Wing Spar Load Test on SkyDock

Hi K.,

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We ran our negative G load test today on our SkyDock prototype.  As the loading was far in excess of the anticipated flight load, this may be considered an ultimate load test.   It was successful with two glitches.

Assumptions:
Gross Weight:  660 pounds
Weight of wing structure 62.5 pounds per wing (outboard of cabin).
Zero Fuel Weight:  570 pounds (90 pounds of fuel; 45 pounds per wing in outboard section).
Testing to 3.8G’s, negative load.
Each sandbag weighed.
Methodology:
1)  Plane was loaded generally in accordance with spreadsheet.  See attached.
2)  Conservative methodology:  stations 0 and 1 weight loads were placed at stations 2 and 3; this increased load on wing.
3)  Glitch 1:  Rivet structure showed signs of failure on both sides on front of “D” cell strap assembly.  Rivets pulling out of carbon fiber “D” cell.  Will be redesigned.
4)  Glitch 2:  center test support structure was placed under carbon fiber in such a way that carbon fiber skin was deflected upwards in tension into internal foam ribs, but just on one side of support structure.  This caused slight permanent compression into foam inside.  Carbon fiber skin is unharmed.  Repair is easy; just inject some expanding foam at affected area.  If / when test is repeated, test support structure will be completely under spar hard points.  This is a testing failure, not a structure failure.
Thanks for your assistance in analyzing the design of this wing.  It’s always fun to see a light thing hold a lot of weight.
Some comments on deflection:  we measured approximate deflection of 4 inches at station 9.5; (this is where we had our safety support stand).  Each section of the wing has a different moment of inertia; the center section is obviously the strongest.  The outboard section has a much lower value; and the third would be on the massive spar strap that connects the outboard and inboard together.  I don’t think we’ve calculated how these play together for predicting deflection; but in any case, the wing is acting like it has an average moment of about 8.  That’s pretty impressive, as I recall the moment of the outboard sections was designed to be around 4.2; the improvement is no doubt because of the carbon fiber bonded to the top and bottom of the spars.  What do you think?
Best Regards
James

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