Fuel Tanks (Part 2)

Continued from Fuel Tanks (Part 1)

After I cut my baffles to size and shape, complete with corners cut for fuel passage, I needed to fill the exposed corners with vinylester and micro so the foam would be sealed off from fuel contact. This was by far the most frustrating task, and took 5 or 6 tries to get right. A few things I discovered during the process:

1. Flox will not work for this.

2. See number one. Instead of trying to prove me wrong, just spend a week standing over your project cursing up a storm (you'll accomplish just as much). Then, use micro and do it right.

3. Micro doesn't work well either. 

Honestly, I have no idea how to do this simply and effectively. Sometimes it worked, sometimes it didn't. However (on to actual discoveries):

1. Vinylester, when exposed to the sticky side of duct tape, cures with a thin sticky film. This is not necessarily a problem, provided you can remove the film and still have a sealed corner.

2. Wax paper *must* be removed before vinylester fully cures, or you will be sanding it off.

3. Cling wrap: same story. 

Part of why I had so much trouble was that I desperately wanted to do all four corners in one batch, so I was coming up with creative ways to channel micro into all four corners without any spilling out. This was *so* ineffective, I spent a week on it and had to resort to doing one corner per baffle per batch. 

As soon as my baffles were sealed, I floxed them into place in the wing. I had a bit of extra flox, so I went ahead and attached my fuel level sender to the tank wall at the same time. I wasn't worried about making the sender removable, since I'd have to do serious damage to the wing in order to replace it anyway.

My next step was to figure out where to put my drain valve. Had I thought a little more about it, I could have easily built a proper sump into this tank; I've done that for the other side. However, this side has no sump at all, so I had to fill it with water and then use a shop vac to suck water out until I could see where the lowest point was. I then popped the bottom out and floxed my drain valve plate in. I had previously drilled and tapped the aluminum plate, and I positioned it as far down in the foam core as possible to give it the best chance of trapping contaminants and water. (picture was taken after lines were run)

I cut and sanded the top of the tank to shape, used duct tape as a release agent, and floxed it onto the baffles in the same way I did the bottom of the tank. After the flox cured, I removed the top and cut a hole for the filler neck. I'm using a standard 1.5" cam style filler neck, because it's several hundred dollars cheaper than the aircraft version. Also, replacement parts are widely available, as they have been since motorcycles and tractors began using this style filler neck in the who-knows-whens.

I also began running my aluminum lines. The left tank is just a holding/transfer tank, so it only needed a pickup line and a vent/return line. Once the lines were flared/attached/floxed in place (as well as the filler neck), the bottom went back on, this time permanently.

After curing, I filled the tank (as much as I could) with water for a preliminary leak test. After leaving it for half an hour, I saw no evidence of water leaking through the foam cores. I utilized my drain valve to remove the water, then give my tank an extra day in the warm garage to dry out. I noticed a few potential pinholes in one of the side walls, which I promptly spent half an hour deciding whether or not I wanted to fix (with the other option of hoping it wasn't actually a leak presenting a pretty solid case in my head.) While debating with myself, and lamenting that I couldn't pressure test the tank until the top was fixed permanently in place, I came up with a simple way to pressure test spots I wasn't sure about. All it takes is a suction cup. Stick it on, and if it falls off there's a leak. So I picked one up, and started trying my method out. Turns out suction cups won't stick to areas without pinholes, either. The only way I was able to find pinholes was visually. I did find a large number, but patched them without much trouble.

One thing I don't see addressed very clearly is bonding procedures for fueling composite aircraft. As best as I can find, the main concern is that fuel sloshing in composite tanks causes a large amount of static buildup on the surface of the fuel. In order to equalize the charges between the fuel surface and the fueling apparatus, the bonding/grounding strap has to indirectly contact the surface of the fuel. I decided that the aluminum fuel lines, which will be in contact with the surface no matter how much fuel is in the tank, were perfect for bonding. The anodized fittings don't impede the conductivity (surprisingly) so I can just attach a wire to the outside fuel lines and run that wire to a common bonding point.

I also floxed a short piece of fuel line to the top of the tank right next to and visible through the filler neck, which hangs down to the 10 gallon fill level. During the first phase of flight testing, I'll fill the tank to this point to match the 10 gallons the other side holds when full.

