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Thread: Low Fuel Pressure

  1. #11

    Join Date
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    Default Re: Low Fuel Pressure

    Skin knows wind chill, airplanes don't.

  2. #12

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    Jun 2011
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    West Columbia, SC
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    Default Re: Low Fuel Pressure

    Quote Originally Posted by Woodennickle View Post
    Skin knows wind chill, airplanes don't.
    Of course an aircraft can't feel wind chill, but this may still be an issue here. Under the cowl, the engine generated heat would tend to warm all attached equipment. If the relative wind is cooling that equipment faster than the engine is heating it, the equipment gets colder. The coldest it can be is the OAT, but it would not get as cold as the OAT without wind blowing the engine heat away.

    I think the wind chill description of "feels like" is a bit misleading too. The reason no wind temps feel warmer than the actual temps, is similar. In no wind, your warm blood is heating a little air at the surface of your skin. If you are not moving, and the wind is totally still, you "feel like" it is warmer because the air at the surface of your skin IS warmer. Coats and blankets just capture body heat. Not very effective if there is no heat source. They describe it the other way, as a lower wind chill, because otherwise people may not take the effects of wind seriously enough.

    Same thing happens in cold water, which is why a wet suit works. Warm a little water next to the skin, and do not let the surrounding cold water carry that little bit of warm water away. Some people carry some large trash bags when flying over water for this reason. Getting inside a bag will have a similar effect, keeping the water in the bag a little warmer than the surrounding cold water. And you can catch air in one or two bags, tie them off, and use them for flotation.

    The reverse is also true. You can boil water in a paper cup with a gas flame, because the water on the inside carries away the heat and keeps the paper at below its ignition temp even though the gas flame is well above. To the paper, the gas flame "feels like" 212* with the water boiling.
    Last edited by DBVZ; 02-07-2012 at 09:00 AM.
    Dwight B. Van Zanen
    Maple Valley, WA and
    West Columbia, SC
    PP/ASEL/IA
    Avid Mk 4 Aerobat

  3. #13
    Senior Member Dave S's Avatar
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    Default Re: Low Fuel Pressure

    Bruce,

    Sorry for not getting back right away - had another deal to do.

    I have to qualify anything I suggest because it is next to impossible to diagnose a problem without having one's hands on the airplane. Take my suggestions with that understanding.

    1) Based on what you have said - the low pressure warning was real - not likely caused by the sensor.

    2) The pressure readings you reported under the various conditions point to either a restriction of some sort; or a mechanical pump which is not quite up to snuff - or a combination of the two - and something transitory when the low pressure light tripped..

    a) The static pressure 4.9 psi (Engine off electric pump on) indicates the electric pump is fine; and, pressure will be normal as long as fuel is not being consumed by the engine ; however, this says nothing about the condition of the mechanical pump and nothing about any restrictions in the line.

    b) The Pressures you observed at Idle (1800 rpm) of 4.6 with the mechanical pump alone and 5.3 with with the electric pump added still indicates the electric pump is fine (note - when a mechanical and electric pump are operated in series it is normal that the pressure will be a bit higher than what the electric pump would provide alone). This Indicates the mechanical pump can provide adequate fuel flow at the low flow rate at idle. Still doesn't rule out a line restriction or a weak but moderately functional mechanical pump.

    c) The pressures you observed of 3.7 psi with the mechanical only and 4.0 psi with the electric under full throttle is sufficient and within spec to operate the engine. The fact that both are reduced from the pressured observed at idle may indicate a developing restriction in the fuel system or a weakening pump(s) - just something to think about. The fact that adding the electric doesn't push the pressure back up to 4.9 or better may indicate the problem is associated with developing line restriction rather than a weak fuel pump - not for certain - I said MAY. FYI - I don't have a fuel pressure gauge on our plane but I did do ground run testing with a new engine and a test gauge there was no drop in pressure under full throttle compared to idle.

    d) Your conjecture on the possible formation of ice crystals on the gascolator screen is certainly worthy of consideration - that is a possible explanation for the drop below the low limit under the specific conditions conditions. FYI - I don't know what your normal flying temperatures are in the part of California where you fly - however - Minnesota has about 5 months of sub freezing weather in the winter and it is done where some folks running 100 LL will add isopropyl to the 100 LL to insure against the formation of screen clogging ice needles in the fuel - I have personally observed the little ice needles suspended in 100 LL sampled from aircraft sitting overnight in sub freezing weather. I would not rule out the ice crystal problem.

    e) Now - what to think about the difference between the pressures you observed of 3.7 psi with the mechanical only and 4.0 psi with the electric under full throttle vs the instance of the low pressure light tripping.... Something was obviously different when the light tripped; however, the other information you developed still indicates there may be some sort of developing restriction in addition to the low pressure indication which could certainly exacerbated the low pressure deal which appears to be transitory.

    I think you are very fortunate having the plane set up with the fuel pressure - I am going to speculate without that you may have never known anything was amiss if you didn't have the dynon.

    For what it worth - I had a PA 28 fuel pressure gauge go to near zero on climb out at full throttle - that one, as it turns out had a pump near the end of its life - reducing the throttle allowed the pump to catch up with the engine and I didn't have to go for "whatever runway you want" as cleared by the tower.....oh well.

