Load testing rig fab

Well that was quick.  An early morning run to Home Depot, a couple hours with the circular saw and drill, and a LOT of sweat.

Just sitting on the grass, this is a bit tippy.  It will need some sandbags placed on the feet to keep the gantry from tipping over.  I had a swing-set of roughly the same design as a kid, but those legs were put into concrete in holes.  A long 2x4 screwed to the bottom of the gantry would work too.

It took two people and some very awkward rope work to flip Goat and pull it up most of the way to the beam.  Next time I will remove the wheel to have another half-foot.

The toughest part now is how to secure the structure to the overhead beam.  I have that yellow strap looped over the beam and looped around the wing carry-through tube, but can't quite get the water knot tied.  There's a foot overlap, but another foot from removing the wheel would make it workable.  How to get the structure all the way up there and tie it may be a trick.  A friend suggested taking one end of the gantry off and lifting it up after tying the knot.  It may also work to tip the gantry over sideways.  It'll be easier with more people.

I want to space those pick-points as wide as possible so it is closer to the ends of the tube instead of near the middle.  The wheel trusses will help transfer the load around if picking from the middle.  Just a lot of bending on that tube...

While it was up, here's how I intend to hang weight.  Each of those gallon jugs is 8lb of water.  Looping the string once around the spar provides some friction, and actually looping it over the rib attachment would be smart too.  I'll need to do the math for splitting up the load into rib-spacing increments.

Since there will be so much riding on this test, I will also do a proof loading of the gantry.  3.5G plus the roughly half-G of weight is 350lb*3.5 + 100lb is 1325lb.  Splitting into 8lb increments is 165 milk jugs.  Woof, that's a lot of jugs.

The weather Sunday is supposed to be gorgeous!  Load testing party at my place on Sunday :-)

Load test rig design

Among other tasks, which I'll get to in a minute, I finally made motions toward a load testing rig.  The Airchair group didn't have any input, so I'm heading into this without others' experience.

I pulled the Goat drawing profiles to make a low-fidelity model of the wing and nose sections.  Flipping this upside down and with an angle of attack, this let me design a loading rig.  I happened to have a pressure-treated 2x10 that is going to be the main beam.  Beyond that, it was a matter of figuring out how to transfer all the load to the ground in the simplest assembly using 2x4's and screws.

Here's what I have...

The legs are made from 10ft 2x4's.  The supports look good enough to me on paper, but I might make adjustments when building the real thing.  There should be zero doubt that it's sturdy enough before getting a green light.

With the 10ft legs and even leaving on the wheel, that leaves about 2ft of clearance from the ground for adding weights.  In my attic are a few hundred empty gallon milk jugs that are about a foot tall, so is a foot of clearance enough?  From previous finite element analysis, the shape of the wing under load should actually lift the tips away from the ground.

Another funny is how to distribute the load to the ground.  I don't have a large enough concrete/asphalt pad, so this rig will be resting on grass.  It's almost 1500lb sitting on the feet.  Adding another brace to help distribute along more surface area might be prudent, or a piece of plywood under either end.

What else?  A bit of work on the nose that started with fabricating the nose skid from some soft Home Depot aluminum.  This is a full 1" width instead of 3/4" called out in the drawings, because it seemed to fit nicely and was what I already had.

After making the nose skid, I really wanted to rivet it, which couldn't be done before covering the nose.  So I covered the nose...

I thought there would be enough material to cover the nose section with one piece, but ended up having to slice off the back and do that separately.

It ended up nice in the end though.  Obviously it's not painted, and still needs to have reinforcement tapes applied.  I was worried the nose would be the most difficult part to cover given how many edges and bolt heads are involved.  It was really not that bad.

It did claim one casualty though ... my trusty old covering iron gave up and broke where the handle attached to the shoe.

Friends who can weld to the rescue!

Wish I could say it was as good as new, but the weld held long enough to finish off the first layer of covering and the handle broke just next to the weld, doh.  It was the right fix at the right schedule though, and I want to say thanks to Trent!

So next up is the load testing rig, sorting out the math for the distributed loading, and then having a load-test party with several close friends.  Assuming it passes, it'll be off to the races for final preps prior to covering the wings.