 Finally, I floxed the top into place.

I blew compressed air through the vent lines periodically while the flox cured to make sure no extra flox clogged my lines.

After allowing a day for curing, I plugged the lines and performed a pressure test on the tank. Naturally, it failed. I believe the flox began curing before the top was fully in place, and I had 4 places air leaked out, all of them at the top of the tank. However, I still had access to all four points, and was able to mix up a runny micro slurry to pour in and around the leak areas. Further testing revealed no leaks, so I removed the wing and set it aside. I'll follow (mostly) the same procedure for the other side.

Continued in Fuel Tanks (Part 3)

Fuel Tanks (Part 1)

I'm not a fan of the header tank location in the plans. Between COG shift, quantity constraint, and location, I've elected to build my tanks into my wings instead. I'll be using my outboard wings, because the inboard wings have aileron cables and landing gear in the space I'd want to use. The outboard wings also have dihedral, which will give me a definite lowest point in each tank. This makes it easy for me to locate my sump drain and fuel pickup.

Again, I'm borrowing very heavily from Mark Langford's fuel system, which you can find at his website.

After my wing spars were attached, I was able to begin by cutting slabs of foam for the top and bottom of the wing. I cut templates for the inside of the wing out of 1x4s, glued the 1x4s to the foam, and sanded the foam to the contours. (Note: After doing this for the transfer tank, I decided to do something a bit difference for the other side. See Fuel Tanks (Part 3) for what I think is a better setup.)

I made a bit of an error during my next step. Vinylester is touted as fuel resistant, and therefore, the best fuel tank resin. Unfortunately, not all vinylesters are created equal, and most manufacturers aren't willing to specify if a given vinylester is "fuel safe." As far as I can tell, it's due to the ethanol that winds up in gasoline today. I purchased vinylester from FGCI, went through the process of laying up my fiberglass, and as it was curing I bothered reading the pamphlet that came with it. "Not recommended for fuel." Guess I'll be adding another layer once I get more vinylester, which I ordered from Aircraft Spruce (sold specifically for fuel tanks).

Once the correct vinylester arrived, I began re-doing my layups. At first, I was laying up entire pieces of foam, then cutting them to shape. But I then realized I was wasting a lot of fabric and resin that way due to the dimensions of the foam. I began drawing the shapes I needed onto the foam first, then cutting fabric slightly larger than necessary and laying up. I finished the pieces I needed for my left wing tank, which is only a holding/transfer tank, then began installing them into the wing. My process is as follows:

1. Draw lines ⅜" down from the top of the inside of the spars, and ⅜" up from the bottom
2. Flox and attach front and rear tank walls to front and rear spars, using previously drawn lines
3. Apply duct tape to the tank bottom (to keep from attaching the bottom permanently at this time), then use tape and supports to hold the bottom in place
4. Use flox and fiberglass tape to build a ledge for the tank bottom to attach to
5. Attach tank sides and baffles using flox, taking care not to attach them to the tank bottom
6. Mark location for tank sump
7. Remove tank bottom, secure tank top in place (with duct tape where tank seams will be)
8. Build ledges for tank top as in step 4
9. Remove tank top, apply flox to tank bottom, and secure tank bottom in place permanently
10. Install all lines and fuel level sender
11. Fill with water for a leak test

Assuming the leak test doesn't reveal any catastrophic leaks, I'll fix anything that needs fixing and then install the tank top the same way I installed the tank bottom. The final leak test will be a mild pressure test; at this point if it fails, I'll have to cut open the tank from the top and try to find the failure.

For my fuel and vent lines, I cut ⅜" aluminum plates and drilled and tapped them for AN fittings. Next I cut away the foam where I wanted the lines to pass into the tank and floxed the plates in place.

 After I attached the front and rear tank walls to the spars, I got ready to lift the tank bottom in place. I attached five pieces of painter's tape with some slack, thinking they would support the bottom.

 As you can kind of see from this picture, five was not enough. However, eleven plus two of my spar-setting jacks was enough. (The funky glare is from four work lights I was using to keep the panels and fresh vinylester warm enough to cure well.) I'm scrambling to finish my fuel tanks before the weather gets too cold.

