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Hi everyone, 

this is my first post on this forum, I am in a team of french engineering students that has been building a Jodel D185 ultralight aircraft for many years now.

It has been equipped with a 80hp 912 UL (brand new), but some parts of the installation weren't very well thought out by the previous teams.

One of the main issues I am facing is that the fuel tank, that is cylindrical and sits behind the pilot and copilot, wasn't originally designed to have a return line, because the plane was designed for another engine, so there is not a port specifically designed to be used for a return line.

The thing is, since the airplane is made of wood and cloth nothing can be welded in situ, and removing the tank would take days of our time and would be risky since the wings have to come off and are not designed to be taken off every other day, and the risk to rip the covering is high.

I have seen people use these riveted return line bungs on performance cars, but I am scared that they wouldn't work very well on the curved surface of the tank.

One of the options I have is to use the vent line right next to the fill cap, but that would mean drilling the fill cap and I wouldn't want the return fuel to drip out of the cap and spray all over the side of the airplane, damaging the covering.

Another option I have is to have the return line hooked up to the bottom of the tank, basically tee'd off the purge lines. The concern I have with this is that the fuel wouldn't be pressurised enough to overcome the hydrostatic pressure of the fuel already in the tank.

There is let's say about 0-50cm of height from bottom to top of tank so about 0-3800 Pascals or 0-0.55 psi in freedom units of back pressure, that would vary with the amount of fuel in the tank and the orientation of the aircraft. I'm not quite sure the restrictor will even be able to push the fuel to the tank.

Perhaps I'm overthinking this and my reasoning is wrong but I would like to get this done right the first time around and not have to worry about fuel supply issues.

I am unsure about the diameter of the restrictor but I can check later in case someone needs to know.

I inserted pictures to illustrate the different options I am considering :

Return line option

(for anyone wondering, all of these fuel lines are not the final product and will be replaced with aluminum hardline even before fuel is inserted and the engine is started)

Any opinions or other ideas ?

Thanks for reading

 

 

  • Re: Fuel return line design obstacle

    by » 6 months ago


    My opinions (based on some experince)

    Vent

    Do not drill the tank filler cap. I had an aircraft that had this feature - as you already know this leads to liquid fuel running down the side of the aircraft.

    If possible, have your vent line/hose rise to the highest point in the aircraft and or a "break tank" before exiting.

    Fuel Return 

    If you can gain access with a drill,  why not have the return line in a similar position as your existing vent point on the tank filler tube? (be sure to have the vent above the return line) If you use a threaded spigot (example shown) you should be able to put in in place, using a wire trace, through the hole you have drilled.  Hold it with long nose pliers and tighten the nut on the outside. Use good quality fuel compatible sealant when installing.

    Fuel Return Line Restrictor.

    Rotax recommend a #35 jet. (On my installation this is too large). You can purchase the jets from Mikuni (VM22/210 Mikuni Pilot Jets - various sizes). The restrictor jet and the boost pump have a relationship  - Rotax recomend Max 5 psi RV recomend Max 7 psi. Metaland Brass 1/4" Hose Barb Bulkhead Fitting Thru-Bulk Straight Adapter for Water Fuel Air

    Fuel Lines

    I would strongly advise against aluminium fuel lines - Cost, many more failure points than "rubber" hose line, much more difficult & time consuming to install and potential for fatigue cracking. Use good quality fuel lines eg Gates Barricade secured with fuel injection clamps (not the serrated hose clamps shown in your photos) or similar and don't do them up too tight. Just replace every 5 years - easy!


  • Re: Fuel return line design obstacle

    by » 6 months ago


    Thanks a lot for the reply, that was very useful. I just noticed on one of the pictures that there actually seems to be another port right behind the vent port that I had never noticed before, you can tell by the hose clamp sticking out slightly. I should check it out before drilling anything, but I'll definitely keep the suggestion in mind in case it's unsable. It's quite hard working on an airplane that you don't always have access to and rely mostly on pictures haha. You are right about the aluminum lines, what I will probably do is use mostly rubber lines but use at least one hardline near the firewall since there are a few tight 45° bends that need to be made, and P clamps are sometimes impossible to install due to very thin wood.


  • Re: Fuel return line design obstacle

    by » 6 months ago


    The return line should originate at the highest point of the fuel system near the carburettors but it is OK for it to enter the tank at the bottom. The hydrostatic pressure is not a problem.

    You mention the possibility of teeing into a purge line - I think by this you mean the water drain line? Exactly this solution is used on many Europa aircraft.

    Personally I think aluminium fuel lines are much better but they must be well secured. I hate the long flexible hoses on the Europa and they are extremely difficult to inspect or replace.


  • Re: Fuel return line design obstacle

    by » 6 months ago


    Good to know it has been done and works, I definitely meant the water drain lines. At least now I know I have a backup solution that doesn't involve modifying the fuel tank !


  • Re: Fuel return line design obstacle

    by » 6 months ago


    I agree, hydrostatic pressure is not a problem. My Kitfox model 7 is a high wing with fuel tanks in the wings, and the return line goes into the side of a header tank behind the seats. This gives a head pressure on the return port of about 2.5 feet, which is no problem to thousands of Kitfoxes out there.


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