August 2009 Archives

Back From Oshkosh

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I got back from Oshkosh late last night.  I had two main goals for my trip to Oshkosh: decide on an engine and decide on an efis.

Engine: I had heard good things about the IO-375 from AeroSport Power.  Fundamentally, it is a parallel valve IO-360 with a redesigned crank that simultaneously lowers compression ratio (8:1) while moving the peak pressure to a point in the crank's rotation that has more mechanical advantage.  The end result is an engine that weighs exactly the same as the parallel valve IO-360, but makes 195 hp.  I spoke at length with Bart LaLonde and am now convinced that this is the engine I'll go with.

EFIS: I have been really impressed with the preliminary info about the Dynon SkyView system, but I wanted to take a close look at Dynon, Advanced Flight Systems, and Grand Rapids.  The Advanced and Grand Rapids systems are available now and look very nice, but I don't need my avionics for a while, so that isn't a big factor for me.  The Dynon system is not yet shipping, but it looks like it will be even nicer than Advanced or Grand Rapids by the time I need to buy anything.  I really like the modular aspect to the system which not only makes it easier to install and upgrade, it makes it easier to extend in the future.  Given that it still has a ways to go before it has all of the promised functionality, I want to wait before making a final decision, but at this point I'm about 95% sure I will go with the Dynon SkyView system.
In preparation for closing the fuel tanks this weekend, Jenn gave me a hand with the six universal rivets holding the fuel tank attach bracket on.  For never having shot rivets before, she did a great job.  Afterward, I installed the remaining flop tube and put sealant over everything.



Sealed Left Fuel Tank

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I got up early this morning before the kids were up to work on the plane a little.  It wasn't enough time to get started on the fuel tanks, so I decided to fabricate these splice plates instead.  These will get riveted to the outboard leading edge wing skins and provide additional nutplates for the fuel tank to attach to.



Here is a closeup of one of the nutplates.  The skin is dimpled here to accept the fuel tank dimples and #8 flush screws.



I didn't get any pictures of how the sealant is applied, but I just followed Van's instructions.  I applied a <3/16" bead just ahead of the skin to baffle rivets (where the long row of silver clecos are below) as well as sealant around each of the rib to baffle rivets and along the outer ribs.  Next, I dropped the baffle into place and clecoed every skin to baffle hole to ensure a tight fit (I'm really glad I have the pneumatic cleco gun).



Next, the upper and lower rib to baffle holes are pop riveted.



Then the z-brackets are pop riveted on.  Double check the orientation of each of these brackets because it varies and it would be really hard to change once they're riveted on.  Just like other builders, I had to grind the nose of my crappy Harbor Freight pop rivet gun down to get to these pop rivets.



At the top of this picture you can see the bead of sealant that gets formed when the baffle gets pushed into position.  I'm really glad I purchased a Semco sealant gun now.  It made it trivial to get a nice bead on the skin.



I came out into the garage a little later in the evening.  It was too late to get started sealing the other tank, so I took down one of outboard leading edge skins and deburred it.  After finishing the tanks, these will be the next pieces to get riveted.



I deburred and dimpled the leading edge skin and ribs tonight (though you can't tell that from this picture).



I only had a short time to work on the plane tonight, but I got the skin and ribs for the left wing's outboard leading edge components dimpled.  The undimpled holes at the top edge of this skin are for attaching the wing tips.



Sealed Right Fuel Tank

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I sealed the right fuel tank tonight.  I didn't take any pictures, but it would look pretty much exactly like the pictures from the left tank that I closed a few days ago.  I'm going to give everything at least a week to cure before leak testing the tank.  In the mean time, I can go ahead and get the outboard leading edges riveted and installed.
I spent a few hours tonight deburring and dimpling the remaining components for the outboard leading edge sections of the wings.  Here are a stack of ribs, ready to be primed.



Here are the two wing skins, ready to rivet.  Hopefully, I can get these both done tomorrow.



My buddy Andre stopped by today and helped me rivet the outboard wing leading edges together.  Here, we're riveting the inboard rib which includes an extra strip of aluminum with nutplates to hold on the tank.  The plans called for AN426AD3-4 rivets, but -4.5 worked better.



Here's the completed left leading edge.



...and completed right leading edge.  I'm really enjoying getting back to basic aluminum work (without having to mess with tank sealant).  It's nice how much faster structures like this go 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".



Countersinking Spars

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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. 



Leak Testing the Tanks

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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.

Tanks are Leak Free!

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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.



Deburring Wing Ribs

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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 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 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.
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 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.



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