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Thread: Why Positive Pressure Fuel Caps on 180hp Conv

  1. #21

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    Default Why Positive Pressure Fuel Caps on 180hp Conv

    I'm trying to remember the formula... I think it is 1 pound fuel per horsepower per hour plus 50 percent... has to be tested at nose up attitude...

    Dad got our Pacer certified on a one time STC for the 0-360 using a stock fuel system BUT all lines converted to 1/2 inch to get the flow needed without fuel pumps... if I remember right the main gear was set on blocks to get the nose up for the FAA requirement... plus we have ram air caps also...

    Eddies system is able to flow for the 0-360 with 3/8 lines mainly because the STC requires the fuel valve be in the both position for takeoff and landing and a better routing of the right side tank... our Pacer does not have a both position, hence the 1/2 inch lines...

    FAA regulations are a PITA anymore... but you have to comply if you want to get your STC approved..

    Keep in mind... Eddies STC with his fuel system DOES NOT use fuel pumps.... MY STC engine conversion for the constant speed prop 0-360 A1P uses fuel pumps to get the required flow... I am pretty sure that Univairs STC also has pumps...

    Brian


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    Last edited by stevesaircraft[bri]; 10-07-2017 at 11:49 PM.

  2. #22

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    Default Re: Why Positive Pressure Fuel Caps on 180hp Conv

    The FAA does not specify an angle.

    Sec. 23.955
    Fuel flow.
    (a) General. The ability of the fuel system to provide fuel at the rates specified in this
    section and at a pressure sufficient for proper engine operation must be shown in the
    attitude that is most critical with respect to fuel feed and quantity of unusable fuel. These
    conditions may be simulated in a suitable mockup. In addition--

    (b) Gravity systems. The fuel flow rate for gravity systems (main and reserve supply)
    must be 150 percent of the takeoff fuel consumption of the engine.


    Please note that the 150% is an FAA mandated safety margin since you can only burn up to 100%. The extra 50% is to keep required pressure to the carb and make up for engine variations, different flight attitudes and minor leaks. As long as the engine is getting more fuel than 100% of what it needs you will never know anything is wrong. Systems with fuel pumps are only required to free flow 125%.
    Last edited by Jeff J; 10-11-2017 at 03:13 PM.
    "Good judgement comes from experience, and experience comes from bad judgement." - Fred Brooks

  3. #23
    Steve Pierce's Avatar
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    Default Re: Why Positive Pressure Fuel Caps on 180hp Conv

    Quote Originally Posted by Jeff J View Post
    The FAA does not specify an angle.
    Both Dakota Cub and Brian both said they had to jack the mains up to obtain the "attitude that is most critical with respect to fuel feed". Not a specific attitude but not level flight either. I wonder how the come up with that. I know in the certification of the Sper 18 they figured out what attitude it would fly at and that is the attitude they used for testing fuel flow.

  4. #24

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    Default Re: Why Positive Pressure Fuel Caps on 180hp Conv

    Among the homebuilders forums I see references for taildraggers ranging from parked on the ramp to putting the tail in a ditch.
    "Good judgement comes from experience, and experience comes from bad judgement." - Fred Brooks

  5. #25

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    Default Re: Why Positive Pressure Fuel Caps on 180hp Conv

    One last reply...

    I lifted this right off AC 23-16

    (b) Fuel Flow Rate for Gravity Feed Systems
    This method is applicable only to airplanes with reciprocating engines. Use this method if
    the airplane is being certificated for use without airplane supplied fuel pumps. The airplane
    may be equipped with optional auxiliary or emergency pumps provided they are installed but
    not used during this test.
    This test demonstrates the adequacy of the engine fuel supply under the most adverse
    operating condition. Measure the flow directly at the carburetor inlet (for example, if a
    reducing nipple is used at the carburetor inlet, measure the fuel flow through the nipple).
    Position the airplane on the ground with the thrust or fuselage level line at the most critical
    attitude for fuel flow. A bench test using the pertinent fuel system components located at
    relative elevations to represent actual airplane critical attitudes may be used. If the airplane is
    tested in the level flight attitude, determine the appropriate fuel head for the critical attitude
    by analysis and test and add this fuel head to the fuel system in the level flight attitude.
    Reduce the fuel pressure to the minimum operating pressure for an acceptable takeoff power
    mixture as recommended by the carburetor and engine manufacturer. An acceptable method
    of simulating the minimum pressure required at the carburetor inlet is to disconnect the fuelfeed
    line at the carburetor and raise the end of the line above the carburetor a distance equal
    to that of the required minimum operating pressure in inches of fuel.
    Compensate for the differential pressure, (calculated as shown below under Normally
    Aspirated, Carbureted, Reciprocating Engine Fuel System Pressure Differential) by
    subtracting it from the required minimum carburetor inlet pressure. For example, if the
    minimum permissible fuel inlet pressure is 19-inches of fuel and a system pressure
    differential is 7 inches (produced between the fuel tank vent space and the carburetor float
    bowl airspace), then run the test with the fuel line assembly raised 12-inches above the fuel
    inlet fitting of the carburetor. (The pressure, in psi, can be calculated at approximately one
    psi for each 40-inches head of fuel.)
    Alternatively, compute the sum of the head pressure associated with best rate-of-climb and
    carburetor inlet requirements, and demonstrate (with the fuel restricted by a valve installed at
    the engine-end of the system to simulate this pressure) that fuel flow is at least equal to the
    regulatory minimum.
    Completely drain the fuel system before beginning the test. Set the system to feed from one
    tank only. Slowly add fuel to the tank until a steady flow is established at the inlet to the
    carburetor or fuel injector unit. Steady flow should be established when approximately the
    usable fuel supply has been added. When steady flow has been established, add an additional
    gallon (or the fuel quantity necessary to complete the flow test) to the tank.
    AC 23-16 9/21/99
    36
    Record the time in seconds for at least one gallon of fuel to flow from the feed line. The time
    for one gallon of fuel to flow should not be more than the figure computed from the
    following equations:
    (1) For reciprocating engine-powered airplanes, certificated under Part 3 of the CAR or
    14 CFR Part 23, at the rate of 150 percent of the actual fuel flow to the engine at
    maximum takeoff power:
    Seconds per gallon = 14,400/(SFC)(TOHP)
    Where SFC = Engine Specific Fuel Consumption (SFC)
    (at Takeoff Power (TOPH)) expressed in (lb./bhp/hr.)
    TOHP = Engine brake horsepower (bhp) at takeoff
    (2) For reciprocating engine-powered airplanes certificated under Part 04a of the CAR, at
    the rate of double the normal flow required for takeoff engine power:
    Seconds per gallon = 10,800/(SFC)(TOHP)
    NOTE. For the purpose of this discussion, engine fuel flow has been defined by the
    calculated term (SFC)(TOHP). If the engine manufacturer has specified a higher maximum
    fuel flow rate, then use that fuel flow rate in place of the calculated term. These calculations
    are based upon a fuel density of 6 lbs. per U.S. gallon.

    There is A LOT more information in the AC...

    Brian

  6. #26
    Steve Pierce's Avatar
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    Default Re: Why Positive Pressure Fuel Caps on 180hp Conv

    Thanks for the additional information Brian.

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