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gravity feed

in 912 / 914 Technical Questions 7 years ago
Looking at the Rotax installation manual for the 912S it appears you will use their mechanical fuel pump backed up with an electric pump, even with a high wing. Why is this? Does the Bing carb need more pressure to operate correctly?

However, I see some Kitfox guys depending on gravity to maintain cruise fuel flow through a broken mechanical pump (not enough flow for t/o), that is, they dispense with the electric back up pump. If so, why have the mechanical pump at all?, just run gravity straight to the carb.

A high wing has plenty of head pressure to supply all the fuel needed for 100 hp. Witness all the Cessnas out there with M-S carbs.

  • Re: gravity feed

    by » 7 years ago

    The simple answer is safety and reliability. What would you give if you lost an engine over some steep Mnts? The answer is you would like to flip a switch or not have an issue at all and be able to go home in one piece with your hide intact along with whomever is with you.

    Some people and companies just do a better job on the safety and reliability aspect,

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: gravity feed

    by » 7 years ago

    True, it sounds as if Kitfox cut a corner here. Still, why does the 912(S) need any pump with a high wing? That's surely less reliable than gravity.

    A pump will need a return line, and this could lessen the chances of vapor lock. However, Cessnas have no particular trouble without a return line.

    Does the operation of the Bing need the extra pressure? I wonder if the peak fuel flow inside an individual carb on the (relatively) high specific output 912 is more than gravity can provide.

    I regret not keeping that recent series in Sport Aviation describing the operation of the Bing. I'll have to go fetch it on the EAA web site.
      
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  • Re: gravity feed

    by » 7 years ago

    Hi Karl,
    Not all high wing fuel delivery systems are created equal. Some flow at different rates. The Bing needs some pressure to perform properly at all altitudes it is tested to. It needs pressure to function well with all aerodynamic forces. With gravity feed the head pressure may be around 1 psi, but I don't know the exact number. Then there is a safety factor. Pressure keeps everything flowing and constant. Without a constant and regular pressure bowl fuel levels would change and not be reliable in all situations.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report
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