Structural analysis preliminary results

Alright, I stayed 1.5hr after work and made a huge dent in the structural work.  Thanks to Trent for helping me get set up on ANSYS.  Here we go...

The primary wing structure was input as BEAM188 elements with a tube shape of varying wall thicknesses.  This includes the LE, TE, and struts.  Where the sleeves are installed inside, the element thickness was increased to match the total wall thickness (no-slip as if it was machined). 

The compression ribs, cables, and jury struts were treated as LINK180 elements with the cables as tension-only elements.  Note that the cables connect across the rib bays as they connect in the real aircraft.

The loading case chosen was a 6g load of a 300lb AUW airframe.  AVL gave the loading distribution across the span, which you'll note tapers off somewhat elliptically from the root to the tip.  To note, the tip sees very little actual loading and the majority is probably the center 1/2 to 2/3 of the wing.

Load was split up 50/50 between the LE and TE, which may not be exactly fair, but is a good first pass.  14 discretized loading locations represented the full half-span of load, some 938lb (conservatively including interpolation error) spread along the rib connection locations.

The LE and TE constraints are pins, fixed in x/y/z space and allowed to transmit moments only in the drag-direction (z if you're counting).  Similarly, the strut connection to the fuselage carry-through tube is set up the same way.  Furthermore, the strut to LE connection can only transfer forces, no moments, which reasonably represents the pinned connection out there.

Do disregard the stray line in the above screenshot; it was taken early while I was still cleaning up the element numbering.

Now the fun stuff... solving the FEA.  The right wing is shown.

Looking at von-Mies contour plots by element starts to quickly illuminate some unexpected results!  First and foremost, the cantilever portion of the wing tip isn't bowing up.  To the contrary, there is hardly much deflection out there at all, and it's ironically down.  That's explained by the significant loading on the inner portion of the wing bowing UP.  Yup, for a positive loading on the wing, the middle of the wing bows up.

Looking at the bottom of the spars shows the maximum stress concentration in the trailing edge.  This is a compression stress.  Turns out that the jury strut connection areas are the most stressed in the wing.  What else is important to note is this location is where the 12' tube meets the 6' tube.  There is only actually a sleeve there.  That is going to change.

This view also shows the minimum stress is at the leading edge attach point.

What you should also be seeing is the struts themselves are bowed.  A lot.  Hm.

I'll be taking another look at the Euler buckling criteria to ensure the relatively smaller strut tubes aren't being pushed to the limits.

Oh also, check out the maximum stress: 8500psi.  According to matweb and wikipedia, the yield stress of 6061-T6 is approximately 35,000psi.  The fatigue limit is 14,000psi.  Meaning, occasional pulls of 6g aren't appreciably fatiguing the structure and we're looking at a safety factor of 4 prior to yield, let alone ultimate.  Take this with a grain of salt ... FEA is an approximation.  But seeing such large margins of safety is really making me smile :-D

Last screenshot is a closeup of the underside of the wing.  Just because electrons are cheap.  And I thought it was a cool picture.

There are several uncompleted tasks / unanswered questions, namely:

• more fidelity in how the load is split between the leading and trailing edges,
• better representation of the spar to spar joint with inner sleeve, now that we know that's an important location,
• what is an appropriate sleeve arrangement for the jury-strut attach point?  This ventures from analysis into design changes.
• check into buckling.

I'm happy to take suggestions or feedback.  I'll post the model in a few days.

Disclaimer: This is not considered structural advice; I consider this post (and all) my personal notes and rhetorical discussion.  This analysis is only for my implementation of a design I found online and does not constitute engineering advice for your project.