Aluminum Dragon

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proppastie

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Finding the CG. First picture balance on the edge of the table for the fore and aft location. Second picture for the span location. Assumption the actual location is close to the center for the Z direction. This includes the ballast weight on the tip. Stay tuned for the load test. (not today though)

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proppastie

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LEARNING EXPERIENCE:....You think you know what you are doing and something happens....sort of like life......better here on the ground than in the air.
FAILED LOAD TEST:....end bottles are water... had to put sandbags in box because they wanted to spread out more than where I wanted to put them. Started at the tip with the light loads. and worked inboard. The failure was lightning quick no warning as I was laying on the sand bag over the failed rib.
Only one rib failed and after the weights were removed the aileron still rotated freely...had it happened in the air I would have been able to still fly, although if the limit load was on the unloaded ribs too, perhaps more would have failed and I would not have been able to fly.

See discussion: https://www.homebuiltairplanes.com/forums/showthread.php?t=30073&p=431974#post431974

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proppastie

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I have started another load test. The plan is to use the bottle jack per Bill's suggestion (see discussion) and hang the weights with the load distributed over the verticals per BB suggestions. I also refined the plates the load was on so the load was distributed over the vertical of the angle element. The plan is to load in increments at 33% 50% 75% 90% and full limit load of 9 PSF. I tried to stabilize the ribs with filament tape and duct tape.

The 33% came out great. Deflection was very close to the model and I thought everything would be alright.

At 50% about 1/2 way bottle jack expected extension... I noticed a rib ready to fail/cripple/twist etc.

Thanks for the help BB, Mcrae, Bill, your suggestions were incorporated in the test.

Apparently I do not know how to predict instability, I am not sure I saw any analytical methods.....This method of incremental load testing should help but I wounder if there is other methods.

To be fair the model and the part are always different....the model is welded beams or meshed surfaces, and the real part is certainly different.

I have tested another vertical member and it looks like at 50% loading more of the same will work.....

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proppastie

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Still working, just not posting…..till now….I had second failure 8/2/18 after reinforcement to .050 thickness of ribs. I decided that my stress analysis was all screwed up. I have done lots of dancing around with different designs and have attempted to make sense of what is happening.
The second failure was done after a more careful load test as from suggestions from the list. The failure was between 50% and 75% of limit load, which gave me a rough allowable for the .050 x .025 angle structure of the rib.
It was also suggested that rather than just crippling strength of the angle I should also look at Euler of the rib.
The failure was at approximately 4578 PSI at 75% limit.
I had designed the structure to withstand a calculated 18,000 PSI crippling at design ultimate load. Not a very good initial stress analysis. Or in other words big mistake and bad design.

What I think happened is:

1. Given the geometry of the ribs and load path…. the least “Moment of Inertia” “I” for an angle is in the 45 degree direction rather than parallel to the legs of the angle. The “I” of the .050 angle in the X and Y is .00126,….the least “I” in the 45 deg. Direction is .00044.
When I plug that “Least I” into my FEA at the approximate 75% limit failure load the axial stress is 4578 PSI.

2. When checking Euler for a 24” long rib, I get 4761 PSI failure. The rib failed at 12 “ on the 24 “ rib. Bingo, least wise if I am wrong it sure makes sense anyway.

The new design is lots stronger in terms of the “Least Moment” .00721 vs. .00044, and in terms of Euler in the failure direction Inbord-Outbord, or Aircraft Y the failure PSI is 36243 vs. 4578.

Shown pictures of added .063 caps and .04 gussets, added .063 angles and .040 doubler to ribs 1-7.
 

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proppastie

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Passed the "Limit Load Test"....9 psf per FAA Glider Criteria....that number includes gust loads.....243 lb on a 17 lb full span aileron. I had estimated the aileron might weigh 12 lb so I have taken somewhat of a hit on weight all aft of the CG...but 10 lb more total weight is better than an in-flight failure.

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proppastie

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Deflections did not match my FEA,...it does not do torque very well for not-round parts. The deflection from the actuator to the tip at 3 psf was 3/4 "...The wing loading for my version is perhaps 2.5 psf.
 

proppastie

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Started the assembly of the LH aileron. I had plotted the RH on Mylar because the vellum was not stable at that length. All I had to do was turn it over for the LH, which was nice. There are reinforcements that I did not make drawings for, so I have the RH part there to refer to.

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proppastie

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IMG_20190325_163210.jpg IMG_20190325_163158.jpg Working on the Spoiler.....looked good on paper....what you see here is Revision 2 of the linkage. I plan on load testing it and may have more revisions if it fails. It is not as stiff as I think it might need to be, but the Never Excede 60 mph limit load on this panel is 15 lb. which it should carry without bending. The problem is the linkage has to move and the play in the linkage causes it to perhaps move too much and I need to see if it will bind up at that load.
 

proppastie

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One of the problems with this is remembering what I designed maybe a year ago......the drawings are not complete with much left to "at assembly" or "when I get there".....

I have been reviewing my rear spar and designing a rear spar attach fitting, which required a slight re-design of the rear spar caps, nothing major.

I needed a "taper attachment tool" for to saw the taper in the caps....trying to follow a scribed line just was not very good.

I made the fixture which allows a .11 degree taper of the cut. I have to flip the sheet every time I do the next cut....it worked out much better.

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