Dec 31, 2011     Fuel Level Sensing - (40 hours)       Category: C21 Strakes

Original builder Chad Robinson intended to install an auto engine and run auto gas. He modified the build of the tanks. All of the work had been removed from the plane when I received the project. The remaining differences are
1) Each tank was built with 5 extra openings through the strake, intended for electronic fuel level sensing using magnetic reed switches, all spec'd as ethanol- resistant. 4 are on angle and sealed by BID. One is at the floor of the tank and holds the stub of a switch intended to track fuel level in the sump. All openings are 1/2" male with a brass pipe thread.
2) The tank sump was made extra large - at least 2 quarts larger than standard. Where the standard Cozy holds 29 gallons each side, #1147 holds 30.
3) The tanks are lined with ProSeal.
4) Chad had installed fuel sight gauges using an ethanol-resistant backing plate of polyphenylene sulfide. I removed those gauges and installed the Vance Atkinson standard, which uses an aluminum backing plate. The Vance package comes with an LED, which I have wired to come on with the master.

  Nov 06, 2021     Fuel lines & flaring tools - (12 hours)       Category: C21 Strakes

It's been a long time since I last made fuel lines, and those were either Aeroquip 471 or automotive hoses. This time I'm using -8 and -6 versatube (3003 soft aluminum) with AN fittings and the 37 degree single flare. It's a fuel injected system, unpressurized from tank through selector valve to pump, then pressurized at about 30 psi for a short run from pump to injectors. The return lines are after a pressure regulator, and are 6-8 psi. The -8 (1/2 inch OD) Versatube is rated at 380 psi, and the -6 (3/8 inch OD) to 520 psi. The tube isn't the issue, it's the quality of the flare and the correct torque on the fittings.

Flaring tools come in a _wide_ range of styles and price points. After two of the lowest grade I moved up one level, and I'm loving it. Below is a photo showing the equipment. After practice, my process is
- cut with a standard cutting tool
- remove burrs with a step bit
- put B nut and sleeve on the tube
- place tube in the correct sized hole of the multi-hole bar, with the end of the tube proud of the surface
- use a course file on the cut end, then check for a clean and perpendicular cut
- level the end of the tube with the top of the bar
- bar goes into the flaring anvil, centering the cone on the tube
- tighten the anvil on the bar; the anvil anvil is not allowed to twist
- add a drop of oil to the cone and spin it until it self-releases
- disassemble and inspect.

The height and inner surface of the flare has to be perfect, with no burrs or scratches. Tool marks on the outer side wall are acceptable. After practice I like the results.

  Jan 25, 2022     Fuel lines - (100 hours)       Category: C21 Strakes

Fuel tank selector, fuel filters & pumps, fuel lines

First is choosing fuel injection and electronic ignition, going with the SDS EM-5 described elsewhere. That choice meant a switchable feed from each tank to the filters and pumps and engine, then a return line flowing back to the same tank. Each strake has the standard outflow, plus a return line in the upper rear corner of the tank. I bought the recommended Andair “6-port” fuel selector and mounted the valve forward of the instrument panel (a cleaner look, more perceived cabin room). I chose to locate the fuel pumps and filters in the hell hole, increasing the distance between people and potential leaks. I also made covers for the hell hole, both a smaller one just aft of the main landing gear, and the larger one just forward.

With tanks, fuel selector and pumps located, it was time to connect them. I chose to follow the sizing of the Andair selector, with -8 feed lines and -6 return lines. If I had it to do over I would have a -6 adapter on the Andair valve and make all lines with the -6. The flow would have been more than sufficient for the intended IO-360 and -6 would have been larger than the finger strainers at the tank exit. More radically, perhaps the tube bending and flaring and fittings could have been sidestepped with an automotive style flex fuel line. I went with stainless braided flex tube for my brake lines, and my next Cozy will explore flex braided fuel lines.

Tanks. I have the plans fuel tanks, with a finger strainer exit. Fuel lines run underneath the aileron control rod, secured to the side wall with click bond studs and Ugolini-style hold down clips (a customized Adel clamp of sorts). I can remove the sidewall cover and access in-line fuel valves – both valves are secured open and were installed only for future convenience. I have a fuel return located in the upper corner of the tank; any bubbles from returning fuel should be far away from the sump.

Fuel filters & pumps. Below are pictures of the fuel pumps and a “cap” made to hold the filters. This package fit neatly in the hell hole, where it can be put behind a barrier screen. Fuel from the selector comes to the rear, through a bulkhead fitting, across the main gear, through another bulkhead fitting, then into the first filter, the pumps, the second filter and then to a steel firewall fitting low on the firewall. The pressure regulator is mounted to the firewall to the left of the pumps + filters. It's hard to see, but the return fuel lines comes in from a steel bulkhead fitting on the lower left, through the regulator, then curves nicely to the bulkhead fitting. It crosses the main gear, then forward to another bulkhead fitting and onward to the fuel selector. Note that other builders have placed the pump and filters under the front seats - I don't know what's easier, but I've reserved that space for remote mounted avionics, whatever they turn out to be.

