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Thread: Low Fuel Pressure

  1. #21
    Senior Member Esser's Avatar
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    Default Re: Low Fuel Pressure

    Quote Originally Posted by jiott View Post
    Just one thing-you say that when you have the wing tanks nearly full you will have "considerably" more head pressure than with only 3 gallons. This is only true to a very small extent since head pressure depends only on height above the measuring point, not volume. Because the wing tanks are fairly thin heightwise, a full vs. empty tank only gives you a couple more inches of head pressure. I believe a rule of thumb is about 1/2 psi per foot of height!

    Jim
    0.43 to be exact. Haha don't mean to be a smart ass but I do this stuff for work all the time and someone might be interested to know.

  2. #22
    Senior Member jiott's Avatar
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    Default Re: Low Fuel Pressure

    Esser, no you arn't a smartass and you are correct. I also did this stuff for a living as a hydraulics engineer. Since we are not dealing with more than a few feet when we talk Kitfox fuel systems, the rule of thumb is easier to remember. Its good we keep each other honest.

    Jim

  3. #23
    Senior Member DanB's Avatar
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    Default Re: Low Fuel Pressure

    Quote Originally Posted by Esser View Post
    0.43 to be exact. Haha don't mean to be a smart ass but I do this stuff for work all the time and someone might be interested to know.
    Love it Esser
    Dan B
    Mesa, AZ

  4. #24
    Senior Member Dorsal's Avatar
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    Default Re: Low Fuel Pressure

    I think if your going to be a smart ass you will have to multiply that by the specific gravity of the fuel which is around 0.7
    Last edited by Dorsal; 05-02-2012 at 06:31 PM.
    Dorsal ~~^~~
    Series 7 - Tri-Gear
    912 ULS Warp Drive

  5. #25
    Senior Member Esser's Avatar
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    Default Re: Low Fuel Pressure

    Quote Originally Posted by Dorsal View Post
    I think if your going to be a smart ass you will have to multiply that by the specific gravity of the fuel which is around 0.7
    Nicely done Dorsal

  6. #26
    Senior Member HighWing's Avatar
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    Default Re: Low Fuel Pressure

    My dad once said - PhD chemistry Columbia University - that the reason we worry about chemicals in the environment is because now we have the technology to measure them. EGT - when they first came out in the 70s they had four lines on the gauge and were designed to indicate relative temps. Now they have °F settings and folks worry about absolute temps. Airplanes have flown - Kitfoxes included - for generations with fuel level gauges and now we can monitor fuel flow and fuel pressure. Are all these numbers really that significant? I used the aux pump once and diagnosing the problem proved a pinched fuel line forward of the firewall. The big indicator was a poorly running engine. I guess a gauge would have shown the problem moments before the engine did, but... Round gauges throughout - I like the looks.

    One other thought, years ago a guy posted a note telling of his rough running engine problem and emergency landing. He had a habit of tightening his hose clamps a bit each annual. you can probably guess the rest.
    Lowell

  7. #27
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    Default Re: Low Fuel Pressure

    Hello again All
    I have some more very interesting observations regarding my new fuel pressure gauge!! ( it is a ROUND gauge to match all the others). Anyway last week I said the gauge read 5 psi with the engine stopped.(Lowell kindly suggested I refresh high school physics)
    I would like to clarify that. Today I went flying and the gauge indicated 5 psi throughout the entire flight. I shut off the fuel as I was taxing in and ran the engine until the fuel pressure bled off to zero. Then restart and immediately got 5 psi again. Shut down and waited about 15 minutes, still 5 psi (engine stopped). BUT after about 30 minutes the reading increased to 10psi (engine stopped)!!
    My take on what is happening is that the fuel cannot escape into the carb (float valve stops this) cannot escape into the fuel pump (check valve) therefore it maintains the 5 psi even though the engine is not running. But after it sits for 30 minutes or so the fuel in the line between the carbs and the pump heats up enough from dissipated heat off the engine that it expands and registers approximately 10 psi. This in turn over powers the float valves and concequently "floods" the engine on a restart about 30 minutes after shutdown!!
    I guess that is one of the reasons why I understand that Rotax is now recommending installation of a return line back to the tank or gascolator.
    Occasionly I have had restart problems after a flight and I always thought it was vapour lock. Now I believe it is flooding due to high pressure hot fuel in the line from the pump.
    Next week. Install a return line!!!
    Comments please.
    Cheers
    Don

  8. #28
    Senior Member Dave S's Avatar
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    Default Re: Low Fuel Pressure

    Don,

    Precisely! Your take provides the exact explanation and reason why Rotax makes this recommendation. So often, manufacturer's instructions say what to do, or what they recommend, but not the details of why.

    Your logic is spot on.

    There are a few (very few) of the older carbureted automobiles that had designed in three spigots on the in line fuel filter. One in, one out and one return via a bleed on the other side of the fuel pump.

    Appreciate you sharing what you discovered on the plane. I can see a return line in my future too - having suffered the same occasional hot soak situation with a flooded start on a summer day. ( FWIW - won't happen when the outside air temp is 5 degrees F).

    Dave S

  9. #29
    Dave Holl's Avatar
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    Default Re: Low Fuel Pressure

    I thought the reason for the return fuel line was to reduce the chances of fuel vaporization?
    I guess it also stops fuel pressure build up with heat when the engine is stopped!
    Dave Holl
    Building Kitfox MK7
    Rotax 912ULS

  10. #30
    Senior Member Dave S's Avatar
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    Default Re: Low Fuel Pressure

    Dave H,

    Sorta both or either - in a hot soak soak shutdown - the bleed line relieves the pressure between the pump and carb so flooding does not occur via the carb needle valve - what starts out as warm fuel can certainly progress to fuel vapor if there is enough heat buildup. It's a matter of degree.

    In any case - without a bleed line excess fuel or vapor or both can be dumped to the carb past the float valve and cause flooding - the pressure buildup will be released through the float valve because it can't go the other way.

    Sincerely,

    Dave S

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