Thoughts about load testing

Various forums online are for or against a load test.  It is my understanding that every single home-built aircraft in Europe must pass a load test prior to getting approval to fly, and those airplanes aren't crashing because of the test.  And a well-managed test to a load condition within the expected operating range should do no more harm than flying normally.  I'd MUCH rather something break or deform on the ground where I have a chance of catching and correcting the error, versus in-flight.  Practically, there are enough Goats flying (ten?) to give some reasonable confidence in the structural design.  But, and I have to keep telling myself this, it's such inexpensive insurance.

For the positive load test, I will be using some heavy-duty sling straps looped around the nose top tube structure where the seat attaches, simulating my body mass being the load.  These straps will be set up to hold the assembled nose and wing sections at a slight angle to simulate an angle of attack.  And then a series of jugs of water will be split along the leading and trailing edge spar tubes and hung using string or maybe bent coat-hangers.  Hanging weights will get more definition after building the hanging rig.

Available to me (four hours away) is an old wooden swing set with plenty of structural margin to hold the mass and plenty of height too.  It's a lot of work to haul Goat that far, and back again.  Or, it's a lot of work to build a big and temporary test rig in the backyard.  I'm torn.  It'll probably be easier to build a copy of the swing set gantry in the back yard here.  Maybe I can give it to a colleague with a youngster afterwards...

Mounting ribs

Ribs, so many ribs.

It's not so much fun repeating a task over and over again.  Once figuring out how to do it, the process does get faster the second, third, and fourth time, however.  This gets easier when coming across weird problems, like this rib that needed to attach right at an aileron hinge bolt.

I finished epoxying the ribs to the right wing and am just six trailing edges away from finishing the left wing too.  It takes about fifteen minutes to prep a few ribs, sanding with dull 220 to give some tooth, cleaning with mineral spirits (for any leftover masking tape adhesive) and isopropyl alcohol, and clamping the fiberglass tape in place.  For the first several, I epoxied the aluminum then wrapped the fiberglass to pull the epoxy up through the fabric like is proper.  The glass is such a loose weave and wets out so nicely that I began to pre-stage the material and then wet out generously to see that the epoxy contacted the aluminum surface.  I can't tell a difference in the bond quality, and this is a non-loaded bond, as the fabric will tend to hold the ribs in place anyway.  It takes another fifteen minutes to mix a small batch of epoxy, wet out the four staged rib attachments, ensure they're nicely soaked in, and clean up.  After four hours or overnight, I come back to trim the excess fiberglass tape.  A quick sanding with a block flattens any raised surface texture, being careful not to cut the underlying fibers.

I also filled in a couple mis-drilled holes with spare bolts.  As I understand, it's improper to leave a hole, and putting a bolt to fill the hole is an acceptable repair.  In both of my cases, there are other bolts nearby that will bump out the covering, so it isn't a big deal to cover over another bolt head / nut.

At some point, I replaced the aft cabane tube.  The upper cabane compression strut needed to be replaced, or so I thought.  The old ones were in perfectly good shape, just needed to be mounted.  Instead of fishmouthing new ones, the old ones were gently wedged into position, mounting end marked, a new hole placed there, and new washers riveted in place.  To install that strut, some minor pressure to pull the aft cabane tube away lets the compression strut to be slid over the washer mounts either end, and the pressure released.  Fits great, and was a lot less work than making new ones.


This Goat project is getting close to being done.  I don't have any additional planned metal work.  The leading edge shells need to be mounted and sanded and glassed, and that should finish off the required composite work.  I could fair the struts with foam and fiberglass, though that is optional for a maiden.  Then it's really just load testing and covering (in that order).

Rib attachment

The ribs are attached with a loop of fiberglass around the rib and leading edge.  It's a clever knot actually.  When it's all cured up, the rib is held firmly in place.  I'm surprised how firm it is actually, so much so, I'm attaching the rest of the ribs prior to the load test. 

Above is the attachment at the leading edge with the CNC'd foam shells taped in position.  After all these are fiberglassed on the back and bonded into position, then I'll block sand the surface smooth and fiberglass the front too.

Right wing almost done!