Load test completed!

Passed!

The load testing of my Goat wing was completed today, with a little help from some friends.  This was a 3G static load, not counting the approximate 0.5G of the structure itself.  I'll give the load distribution in a table at the bottom of the post for completeness.

Mary helped me move the equipment to the back yard, set up the test rig (again) and the Goat nose/wing.  We lowered the rig onto the upside down Goat, supporting the gantry with two ladders, tied the water knot at head-height, and then raised the gantry plus Goat back into the air.  Raising was difficult with that much more weight hanging in the balance.

We then moved all the distributed weights out to stage them next to their location, checking each and every jug for correct weight to within a half-ounce.  This task took about a half-hour with two people, and was a surprising amount of walking.

With Goat hanging upside down and staged, Trent and his family came over to help.  We loaded weights in tandem, each placing weight at the same time with a little nod when ready to let go.  We alternated between the leading and trailing edges, usually two leading edge stations, then three trailing edge stations.

About half-way through loading from the root toward the tips, we noticed the trailing edge spar had a noticeable bow to it.  Not unreasonable, but definitely visible.  Nothing looked out of place or any weird sounds, so we finished the loading.

Despite the 75/25 leading to trailing edge distribution, the trailing edge spar had much more visible bend between the strut and root connection.  The leading edge spar did not have appreciable bend, likely supported by the nose flying wires.  This matches the FEA runs from before.

The tips outboard of the strut connection likely bowed up, but were pulled back down by the load.  Looking down the spar showed a distinctive gull-shape on each half.

We let the weights hang for about two minutes, taking several pictures and inspecting the structure for any deformations.  We then took weight off, from the tips toward the root.

After a small celebration that a decade of work passed its 3G load test, we looked over the structure for any permanent deformation.  The compression strut where the wing struts attach has a noticeable bow to it now.  This is the same strut where the aileron slide tube attaches.  The trailing edge strut itself had a bow to it now, almost like the tube separating the leading from trailing edge strut was too long by about a half-inch to inch.  More investigation needed on that one to identify if that's the cause, or it's permanent deformation from the test.  Nothing else was identified visually as deformed.  Even the nose carry-through tube, where all the weight was hanging from the strap, looks fine.

Disassembly was uneventful, other than realizing it will be much harder to do once covered.  The gantry is now dismantled and all those milk jugs are empty and awaiting recycling.

It's fitting that Trent was available to help.  He was the one who helped set up and run the FEA.  Trent's comment was "Looks just like it did in ANSYS."  Thanks man!

3G load distribution

Rib     LE      TE

13      15.9     05.3 (tip)
12      23.3     07.8
11      27.6     09.2
10      30.2     10.1
09      32.0     10.7
08      33.3     11.1
07      34.3     11.4
06      35.0     11.7
05      35.5     11.8
04      35.9     12.0
03      36.1     12.0
02      36.3     12.1
01      36.5     12.2 (root)

Load distribution

I tried to do a proof load, but couldn't figure out a great way to get over a hundred milk jugs hanging from a central point.  A friend recommended trying a beam just like the spars would be, and placing the weight along the beam.  That just wasn't as balanced as I'd hoped, so instead I've decided to trust that this structure is okay and push toward the actual loading.  It didn't budge when both my wife and I hung from it, which is certainly not +1000lb, but it's sturdy.

Yesterday morning while it was raining, I pulled up the AVL model and set up the model to have 50 strips.  These were merged into 25 pairs and the strip forces calculated.  The strip forces give load distribution along the span, and that was scaled up to 3.0G's.  I could always add more mass in a second test based on this same scaling.  I had been saying 3.5G's for a long time, and now that I've done all this work, the extra half-G probably should have been put into this water weight.  Oops.

Anyway, here's the pile of water all ready to be hung.  Every jug was individually measured and matches the load within a few tenths of an ounce.  If you look closely, you can see the masses written on the jugs.  It's a 75% leading edge, 25% trailing edge split, using the actual distribution from the AVL model.

Last night I spent two hours cutting and tying lines between pairs of jugs for looping those over the spars, and bending clothes-hangers into makeshift hooks for the singles.  It's ready to go.

I took off the wheel from the nose section for getting some additional height, and the aileron crank arm for setting up the sling without that in the way.  The pick point looks good.  I can either push the slings out toward the strut connection points or inward toward the wheel support struts.  Outward is more similar to the loading point from the pilot sitting in the nose, but riskier if the slings try to slide inward.  Inward is easier and directly transfers load to the wheel struts which then transfers to the strut connection points, but less representative of how the pilot's weight transfers to the struts.  Either work in my assessment and will depend on feeling out the setup when hanging.

So it's Sunday now and it's quite windy and gusty outside.  This just isn't conducive to a large-scale load test with literally a decade of work hanging in the balance.  Everything is back staged in the garage waiting for better weather and my availability.  Hoping for next weekend, or it'll have to wait until June...

Load. Mass that is...

Soooo many water jugs.  Count 'em if you'd like.  150x.

Looking toward proof loading, perhaps tomorrow night.  No clue how to hang this much weight off a common strap.