Aluminum Dragon

[proppastie]

Load test failure....check out the link below

Wing Load Test Failure Aluminum Dragon

Better to fail on the ground than in the air.....and why we test. It looks like classic Euler buckling of the top cap strip......So after my last test where there was buckling of the skin and adding stiffeners I mounted the wing properly upside down and at 15 deg. .... I made shelves so the...

www.homebuiltairplanes.com

 

[proppastie]

turning over another look at the failure looking at the top .....kind of ugly....
bent part is cut out,,,, angle and new center cap and splice added to 1/2 between the rib 1 and 2....all that is left is to re-rivet a new z
stiffener.....then I can turn it over and do the bottom cap the same. ...

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[proppastie]

so ready to put back on the tractor for another try at the load test...The long angles are (I hope) permanently bolted so being able to remove or attach one wing at a time required the split and bar at the center in order to maintain the large moment of inertia in the fore and aft direction required. This is really stiff compared to my former poor design .

 

[proppastie]

So the wing has been back on the load fixture/tractor for a while.....I re-did the distribution to elliptical and added 10% for tail down load....the loading went from aprox. 700 to 800 lb. I decided I wanted to keep track of the deflections given all the problems I have been having....using the 7" deflection from an old FEA as a conservative estimate...it is not really accurate because it does not have the carry through fix, the stiffeners fix, but it also does not have the tail download additional weight. .......I am getting lazy in my old age. I decided the mount was very weak in the fore and aft direction ( the setup was talking to me and I listened this time) and so I added aluminum reinforcements as shown to the other side of the tractor......I also blocked up the tractor to get it off the rubber.....the mount was still moving in the vertical direction and I added the laser from the center of the wing to a reference out at the tip. I got to 50% load and was getting wrinkles from where the stiffeners ended to where they started on the other side of the rib, the deflections were too high also.......more bad design. I was very discouraged and what made it worse was my calculations to figure out what was going on did not find the problem....However I discovered I was off in my input of E (1.05x10^8 and not 1.05x10^7 makes a big difference) and now I am happy on the road to another fix and learning lots of stuff...... good things: I did not permanently bend anything....loading in 10% increments worked this time .....and even though the mount slipped.....when I un-loaded the wing the zero point movement along with the deflections measured gave me a net deflection very close to my estimate.

 

[proppastie]

did a bunch of calculations .....its not that I had not checked these locations.....I had just checked it wrong......the good thing about a failure at a known stress is now one can hopefully come up with the correct method to stress analysis a section.....with all my mistakes I am getting good at hind-sight-stress-analysis.....anyway here is the reinforcements which will bridge the z stiffeners across the ribs.....We will find out in a week or two if I did it right.