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912 in a Murphy rebel

in General Tech Discussion 8 years ago
Hello all. Love the knowledge here. I have a high wing 912 ul rebel and it has no fuel pump. I'm not the builder and these machines are rare. Other forums seem to agree that with high wing in the event of fuel pump failure gravity will suffice. Should I be doing a flow test at the carbs to confirm no aux pump is required? I'm installing the eis system pretty soon so now would be a good time...
Best
Sheldon

  • Re: 912 in a Murphy rebel

    by » 8 years ago

    HI Sheldon.

    I have had a 912ULS in a Highlander for over 10 years with no auxiliary pump. I checked fuel flow at the mechanical fuel pump before first engine run and found a more than adequate flow.

    Good luck with that Rebel. They are nice planes.

    Alan
      
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  • Re: 912 in a Murphy rebel

    by » 8 years ago

    If you are concerned about redundancy, There is no guarantee that gravity would be able to push fuel to the carbs in the event of the primary mechanical pump failure.
    If one of the valves in the mech fuel pump sticks closed, no fuel is going through.
    If the diaphragm fails, you will be pumping fuel overboard.

    If you install an auxiliary pump, be sure to also install a valve that allows the aux pump to bypass the primary pump.

    Don't assume that you will be able to pump fuel through a failed pump.

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

      
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  • Re: 912 in a Murphy rebel

    by » 8 years ago

    Very interesting. What is the general consensus? Is it a common thing to add a pump and a check valve to bypass the engine driven pump? If there has been one failure in a million hours I'd pass. If there have been lots perhaps I'd do a mod. Does anyone have any pics or part numbers and sources for a bypass? I'm guessing a couple T fittings, and a check valve?
    Thoughts on a pump best suited?
    Best
    Sheldon
      
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  • Re: 912 in a Murphy rebel

    by » 8 years ago

    YOUR RESULTS COULD BE DIFFERENT FOR YOUR PLANE SO IT WOULD NEED TO BE TESTED.


    I posted this before, but can't find it now. This was another research project for Rotax people.
    The CTSW high wing was used.
    I took the hoses off the fuel pump and joined them with a piece of tubing. This is now only gravity feed and unobstructed. It will run the engine at full rpm without a hiccup.
    The second test was with the fuel hoses back on the pump, but the pump was removed from the engine and left to hang. It got nothing through other than gravity feed. The pump has a free floating check valve and opens and closes with fuel flow caused by the diaphragm. It only works when the pump is pumping (more or less). So with the pump off the engine it could sustain 4600 rpm in a static test, but somewhere just above that the engine stumbles and coughs. 4600 static should be around 5K in flight. The check valve does cause some restriction, but still flows adequately. Fuel pumps rarely ever totally fail. It is usually a slow death and leakage occurs and should easily be seen .

    The Rotax Manual says bottom fuel pressure for carbed 912's is 2.2 psi. It's a good reference point. You'll never have a fuel flow issue at 2.2 psi, but it does make you feel better when you see it from 3.5-5.5 psi.

    Low wings are in a totally different boat and absolutely should have a back up electric pump, but it doesn't need to run all the time if the mechanical pump is doing its job..

    p.s.
    Nothing wrong with adding an electric back up pump.

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

      
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  • Re: 912 in a Murphy rebel

    by » 8 years ago

    I believe a fuel flow test is mandatory on any new installation; the FAA also strongly recommends this. Not only do you need to check that gravity is adequate to run the engine in all critical attitudes, but you also need to verify that there are no surprise restrictions in the lines. As a side note, when I did my fuel flow test I found that there was way more than adequate flow with the mechanical fuel pump bypassed; but with the pump in the circuit but not running, the gravity fuel flow was very marginal.
      
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