(I chose to install cut off valves close to the tanks - the black hexagonal interruptions in the tubing.. They are wired open under the side panels, available only for an annual when I want to service the lines and spill as little as possible (!).)

Materials, techniques & tools. The lines are made of VersaTube, a soft aluminum commonly used for this purpose. A 50 foot length each of -6 and -8 was sufficient. Burst strength varies with size (smaller is stronger), and all sizes are at least 10x the maximum pressure that can be generated by the fuel pumps. Experimenting with the tubing found that smaller sizes (-3) bend easily - I had a 8 foot length I used to template the bending of -6 and -8 tubing. I had _no_ luck with the standard "triple header tubing bender" sold everywhere - I got crushed spots, inconsistent bends and very difficult to get a finished bend out of the tool. I ended up using bending springs - the -6 bends well with an outer spring and the -8 requires both an inner and outer spring. The outer springs come as a set for a wide range of sizes and I found a -8 inner bending spring on eBay. (The inner bending spring for PVC will _not_ work on VersaTube.) My preferred technique came to be putting the tube in a spring and enclosing in a bending jib made from scrap plywood. I recommend you _not_ use the cheapest flaring set, but go up one grade to the $40-50 one. Below is a picture of the tools I used and the scrap I trimmed off when I decided to remake a part. The most common error was to make the tube just a bit too long, so I got to trim and remake the piece. The next most common was to flare before putting the nut and sleeve on the tube!

Most everyone does the flares the same way. My only tip is to add a drop or two of cutting oil on the flare cone - I got super smooth faces. When it comes to fitting pieces, when I couldn't get the nut to to mate with the opposing threads it was where I had the one end secured and was fighting the other. Undo that first end and get alignment at the problem end, after which I was still able to reattach at the first.

Fittings. I underestimated VersaTube, and assumed I would need a lot of 90 degree fittings to get good corners. Instead I was usually able to bend the tubing and get the job done without fittings. (This is preferred by SDS, where they want bends over fittings.) I hit my limit in the 180 degree bends of -8 used between fuel filters and fuel pumps. An inner spring plus an outer spring plus use of the bending jig got my parts aligned and fitted. Deciding how to piece the fuel lines then turns on design for future service. I have 3 pieces in each of the lines between fuel selector and tank, a count that made reasonable the install-remove cycle. 3 x 6 lines = 18 pieces, each with 2 ends. Add in the plumbing of filters and the pumps and you get practiced at making and appropriately flaring the tubing.

Sealants. We pretty much all use Gasoila as the thread sealant for fuel lines. I also used a film of "EZ Turn" fuel lube on the faces of the fittings. A small tube of each is more than enough for the project.

  Nov 06, 2021     Fuel Pumps       Category: C21 Strakes

I'm using fuel pumps supplied by SDS as appropriate to their system. The twin pumps are Walbro GSL-393 pumps set in a machined alodined frame. Heck, they look so nice I hate to bury them in the back of the plane. The pump can provide 155 liters per hour, or 41 gph. They draw 4-5 amps in normal ops, 5.5 at the max expected draw (20 gph, a pessimistic read of full rich full power) but are to be fused at 20 amps. The pumps run one at time, and I've put the pump selector switch next to the tank selector (switch tanks, switch pumps) hoping to achieve even wear and to always have the backup at hand. From pump selector switch to the pumps is 7 feet each way, 14 feet for the round trip. The wire size calculator says I can use 14 AWG, so another order to ACS... Planning for 20 amps seems crazy, given that the draw is measured at 4.5a. But I'd sure hate to lose a fuel pump on an EI/EFI system.

Walbro's comment on filters: "Note! This pump should never be operated without a fuel filter between the tank and the pump. This pre-filter should be 20- to 40-microns. Additionally, almost all applications require a 10-micron post-filter between the fuel pump and the fuel rail. Putting just a 10-micron filter before the pump is not recommended. We offer a good quality metal cannister filter (disposable) with 3/8-inch hose barb ends as well as the outstanding Holley aluminum billet 390 LPH filter with -6 and -8 AN fittings. The Holley filter can be disassembled and cleaned or the entire filter media renewed." I am using a 30 micron filter pre-pump and a 30 micron after. Both filters are as spec'd by SDS and in SDS enclosures, again beautifully machined aluminum anodized gold.

