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.

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!

Assembly, because it's Spring!

I'm a fair-weather building for sure.  It was 75 and partly sunny on a Saturday, perfect for assembling Goat with the newly covered tail and seeing how everything is fitting together.  First, some wide-angle pictures...

 

The newly covered control surfaces (except that right aileron) look awesome.  Even in a light breeze, I could feel the aileron and elevator putting some slight control forces to work.  Sandlin's idea of "wind jamming," referring to balancing/steering with the control surfaces in wind, makes perfect sense now.  And if the wind direction is right, I can probably do this myself in the back yard with minimal risk.  That will help me figure out the stick deflections needed for positive control.  Coming from an RC background, my thumbs are used to moving, not my whole arm and both legs.  I also remember how the stick felt very heavy in roll and very sensitive in pitch in a Blanik, but less so in a Cessna 172 (well that was a yoke).  Wind jamming seems a great and safe way to get familiar with the Goat.  There is a good video of this somewhere, but this is the best I could find in a pinch.

The tail is all covered, but still needs to be painted.  There were no issues with assembly here with access being different ... with the wing, I tend to reach through the structure where covering will later prevent that access. 

Perhaps you already understood this from the drawings, but it seemed worth sharing a closeup of how the aileron cables route at the wing root while I was taking pictures already.  Several builders have replaced this arrangement with solid pushrods and torque tubes for the ailerons.  For me, this cable arrangement still seems okay.  The blue painters tape marks a tube I'm going to replace because of that stray hole.

 

I tried leaving the control surfaces, cabane, and strut on this wing panel to move it back into the garage.  It's quite awkward.  I normally find the balance point and hoist it over my head, using a good grip on the internal struts to balance it.  But, that won't be possible with the covering on, so it makes sense to resolve how to move it differently before covering.  I usually move the strut separately, and it does have quick links to support that.

The only new major lessons were that the wing control surfaces will definitely need to be held in place somehow prior to assembly and post disconnection.  I had removed the tail section and had the nose section balanced on the skid forward.  This large assembly caught some wind and rotated backward without me controlling it, with the ailerons streamlined (still connected to pushrods).  The ailerons were the first item to hit, transferring force back into the control pushrods, which both bent.  Those need replacement now.  I'm thinking a really long velcro strap to hold the control surfaces folded up against the wing.

Rib attachment part 2

The leading edge fiberglass wrap has had a couple weeks to cure (ha, meaning I haven't been working).  Sanded down with a small block and 80 grit paper, it is great.  I'm pleased.  The rib seems firmly attached, which is the whole point after all.  It should also cover nicely.

I shifted the whole wing in the garage and bonded on the trailing edge.  An addition was a secondary support for the last rib, using the tip support.  It was bonded too, and lashed using a single s-glass thread.  Should be secure.

That's it for the ribs for now.  I really don't want to do them all before the load test, but wanted to see how that process was going to go.

Rib attachment

Not much work today, mostly experimenting with the rib attachment and leading edge shells. 

I attached only the outermost two ribs and used the method as described in the drawings.  I first pulled the fiberglass tape around dry, situated the fabric, and lightly marked the edges with a thin sharpie.  After removing the dry glass, a quick rough using 220 grit sandpaper within the lines gave some tooth for the epoxy to grab.  When I mentioned sanding aluminum to a colleague, he warned about removing the oxide protective layer and inviting corrosion.  With the epoxy sealing out the surface scratches, hopefully surface corrosion is no longer an issue.  Anyway, a good wipe with alcohol cleaned the surface and prepped for epoxy.  After adding the epoxy and clamping the fiberglass tape to hold it all in place, it cured for a few hours in the warm sun.

Several leading edge shells were added temporarily just to take another look at this process.  The outermost one had to be cut down an inch or so to fit the reduced width spacing, which was easy enough with a new, sharp Exacto blade.  After it was in place, here's a look at a few of the leading edge shells held in place with masking tape.

I'm considering to put a layer of fiberglass on the back of the foam prior to bonding it in place.  If I also add a layer on the front (after sanding), that will stiffen the foam tremendously, providing some ding resistance and hopefully keeping the foam from getting cracked from any poor handling.  Sandlin calls out for a layer of epoxy seal coat for the foam. The Stewart Systems process I'm using to cover does not eat foam like Polyfiber adhesives, but it is a good idea anyway.  I have a bunch of 2.75oz/yd2 cloth that is probably a bit light.  4oz unidirectional fiber seems about right.

