Rubber fuel cells

[tonyslongez]

Has anyone looked into using custom rubber made fuel cells? When I was at Oshkosh this year I talked to a gentleman that made custom rubber fuel cells. The only reason to consider bladders is because they seem to be more sterile than just a fiberglass tank. I seem to recall a few forced landings do to clogged fuel filters from the strake tanks despite the builders best efforts to keep the tank clean prior to closing. A fuel bladder seems like it would be the way to go. Certainly wouldn't have to worry about leaks when closing the strake. weight may be a problem though. Try not to beat me up I'm just thinking out loud here.

 

any thoghts ?

 

Tony

[Jon Matcho]

A common thread in a substantial portion of the forced landings is debris in the fuel system. Any discussion on how to avoid this is worthwhile.

 

The weight concern you mentioned was my first concern as well. I'm also not sure I'd care to button up my strakes with a rubber balloon in there.

 

I'm sure the plans approach can work, provided you take care to be very clean and careful.

 

My $0.02...

[tonyslongez]

"The weight concern you mentioned was my first concern as well. I'm also not sure I'd care to button up my strakes with a rubber balloon in there."

 

Jon, the company that I looked at had a formed rubber tank they said they could make any shape and size. Ya know certified aircraft have been doing the rubber balloon thing for years with pretty good success. I'm wondering if I would still need to use the baffles or could I get away with making the strake out of carbon fiber. maybe a "Plug in " type tank like the Rv guys use supported on both sides with a bulkhead and secured to those bulkheads. Maybe?

[Chairboy]

It's true, there have been many bladder tank planes, but the ones I hear about are usually in the context of problems. For example, I know that for a while, there was a whole generation of 182 with deteriorated bladder tanks, if I recall correctly. I think there are also new requirements to keep tanks full during storage or something, not sure.

 

I haven't built a plane yet, so take this with a grain of salt, but it seems like using a bladder tank is adding weight and complexity to a problem that could be solved through cleanliness during the final assembly.

[tonyslongez]

Well its not just a cleanliness issue, it's also an assembly issue, and a leak issue, and so far as I can tell we only have one example of aircraft (182) that had known bladder problems. There are alot of aircraft that have bladders that have never had a problem. As far as the keeping the tanks full during storage. I understood that to be a condensation issue not a deterioration one.

 

Tony

[Waiter]

I believe this issue is a "non-issue" if the correct Assembly techniques are utilized:

 

1) Use fine mesh filter screens before the sumps. I installed brass screens designed as filter screens for shallow well pumps, I think they are 40 mesh (40 wires per inch). These will trap all but the smallest stuff.

 

2) Clean the inside of the tanks throughly before final closing.

 

3) Do NOT make the fuel cap cutout until just before prime and paint. This keeps the majority of the crap out of the tanks.

 

4) Do the fuel cap cut outs using the updated method. This significantly reduces the intrusion of debris while performing the cut out.

 

5) Perform a final clean out (vacuum) using a very small diameter tube ( I use a 1/4 inch tube tapped to the vacuum hose. DO NOT use the regular vacuum hose, it will blow the debris into corners and other places that are totally inaccessible, I GUARANTEE IT.

 

6) Monitor the fuel filter frequently, especially during the first ten hours of operation.

 

 

If you follow these procedures, you will have no problems with debris in the fuel lines

 

Waiter

[ezelady]

Here's my two cents. As stated before me leaks are a pain, cleaning is essential and good filters. Check the filters every hour or flight for the first ten hours.

 

As for removing the baffles they are structure. Darn it. I could have gotten a couple of more gallons of jet fuel.

 

Maintaining rubber bladders is a pain in the ...... condensation and break down is a BIG problem just ask Mooney.

 

ezelady

[Jon Matcho]

I'm wondering if I would still need to use the baffles or could I get away with making the strake out of carbon fiber.

I understand that the baffles are there to prevent fuel in a half-filled tank from sloshing around so to change the airplane's CG dramatically while in flight. For example, imagine a high AoA and all of the sudden your a$$ drops out and you pancake hard into the trees just before the runway.

