General Forums => The O' Club => Topic started by: nrshida on April 22, 2017, 03:45:51 AM
Title: Esoteric design question about wet wings
Post by: nrshida on April 22, 2017, 03:45:51 AM
I figure there must be some people here with maintenance knowledge working on aircraft with wet wings or fuel tanks otherwise incorporated into the structure of aircraft.
What are the typical issues with this design solution? Are leaks an issue? How are the tanks sealed, with flexible bladders? How is feed and return handled? Do they require periodical refurbishment because of corrosives in the fuel? Is it in your opinion a good solution or are the ongoing issues a pain in the a**e?
Title: Re: Esoteric design question about wet wings
Post by: icepac on April 22, 2017, 07:29:52 AM
Fuel is tightly controlled by the FAA regulations so you should never run into any fuel that is "corrosive".
Most issues with either bladder or true wet wings are with water intrusion.........and it's usually the fuel caps.
A bladder is best left full of fuel or it may wrinkle and possibly get brittle but they are made by super exacting standards and usually last at least 12 to 15 years with some lasting over 30.
With wet wings, you will see one plane that never leaks and maybe an identical plane parked next to it that is always having leaks fixed.
Sealant is only as good as the preparation and care in putting it on.
Things to weigh are how prices for labor and parts are changing relative to each other which could make one option better than the other when before it was the opposite.
Title: Re: Esoteric design question about wet wings
Post by: zack1234 on April 22, 2017, 09:17:18 AM
I knew a aircraft engineer who mentioned that Middle Eastern sanitary standards on the planes toilets were causing structural issues in the planes structures.
Apparently getting to these compartments in the plane was impossible.
Title: Re: Esoteric design question about wet wings
Post by: Maverick on April 22, 2017, 10:09:16 AM
The Piper Comanche I had for over 10 years had "wet" wings. The fuel tanks were flexible bladders installed inside. When I got it there was always the odor of fuel in the cockpit. The mechanic I had said the bladders had gone porous over the years due to having been left sitting with less than full tanks. We pulled the bladders and I sent them off to a repair station and got a set of replacements. AFAIK they rebuild (re vulcanize?) them and use them again. In the next 9+ years of ownership I never had a fuel odor or leak issue from them. I ALWAYS kept them full. As soon as I landed at the end of the flight day I had them topped off.
Installing them is kind of tedious as there are clips built into loops on the bladders that fit into slots in the wing structure to hold the shape. You also need to install fresh chafe tape in areas that tend to abrade the bladders or you will be doing the job again sooner than you need to otherwise.
Title: Re: Esoteric design question about wet wings
Post by: Vraciu on April 22, 2017, 10:31:13 AM
The biggest problems with wet wings are microbials. Jet fuel has living organisms in it and if you don't keep up with it the stuff will eat the airplane. True story.
Sealing is pretty straightforward. Also, maintenance schedules have items like draining water and running Biobore at intervals to address these issues.
If your maintenance is sound you'll be fine. For a non-combat airplane it's the way to go.
Title: Re: Esoteric design question about wet wings
Post by: colmbo on April 22, 2017, 12:38:24 PM
The biggest problems with wet wings are microbials. Jet fuel has living organisms in it and if you don't keep up with it the stuff will eat the airplane. True story.
Avgas as well. We found a 180 with a big wrinkle in the bladder trapping some water. The accumulated dirt and algae in the "pond" looked like a garden.
Title: Re: Esoteric design question about wet wings
Post by: nrshida on April 22, 2017, 12:39:47 PM
Ah this is perfect information. Exactly what I was looking for.
Many thanks all :salute
Title: Re: Esoteric design question about wet wings
Post by: DaveBB on April 22, 2017, 01:49:30 PM
Petrochemical inspector here. Yes there are microorganisms that do metabolize oil and various types of fuel. Once a tank or pipeline gets colonized with these microorganisms, they never go away. Even with biocides. Most likely all fuel storage tanks for avgas are colonized with these microbes. The most harmful type are the sulfur-reducing bacteria (to metal). They create distinctive cup shaped pits.
Title: Re: Esoteric design question about wet wings
Post by: Vraciu on April 22, 2017, 02:56:25 PM
Petrochemical inspector here. Yes there are microorganisms that do metabolize oil and various types of fuel. Once a tank or pipeline gets colonized with these microorganisms, they never go away. Even with biocides. Most likely all fuel storage tanks for avgas are colonized with these microbes. The most harmful type are the sulfur-reducing bacteria (to metal). They create distinctive cup shaped pits.
Yes, but left untreated they will eat the airplane. I saw the inside of a Falcon or Citation where it was damaged badly enough it had to be scrapped.
Title: Re: Esoteric design question about wet wings
Post by: Vraciu on April 22, 2017, 02:59:55 PM
Avgas as well. We found a 180 with a big wrinkle in the bladder trapping some water. The accumulated dirt and algae in the "pond" looked like a garden.