Remaining covering is the right aileron, the cabanes, the nose, and then the wings!  I'll be stopping for a load test prior to beginning the wing covering.

Tail covering

The vertical tail took quite literally all day.  I cut separate left/right pieces.

The first half went reasonably well.  It wasn't hard, just a lot of bolts to go around near the trailing edge in particular.  The cutout for the control linkage was a particularly challenging area.  I ended up making a grommet using a piece of thin metal roof flashing.

The flashing was bonded onto the skin and then a larger patch put on top.  Perhaps the slit could have been shorter, which I'll know after a full reassembly.  With the long slit cut, the grommet deformed a bit relieving the tension in the aft portion of the covering.  Meh, it'll be okay.

 After an evening of covering the second side, I got to the point of match drilling the new tail bracket to fit the tail on.  It's now fixed in place and fully remounted.  The controls have been re-rigged as well, and they work fine.  It's weird having the skins attached.  Just different.

To another day...

A little more covering

I knocked out covering of the two flaps.  They were quick, being rectangular.  Took just one evening each, and that was just enough time between getting home from work and needing to head to bed.  I forgot how the covering task wasn't hard, just takes some time.  Here's a mid-process shot.

The ailerons are now on the bench awaiting covering. Those should take a couple evenings since they're just a bit larger and also have the nice long curve near the tips.  But I'll get to those later.

The tail needed some cleanup work before covering.  It was dusty from sitting in the garage and was easy enough to wipe clean.  I took off the flying wires, control cables, and brackets in preparation for installing the covering.  Then I took a closer look at the foam spacers during this cleanup and just wasn't happy with them.  The epoxy was cracking off the aluminum in places and just wasn't carefully applied back in May 2010.  Instead of covering over this, it made more sense to take care of replacing it now.

After a punching out the foam and a bunch of scraping to remove the flaking epoxy, and some additional clean-up with isopropyl, the tail section looks much, much cleaner, and almost ready for covering.

This part needs to be replaced.  It might be fine, but there is no reason to stick with a part I'm not satisfied risking my life on.  I built this bracket to the specifications in the drawings, but apparently the hole in the horizontal stabilizer didn't align perfectly.  Adding a shim washer made the fit work, but, yuck.  I'll order some more aluminum channel and match drill the holes next time to not need the washers.

That's all for today.

Some simple covering

I took a long break from working on Goat to do other things.  Life.  The EC Goat guys beginning to cover helped encourage how far behind I have gotten.

One elevator half was prepped for covering a long while ago.  Unfortunately, I put the adhesive onto the surface and didn't do the covering.  After months of sitting, it seemed prudent to remove it and start over.  It took a LOT of rubbing with my fingers and some scraping to remove it all. Hours of work.  Don't leave work half completed...  Interestingly, the adhesive is like a very sticky goo.  You can get it off the surface of aluminum one little bit at a time.  It appears the adhesive works by being in shear and having ample overlapping surface area.

After clearing the old adhesive yesterday afternoon, I started anew this morning.  Put cloth tape over the sharp corners of rivets, tube edges, and bolts.  Add adhesive around the perimeter and let dry.  Place the fabric down gently into the dry adhesive on the first side and smooth out all wrinkles.  Iron down (~225F) around the edges, working out wrinkles.  This is the stage of the first photo.  Add adhesive around the edges again, letting dry.

 

Come back and trim the excess, then iron it down inside the perimeter.  Flip to the second side.  Add adhesive about 1" around the perimeter of the first side, let dry.  This is the second photo

Once that perimeter is dry, pull the fabric around and start working carefully to remove all wrinkles, using the iron to help shrink the edges.  Come back with pinked shears and trim to that same 1" overlap.  Iron down, and definitely remove all wrinkles.  Finally, add another layer of adhesive and wipe off while still aggressively wet.  Let dry, then do a final ironing over the overlap to set the adhesive, and do a final shrinking over the whole thing.

I also added reinforcements over the hinge eyebolt penetrations through the covering.  These were a circle traced onto fabric, cut out with pinking shears, and a small slit to slide over the eyebolt.  First slid this into position and traced the outline, then added adhesive to the skin fabric, then pushed the patch down into the wet fabric, wiping up excess.  After that dried, ironed down the patch into the adhesive, then add adhesive and wipe away excess.

This now completes the horizontal tail to include both the fixed and moving portions.  These need to be painted and they'll be ready to go.

Next up is probably the flaps.  Those should be easy, since they're just large rectangles.  Next is probably the ailerons, again because they're mostly just large rectangles.  No formal plans beyond that, lest I get ahead of myself!