 

The fuel system is definitely an area to be clean and careful with, but I don't think it's an area that needs a design improvement -- it's proven itself for thousands of aircraft.

[argoldman]

Well its not just a cleanliness issue, it's also an assembly issue, and a leak issue, and so far as I can tell we only have one example of aircraft (182) that had known bladder problems. There are alot of aircraft that have bladders that have never had a problem. As far as the keeping the tanks full during storage. I understood that to be a condensation issue not a deterioration one.

 

Tony

Greetings Tonyslo,

 

Fuel bladders are a solution to metal tanks that, for one reason or another can't seal. When good, they work well, however there are many problems.

 

The cessna AD on bladders, I believe was because the way they were attached (or misattached) to the wing allowed wrinkling of the bottom of the bladder, which trapped water, made it impossible to drain at the lowest place, and water injestion happened when the plane was josteled in flight.

 

One has only to look at commercial concerns that are born out of fixing and replacing these devices. A semi-major industry is buildt around reconditioning, repairing and/or replacing these bladders. (at OSH there are atleast 2 companies which display.) These bladders are subject to drying out (if left other than totally full) chafing damage if not installed properly, aiging and loosing their elasticity, and damage during fueling due to the filler cutting the bladder around the neck or at the bottom of the bladder, among many other maladies.

 

If you look at bladderinclusive aircraft, you will notice that there is always a largish inspection-type plate around the filler neck and other possible sites. This is necessary because the bladder must be placed in this hole, and using very long arms, the bladder is clipped to the structure in which it fits. This clipping is necessary to keep the tank from folding over on itself and decreasing the amount of available fuel. Look at the bladder access panels on store-builts, and just imagine the skinned knuckels, cut arms, and expansion of the english- or other- language while attempting to install the bladder through these all too small access ports. They are also quite expensive and weighty.

 

In terms of fuel hygiene, what I did with my dragofly, and intendto do with my aerocanard, is to reflux fuel from the tank, through a filter, returning it back to the tank. Start with full tanks,with only a little air space above to allow liquid adgitation in the tanks. I did this with a facet pump for a period of about 3 solid days, periodically vigorously shaking the craft to dislodge any glass or foam detritis that may have remained in the tank. Check the filter frequently. If ther is still stuff seen, after the first couple of cleanings, continue until clear. Don't cheap out and fill the tanks only partially full, the bad stuff may be hanging near the top. For economy, perhaps use auto fuel, then use it iin your car later ( If you don't want to use it in your aircraft. Since the fuel is constantly being filtered, it should be good. This, of course, assumes that in your construction, you maintain strict hygienic control of the tanks.

 

Ah, but you say...... I can put these things in before sealing the strakes.

That will work for a while.. But the reason why the big boys make them removable is because they will eventually need service.

 

If you glass tanks are made properly, they are much better than bladders.

 

Save bladders for the subject of another string which occurred earlier--- relief tubes.

[tonyslongez]

Argoldman

 

thaks for the reply, very informative. the bladder thing was an idea that I was thinking outloud about. So back to the grind of the original wing strakes. I'll folllow the plans on the assembly . I love your idea of the syphon and return pump seems like a good way to go I'll definately be using that one.

 

Thanks Tony

[argoldman]

Argoldman

 

thaks for the reply, very informative. the bladder thing was an idea that I was thinking outloud about. So back to the grind of the original wing strakes. I'll folllow the plans on the assembly . I love your idea of the syphon and return pump seems like a good way to go I'll definately be using that one.

 

Thanks Tony

Tony, I would not use a syphon as that does not simulate the actual situation. The syphon could be anywhere in the tank and miss some of the material that would eventually find its way where the sun don't shine.

 

Instead feed to the pump from the fitting that will eventually feed your filter. This way, the fuel will circulate the way that the operating engine and pump will do it.

 

the output of the pump can be routed to the fuel filler, or to the vent.

 

Don't smoke or use a match to see fuel level while doing this. do only in a well ventelated area. Gasolend vapors are heavier than air, I believe so crack the hanger door