Continued in Fuel Tanks (Part 2)

Rudder Pedals (Part 1)

It's mid September, and I'm trying to use the rest of the warm weather to get as much fiberglass done as possible. However, it's raining and cold today, so I'm working on something the weather won't affect. I stole Mark Langford's design for rudder pedals. You can steal them too, by visiting his site.  These designs call for 4" aluminum angle stock, ⅛" thick. I was unable to find anything that large any thinner than ¼", so that's what I'm using. As a result, I had to slightly adjust the templates on the smaller piece of aluminum angle so the pedals would line up correctly. I printed the templates out on full-sheet labels, cut them and stuck them right on the aluminum.

My shop is set up for cabinetry, and I work with a lot of exotic woods on a semi-regular basis. Because of this, I didn't want to use my bandsaw to cut aluminum and risk getting aluminum shavings stuck in my expensive wood the next time I work with it. So I resorted to using a sawsall with a bimetal blade. I was able to rough out the shapes that way (along with some cheating with a drill press.)

From there, I used a sander to finish shaping the pedals and round the edges. I then drilled out a number of holes (as laid out in the drawing) to save weight.

I'll use the pedal swingarms to line up the pedals with the brackets, then attach the brackets and pedals with JB Weld. I may add grip tape if I feel it'll be beneficial. I have enough material to do two sets of pedals, but I haven't decided if I'll install them both or only the pilot's side. The passenger/"copilot" won't have brakes, so pedals would really be superfluous. Once I build my brake cylinder bracket, and have my seats installed, I'll attach the pedals to (and most likely relocate) the swingarms with PVC pipe and a screw (again a la Mark Langford).

Wing Spars

I decided to take advantage of the warm weather and get as much fiberglass work done as possible. Since I can't build the cowl until I have the engine mounted, and I can't build the turtledeck until I at least decide what I want to do for a canopy, the wings were next. The wings consist of two wooden spars that span the length of the wing, foam glued to the spars and shaped to the airfoil, and fiberglass covering everything. The spars attach to the fuselage by way of steel fittings and bolts. I built a handful of stands to adjust and hold the spars where I needed them. Bolts and internal nuts allowed me to raise and lower the stands to achieve the desired result. Because of space constraints, I have to build one wing at a time. First order of business was the front spar. For this particular wing design, I needed to raise the bottom tip of the spar 5" above the bottom root of the spar. Once the spar was in the correct location, I attached the outboard wing attach fittings (WAFs) to the inboard WAFs, positioned the outboard WAFs where I wanted them on the spar, and clamped them on tight.

After checking my height and dihedral again, I began using a jig to drill straight through the spar for the WAF bolts. After the holes were drilled, bolts, washers and nuts were inserted and torqued.

Next, the rear spar. This wing is designed with 3˚ of washout, from 3.5˚ at the root to 0.5˚ at the tip. In order to achieve this, a plywood template was nailed onto the forward spar, then onto the rear spar, in the proper locations. The rear spar was then adjusted until the template showed 0.5˚ along the chord line. I then turned my attention to the root end of the rear spar, lining it up with the fuselage. I had to adjust the root and tip a few times to get it perfect. The next step was bending the rear WAFs to compensate for the angle of the rear spar. Once these lined up well with the spar and the inboard WAF, they were attached the same way the forward WAFs were.

The plywood template that's not attached will be attached at the root prior to gluing the foam in, and I'll shape the foam to those templates.

The final adjustment was reaming the WAF to WAF holes to the correct size. Each wing is held on by 4 bolts, and any amount of slop at the WAFs means play at the wing tip. I selected bolts that were the same size according to my calipers, then ordered reamers that were .001 smaller. This gave me a little wiggle room, since I'd be reaming with a cordless drill. Using a jig, I was able to (CAREFULLY!) drill and then ream the holes to exactly the size I wanted. The bolts were then inserted and secured. I'm using socket head cap screws and captive nuts, so I only need small holes drilled in my wings for the bolts to be inserted. Once the rest of my hardware shows up I'll be making a plate to rivet the nuts to. After that, it's on to foam and fiberglass.