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
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