be 4 lbs lighter.hull modification 041.jpg
I plan to build a new wing even if I still use 6061 T4 it will be much lighter than this one
A new wing with 6061 T4 different air foil 13 half span with a 14 ft spar will only weigh 28 lbs
full span 27 ft Hornier tips as before will 56 lbs uncovered . Saving an additional 4 lbs would be nice.
 
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pictsidhe

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I did some quick numbers for my cantilever 103 project a few months back. I think with a 12" deep spar, a sheet web was a shear buckling nightmare unless I made it very thick (and heavy). Stiffening it adequately didn't appeal as it needed very close stiffeners. I am currently planning to use a truss.
 

Norm Langlois

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hull modification 055.jpg
This is the second test failure, without the flying wire. In the use of a new spar . This spar has 11 inch diaphragm . Failure begins with compression of the 1 inch wide apex .
Any attempt to cantilever would need to be reversing the load by inverting the spar.
This spar has a flat thinner material .020 just to prevent splay . I intend to install a formed third side, a hat shaped with matching angle bends , recessed diaphragm. Riveted side and top flange . This would be 2 legs 2 thicknesses of compounded angles,equal materials in compression.
 

plncraze

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Is there a chance as you load test when you could stop, remove the load, reinforce and load again. This failure looks as if it could be stopped with a strap like the RV's have for reinforcement.
 

BBerson

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Usually, the caps are many times thicker than the vertical skins (webs). The webs could likely be .020" if the cap corners are .125".
 

Norm Langlois

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In the picture above the compression of 6061 T4 .050 This shape \_/ the shape 60 degree triangle is all that was resisting 700 lbs per side over a 13 ft half span

The above test was the second . This is apparent on both sides of a reinforced apex spanning the mid section for about 3 ft.
The first test was at wing center. see belowhull modification 047.jpg
Here you can also see the apex is a 1 inch wide flat.
This very tall spar was minimally cantilever-able at 1.5 G
I propose it was not the height nor the large diaphragm . But simply just to thin to resist compression. By inversion there would be 2 legs of double thickness in compression. the compound riveted in structure would have two more legs outward, caps to match the main \__________/
Uni-body construction of a sort.
Thinning down would I suppose just exacerbate . If made of 2024 T3 instead of 6061 T4 and remain At .050 through out. Only slightly heavier, than the implied 28 lb. wing half span, uncovered.
I will do a sample flare on the 2024 and see if that is possible.
 

Norm Langlois

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Pincraze
the floor was my stop . Any deformation during a test would be a permanent compromise. movement in the diaphragms, wasn't detected the failure was sudden.
 

BBerson

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Reinforcing caps only need to go out about half way. Thin metal is enough at outer wing. Thin webs need stiffeners, however. So your thick webs might be best for you. An optimal part usually takes more hours to build
 

Mad MAC

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Given that you are using 6061-t4, and the failures look to possibly be flange crippling (one needs to run numbers to check), changing material or temper should be closely considered. Flange crippling is driven by geometry but is also limited by compressive yield & for
6061-T4 Fcy = 16 ksi
6061-T6 Fcy = 36 ksi
2024-T3 Clad Fcy = 37 ksi
 

BBerson

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Yes, that 6061-t4 is low yield. It is crippled beyond yield.
Flipping the spar would help add more compression area, but then the tension side would yield.
It needs stronger alloy. But 2024-t3 might be a challenge to bend
 

Norm Langlois

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These tests were Cantilevered . That's a lot of concentrated pressure for .050 material. certainly the first test was a few less bags of sand,than the second. With all the pressure focused at the center. The second test divided to two points 30 inches apart by the application of reinforced apex. Compression can be enhanced or eliminated by boxing the apex . I did not like the idea of riveting some component along the apex. I though the many rivet holes may also compromise.
I have given the spar much thought . I use it with a single 1/8th flying wire . Its far more than needed. I am chasing after the cantilevered wing hoping to find a folded or simple remove option. for use with any type UL Though this triangle could be used LSA if made thicker.
The flare to the lighten holes is compound. I need also to be able to do that to the 2024 T3 to use that material. I also propose to make that a smaller internal hole making more of a return further enhancing the diaphragm.
 
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BBerson

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The triangle wouldn't fit the airfoil well if inverted. Might leave it as it is but make the upper apex wider? Say 2" or something. That would get the upper and lower cap area more symmetrical, I think.
 

Norm Langlois

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Bill
2024 T3 is only challenged to require a 3/16 radius in this case . One if the important feature of this spar shape is none of the bends exceed 30 degrees . Had I tried to exceed 90 even the 6061 T4 would have developed cracks. That is the reason for the flat zone apex. It also provides a rivet point for the wing ribs. Like wise the other flanges also provide rib fastening zone.
Any way My gut says DO not thin down . I just thought I would ask all of you what you think.

Have any of you noticed the Icon A 5 has a single pocket retaining the wing with a lock.I know it is composite .It also appears to have some single spar.The other points are minor for the controls.
 

Norm Langlois

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Bill
The new rib design will all be foam core. where they attach to the spar will have an inner cap brace.
If I were to leave it in this wide side down. I would run an inner apex boxing it and riveting along the side diaphragm. This option would add some weight 2 1/2 inch wide material 14 ft per half span. Might do it anyway.
 
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Norm Langlois

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The original UL wing has half of a 3 inch .035 tube for a leading edge. Any new wing will use 18 inch wide .016 2024 . I made my last TE from 2024 . I was able to fold that 2024 into a TE .The same size TE you get from Aircraft Spruce. There's is 5052
 

poormansairforce

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Two 30° bends on the front and back of the spar cap would be stronger since it leaves less flat area under high stress. More like a tube. Don't forget that aluminum has a fatigue factor so you'll probably need more material there.
 

Dana

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Norm, did you ever do any kind of stress analysis on the existing spar? That triangular shape with all the holes is not very efficient structurally...
 
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