Ewww.
Title: Re: Esoteric design question about wet wings
Post by: colmbo on April 22, 2017, 03:34:24 PM
It was a good thing really. At least it wasn't sloshing out and going through the engine. Had that happen on takeoff one fine day at about 50'. Luckily the Continental was able to choke it all down before we hit the trees.
Title: Re: Esoteric design question about wet wings
Post by: colmbo on April 22, 2017, 03:35:21 PM
I don't think a true wet wing has a bladder though does it?
Title: Re: Esoteric design question about wet wings
Post by: Vraciu on April 22, 2017, 04:39:01 PM
It was a good thing really. At least it wasn't sloshing out and going through the engine. Had that happen on takeoff one fine day at about 50'. Luckily the Continental was able to choke it all down before we hit the trees.
Yikes. I have been supremely lucky in that regard. I count my blessings for it.
Title: Re: Esoteric design question about wet wings
Post by: Vraciu on April 22, 2017, 04:43:00 PM
I don't think a true wet wing has a bladder though does it?
The jets I have seen in MX do not have bladders in the wings (or belly tanks best I can remember). They have numerous sealed access panels on the underside of the wing and sides of the belly tanks. They're designed in such a way that they're pretty easy to seal so no leaks occur. I had my engineers (mechanics, techs) show me once and it was a pretty nifty design.
One of the guys I knew used to work on BIIIIIIIIG jets. He had to go up inside the fuel tank with a respirator to do work in it. Said it scared the hell out of him. Lol
Title: Re: Esoteric design question about wet wings
Post by: saggs on April 22, 2017, 06:20:03 PM
I'm an A&P who works on Douglas DC-6 and Curtiss C-46 aircraft most of the time. I'll use the DC-6 as an example because it has a combination of wet wing and bladders.
The DC-6 has a combination of wet wing, and bladders in the wing. The main tanks are wet wing between the front and center spar, with bladders behind the center spar. The outboard alt tanks are the same, and the inboard alt tanks are all bladders. However some in our fleet we have deleted the alt tanks, and some we have even deleted the main tank bladders.
As far as wet wing vs bladders when it comes to maintenance, I'll take wet wing any day. When there are damaged stringers, or corrosion or popped rivets or anything that needs fixed in the wet wing, it's drain the tank open up a couple panels, ventilate for several hours, get in and fix what needs fixed. (you may need to scrape away some sealant depending on what needs fixed.)
With the bladders, it's drain the tank, open panels, remove the bladder cap panels (a PITA), ventilate for several hours, get in and disconnect all the bladder interconnect points(another PITA), unsnap and remove the bladder (being very careful not to crease or damage it) then you have a phenolic liner underneath the bladder that protects it from sharp edges on rivet shop heads and stringers that you have to un-tape the seams of, then unscrew and remove... then you are finally down to the structure where you can repair.
It takes probably 5-10 times as long to gain access for repairs when we have to pull a bladder vs. just the wet wing. As for leaks, the wet wing does tend to leak more. But with Avgas small leaks evaporate away so quick we don't even worry about them (just leaves a small blue stain) For bigger leaks that we do have to fix it is much easier to fix then with a leaky bladder. Just get in and reseal wherever it's leaking from, vs. tearing out bladders and replacing gaskets.
You asked about sealant. Wet wings (AVgas wet wings anyway) are sealed wherever there are rivets or bolts through the skin with PRC (commonly called Proseal) and then top-coated with a fuel resistant product called Scotchweld EC-776. Any rivets in the wet wing are shot "wet" (meaning coated in proseal before they are shot) to eliminate leaks.
Now the Curtiss is different altogether. It has actual aluminum tanks built inside the wings, and they are pretty much maintenance-free. For other repairs on the wings you just have to work around them. The downside is they do not fill all of the available space with fuel like with a wet wing.
I don't think a true wet wing has a bladder though does it?
This is correct. "Wet wing" refers to the fuel actually filling the structure of the wing, nothing but a skin panel between you and the fuel, vs having a bladder or welded tank or something else as the container inside the wing.
Title: Re: Esoteric design question about wet wings
Post by: Oldman731 on April 22, 2017, 11:01:41 PM
I'm an A&P who works on Douglas DC-6 and Curtiss C-46 aircraft most of the time.
Thanks, Saggs, very interesting information there.
- oldman
Title: Re: Esoteric design question about wet wings
Post by: nrshida on April 23, 2017, 03:28:15 AM
Fascinating. Life everywhere even in aviation fuel!
So these rubber bladders don't seem to bring much to the party. True wet wings seem a far better solution. But these microorganisms and other nasties don't attack the aluminium-alloy structures? Is the whole surface protected, anodized or coated with Proseal or is it only on areas where a leak might be anticipated? Are magnesium-alloys a no-no for raw fuel?
