MONTY THE ANSWER MAN ARCHIVE...
HE'S DONE THE WET
WING THING... (030500)
The other two planes that I know of that are wet-wing are flying. One has changed hands and leaks, even after some re-seal attempts. The other is still undergoing some work to clobber stubborn seeps, but is flying and the leaks are small. Both were completed with the center section still on the a/c, and both were done by raising one skin and bead-sealing the joints. I took my center section out and took it apart (about 700 rivets.) I did this because I had some other issues in the gear bulkheads [cracks], dinged up leading edges, etc. so I took the center section out and completely disassembled. All but one rib was removed, both skins were removed. Doing so permitted me to do a comprehensive job of seal preparation (cleaning!) and to fay-seal most joints. I have a lot of pro-sealed clothing in my rag bin.
The jury's out on whether I'll have leak-free system, but I suspect either it'll leak a bit less or the leaks will be much easier to track down. This is because the fay seal stops fuel migration between ribs and skins. In the bead-sealed systems, fuel that passes through the seal finds itself between layers of metal, where it can freely migrate until it finds a loose rivet. The fuel may appear to be leaking somewhere distant from where the actual leaking seal is...).
And the punch line: I wouldn't recommend this mod., unless 1) you just happen to have the center section out, and 2) you like working on sheet metal, and/or 3) you have another a/c to fly ( I do).
Time: Including disassembly of center section, making a work rack to hold the c/c in my shop, disassembly, reassembly, sealing, new skins top and bottom of the tank, leading edge reskin, gear bulkhead crack repairs has taken me a year and a half. That's working an average of 8 hours per week with a few weeks off. Total job was about 350-400 hours, and I'm reasonably proficient with riveting and jigging. If I hadn't needed to do the gear bulkheads and reskin the leading edge, I'd see something like 200 hours. Its a big job and hard to do right.
Factors: I went the extra mile on some parts. I milled bosses to hold flush tank drains, I made hand-holes that were flush and rubber-sealed to permit re-entry for leak fixing, and I made new skins for the top, bottom and leading edge of the C/C. I am a somewhat slow worker, but reasonably equipped (shear, brake, compressor, lots of bucking bars, and helpful friends). I'm sure a professional could cut this time down quite a bit, but it's still a big job. -- Bill Jacobs p.s. my bird's in Denver, if you're ever up this way.
OZZIE ON "WET WINGS"...
In my case, I did it according to the Temco drawing, going to the second rib inboard from the outside of the center section. This also made the approval process easier. My center section was completely disassembled. All skins were removed, all gear boxes replaced, many ribs replaced (including both inboard ribs), all hardware replaced. I block formed the solid ribs to replace second ribs on both sides, and they were beaded for strength since they now formed the endcaps of the wet wing. One intermediate rib on each side had cover plates installed to act as slosh plates, then all intermediate ribs had notches cut to allow fuel drainage/transfer. Cover plates were made and riveted with pro-seal to seal the aft spar and inboard rib lightening holes, any small holes in the web of the spar were also sealed. One large cover plate was attached in bolts and nut plates on each inboard rib for an access panel. It should be noted that special nut plates are required for fuel tanks (they have sealed rubber backings and o-ring gaskets). The fuel filler cap panel is the other access point, which is why they use the large flushmount fuel caps. Don used his auto-cad computer programs to make templates for all bolt, rivet, and screw patterns on the cover plates access panels, and fuel cap mounts. To make the assembly process even harder, I decided to flush rivet the entire center section. No countersinking was used. So now we had to dimple all the ribs, all the skins, make shims to even everything out, and also make shims for the sparcaps since we didn't want to countersink into the spar. When everything was put together with cleco's, all skins were check to be completely flat with all rib flanges (used thick paper to check for gaps). Then the nasty job of putting it all together with pro-seal. I was surprised how little was actually used and even questioned Don about that. He said it is very common to go overboard with the pro-seal and that the key is to have everything properly fitted prior to assembly. Each and every rivet had a bit of the pro-seal, along with a thin layer between all rib flanges and skins, followed by thin beading. The center section was leak checked with alcohol (due to lower density than fuel) and we found one small leak which was found and plugged. The center section was leak checked with alcohol a second time and was good to go. The center section was then installed back into the aircraft.
