I finished deburring and dimpling the right wing’s rear spar and then started working on the leading edge to tank joint. If you recall from a week or so ago, the tank and leading edge skins aren’t perfectly flush along the joint. To fix this, I needed to shim out the outboard leading edge skin to be flush with the tank skin.
I took some measurements along the joint to see how much shimming needed to be done and then drilled out all of the rivets on the inboard rib of the outboard leading edge section (the one containing the tank splice plate). Next, I fabricated a few shims shown below for one side of one of the joints. If you click the picture to zoom in, you can see that parts of the shims have been run against the scotchbrite wheel to vary the thickness along the shim.
Here you can see one of the shims in place on the inside of the outboard leading edge skin. These still need some fine tuning and will need to be primed before they’re installed. These shims are only for the bottom side of the right wing, so there are quite a few more shims to be fabricated before I can rivet this all back together and install the leading edges on the wings.
No pictures again today since all I did was deburr and dimple the right wing’s main ribs and the pictures wouldn’t look any different than the one from a couple of days ago. I ran out of steam before I could do the rear spar so that will have to wait until tomorrow.
I deburred and dimpled the rear spar on the left wing. The rear spar is much thicker than the ribs, so after dimpling the spar with the squeezer, I had to use the microstop countersink to make the dimples a little bit deeper so that the skin will seat fully.
I also received an order today from SteinAir with my roll servo wire and an assortment of snap bushings and grommets.
I finished deburring the inside edges of all of the left wing’s main ribs and then dimpled all of them. There are over 500 main rib holes per wing, so this took about two hours. I don’t have any pictures of it, but I also finished countersinking the main spars for the skin attach rivets.
I can see why people say that if they were to build again, they would opt for quick build wings. The wings are definitely boring in many spots. It’s not that each individual task is boring, it’s just that there are many spots where a task needs to be repeated dozens or hundreds of times. If the quick build wings had been available with capacitive fuel senders, then I might have considered them, but I really didn’t want float senders.
I countersunk one more group of holes on the main spars and then took a break from that to start deburring the holes in the main ribs. I did all of the outer sides of the holes and then used a couple of drill extensions to get the inner sides of the holes.
I came out early this morning and the water level had dropped nearly 4″ which is exactly as calculated for a nearly 10º drop (I was more precise than that, but that’s not important). I can now claim the tanks are leak free! I’m really glad that’s over with and there are no leaks. The tanks weren’t especially hard to build, nor was the tank sealant nearly as bad to work with as I had heard. I was more concerned that if I had a leak, I might have to open the tanks back up to fix it (requiring cutting access holes in the back baffle).
I stopped by OSH tonight and bought some 3/8″ 16tpi, 1/4″ long threaded plugs to use as stops in the tie-down brackets. The tie-downs that Van’s provides can screw in until they contact the wing skin and then crush it. These plugs provide a hard stop for the tie-downs to tighten against.
Here you can see that they simply thread into the tie-down brackets.
The lighting is poor, but you can see the plug deep inside the bracket. The tie-down rings thread into the bracket and stop against these plugs with about 1/8″ clearance between the tie-down ring and the wing skin.
It’s been a week and a half or so since I finished sealing the last tank, so I thought now would be a good time to leak test the tanks. I put some packing tape over the filler hole, put some caps over the return line and fuel pick up lines, and made a little elbow out of some 1/4″ soft aluminum tubing and attached it to the vent port along with some 1/4″ ID vinyl tubing to make a simple water manometer.
The fuel drain gets a special plug with a Schrader valve so that air can be pumped in with a bike pump.
After making sure everything was torqued down and sealed properly, I put in some water in the tube and pumped the tank up to just under 1 psi. 1 psi is about 27″ of water, so I pumped it up to about 24″. Of course, as you pump air in such that the water rises in one part of the tube, it falls in the other part, so you only need to get a rise of about 12″ to create a 24″ water column. Here you can see that I’ve made a mark on the tube right at the level of the water.
I’ve also grabbed our digital thermometer to get an accurate temperature reading in the garage since the pressure in the tank will vary with temperature. The temperature at the start of the test was actually 82.9º, but I didn’t grab the camera until a few minutes later.
As the garage started to warm up in the afternoon, the water started to rise which is a good sign that the tank is not leaking.
Later this evening, I came out to the garage to see how the tank was holding up. The water level is down about 3.38″ (6.76″ lower water column). Running a quick pV=nRT calculation shows that this would be the correct water level for an 8.24º temperature drop.
An 8.24º temperature drop from the starting temperature of 82.9º would be 74.66º. Given that a final temperature of 74.4º (shown below) would actually have made the water fall even further, I think we can safely conclude that the tank is leak free! I didn’t actually expect it to even be this close since the temperature inside the tank wouldn’t precisely track the temperature outside the tank if it were changing very rapidly.
The test is running on the right tank now and it’s looking good so far, but it’s only been about 45 minutes so I’m not ready to claim it’s leak free. I’ll check tomorrow morning and see how it held up overnight.
I started countersinking the spars for the rivets that will hold on the skins. First up is to rivet the ribs to the spars using the forward holes in the tank attach section. You can see one rivet set just above the cleco to the left and another countersink to the right that is ready to receive a rivet. The plans don’t mention anything about these holes, but the tank will cover up this area, so it only makes sense to rivet these.
Here you can see all of the holes that must be countersunk along one side of one of the spars. There are over 600 holes to countersink on both sides of both spars. I’ll shoot some primer on these before riveting everything together.
I installed the outboard leading edges and the tanks to evaluate the joint between the two. First the good news: the gap between the two is virtually nonexistent and the leading edges align perfectly.
Now the bad news. Like many people have encountered, the flushness of the two skins is rather poor. I need to probably shim the outboard skins out a bit to bring them flush with the tanks. The worst is about 0.042″, but there are several places that are over 0.020″.