I am hoping there are some experts in the house on this. Building a pusher the fuel tank is in the fuse behind the seats [top to bottom firewall]. Not wanting to use the twin plastic 5 gallon cans in the original design I plan an aluminum rectangle (33wide X 16tall X 13length, in inches) of around 27 gallons with weight and balance understood. This will be an LSA weight bushplane with a Rotax 912 making it very weight sensitive.

I submitted a rough drawing of the tank to a custom welder I found on the internet that builds to suit. They came back with a plan for a tank made out of .125 sheet (5052) claiming anything thinner would fail. That puts the tank at close to 40 pounds! I had assumed the more common aviation tank thicknesses of .050 and up would be fine. I planned two baffles, front to back, and 4 mounting tabs -on the side- adjacent to where each baffle welds in. This tank will be mounted on it's side to a vertical bulkhead. I have much more detail sorted if anyone is interested, but just want to get this out there if we have any experts on aluminum tank construction for aviation.

Can I [should I] go thinner than .125?

Any referrals to custom tank welders for Experimental aircraft?


-M3X

Have you considered building the tank using rivets and proseal. I suggest there are significant weight savings using this method. If you have not already done so, have a look at how vans aircraft has designed the tank for the RV12. That tank appears to be a little smaller than you require at 20 gallons or you could use it.

CaptKirk wrote:Have you considered building the tank using rivets and proseal. I suggest there are significant weight savings using this method. If you have not already done so, have a look at how vans aircraft has designed the tank for the RV12. That tank appears to be a little smaller than you require at 20 gallons or you could use it.

Hi Kirk,

Thanks for the reply. I am aware of that method of construction, and find it pretty interesting, however not sure it is a fit for this project for a couple of reasons: I am mounting via hard points on the side of the tank, meaning the tank must be self supporting, under G-loads full of fuel, towards that end I need two baffles functioning as shear-support to maintain rigid form, and riveting baffles would involve a 90 degree fold to allow rivets into the skin, making them not in shear and therefore a weak element.

Of course I could make a tray to sit a tank on, and I may do so if I have to, but my preference is a welded tank, side mounted, for some solid reasons around crashworthiness with this frame, so I will reserve the riveted tank as my plan B.

-M3X

I'm pretty sure that a properly designed tank made out of .032" aluminum will be sufficient. Ask the properly degreed engineers on the forum (I ain't one), but I'm guessing that the key is having structural "ribs" inside the tank to brace it against being split from the fuel inside.

The "ribs" can be perforated with a few 1/2"holes, serving double duty as "baffles" to limit the fuel sloshing around too wildly.

M3X I have a question for you... do you have any direct knowledge of whether the M3X 2 seater (with only one pilot on board) has any better or worse STOL capability than a Firestar 2, assuming that both airplanes have the same engine? It is my understanding that the M3X has more wing area and more weight than the FS2. I have zero experience in either aircraft.

EZFlap wrote:M3X I have a question for you... do you have any direct knowledge of whether the M3X 2 seater (with only one pilot on board) has any better or worse STOL capability than a Firestar 2, assuming that both airplanes have the same engine? It is my understanding that the M3X has more wing area and more weight than the FS2. I have zero experience in either aircraft.

No experience with the 2, however as they have a similar wing cross section of differing sizes, I would simply look at wing area Vs gross weight for a sense of the STOL experience. Coupling that to the power to weight and you have a few bits to guide you.

-M3X

The Bearhawk wing tanks are built from .050" alum, according to a Bearttracks newsletter. I'm not sure if that's what my factory tanks are, I'd have to measure.

I think with baffles acting like gussets, it would be structurally sound no matter the shape.

MX3

5052-H32 Is "relatively" soft and can not be heat treated to original strength.

6061-T4 or T6 is relatively harder and CAN BE heat treated after welding.

Are you planning on square corners?

Tanks with TAB mountings have been problematical in the past.
Maybe make one surface of the tank large enough to include "tabs" running full length without welding them on separately. You would probably have to grind down the tab welds on the side up against the firewall.

