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Thread: Uninterrupted fuel supply in high bank angle and negative G

  1. #1

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    Uninterrupted fuel supply in high bank angle and negative G

    Should a normal category airplane (this is my idea of airplane, move from point A to point B quickly, not to do somersault in the air.) maintain continuous fuel supply under long-duration negative G condition?

    My understanding is that a normal category airplane will not have long-duration negative G operation, thus the fuel system is not required to supply fuel under such operation. But I am not sure.

    A related condition is high bank angle, or more precisely, high side-slip angle which moves the fuel to a side. What are the fuel system design requirement for a normal category airplane under such condition?
    Last edited by wantobe; 12-14-2012 at 08:45 AM.

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    Sounds like a near aerobatic maneuver to me. An aquaintence wrecked a Baron in a somewhat similar condition. He did a hasty takeoff with the fuel selecters in "aux" position. Mains full, Aux's less than full. The airplane is placarded against this and its also covered in the AFM. The mains have two fuel lines, but the aux tanks only one.

    He took the active accelerating while turning 90 deg to line up for an intersection take off. Centrifigal force was no factor for #2 tank, unfortunately the #1 tank unported during the turn. It ran just fine until he rotated. Then the air bubble hit #1 engine. You cannot believe how fast it hung a sharp left. Several witnesses. New airplane with less than 100 hrs. I'm pretty sure that the engine would have resumed normal operation on its own if only they were at cruising altitude. They made Barons for a long time. Single engine planes unport too. FAR 23.955. covers this for normal catagory.

    Is this the sort of condition that you are describing?

    Bob

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    The short answer is no. Normal and utility category airframes will handle -1.5 G but a carbureted engine stops almost immediately if any negative G is encountered. Fuel injected engines will run a moment longer. The airframe design requirement is to handle turbulence. But the powerplant and fuel system designs are not required to function under negative G.

    Hope this info is useful,

    Wes
    N78PS

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    prasmussen's Avatar
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    Whoops! I thought fuel injection was the answer to inverted flight? Along with a dry sump and floppy tube what else could an old pilot ask for Christmas?
    The journey is the reward.

  5. #5

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    Quote Originally Posted by Bob Dingley View Post
    ... Centrifigal force was no factor for #2 tank, unfortunately the #1 tank unported during the turn. ... Single engine planes unport too.
    I looked up "unport" in Merriam-Webster which said this word did not exist. What does it mean?

    Quote Originally Posted by Bob Dingley View Post
    FAR 23.955. covers this for normal catagory.
    FAR 23.955 does not mention negative G, only requires proper fuel flow in most critical attitude. This is a surprise for me since I expect it to impose fuel flow requirement under negative G for aerobatic airplanes (FAR 23 covers normal category and aerobatic category).

    Quote Originally Posted by Bob Dingley View Post
    Is this the sort of condition that you are describing?
    The condition I am concerned is long-duration negative G.

    Thanx!

  6. #6
    steveinindy's Avatar
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    I looked up "unport" in Merriam-Webster which said this word did not exist. What does it mean?
    It means that the fuel moves (by gravity or the inertia of the fuel in a sharp turn) from the ports of the line leading from the fuel tank.

    FAR 23.955 does not mention negative G, only requires proper fuel flow in most critical attitude. This is a surprise for me since I expect it to impose fuel flow requirement under negative G for aerobatic airplanes (FAR 23 covers normal category and aerobatic category).
    The most critical attitude is going to vary from aircraft to aircraft so it is up to the designer to define that. The FAR is necessarily vague about that. A reasonable person would read that as "If you're going to fly upside down or pull negative G, you have to make sure you won't kill your engine". It shouldn't be surprising that a category that covers everything from a Cub to an Extra 300 is not going to spell out every last minutiae detail.

    The condition I am concerned is long-duration negative G.
    "Long duration" (more than a couple of seconds) negative G also winds up often causing equally long duration mental impairment. Pilots- especially with the level of physical fitness of the average GA pilot- tend to poorly tolerate negative G and it can cause "red out" or full unconsciousness.

    Fuel injected engines will run a moment longer. The airframe design requirement is to handle turbulence. But the powerplant and fuel system designs are not required to function under negative G.
    Not being the most well-versed in advanced engine and fuel systems, how would one overcome this so if you were to encounter turbulence or something else that imparts dramatic but temporary negative G that you can keep your fuel injected engine running? As for the carburetor-equipped versions, I would not even consider such an engine for a modern aircraft because of the various problems (icing being the big one but apparently intolerance to negative G is another) with the system when better options exist
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



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    Quote Originally Posted by wantobe View Post
    FAR 23.955 does not mention negative G, only requires proper fuel flow in most critical attitude. This is a surprise for me since I expect it to impose fuel flow requirement under negative G for aerobatic airplanes (FAR 23 covers normal category and aerobatic category).
    Might have to poke around a bit, like in 23.951.

  8. #8

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    Bravo Steve for explainining "unport." First time that I remember seeing it was in the accident report. Which recomended an ammendment to an AD thats been around forever:

    http://rgl.faa.gov/Regulatory_and_Gu...B?OpenDocument

    (BEECH Models 95-55, 95-A55, B95A, D95A, E95, 95-B55, calls for a placard and flight manual ammendment)

    No need to defy physics when designing systems. You just slap a placard on the panel, ammend the AFM and use common sense.

    Bob

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    steveinindy's Avatar
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    Quote Originally Posted by Bob Dingley View Post
    Bravo Steve for explainining "unport."
    Thank you. The fact that you approved of/appreciated my explanation means a lot since I respect your opinion a great deal.


    No need to defy physics when designing systems. You just slap a placard on the panel, ammend the AFM and use common sense.
    That's kind of what I figured but then again my thinking has always been that if you can design a problem out with a practical approach, all the better.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  10. #10

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    I will suggest that the statement "negative G also winds up often causing equally long duration mental impairment. Pilots- especially with the level of physical fitness of the average GA pilot- tend to poorly tolerate negative G and it can cause "red out" or full unconsciousness." indicates too much time spent reading and no time spent in inverted flight.

    As an active aerobatic competitor, I can report that the compulsory Known programs include figures like inverted 270 steep turns that are not very short in duration. Outside snap rolls ARE short duration but can expose you to high -Gz. The Advanced and Unlimited guys come down with -7Gz on the meter some days.

    The only situations where I am aware that we see "red out" are in ground based centrifuges.

    For what its worth, what will put a pilot to sleep is transition from a period of negative G to high positive G. You will wake up woozy but that has not prevented some really competitive guys from turning back to the contest box and finishing their flight programs.

    Getting back to powerplant behavior under negative G, the Czech Zlin has a carburetor that has a double float bowl, one upright and one inverted. When the airplane is rolled from upright to inverted, or back, the engine will cough as the carb switches between the float bowl that supplies fuel to the venturi and the fuel pump refills the float bowl. Very clever design.

    A carburetor stops working at negative G as soon as the float moves to close off fuel flow into the bowl. Normally the boyancy of the float in a full bowl pushes the needle valve closed. Negative G also causes the float to move the needle valve to close off fuel flow. Plus the flow of fuel from the float bowl depends on +G. So the engine stops making power.

    Fuel injection requires more fuel supply pressure to operate. But that higher pressure takes longer to decay when flow from the supply tank stops. So a fuel injected engine runs a moment longer under negative G. As noted above, if you have a flop tube in the fuel tank, you can keep fuel flowing and the engine making power. But you better also have an inverted system on the engine oil supply or making power just makes bad wear in the engine.....

    Best of luck,

    Wes
    N78PS

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