Here's a link to the Walbro specifications page: https://walbrofuelpumps.com/walbro-gsl393-fuel-pump.html

  Apr 03, 2009     Fuel Valve, Electric Pump, and Gascolator p1 - (5 hours)       Category: C21 Strakes

Note from Chad, the original builder:
"My engine will be a Mazda rotary, which uses a high-pressure fuel injection system with a return. After much debate, I'm following in John's footsteps. Each tank will have a pump underneath each rear seat, which will be tied together at the fuel rail. A return will feed a solenoid that will pass the return fuel to the selected tank. Only one pump will be on at a time. This is a useful setup with some interesting side effects. First, its controls can be simplified to the point that feed and return are switched together, so there is no need for the pilot to manage both devices, and accidental cross-feed is prevented. However, deliberate cross-feed can be engaged when desired. This is a nice feature if you're on the ground and about to refuel. You know the fuel you have remaining is good, so you can pump it all into one tank (which is now a "known good" tank) and refuel ONLY the other tank. You can then take off and perform critical maneuvers on known-good fuel, and only use the new fuel when it is safe to make the switch. If there is contamination, this gives you a response option. You can also use this technique to empty a tank for inspection."

"In the first picture below you can see several of the components in my fuel system, including the pump, tank fittings, and sump level sensor."

PK note 6/8/2020. The project arrived without fuel pump, fuel valve or gascolator. The prior builder even stripped off the AN fittings. I plan on using the fuel flows standard to aircraft fuel injection, with return back to the source tank, and that requires a dual switched fuel valve. I have one of the 6-port Andair valves from a Cirrus, and need to install hard lines bringing the fuel through filters and pumps forward to the dash-mounted valve, then return lines to the source tank and the engine. This is going to be a challenge and learning experience.

(There are wires going into the tank, located at about where the sump would be. This is the fuel level sensor, an array of 5 gas float switches to each side, with a resistor network designed to give output varying with fuel level. It was a clever, if overly complex solution to fuel monitoring, described in C21 "Fuel Level Sensors". I've tied off the wires and will put them under a protective cover for future use by someone else.)

  Oct 17, 2021     Fuel system - (100 hours)       Category: C21 Strakes

Variance from plans - EFI/EI, return lines and location of fuel selector valve. The plans fuel system is designed for a normally aspirated parallel valve O-360; I'm building towards an EFI / EI system from SDS (their EM-5) feeding an IO-360 angle valve engine. This system has a pressurized fuel system with the fuel return flowing back to the originating tank (no sump). The Cozy plans put the fuel valve in the seat back, just like the Long EZ and notoriously difficult to operate in flight. I moved the valve to the instrument panel ("IP").

The SDS system that I'm building towards requires a return fuel line, and getting that return fuel to the correct tank means a 6-port fuel selector. Like most others, I'm using the Andair valve. In my case I got one out of a Cirrus, but it's all the same 6-port valve with permanently lubricated ceramic disks. To fit this large-ish valve in the Cozy, most put the valve on a center console, between the throttle quadrant and the foot of the instrument panel. The SDS FI means that I don't have a mix lever, so all I have is a throttle cable. With only that one control to move, I am trying to have everything on the panel, and avoid having the throttle quadrant taking up people space. That means shoehorning the fuel selector onto the panel. I think I have it done, but it did cost me 100 hours of fiddling, fitting, repairing and replacing. I ended up with a standard Andair face plate, placed low on the IP. Where the Cirrus uses an extension from face plate to valve, I trimmed that to as short as possible, while keeping the extension and the stiffness of that specialty shape. The valve itself is supported at the front by the face plate, and at the rear by a bracket shaped to the nose gear wheel well. There are nutplates on the back of the IP, inside the wheel well, and rivet nuts on the back of the valve. Note how the banjo valves were re-clocked to permit a fit under the rod for the nose gear manual extension. Valve surfaces now have a film of EX-Turn fuel lube. The banjos are safety wired using the same pattern as Andair had on the part when shipped to Cirrus.

Below is the schematic provided by SDS in their implementation guide. Mark Rieger, Cozy builder in California, was kind enough to share his more detailed schematic, with a parts call out. Mark's system is 100% AN-6, but otherwise similar to this build. My tanks have an AN-8 out and a -6 return, and so does the fuel selector valve (as above). I got frustrated with the organization of all the fiddly little AN parts, so I ended up making a paper layout, with post-it notes for each of the AN components (see below). I called out to multiple vendors before finding a combination of 3 that covered the parts I need at a price I can afford. Here is a walk through of the line sizing, as the fuel travels:

Tank to shut-off valve - AN-8 to AN-8
Shutoff to selector valve - AN-8 to AN-8
Selector valve to filter - AN-8 to 3/8th inch NPT
Filter to pump - 3/8th inch NPT to 3/8th inch NPT
Pump to filter - 3/8th inch NPT to 3/8th inch NPT
Filter to bulkhead fitting - 3/8th inch NPT to 3/8th inch NPT
Bulkhead fitting to fuel block - 3/8th inch NPT to 3/8th inch NPT
Fuel block to injectors to fuel block - TBD
Fuel block to Bulkhead fitting - 3/8th inch NPT to 3/8th inch NPT
Bulkhead fitting to selector valve – AN-6
Selector valve to tank – AN-6

Note that fuel lines are segmented for future servicing. (The more segments the more opportunities for leaks, but also the easier to service.