Metal tanks will crack and rupture where "plastics" are often pliable enough to give and bounce/pop back without a crack.

The aluminum tanks I have watched formed placed long dimples / grooves in outer surfaces. IE: Cessna 170 tanks.

Most successful aluminum mounts have used straps instead of tabs. Some have even changed from tabs to straps. Granted they were all inside of the wings.

Confession: The most troublesome tank for me was in a 1939 wing. ~~55 yrs.

You should be able to find a shop that will work with you while you are there. You might have to travel over one more valley.

Usually the EAA is able and willing. www.eaa663.org/ Livermore Chapter.

While I was helping to salvage an old white AN-2 there used to be a once a month on a Saturday that pilots would put their older planes "on display" down by the golf course club house. Ya might even get to see the fellow that had been taxiing his red Fairchild 24 - round engine - NO WINGS for display.

Hope it helps
Chris C

Belly fuel tanks in a beaver are made of .040 5052. Big 35 gallon tanks with no internal (maybe 1?) rib, works good lasts long time.

Thanks everyone for the helpful replies.

Zzz wrote:I think with baffles acting like gussets, it would be structurally sound no matter the shape.

I think your right; I plan to use at least two baffles.

wannabe wrote:Tanks with TAB mountings have been problematical in the past.
Maybe make one surface of the tank large enough to include "tabs" running full length without welding them on separately. You would probably have to grind down the tab welds on the side up against the firewall.
.......
Most successful aluminum mounts have used straps instead of tabs. Some have even changed from tabs to straps. Granted they were all inside of the wings.

.......
You should be able to find a shop that will work with you while you are there. You might have to travel over one more valley.

Usually the EAA is able and willing. http://www.eaa663.org/ Livermore Chapter.

........

Hope it helps
Chris C

Excellent points all, in particular the tabs verses straps question.

Halestorm wrote:Belly fuel tanks in a beaver are made of .040 5052. Big 35 gallon tanks with no internal (maybe 1?) rib, works good lasts long time.

The pattern I am seeing here is thinner walled tanks, strapped into some kind of tray or other structure that provides structural integrity, maintaining tank shape via that tray, with the straps spreading the load over the entire structure, avoiding stress points tabs can introduce.

Thought I might save weight with the tabs and self supporting side mount, clean with minimum parts count. However the proven designs seem to go with a lighter tank and a tray with straps.

I guess my next move is to ask for referrals to master aluminum tank welders, preferably within a few hours drive of the SF bay Area. Any suggestions?


-M3X

M3X wrote:
The pattern I am seeing here is thinner walled tanks, strapped into some kind of tray or other structure that provides structural integrity, maintaining tank shape via that tray, with the straps spreading the load over the entire structure, avoiding stress points tabs can introduce.

I think there is POSSIBLY an easier way, but I don't have a M3X fuselage to look at to be sure. If there is a reasonable amount of cross-brace and diagonal brace tube structure across the bottom of the fuselage, put a piece of honeycomb sandwich material (surplus airliner flooring, available without too much fuss) down across those tubes, and use a strap to hold the tank down in place just like the stock plastic cans. You can easily make up some padded wooden blocks for the sides and against the front/back of the fuel compartment.

Just for grins and giggles, why is it that you want to build a custom aluminum tank instead of using one of the hundreds of different size/shape plastic tanks available?

Here's a link to an article on some aluminum tank failures and testing.

http://seqair.com/skunkworks/FuelSystem ... blems.html

Tanks are 0.050 5052 detailed plans for the tanks (and the rest of the aircraft) are available free on the Sequoia Aircraft website

Wow... interesting article. Thank you for posting the link. I would love to see whether a molded polypropylene tank would do better or worse on those torture tests!

M3X wrote:Thanks everyone for the helpful replies.

Zzz wrote:I think with baffles acting like gussets, it would be structurally sound no matter the shape.

