Tech question: Why are tanks not welded?

[Bacachero]

One question for those with knowledge on aircfraft manufacturing....Ive always wondered why the constant problem of the leaking tanks has not been permanently solved...other tan bladders....Cant the tanks be welded at the joints for a permanent bond without sealants? 

[clh]

I am not an expert, but I believe that Since they are integral to the wing skins, I would presume that if they were welded, it would be susceptible to fatigue fracturing due to flexing of the wing during flight and landing.  If you have a flexible sealant, it will allow the panels to flex.

 

Rigid tanks are usually supported in the wing structure.

[Hank]

That's right, our tanks aren't welded because we don't have "tanks," just sealed up spaces between the ribs. The wing skin itself is the outer tank surface; the back is the spar; and wing ribs form the ends.

 

Cessnas have removable aluminum tanks, and they are welded up and inserted into the wings. Not so with our planes.

[carusoam]

Old tech...

Build the wings, then seal them to make them tanks too!

With modern manufacturing techniques....?

Ford is or will be manufacturing large parts for the F150 out of aluminum...

Shaping and welding aluminum in the past was too challenging.

It will be interesting to see how durable the Ford products are. Essentially proving the new technology.

It's a long way to go to get proven technology into GA...

We have new sealants, sort of... Yaaaaay!

Best regards,

-a-

[N601RX]

Some aircraft have welded tanks, but it is a separate tank that fits down inside the wing bay. My guess is that the sealant and labor is cheaper than the aluminum and a tig welder labor. It would be nice if they were made like the rv tanks so that they could be removed from the wing by removing several screws. They use a sealed and riveted tank that is bolted onto the spar and the tank is the leading edge of the wing. Also any time you weld any of the heat treated aluminums such as 2024 it looses 1/3 of its strength in the heat affected region down each side of the weld. It would have to be reheat treated and aged to regain its full strength.

[cliffy]

Wing skins are made of 2024 aluminum, It's not weldable due to the copper content. Because the skins are the structural strength of the wing, welding would compromise the skin strength.

[bonal]

I agree the fact that the tank is the wing would mean that welding would weaken the strength of the metal. Also all that flexing would cause the welds to break down and fail. With regards to safety it seems that often when a Mooney goes down off field the wings are opened releasing fuel causing potential for fire. It's great to have a steel cage around the cabin but not so great to survive the crash only to be killed by fire. Has there been any data supporting the possibility that bladders provide more protection from fuel escaping from a damaged wing? Personally I'm glad I have those bladders I don't know how tuff that rubber is but it could be strong enough to help keep the fuel from escaping in the event of a crash.

[Piloto]

You may ask why isn't the whole plane welded instead of riveted. Like Clh mention welding on aluminum wing skins is prone to fatigue cracks. Also more subject to expensive assembly rejects. If the welder makes a mistake the whole skin panel needs to be replaced. while on a riveted panel you just drill out the rivets. If the skin panels were welded how would you replace a wing panel after it was damage. There is also the issue of welding two different material thickness, like the spar cap (0.25") with a skin panel (0.025"). Too much current will just make a hole on the skin panel.

 

Another option that has been used in aviation is gluing the panels like on the Grumman AA5. But after twenty years on planes exposed to sunlight (never hangar) the glue becomes brittle and those skin sections need to be riveted.

 

Like screws, rivets still a good option as fasteners after more than 200 years of their invention.

 

José       

[kerry]

My m20a which has a wood wing uses aluminum tanks.  17.5 gals in each wing and 14 gal under the rear bench for a total of 49 gallons.  I had a right tank develop a leak at one time and it was about a 40 minute job to have the tank in and out of the plane.  I took it to a weld shop and I think they charged me about $20.00 for the repair.  Every time I read about the cost involved with fixing a sealed tank I'm thankful I have my wood wing.  My guess why Mooney went with wet tanks had to do with how much fuel could be carried in each wing.

[aviatoreb]

My previous airplane was a Diamond DA40 - and it had fuel cells.  The DA40 has never had a post crash fire.

[rbridges]

Some aircraft have welded tanks, but it is a separate tank that fits down inside the wing bay. My guess is that the sealant and labor is cheaper than the aluminum and a tig welder labor. It would be nice if they were made like the rv tanks so that they could be removed from the wing by removing several screws. They use a sealed and riveted tank that is bolted onto the spar and the tank is the leading edge of the wing. Also any time you weld any of the heat treated aluminums such as 2024 it looses 1/3 of its strength in the heat affected region down each side of the weld. It would have to be reheat treated and aged to regain its full strength.

 

I was wondering why they didn't go that route.  maybe trying to avoid the extra weight of a metal tank in both wings?

[Mooneymite]

Given Mooney's wing construction technique, wet wings were a great idea. They are light-weight and generally durable. However, I'm not sure the engineers considered 30-50 year-old tank sealant! Even if Mooney had put bladders in the wings, nothing lasts forever. Despite all the Mooneyspace words devoted to leaking tanks, the problem (with a few notable exceptions) seems to be aging, not the basic concept. In terms of weight savings, cost of maintenance, reliability and durability, Mooney's wet wing is probably a better choice than welded tanks, or bladders.

A lot of airliners use tank sealant, not welded tanks. It works for them, but most airliners are pulled off-line within 25 years.

[Rhumbline]

However, I'm not sure the engineers considered 30-50 year-old tank sealant!

 

Yup.  Whether engineering or marketing, I read somewhere (probably in Those Remarkable Mooneys) that the Mooney management folks back in the '60's estimated a typical service life of 22 years for their airplanes.  I doubt they'd ever have believed that large numbers of their planes would still be kept in airworthy condition well over a half century later.