  Jan 12, 2009     Fabrication of Parts - (4 hours) Category: C21 Strakes

I didn't think to get good pictures of this step. Basically, I followed the CozyGirrls' method to make the strake ribs. It's not hard. You just edge-glue two sheets of foam into one large sheet, glass both sides, then trace and cut out the various strake rib pieces. A band saw followed by some light sanding made quick work of this step. The only complicated part is that it turns the process into a REALLY big layup - I mean, you're stretching very large pieces of fabric out and wetting out/stippling into the center of a large sheet of foam. It really helps to have an assistant for this step, and to use a slower hardener mix. I actually completed this step over a year ago, but hadn't cut all the pieces out until I moved to the hangar. I marked it complete on this date only because on this date both strakes are officially sealed, which means the parts must be done. =D

  Jan 12, 2009     Sealing Fuel Tanks       Category: C21 Strakes

If you want to fly with mogas, don't use MGS-335 in your tanks. John Slade has proven that MGS-335 deteriorates over time (it takes years, but it does happen) when exposed to mogas, most likely due to the ethanol that is in it now. Various solutions have been proposed, including a range of tank sealants, the use of alternate epoxy systems like EZ-Poxy, and even building internal tank layers with vinyl-ester (which is designed specifically to resist chemicals, including alcohols). In my case I chose to seal my tanks with Pro-Seal. This process isn't hard, although it is expensive. At the time I wrote this, Aircraft Spruce was selling a quart kit for $150, and I ended up needing two.
The tanks were built per plans, but before attaching the top skin they were liberally coated with the Pro-Seal. This included the sump cover and the inside of the top skin, excluding the area around the T-hats. Pro-Seal is a manganese dioxide-cured polysulfide compound. Polysulfides (which, incidentally, my tank level sensors described below are also made of) are particularly resistant to chemical attack, and ethanol in particular. This solution is untested. However, I believe it to be the best choice for my aircraft. Pick a solution, but whatever you do, do something.

  Apr 16, 2009     Leading Edge and Outboard Fairing Blocks - (8 hours)       Category: C21 Strakes

04/02/2009 (3 hours): With my strakes swept forward, their leading edges fall just past the instrument panel. This was too good of a coincidence to pass up. Along the leading edge, just under where the foam blocks will be, I installed electrical channels made of shower curtain rod covers. These are great - wires slide very easily along them. I carefully tucked them inside one another at the joints (where the sweep of the strakes changes), and duct-taped over them wherever there were holes to prevent micro from getting in when the leading edge foam blocks are attached.

In each channel I installed two pairs of 16ga wires (one each for NAV and strobe lights), and an antenna wire for the wing COM antenna. If I run into trouble with the strobe disrupting antenna signals I can always move one of the wire pairs, but I'll wait and see before I make any changes.

04/09/2009 (1 hour): Tonight I installed SOME of the leading-edge fairing blocks. I've left two off so I can do any fixes required if the left tank fails its next pressure check.

04-16-2009 (4 hours): The rest of the blocks went on tonight. Fairing into the fuselage was a little tricky but they look like they match, as near as we could tell. The leading edge layup was extended to cover the holes drilled to find the tank leaks.

  Apr 05, 2009     Pressure Check - (11 hours)       Category: C21 Strakes

This is a long section because it took many steps. However, I believe the overall method for finding and fixing the leaks was sound, and I'd like to repeat it here for other builders:

1. Connect a hose barb fitting to an air compressor with a good (easily turned) pressure valve. Close the valve all the way. Hook a line up to a T fitting with one leg to an altimeter and the other to a tank vent line.

2. Break apart a standard butane BBQ lighter, the kind with the long nose. This will yield a largish butane cylinder and a tiny plastic hose. Just before hooking up the lines above, insert this hose into the tank's vent line and pull the cylinder's valve up for 3 seconds to release a shot of butane into the tank. You don't need much!

3. Turn on the air compressor and slowly pressurize the tank until the altimeter reads 1500 to -1800 ft. (We found -1800 was safe and made it easier to find the leaks.) Be patient. You have to FILL the tank with air, and the vent pipe is not large. Go slowly and let the tank settle every few seconds until you get it stable.

4. Use a gas sniffer around all joints, the leading edge, the end rib, the sump, the baggage compartment, and all fittings. Pay particular attention to seams, but don't assume the seam IS the leak. Do NOT throw soap liquid around yet! We found out the hard way that the gas sniffer is sensitive to the soap! (Try sticking the sensor of your gas sniffer into your soap bottle.)

5. If you think you've found a leak in bare glass, you can use soap to confirm it. This is useful around the sump. But if it's near the leading edge or ribs, it's probably a pinhole that's escaping through one of the kerfs in the leading edge curve. Drill small pilot holes through the TOP glass (don't go all the way through!) and use the gas sniffer to home in on the exact location. Then dremel away the likely area and use the sniffer and/or soap to confirm.