Since I wanted to know the exact amount of usable fuel, we put the airplane in the level flight attitude and put ten gallons. We then used the electric boost pump to pump out fuel until it started to suck air. We pumped out 9.5 gallons, which resulted in 0.5 gallons of unusable fuel! I then wanted to know what the usable fuel would be in the three point attitude, so we repeated the process in that configuration. This resulted in about 4 gallons unusable in the three point attitude. Total fuel came to 52.0 gallons, with 51.5 usable. I personally see no necessity of going to the outboard rib for 62 gallons, and I rarely top off my wet wing except for a long cross country.
The net result after flying my wet wing Swift around for almost two years is that I LOVE IT! The center section has not leaked one drop, and I have bent my airplane around the sky quite a bit. I had a fear (unjustified) that I might develop leaks after doing aerobatics, but Don said that the pro-seal has a very high tensile strength and would actually make the wing stronger. He was right. Since I had started with auxiliary wing tanks, I now found my plane about 30 pounds lighter after removing 4 metal tanks and plumbing. I later sold the aux tanks to Mike Sweeley who had Don install them in his Swift. In addition the fuel system was simplified to ON/OFF rather than a four way selector valve. I might also add that I did not install drain valves in the wet wing, I viewed this a just another potential place to leak. I still have the drain in the sump, which must be used with the plane in a level flight attitude (i.e. lift the tail).
I personally think that the wet wing mod is the best thing since sliced bread, and it is obviously good enough for Boeing. Having said that, I must agree with Bill's comments about it being very labor intensive. There is no reason to consider this mod unless you are completely rebuilding a center section (for whatever reasons). If you decide to do it, be prepared to spend double your estimated cost and time. I usually make most of the West Coast Swift fly-ins if anyone wants to see my conversion. Blue skies -- Ozzie
DON BARTHOLOMEW HANDLES
A WET WING CONCERN FROM ED LLOYD... (060500)
First, let's look at the original tanks. They can, and DO trap LOTS of water. Water enters the tanks from either the vent or the filler. The plane sits in a tail low attitude which has the rear, inboard corner of the tank lower than the drain to the sump. During these conditions, you can drain the front drain and the rear sump drain (if the plane is equipped with one) until nothing comes out and still have water in the tanks that will not get to the sump until the tail is lifted (as in after takeoff). I have personally seen this happen. The way to prevent this is to lift the tail to a level attitude and shake it and hold it there for awhile to let the water run to the sump where it can be drained from the REAR sump drain.
Now to the wet center section. How the individual one is built has a lot to do with if or how much water it will collect. There are drains cut in the bottom of the ribs to allow the fuel (and water) to flow to the lowest point. The tank has to have sealer put in it to seal the tank. HOW this sealer is installed and blended makes a BIG difference in if or how much water can be trapped. A sump drain is installed at the inboard, aft corner of the tank so if the sealing is correct, this drain will empty the water actually better than the stock tank. Of course, if the sealing is sloppy or there is no drain, then a lot of water could be trapped.
My suggestion is anytime the plane has been exposed to possible water due to weather or washing, raise the tail and shake it for at least 30 seconds to allow the water to be sloshed around by the fuel move to the sump area. Then, let the tail down and drain the rear sump drain. If the plane does not have a rear drain, put one in.
The article referenced a Cessna wet wing. An AD was issued for Cessna 182's with bladder tanks some years back. Seems wrinkles may develop in the tank and trap water. To comply with the AD, you had to try to blend the wrinkles. This process destroyed many tanks because they were not flexible enough to allow the blending. After you put a new tank in and still had problems, the AD allowed to placard the plane that after rain conditions, lower the tail and shake to get the water to slosh around the wrinkles to the drain. Another case that you don't have to really fix the problem as long as you spend money with the manufacturer. Sort of like replacing a Continental air melt crank (with a low failure rate) with a new VAR crank (that by now has a quite high failure rate). The last was my personal soapbox, but don't get me going there. Take care, Don and Helo