I saw a video about what you were talking about Vraciu. Saw a big dude wriggle into the oval access hatch of a jet airliner. He had to put his hands at his sides and go head-first. Must have been a miniature gynochologist in a former life :rofl
Thank for all the input fellas, this information isn't easy to find through ordinary channels. :salute
Title: Re: Esoteric design question about wet wings
Post by: saggs on April 23, 2017, 05:08:45 AM
So these rubber bladders don't seem to bring much to the party. True wet wings seem a far better solution.
It all depends... most small planes have bladders or built in tanks so they can put other stuff through the wing around the tanks, stuff like aileron push rods, pitot lines and electrical wiring, and they don't need that much fuel anyway, the fuel tanks in a typical Cessna 1xx wing don't take up that much room compared the size of the whole wing.
Most big planes are wet wing or a mix, because they want to carry as much fuel as possible, and the wings are designed with that in mind. Also they have room in front of the fwd spar or behind the center or aft spar to run wiring and hydraulics and air ducts and all that. I know on a 6 the reason for the bladders behind the center spar is because they run air ducts, fuel crossfeed lines and aileron control cables through there. Engine control cables, fire suppression lines and wiring are all run in the leading edge area in front of the fwd spar. There is nothing in the wet wing area between the fwd and center spar except boost pumps and capacitive fuel probes.
But these microorganisms and other nasties don't attack the aluminium-alloy structures?
This is really only a problem with planes that sit idle for long periods. Working planes which fly often burn through the fuel quick enough that microbials don't have a chance to grow. (also should be sumping tanks often to ensure there is no water in the bottom) Also there are fuel additives designed to prevent such growth. As for corrosion, they can be corrosive, but that is of secondary concern to them causing fuel starvation, and the aluminum would be alodined against corrosion.
Is the whole surface protected, anodized or coated with Proseal or is it only on areas where a leak might be anticipated? Are magnesium-alloys a no-no for raw fuel?
Proseal just the rivet lines and fittings. Top coat everywhere (the top coat is only as thick as paint and brushes on like it) The aluminum is also all alodined. There are many flavors of proseal too, you used different stuff for avgas and jet A, different consistencies, set times etc.. I don't believe that fuel is bad for magnesium, but magnesium alloys are not used for skin, there are often many magnesium parts on aircraft, but they are usually fittings and brackets and wheels, big thick pieces. Skins on metal airplanes are mostly aluminum, with stainless steel and titanium also used in some places as needed.
Now, in modern planes with composite wings... I admit I have no idea how the wet wings are built or sealed.
I saw a video about what you were talking about Vraciu. Saw a big dude wriggle into the oval access hatch of a jet airliner. He had to put his hands at his sides and go head-first. Must have been a miniature gynochologist in a former life :rofl
It is usually the small guys who get drafted into "tank diving". You definitely can't be claustrophobic and do it. I'm big enough that I can only fit my head and one shoulder into a DC-6 oval panel, it's the skinny guys who draw that job.
Title: Re: Esoteric design question about wet wings
Post by: icepac on April 23, 2017, 07:39:05 AM
Just make sure any plane with an autogas STC doesn't use any gas with ethanol in it.
MTBE additives are fine as long as you realize they lean the mixture and that your favorite mixture lever setting will change.
Title: Re: Esoteric design question about wet wings
Post by: nrshida on April 24, 2017, 01:45:15 AM
It is usually the small guys who get drafted into "tank diving". You definitely can't be claustrophobic and do it. I'm big enough that I can only fit my head and one shoulder into a DC-6 oval panel, it's the skinny guys who draw that job.
You need to grow a wide bicycle moustache Saggs, then use it like a cats whiskers to see if you fit!
Again many thanks for all the responses. Answered all my questions about the topic. Thanks for sharing your experiences and expert opinions :salute
Title: Re: Esoteric design question about wet wings
Post by: 63tb on April 24, 2017, 06:52:03 AM
Wasn't there an episode of Dirty Jobs, where Mike Rowe went inside the wing of a KC-135 to repair a leak?
63tb
Title: Re: Esoteric design question about wet wings
Post by: nrshida on April 24, 2017, 07:27:52 AM
Wasn't there an episode of Dirty Jobs, where Mike Rowe went inside the wing of a KC-135 to repair a leak?
Yup. Found it. Both wet wings and bladders:-
Title: Re: Esoteric design question about wet wings
Post by: DaveBB on April 24, 2017, 11:25:14 AM
Be careful about entering confined spaces (fuel tanks). Unless you have supplied breathing air, cartridge respirators won't help if the O2 levels are below life sustaining abilities. There have been thousands of people killed entering confined spaces, so hopefully you all have been trained on this.
Title: Re: Esoteric design question about wet wings
Post by: G0ALY on April 27, 2017, 05:28:16 PM
I am embarrassed to admit that when I read the title “Wet wings” I thought you were asking if people prefer them wet or dry…