2009-04-02 (2 hours):

John and I played with an altimeter for a while, checking out both tanks. He managed to rig an air compressor to JUST BARELY pressurize the tank. There's definitely a leak somewhere in both tanks - a slow one, but worth finding. I had the great idea to duct tape plastic wrap loosely around all the typical leak spots - sump, outboard rib, inside wall, etc. My hope was that the leak would gradually inflate the area, effectively making a visible "balloon". My hopes were dashed! Either the leak isn't enough to noticeably inflate the plastic wrap, the idea itself is flawed (the tape around the wrap is leaking), or we just didn't tape the right areas. Whatever the reason, we ditched that idea.

I have two more ideas I intend to pursue. This weekend I'll be bringing a leak detector - a professional model my neighbor used in Navy housing to find gas leaks. My plan is to inject a tracer gas like butane into each tank and look for each leak. Yes, I know, I will have to be VERY careful once this is done. But the leak detector is extremely sensitive, and even a tiny bit of gas will set it off, so I don't need to inject enough to reach explosive levels!

My second idea is similar to finding A/C leaks. I have a canister of UV dye. Injecting that will show any leaks under black light (and the leak detector will sense this dye, as well). It's relatively inert and safe for fuel systems, but it's my second choice because I'm not sure the dye will properly aerosolize enough, and STAY aerosolized enough, to trace. Still, it's good to have a backup plan. Worst case this dye could be mixed with a light machine oil and it would definitely work then, it's just that I'd need like 25 gallons of it! I guess I could put it into fuel, but that's no cleaner than just using fuel and your nose to find the leaks. The one nice thing about this dye, though, is that it marks whatever it leaks into, so with a black light you can search around for even tiny, slow leaks that would otherwise escape notice. It's conceivable you could leave the tanks sitting for a week to try to get even the slightest leaks to show up.

2009-04-05 (4 hours):

Today we followed up on the pressure check. The UV dye was basically worthless - it didn't aerosolize enough to make it useful. I do think if it was added to fuel it might be handy if you were planning to fill the tank. It permanently marks anything it touches and glows bright yellow under a black light.

What did the trick was the butane and the gas leak detector. This is an expensive, professional unit - I don't know if a cheap, off-the-shelf device would work. I scavenged a tank of butane from a disposable grill lighter, which was nice because it included a handy plastic tube that fit over its nozzle. I ran this a few inches into the vent tube, got John to douse his cigarette, then pulled on the little nozzle until I could feel the gas come out (it gets quite cold). We quickly attached the air compressor, ran a bit of pressure in (about -1500ft on the altimeter), then started checking for leaks.

The left strake was a bit tricky. One leak showed up right away - it was a pinhole in the sump, which the leak detector localized then we found with soap bubbles. (The soap for kids' blowingbubbles worked great - it's specifically designed to produce large bubbles with minimal pressure. Dish soap is fine too but it helps if you dilute it.)

The second hole was more difficult to find. As noted above, we had previously duct-taped large sheets of saran wrap over various common leak areas. The leak detector was confirming that the hidey hole between the left two outboard ribs was filling slowly with the butane/air mix. But though we both tried (for almost two hours, I think), we couldn't find the area with the soap bubbles.

Switching to the right strake, we found that leak very quickly. If you're going to use this method, it helps to have fresh air coming in, but not a breeze. The detector is VERY sensitive, and it can pick up on the CO in human breath, smoke from a burning cigarette, or even fumes outgassing from your hangar mate's gas tank. Or anything else that might emit a vapor. Since it's basically a particle counter, even slight contaminants in the air can make it react, which forces you to turn down the sensitivity, which makes it harder to find the leaks. The right strake was coming up blank until we opened the hangar doors. (We hadn't done that before because it's still quite cold and windy here - fortunately, we were blessed with a sunny and relatively still evening.)

Bingo. As soon as we did that, and let the air clear out and settle a bit, I found a strong leak around the forward drain port. John tweaked the teflon compound around the screw in there, and eventually switched to teflon tape, and the tank is now sealed. We left an altimeter on it, pressurized, and we'll check it in a few days.

Back to the left tank, I think I just got lucky. John had suggested that many leaks creep along the strake skin kerfs - he'd had that problem himself. And the way the butane was filling the hidey hole made it seem like it had some expansion room ALONG the leak point. That is, it wasn't reacting the way the pinhole in the sump was. With that one, every time we pressurized the tank a bit more, the leak detector immediately went into overdrive. In the hidey hole, the buildup was much slower. It seemed like it had a delayed reaction - like it was creeping along something. I did finally notice with the leak detector that the leak was strongest along the leading edge - not against the tank wall, as I had intuitively been probing, but actually halfway OUTBOARD from that rib.

John suggested a binary search by drilling holes along the leading edge. Bingo. At the first hole he drilled, the leak detector went nuts. We then drilled additional holes at various intervals, noting that whenever we drilled a hole closer to the source, that hole had a stronger emission, but farther holes were reduced. As we zeroed in, we began using the Dremel to open up sections of the upper skin until we found what we were looking for - a hole in the inner side of the upper strake skin. This opened up very near one of the kerf lines, and was allowing the leak to follow that line. Incidentally, this was VERY near the outboard tank rib, which was why the ProSeal hadn't helped there wasn't any there because we had to allow a space for the flox on the T-hats to adhere!

