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Load Test Failure Main Wing Aluminum Dragon

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proppastie

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https://www.homebuiltairplanes.com/forums/threads/aluminum-dragon.24732/post-615015
In the interest of education and completeness as regard the Aluminum Dragon I offer up this discussion of my most recent load test failure.

This is the last load test of this design as the wing is too heavy (65# LH wing less fabric and finish )....I have run out of fixes for my EXCEDDINGLY BAD DESIGN......There is no one else to blame but myself.....Being a cheap home-builder I did not want to spend the money to have an experienced stress expert check my design and it is unreasonable to expect someone to do it for free. I have had a huge amount of fun doing this and expect to continue....I am currently working on a new structural design.....Hopefully after 11 years I have learned enough to be successful.

I believe that the web of the spar buckled and the spar stopped acting like a beam.....I will show analysis to support this theory …..but not being experienced enough in stress analysis and not fully understanding what is a highly specialized field. I open the discussion up for others more experienced to comment.

For those of you that have not followed my design/build....this is a one-off design with no plans or kits being considered. I do it strictly for education and as a retirement project....for the fun of it, and to keep busy and active.

The wing failed at between 50% and 60% ….it did not survive the 60% load.....

The analysis is of a loading without the 15 degree tilt but in any case the numbers are close enough for my conclusions.

Here are the two relevant areas of the attached spread sheet.....

The first screen shot is of a Z stiffened panel which represents my stiffened spar web....the example is from Bruhn 75 and was used to construct that portion of the spread sheet.

It shows a crippling stress of 10197 psi.

The second screen shot is of the stress on the full spar at the Wing Station 51 where the failure was.

The stress at the extreme fiber at 60% load is shown as 10725 psi.

The spread sheet also has screen shots of the Autocad Mass Properties of the Spar from which the “I” second moment of inertia was obtained and a screen shot of he elliptical load distribution and centroid of the loading at WS 51

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