I think your right; I plan to use at least two baffles.

wannabe wrote:Tanks with TAB mountings have been problematical in the past.
Maybe make one surface of the tank large enough to include "tabs" running full length without welding them on separately. You would probably have to grind down the tab welds on the side up against the firewall.
.......
Most successful aluminum mounts have used straps instead of tabs. Some have even changed from tabs to straps. Granted they were all inside of the wings.

.......
You should be able to find a shop that will work with you while you are there. You might have to travel over one more valley.

Usually the EAA is able and willing. http://www.eaa663.org/ Livermore Chapter.

........

Hope it helps
Chris C

Excellent points all, in particular the tabs verses straps question.

Halestorm wrote:Belly fuel tanks in a beaver are made of .040 5052. Big 35 gallon tanks with no internal (maybe 1?) rib, works good lasts long time.

The pattern I am seeing here is thinner walled tanks, strapped into some kind of tray or other structure that provides structural integrity, maintaining tank shape via that tray, with the straps spreading the load over the entire structure, avoiding stress points tabs can introduce.

Thought I might save weight with the tabs and self supporting side mount, clean with minimum parts count. However the proven designs seem to go with a lighter tank and a tray with straps.

I guess my next move is to ask for referrals to master aluminum tank welders, preferably within a few hours drive of the SF bay Area. Any suggestions?


-M3X

My Stinson aux tanks are made of .050. That's plenty thick. They also have 2 internal baffels of .032, with flanged lightening holes in them. Makes for a nice, light and super strong tank. Structural integrity was a big deal to me on the design of these, as they go between the front of the baggage compartment, and the rear of the cabin. I forget the actual numbers, but the engineer I had look it over said if you damaged those tanks by an impact or crash, they were going to be the least of your worries!
John

hardtailjohn wrote: My Stinson aux tanks are made of .050. That's plenty thick. They also have 2 internal baffels of .032, with flanged lightening holes in them. Makes for a nice, light and super strong tank. Structural integrity was a big deal to me on the design of these, as they go between the front of the baggage compartment, and the rear of the cabin. I forget the actual numbers, but the engineer I had look it over said if you damaged those tanks by an impact or crash, they were going to be the least of your worries!
John

Interesting. Your use case seems similar to mine. Again, .050 keeps popping up as a workable thickness.
Question: was your tank welded together, or riveted/sealed, or something else?

-M3X

c170pete wrote:Here's a link to an article on some aluminum tank failures and testing.

http://seqair.com/skunkworks/FuelSystem ... blems.html

Tanks are 0.050 5052 detailed plans for the tanks (and the rest of the aircraft) are available free on the Sequoia Aircraft website

Appreciate this, thanks!

EZFlap wrote:Just for grins and giggles, why is it that you want to build a custom aluminum tank instead of using one of the hundreds of different size/shape plastic tanks available?

I see a huge value in plastic tanks, for all the reasons you cite. Unfortunately they are not a fit for my project for a few reasons.

Short answer: No baffles, along with unsatisfactory automotive baffle replacements (foam-ping pong balls etc); Most are designed for automotive style top-suction feed, meaning no water trapping sump, else a metal pinch bolt assembly with a seal of dubious durability; I was unable to find a shape that suited me: tall, wide, and short length, is a requirement, and the standards of large gasoline tanks is short wide and long.

There is a paragraph or two of discussion backing up the claims above, but can't imagine they would be of interest to anyone so I will save the electrons

I am leaning towards an al tank, around .050, with multiple baffles, welded by an expert if I can find one, sitting in a tray with straps. I am pretty handy with a tig torch and 4130, but certainly not ready for taking on Aluminum. So expert welded, else I will rivet it as suggested and seal it.


-M3X

M3X wrote:Question: was your tank welded together, or riveted/sealed, or something else?

-M3X

All welded construction. They sit in a cradle on the bottom, and have abig welded tab on top, with a silicone rubber isolated mount to the tube structure. The baffles are welded to the outer skins of the tanks and become a structural member as well.
John