The fix is easy. I stuffed a tiny bit of flox into this hole and the one in the sump, then put a very wet 2-BID patch on top, with some plastic to help eliminate pinholes, and worked the epoxy carefully with a hair dryer to remove all traces of bubbles. On Thursday when I go back out we'll recheck the tank pressure.

Despite the work involved, we were both pretty pleased. The right tank sealed on the first try - we don't really count the drain hole as a "leak" since it wasn't a problem in a layup, we just had to tighten the drain plug. The left tank only had two small issues, and both were easily fixed once we hit on the right combination to find the leaks. In any case, it was a 1-day effort. The big deal here is the ProSeal, which let us eliminate 95% of the bottom skin as leak sources. That's VERY helpful to know!

2009-04-07 (1 hour):

So the pressure went up! The secret, of course, is the temperature change, and also outside air pressure changing the pressure on the outside skin of the tank and the test tubing. This is amazingly important - even pressing on or squeezing slightly the strake skin or tubing makes a big jump in the altimeter. The right tank is sealed!

The left tank, not so much. I tested it again and was sad to see the leak was just as bad as before, despite fixing two big holes. Well, nothing for it. I started drilling holes again, this time starting at the kerf line that caused so much trouble before. It took a LOT more holes to find it this time, but it was right where I predicted - within an inch of a kerf line. Those kerfs are trouble. If you're reading this, and you haven't yet done your strake skin layups, make sure there's plenty of micro in there!

Anyway, this leak turned out to be a weak point in the skin that was letting air bleed out, find its way to a kerf line, then progress along the leading edge that way. Another patch, and I'll test again in a few days when I get back out. What I SHOULD have done is duct-taped the hole so I could test for any MORE leaks, but I stupidly didn't think about that. Oh well.

By the way: This leak testing system is working very well. The dye was a total waste of time, but the gas leak detector is so sensitive that it picks up even a tiny amount of butane. I'm injecting only a small amount - about equivalent to holding a butane lighter open for 3 seconds - in the entire 25-gallon tank. I've cautiously tested it to see if it will burn. It doesn't. But the leak detector definitely finds it, and screams its head off when you're right at the source of the leak. The only frustrating part has been adjusting the sensitivity of the unit to tolerate the other hydrocarbons we have in the hangar. John is working on his fuel tanks, and we're still heating with propane heaters, so those things are throwing the detector off a lot.

To make this system work we're pressurizing the tank to between 1500-1800ft (altimeter reads negative). At sea level (which we're at) that's about 1psi - not a lot of pressure to get from an air compressor. To achieve it we're just BARELY cracking the valve open. The air compressor helps maintain the pressure despite the leak, which is helpful for finding it. Unfortunately, what it also does is kick in every few seconds (because there IS a leak). The leak detector isn't loud enough to hear over the air compressor, so it slows down the detection a bit. If you can, get a detector with headphones!

Also, pay CLOSE attention to your pressure! It tends to creep up or down. Down and your leak goes away, which is not so good if you're trying very hard to find it! At least it's not harmful. But up, and you'll eventually crack something open elsewhere, just making more leaks. Not good!

04-09-2009 (2 hours):

I'm not sure exactly how tightly sealed these tanks need to be. I'm finding a number of leaks in the left tank, but they're all pinholes in the top skin. Only a very small amount of air is escaping, and only because I'm digging along the skin to find them... giving them an escape hatch as I do so. At least I'm getting faster at this.

04-12-2009 (1 hour):

The leaks continue. I managed to find and seal three more upper-skin pinholes. There's a pattern here - they're almost all along the middle tank rib, where there was no proseal (because it had to be bare glass for the flox to stick). The good news is the tank is definitely leaking a lot more slowly - as I find and fix each one, I can tell it's working. One lesson I learned the hard way is the gas sniffer detects soap! I never would have thought this, but it does. DAMHIKT.

04-16-2009 (1 hour): Finished! Both tanks now hold a seal. Onward!

  Mar 22, 2009     Top Skin - (20 hours)       Category: C21 Strakes

My top skin is more or less to plans, but there are a few things to note from this step. First, there is an access hole in the outboard rib, normally an area filled with foam. After some discussion, John and I decided to leave this area open, and fill it later. For now it will provide an easy access point to one of the common sources of fuel tank leaks. After the tanks are fully sealed we'll discuss what to do with it.

Second, you can now see the accessories installed in my tank. In addition to the usual fuel line elbow in the sump, there is a return at the top of the tank. I'll use a solenoid to switch the return, which is the same thing John does. There has been a LOT of discussion of how to do this - I've agonized over it for THREE YEARS, and discussed various solutions with all the experts. My final conclusion has been that there IS no "perfect" way - every method has pros and cons. I chose this method because it seems the most natural to me, and thus I believe that during a failure I will be more likely to intuitively select the right response than if I'm relying on a system that makes less sense to me in the first place. Designed for Humans.

