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I'm wondering what the best fuel flow test is for the 914, which is very similar to a fuel injected engine in that it requires fuel pumps that push a lot of fuel to a regulator, with most of it being returned to the header tank.

A "typical" fuel flow test whereby one puts the aircraft in an extreme nose up attitude and then disconnects the hose at the carburetor, does not seem logical because the pumps will need to be turned on, and the fuel flow will likely be in the 35 gph range (the 914 requires only 8.7 gph at takeoff performance).

Doing a gravity test at the pump inlets also doesn't seem all that helpful because the pumps have considerable suction and can undoubtedly deliver far more fuel than gravity alone.

Suggestions?
  • Re: fuel flow test 914

    by » 6 years ago


    If this is not a new build of a new design the only reason for testing the flow is just out of curiosity.
    This should have been done by the original designer of the fuel system.
    But, as inquiring minds want to know...

    Remove the return hose from the fuel pressure regulator to the Fuel Tank at the tank end.
    Place it in a clean Gallon container.
    Turn ON the Fuel Pump(s) for 60 seconds.
    The Gallon Container should now have about a half gallon of fuel in it.
    30g/h = 0.5 gal per minute.

    Other parameters that you may want to consider...
    The Fuel Pressure must be 3-4 psi above the Airbox pressure.
    The Pressure in the Return Hose must be less than 1.5psi at the regulator end. (Free Flowing)
    The Intake ends of the Fuel Pumps must be no more than 10 inches above the Bottom of the Fuel tank. (A Low Wing concern.)

    And again, this should have all been verified by the original designer of the fuel system.

    If you are checking the flow because of an engine problem,
    Consider that if the fuel is being returned to a Header tank, that tank needs to be vented."
    The fuel gets "Abused" as it passes through the fuel pumps and regulator and generates small vapor bubbles.
    If these bubbles are allowed to accumulate un-vented within the fuel system, they will eventually find their way to the carbs causing intermittent problems.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


  • Re: fuel flow test 914

    by » 6 years ago


    Thanks. This is for a fresh design. One thing you pointed out that I didn't think about hard enough was the free flowing return hose. I just made it 3/8 with no sharp bends to my header tank, thinking that should be fine. But my header tank is about 18 inches below my wing tanks. Now I'm wondering if the weight of the fuel in my wing tanks might cause too much resistance in the return hose. Seems like the 912iS installation in the Zenith that is featured in the Rotax-Owner videos does it that way without problems, so hopefully it will be okay.

  • Re: fuel flow test 914

    by » 6 years ago


    If this is a new design; you need to be following the Installation Manual.
    The Fuel Pressure and the Return Hose considerations are all covered in the Manual along with a few other "Gotchas" that you haven't considered yet.

    The UpHill Path back to the tank is part of the 1.5 psi pressure budget.
    Fuel will produce about 1psi for every 44 inches of rise.
    So, your 18-inch rise will eat up about 0.4psi of the allowance.
    3/8" hose and 18" should be well with the limits but, as the designer, you will still need to actually measure it in order to sign it off.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


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