Internal aux tank

[BravoGolf]

After reading several posts about wing auxiliary fuel tanks I decided to eventually build an internal aux tank. My initial thoughts are to (when needed) replace the passenger seat with a fuel tank. Using 25 - 30 gallons as a target it should be in the 200lb range with the weight of the tank. At that size I gain about 3 hours of fuel depending on burn rate. I'm in the process of fabricating the fuselage and plan on adding the hard points to support securing the tank. Has anyone else done this?

Unanswered questions...

I am thinking the easiest method for fuel transfer is feed directly into the fuel selector valve. By that I mean a separate connection at the valve not a T connection into one of the main tank line. But this would require a fuel pump as the tank won't have enough gravity pressure to feed the engine. Given the discussions about fuel pumps is this plan a problem?

The fuel lines would be under the floor. All connections, power and fuel would be accessible at the floor (under a cover plate?) With a closed connection at the floor and the fuel selector out of the aux position there should not be any fuel leakage. If the aux position did leak it would only allow a small about of fuel into the line, the closed connection would prevent leaking into the cabin.

I think this will give me the additional range when I want it without adding the weight of the wing aux fuel tanks. I would only install and use the fuel tank on long journeys like flying to Oshkosh.

Any thoughts or advice would be greatly appreciated.

Scott

[kestrel]

I would pump into the main tanks as the normal aux tanks system does. My advice, your choice.

You will spend more time installing and removing the aux tank for a trip to OSH than it will save. There are cases where longer range is very useful, but in my opinion, this isn't one of them. ...again, my advice, your choice and you may have other more compelling cases.

[BravoGolf]

I thought about pumping into one of the mains. I’m not sure which method to use; pump up to a T connector at the fuel gauge area or just straight into the fuel selector. From an installation point of view it is a difference in the length of the supply line. Clearly I need to do more research

[alaskabearhawk]

Ron Jones put one in his 4-place:

http://www.mykitlog.com/users/display_log.php?user=jonesronc&project=191&category=2003&log=107218&row=10

http://www.mykitlog.com/users/display_log.php?user=jonesronc&project=191&category=2003&log=115186&row=9

[kestrel]

Take a look at this video. This guy had an aux tank that pumped into his feed lines and that is the reason his Cub went for a swim.

With very careful operation, the mistake can be avoided. ...or just don't pump into the fuel feed lines.

https://youtu.be/QJrVUZKcxrU

[BravoGolf]

Very interesting video, thanks for posting it here. It is interesting that his implementation pumped fuel from a bladder on the floor up to the fuel tank and then using the normal fuel flow to the engine. The Bearhawk wing aux tanks use a similar procedure except it is recommended the fuel is plumbed into a T at the sight gauge. I have a three way fuel valve that would allow selecting the aux tank to feed the engine directly instead of filling one of the tanks. As it will be a single tank it can either be pumped to one of the mains or flow directly to the engine.

It seems a direct line to the fuel selector instead of running a line up to a tank would be a easier installation. However I would need to provide for a fuel level indication on the aux tank. That way I can manage the fuel to avoid running out and stopping the engine. Maybe placard the fuel system aux tank not to be used for takeoff or landing? Normal ops would be take off and once in cruise switch to the aux tank until empty then switch to normal left/right management? But that does increase the workload.

I am not sure this is the best design. Still plenty of time to consider and reconsider. Thanks for your input.

[kestrel]

I'm not sure what you mean by, "pumped fuel from a bladder on the floor up to the fuel tank and then using the normal fuel flow to the engine." I believe he was pumping into the fuel feed line from the tank. If pumping into a non-selected tank, the fuel will flow into the tank. In this case, he was pumping fuel to the engine with the tank taking what wasn't burned by the engine. ...until it was pumping air into the fuel feed line. The Bearhawk system doesn't pump into the fuel supply line, so it won't suffer from this problem. The main tank will act as an air separator and feed fuel to the engine.

If pumping directly to the engine, be careful that the pump operates in the correct pressure range and don't use it below 2,000 ft (or something like that). If you would never select any tank low on fuel when low on altitude, this may work fine. With sufficient altitude, running the aux/ferry tank dry shouldn't be an issue. ...but I prefer pumping into the mains even though it requires another fuel line up to the tanks.

The the case of the Cub, it felt OK to to transfer at low altitude, but he screwed up by pumping into the selected tank.

[BravoGolf]

I meant that in my installation there is a choice. I could run a fuel line to a fuel selector so fuel could flow directly to the engine or pump fuel up to a connection at one of the main tanks. By pumping up to a main tank the fuel would flow using the lines from that main tank, no additional port on the fuel selector would be required.

He made several good points about cockpit management, being in a hurry and making mistakes. It is to his credit for posting the video so we can all learn from his incident.

[Bcone1381]

It seems to me that an error tolerant fuel system design ought to be a very high priority. A good fuel system design is one where forgetting something will not instigate a fuel feed issue. Pushing the fuel up to the top sight gauge is tolerant of forgetting to turn off the aux tank pump and has value also in that it empties the aux tank 100% with zero risk.

[kestrel]

It seems to me that an error tolerant fuel system design ought to be a very high priority. A good fuel system design is one where forgetting something will not instigate a fuel feed issue. Pushing the fuel up to the top sight gauge is tolerant of forgetting to turn off the aux tank pump and has value also in that it empties the aux tank 100% with zero risk.

I generally agree with all of this and prefer this approach. However, IFF you can create a 3 main tank setup (counting the ferry tank as a removable "main") where all 3 of them are capable of reliably supplying fuel to the engine through the main fuel selector, then there is no operational difference in the 3 tanks. If you are are low altitude while with a tank that is low on fuel selected, you may be in trouble. At altitude where there is time to switch tanks shouldn't be an issue.

The main concern is that this ferry tank is a very different beast that feeds by a fuel pump instead of gravity. Making sure that both types of system work together could have some traps. ...or it may be as simple as making sure that the pump matches the pressure of the gravity fed tanks at the needed range of flow rates.

I see it as very easy and fool proof to pump from the ferry tank to the mains so long as you don't pump into the feed line, so that would be my choice.