You can also see the plates installed for the level sensors. There are five in each tank - four in the tank itself, and one in the sump. The original Cozy just has sight gauges. Many builders install level sensors, but there are accuracy problems, especially with ethanol-based fuels. I chose a middle ground. In case you're curious, the part I used is a Cynergy 3 RSF86Y100R. Digikey carries them for $14.83 apiece, with a bit of a discount if you buy 10. But apparently Digikey won't be carrying them much longer. If they run out, both Allied Electronics and Newark show stock for similar prices.

I'll measure the weight next time I think of it. But as for that, they feel pretty light, and capacitive senders aren't exactly feathers anyway. The cost is probably comparable, if a bit more because I used five sensors.

  Mar 22, 2009     Bottom Outside Skin and Sump - (16 hours)       Category: C21 Strakes

My sump blisters contain an added accessory, a liquid level sensor designed for fuel tank installation. It's made of polysulfone, which is both fuel and ethanol resistant, and has a 1/4" NPT tap so it's easily installed and completely accessible from outside the tank in case it ever needs to be serviced or replaced.

I actually installed a series of these sensors at even spacing up the tank wall (which was a bit of doing, actually - John saved my wrists by doing all the tapping in the aluminum plates installed in the tank walls, and I owe him - it was a lot of work doing that tapping!

The idea here is that people have had a lot of trouble with tank level sensors, especially when ethanol is in the fuel (as it will be in mine). But a mechanical sensor has fewer failure modes if it's properly installed. These units add an additional safety factor in that they use magnetic switches, so there is no risk of spark inside the tank. They're not cheap, at about $11 per, but they work very well and I'm looking forward to using them. I'll hook them up to a simple LED bar graph indicator, with an additional annunciator warning on the lowest tank sensor, and the sensor in the sump.

  Jan 12, 2009     End Rib and Outlet Piping - (6 hours)       Category: C21 Strakes

My "end rib" step is a little weird because I followed the CozyGirrrls' technique and installed the rib with the rest of the strake parts. This was done while the wings were attached, then scribed and cut to the profile of the inboard edges of the wings. This will produce a very good match to the wing profile, and, ideally, minimize the finishing work in this area. This area is normally filled with foam but I left it open for now. The plan is to put storage compartments in here for spare bulbs, tire tubes, etc.

  Jan 12, 2009     Top Skin Cores - (20 hours)       Category: C21 Strakes

I haven't had as much hangar time as I'd like, and had a bunch of dental work to recover from on top of it all, so John gets credit for the strakes. I just helped hold the skins as final assembly was completed. In these pictures you can see the dark ProSeal. It goes on VERY thick and stiff, but looks like it does a good job of forming a solid barrier. Between that and the hat method for installing the top skin, I think there's a good chance of either getting a seal on the first try, or coming very close. We'll see!

These pictures are very dark because the hangar is very dark. It's winter right now, and we're having to use two propane heaters to get the hangar warm enough to do any work at all. John asked me to get some electric blankets to help the strakes stay warm enough to cure. I wasn't able to find any, but it worked out OK anyway. The 50-50 Fast/Slow mix we are using for our MGS managed to cure rock-hard overnight.

  Jan 12, 2009     Vent, Screen, and Drain - (10 hours)       Category: C21 Strakes

Some of these pictures are dark because it's winter, the hangar doors are closed, and lighting is pretty poor in the hangar right now. Here you can see here one of the fuel sump screens. It's a sink strainer, obviously, but hey, that's what the plans call for and it does the job very well. You can also see the vent and grounding lines, with the recommended additional vent holes near the back corner of the strake drilled into it. The water drains are also installed - I'll try to dig up some better pictures of them later.

  Jan 12, 2009     Inside Layups - (20 hours)       Category: C21 Strakes

The inside layups went more or less according to plan. However, because I'll have ethanol in my fuel, I paid the extra bucks (lots of extra bucks) for ProSeal, which is supposedly proof against a range of alcohols and fuels. We'll see. It sure is expensive. It's also a hell of a mess - it's very thick, hard to mix, and hard to spread, even when heated. It's definitely not like micro - it doesn't spread smoothly and you end up with lots of little icicles and frosting tips. It cures to a rubber-like coating. We'll see!