    Sincerely,

    Dave S
    KF7 Trigear
    912ULS Warp Drive

  4. #14
    Senior Member
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    Default Re: Low Fuel Pressure

    Hello All
    This is a bit of an old thread but it seems to relate to my story today.
    I finally got around to installing a fuel pressure gauge in my KF Vixen 912UL. I purchased a 2" Holley electrical fuel pressure gauge (P/N 26-503) from Side Street Auto and mounted the sender about 3 inches down stream from the fuel pump. The gauge is graduated from 0 to15 psi.
    With 2/3 tanks of fuel the gauge reads slightly on the low side of the 5 psi mark with a engine SHUT OFF. With engine at normal idle there is very little difference in the gauge reading. On full throttle climb out (5500rpm) or cruise at roughly 5000 the gauge seems to read right in the middle of the 5 psi mark.
    The conclusion I am drawing from this is in the event of a fuel pump failure there appears to be adequate fuel pressure to run a 912UL at full throttle!!
    I find this very reassuring. My engine is a mid 90`s with 330 hours and the original pump with no return line.
    Thought you may find this interesting.
    Cheers
    Don

  5. #15
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    Default Re: Low Fuel Pressure

    Hello all
    I just received a private email from Lowell that seems to blow a hole in my "reassurance" about 5 psi at engine OFF. Seems that I have forgotten my high school physics.
    It will be very interesting to read everyone`s comments about this one!!!
    Oh well, another useless gauge to look at!
    Have a great day.
    Don

  6. #16
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    Default Re: Low Fuel Pressure

    The problem here is some of you equate good pressure with good flow. Think of a garden hose. When you hold your thumb over the end the pressure goes up and flow is zero. When you take your thumb off the flow is high and the pressure is only the resistance of the hose.

    Of course, at the extremes no pressure is no flow as well as high pressure can be no flow.

    Add a fuel cube flow sensor to your Dynon.
    Last edited by TahoeTim; 05-02-2012 at 05:06 AM.

  7. #17
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    Default Re: Low Fuel Pressure

    G`morning All
    My original idea with the gauge was to be able to monitor fuel pressure in flight. A drop in pressure should indicate a possible pump failure. As Lowell pointed out there should be very little pressure (less than 1 psi) with the engine stopped. Obviously some of these senders do not appear to be very accurate!
    Well at least I now have a 1/8"NTP outlet in order to connect a test gauge to and find out what is actually happening. Hopefully I will actually be able to find a practical use for this instrument. After testing I plan on replacing the sender.
    Cheers
    Don

  8. #18
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    Default Re: Low Fuel Pressure

    hi guys, just saw this thread and want to share my fuel pressure adventure. while flying off time on our ss7 with a 912uls.
    after climbing to 7500' and setting power to 5300 rpm just set back and flew the box to build time, had never ran the engine at a high power setting for a very long time in the earlier testing so at about 2.5 hr, i saw a fuel pressure fluctuation on the garmin g3x. at first i thought it might be anouther sender problem as with the oil pressure sender after only 3.0.hr" a story for later". after watching the pressure it was obvious it was trending lower. reducing the power to about 4900. rpm stoped the droping pressure at about 2.5 psi.
    i made it back home at that power setting still not sure i realiy had a fuel pressure problem.
    after landing i did a full power run up, the pressure fluctuated as before allways trending down. after about 2 min. at full power the pressure reached 0.2 psi and the engine started shuting down.
    after looking at all hoses, filters, fittings i found a tywrap i had used to support the vent line on the right tank pulled to tite on the #1 rib it took a long time for this restricted vent to show up just glad it wasnt at night on my first long cross country. hope this helps someone

  9. #19
    Senior Member DanB's Avatar
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    Wink Re: Low Fuel Pressure

    Quote Originally Posted by TahoeTim View Post
    The problem here is some of you equate good pressure with good flow. Think of a garden hose. When you hold your thumb over the end the pressure goes up and flow is zero. When you take your thumb off the flow is high and the pressure is only the resistance of the hose.

    Of course, at the extremes no pressure is no flow as well as high pressure can be no flow.

    Add a fuel cube flow sensor to your Dynon.
    This has been a good thread as it promotes a little thought and (what if) scenarios as we should all be doing. What Tim has mentioned about Flow vs. Pressure has been my thought process as i built my plane. I currently have a Grand Rapids EIS to monitor my sensors and I went with the "Red Cube" fuel flow sensor (which by the way will also work with a Dynon engine monitor). Certainly, there is reason to want to know your fuel presure (which I do not have to date), however, I can and do monitor my fuel flow. Reasoning (and I know I will be shot down if there are holes in it and please do as I am still testing and evaluating :
    1. I have done a static fuel flow (no engine running) to the fuel pump and found to have 15 GPH with a head pressure in the wings loaded to only 3 gallons on each side (no, I didn't check pressure). This is more than enough considering my 912s will ask for no more than 6 or so on climb out and I will have considerably more head pressure (more than 3 gal per side).
    2. During my testing phase ( that would be now)...I am beginning to develop standards (or primary benchmarks) for what my fuel flows are at given RPM's / altitudes, speeds, etc. May make up a little chart to keep handy in my knee pad.
    3. If my "standard" flow begins slowing considerably, I could start thinking blockage building up somewhere? If it stays the same and my airspeed and or RMP begins droping (given no head or tailwind), that would be a possible leak to look for.

    I am not yet sold on the need for a fuel pressure sensor (give me a list of reasons of what more it will do for me), but I am now re-thinking about installing an electric fuel pump. I know many have them and does make sence as a back-up pump or I think even more importantly to force that fuel up (or from behind) through a possible choking fuel line or filter. Could be enough to help hang on long enough to get on the ground as in the case of Cardinal Flyer's initial post. Go ahead...start throwing flaming arrows . Good stuff here
    Dan B
    Mesa, AZ

  10. #20
    Senior Member jiott's Avatar
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    Default Re: Low Fuel Pressure

    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

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