Re: no fuel flow during test
Going without a header tank is not advisable.
Let’s suppose you decide to forego the header tank and put in two lines, one from each tank and join them at a “T”.
As you make your landing approach you are a bit high so you decide to slip a bit.
The fuel in the lower wing sloshes to the lowest point away from the fuel supply port in the tank.
The fuel in the higher wing flows down to the “T” fitting and tries to go back up the opposite fuel line into the low tank. In doing so it creates a siphon action at the “T” fitting and fuel may be restricted to the engine.
Adding a vented header tank where the two fuel supply lines join prevents the siphon action. In the above slip to landing scenario, fuel will flow from the high wing tank into the header, and at the same time some fuel will try to flow across from the high tank to the low tank. However, with the header tank the siphon action is broken with a vent line. Fuel flows unrestricted to the engine.
The other scenario is where you are doing a long and steep descent with low fuel quantity in the wings. Although the Kitfox header tank allows for several minutes of steep descent with the engine at idle, it is still possible in rare circumstances to run the header tank out of fuel. One builder reported such an occurrence after crossing the Sierra and making a long steep descent to the central California valley. Raising the nose to a normal descent allowed the header tank to refill. This appears to occur only with very low fuel in the wings.
Installing a header tank low fuel warning light may be helpful.
To answer your other question about the free flow of fuel through the pump: That depends on the pump design. Many fuel pumps have check valves that prevent backwards flow. Mechanicals almost always and electrics maybe yes and maybe no. The head pressure from the tank must be sufficient to push the check valve spring open. Some pumps are designed to close the check valve only when the pump is running, the rest of the time the pump is in free flow mode. You'll need to check your fuel pump specifications to see if it is a free flow design.
Because of the position of the carburetors on top of the Rotax engine it is possible to get the carbs higher than the fuel tanks when in a very steep climb. That's why the Rotax installation should include a low mounted fuel pump.
For engine installations with low mounted carburetors like the Lycoming or Continental it is almost impossible to get the carburetor higher than the wing tanks in a steep climb.
John Pitkin
Greenville, TX
Re: no fuel flow during test
Do you get good flow if you pull the line before the pump?
Re: no fuel flow during test
John's comment is on the mark. In the old days, before the vent line was put in the system there were several instances where there were fuel flow issues in uncoordinated flight. The vent line corrected that. Can't comment on the mechanical pump. One question I have always had though, if the mechanical pump fails, will it pass the aux pump fuel to the carburetors, or will it simply allow it to run into the crank case. I have heard stories like that with regard to diaphragm pumps.
Lowell
Re: no fuel flow during test
Thank you John. That makes total sense. I will keep the header tank. It is vented to the right wing tank. There is a "free" nipple at the bottom of the header and I can run a fuel return line to it easily but was wondering if it's ok to put a fuel return into a "full" header tank or should I run it to the right tank like the vent from the header. It just seems like a vent line to the header tank would have to overcome the weight of fuel in the header and the wing tank, and with the vent going into the bottom of the tank I would need a check valve and the restrictor fitting to keep from backflowing fuel into the carburators. This is all part of my months long search to the problem of fuel venting from the carburators at low idle.
The new rubber isolators are going into the mounts today. That should quiet some vibration.
Re: no fuel flow during test
Fuel return can go directly to the header tank, its a closed system so the pressure is balanced.
Re: no fuel flow during test
Thank you Dorsal. I am ignorant of the term "closed system" can you explain it ... "as if to a child."
Re: no fuel flow during test
Sorry, that just means that the fuel is coming from the header tank and returning to the header tank in a closed loop. Given the pressure at the supply side and return are the same the pump will see no static differential pressure. This would be true even if you return to the wing tank as then the closed loop includes the headertank and lines to the main tanks.
Re: no fuel flow during test
Ok, Got it.
I think I will go ahead and get the facet fuel pump on order. I'll have my mechanic research the requirements for check valve placement and power supply requirements, pressure regulation, star alignment etc. etc.
Thank you so much for your reply.
Jay
Re: no fuel flow during test
I am still curios if you get good flow when you pull the line going into the fuel pump?