  Jan 12, 2009     Jigging and Assembly of Parts - (30 hours)       Category: C21 Strakes

I screwed this step up pretty good, and John's help was immeasurably valuable in catching and fixing the mistake. Basically, I started off OK. I rigged a table under the left strake area using plywood and 2x4s to rig up a relatively stiff and sturdy (and perfectly level) jig table. I didn't know at the time that I could really have just skipped this step, but c'est la vie. Using the table as a guide turns out to not be as convenient as using the string-line method to align the rib leading edges, so I ended up doing that. My case was a little complicated because I'm also doing the extended strake modification, so there are actually two important lines to deal with. But it sure is an easy method to do. Sort of. I must have either been tired or rushed when I set this up because when John double-checked the left and right sides they didn't match. He ended up having to redo a fair bit of the layout work. It all worked out OK in the end, but this does underscore the importance of taking it slow and having as many eyes on a project as possible to catch mistakes. I'm sure I would have caught it, too - it wasn't exactly a subtle mistake. But it could have been avoided if I hadn't been in such a rush.

  Jun 06, 2009     Fuel Tanks and Strake Sealing - (30 hours) Category: C21 Strakes

Sealing the strakes is always a tricky job, and finding leaks can be time consuming and frustrating. I managed to work out one method that worked pretty well for me. Hopefully it will for you, too! Start with an altimeter, a length of flexible tubing, and a Tee fitting. You'll also need some way to reduce the size of your air compressor's output tubing. A short length cut from a rubber air hose seems to work well if you use the right flexible tubing - the tubing slides inside the air hose and makes a decent seal, and the air hose goes over a hose barb on the air compressor. You must have an air compressor with a good valve - you'll only be tapping 1psi or so from it!

Use this setup to pressurize the tank by hooking up the third leg of the Tee fitting to the vent line. Remember that air takes time to flow through such a small line. I've found it worked best if I pressurized the tank slowly (2 minutes or so) until the altimeter read about -1500ft. Stop here and clamp off the air hose so it can't leak through the air compressor fittings - they're a common leak source! See if your tank holds pressure. If it does, you're done - great! If not, keep reading.

Forget the soap. You have a lot of surface and joint length to hunt for leaks, and the soap just makes a mess. Save it for later confirmation once you think you have one. Instead, go get a "TIF 8800A" gas leak detector. Don't get a cheap one - this is the real deal. It's very sensitive, has adjustable sensitivity, and detects a wide range of gases. These sell for $100-$200 online, but I've seen them on eBay for as little as $40. In my opinion, this tool is indispensible! While you're at it, get a disposable BBQ lighter, the kind about a foot long, that your local market probably sells for a few bucks.

Back in the hangar, carefully crack open the BBQ lighter and you'll find a hefty cylinder of butane and a short length of tubing. You could get a butane lighter refill cartridge, instead, but they're more expensive and you really don't need much here. Stick the butane cylinder's tube up the vent line, and pull up on the cylinder's valve to open it for a few seconds. (Again, it doesn't take much!)

Now re-pressurize the tank. Using your gas sniffer, probe slowly around likely leak areas. You'll find that you can use very high sensitivity levels if you move slowly. (It also helps not to be using propane heaters or have open fuel or solvent cans around your test area.) Likely leak areas include the sump, water drain fittings, and along the kerfs in the leading edge.

I didn't fill in the outboard dead spaces with foam - I left them open. I'm glad I did, because a major leak showed up here. If you did, you may find as other builders have that you have a leak elsewhere that migrates along the kerf lines and shows up here. Instead of drilling into this area, if you've filled it, I'd suggest drilling 2-3 small holes through the outer (only!) skin of the strake, near the leading edge, and right on top of one or more of the kerf lines. Butane is a very light gas, and even though Divinycell is supposedly "closed-cell", I've found the leaks migrate through the top skin pretty easily. Making a few test holes is a quick, painless check, and it will get covered over when you glass the leading edge. You might as well find out early!

As you explore, keep checking your tank pressure. Make sure you have enough to keep forcing air (and butane) out any leaks, but don't overpressurize the tanks either! I found -1500 to -1800ft worked well.

Once you know you have a leak in this area, finding it is a matter of drilling more test holes to zero in on it until the leak detector goes nuts. Then use the Dremel and a sanding drum to open up a small portion of the skin, and carefully sand down to the inner skin. If you're right on the leak, the detector will go nuts, and you can use some soap to verify that you have a spot. Open it up enough to do a proper layup, and put two wet plies of BID over it - an inch square is fine. It's not structural. Fill the area back up with foam, sand flush to the outside skin, and it'll disappear when you glass the leading edge.

Perfecting this method took several work days and probably 25-30 hours, all total. Had I had this method written down ahead of time, and known it would work, I could probably have found AND fixed all of the leaks in just a few hours. It's that easy.

  Nov 27, 2011     Fuel Fittings Category: C21 Strakes

Drill holes in the flange of a 3/8 NPT threaded fitting to allow flox to grip it, then flox it into the side wall of each sump. Glass over the area with 2 BID, then carefully cut away the BID to uncover the hole. Cutting it with a razor blade is a better idea than using a Dremel, to avoid tank contamination from fiberglass dust.

These flanges will support the installation of a finger strainer in each sump (see Ingredients, Aircraft Spruce). Alternatively, you could drill and tap a piece of aluminum to do the same thing, but unless you need more 3/8 NPT holes tapped, two flanges are cheaper and faster than the tap.