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Thread: Info Needed A23-24 Issues

  1. #1

    Info Needed A23-24 Issues

    <We have a 1966 A23-24 which needs a cruise prop to replace its climb prop.>

    From the Type Certificate Data Sheet for the A23-24, downloadable from BAC:

    Propeller and propeller limits

    McCauley 1B235/BFA 7862 (with 4 in. spacer, P/N B3637)
    Diameter: not over 78 in., not under 76.5 in.
    Static rpm. at maximum throttle setting:
    not over 2350, not under 2250

    McCauley 1B235/BFA 7762 (with 4 in. spacer P/N B3637)
    Diameter: not over 77 in., not under 76.5 in.
    Static rpm. at maximum throttle setting:
    not over 2350, not under 2250

    There are only 2 choices for fixed pitch prop on the A23-24 model.

    A 78" diameter x 62" pitch prop, and a 77" diameter x 62" pitch prop.

    Since the pitch is the same, only the diameter is changed, so there isn't
    much "cruise" or "climb" variation available.

    Step one would be to establish some benchmarks and see what the actual performance of the engine and prop currently are. A flight to 8000' DA for a full throttle run with an accurate tach (or some means of calibrating the readings) would tell you if the prop is well matched to the engine and
    airframe.

    You want to see 2700-2750 RPM at wide open throttle and the 8000 feet Density Altitude. If you have significantly less RPM, the pitch needs to be reduced, if substantially MORE, then you need to increase the pitch. The rule of thumb is about 50 RPM change per inch of pitch change.

    Step two would be to take your prop to the prop shop and verify its settings. After 40 years, one would not be surprised to find it is not pitched per the Type Certificate.

    Since you don't HAVE any options for pitch (62" is the only legal pitch) you have to confirm yours is at that pitch or within the manufacturer's tolerance for measuring pitch. There is a 1" tolerance in most prop manufacturer's pitch measurements, so you can have a true 61" to a true 3" and be a legal 62" prop.

    Cutting the tips will also act as a pitch reduction, and there is a minimum
    diameter of 76.5" to work with.

    Bob Steward, A&P IA
    Birmingham, AL

  2. #2
    Oh dear! My prop is a BFA7764/1B235. I guess its off to the Big House for me.

  3. #3

    info needed A23-24 issues

    Sounds like you already have a cruise prop. Realize that over the years of running, propellers tend to twist towards a flatter pitch, and occasionally need and overhaul and repitching. Maybe you can get your local prop shop to measure your blade pitches and advise you. I'm ignoring that different pitch number in the model of the prop, maybe you should ask about that off the record with the prop shop.
    I chewed through my restraints for this?

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