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Thread: Constant speed prop question

  1. #11
    Jim Hann's Avatar
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    Todd,

    Not to sound preachy, but if the performance changed that much with CG shift, I'd recheck all of your numbers for your weight and balance! That is a huge change, be careful!

    Jim
    Jim Hann
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  2. #12
    Chad Jensen's Avatar
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    We'd all love to see more reports of your improved performance after the switch! I think Jim is spot on with the swap you made. Keep the numbers flowing!
    Chad Jensen
    EAA #755575

  3. #13
    Hank's Avatar
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    Just be careful adding lead to the tail. Nose heavy flies poorly, but tail heavy flies ONCE! double-check your math, and reweigh to make sure you got it right. And attach it firmly, too.
    — Hank
    1970 M20-C

  4. #14

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    Todd, Hank is absolutely right. If you are a little nose heavy it just flies slower, climbs slower, and is nose heavy in the landing flare.
    If you make a mistake on your cg figures and get it past the aft cg safe limit, it is dangerous. It may fly fast, be light on the stick force, and land easy, BUT IT MAY NOT RECOVER FROM A STALL EASILY AND ALMOST CERTAINLY WILL NOT RECOVER FROM A SPIN. You can't get the nose down to get the airflow over the controls.

    So proceed very carefully, perhaps do several test flights and stalls, starting just a little less nose heavy, and slowly move the cg aft. Leave some margin. And maybe wear a chute until you have it worked out.

  5. #15
    Todd copeland's Avatar
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    Good advice all. I agree with the aft CG worries. Frankly, I wasn't that concerned with the forward gc as it is not nearly as dangerous. The figures I have now with no change is still within 2 inches of the forward limit. Still, there is a lage difference in feel of the plane and the performance as described. There are glisters out there with 180hp and constant speed props that must be moving there battery much further aft than I have it. Hopefully this weekend I get a new set of wieght numbers that I am 100% confident in and the I will keep it on the scales and add the appropriate weight to the tail to get a more ideal CG. My calculations show it will take about ten ponds but I will be absolutely sure and the lead will be well secured. I have read stories about weight moving around in a flying airplane and they never end well. I will post the results here.Todd

  6. #16

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    Hello and any updates Todd?

  7. #17
    Todd copeland's Avatar
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    Now that you ask, I do have an update. I bought new scales to use with the "aircraft scales kit" from aircraft spruce. The design of them is fine but the scales themselves were worthless. We were never happy with the results from them when we did the original w&b as they seemed to be unreliable and inaccurate. The end result is that our glastar is about 25 lbs heavier than we thought it was. As far as the balance issue we were within the forward limit but we were close to it. We experimented with weight in the tail while it was on the scales and ended up putting 12 lbs of lead back there. We are still toward the front on the CG but it gives us a generous baggage weight capacity. Performance is excellent now. Performance takeoff in about 400 feet at gross, climb between 1000-1400 fpm depending on condition and weight. The biggest improvement is in cruise as we hit the book numbers now and can cruise at 132knots. Bottom line is that while we were within CG limits we should have know exactly where it was before testing the new prop. Also, I have learned how much the CG can affect performance in an aircraft even within the acceptable range.Todd

  8. #18
    FlyingRon's Avatar
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    Hopefully you were doing the weighing inside?
    One of the things that bothered me about Cringley's attempt to build a homebuilt (the second time when it was a Fisher) was he did all his weighting outside in a breeze.

  9. #19
    Todd copeland's Avatar
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    Inside and leveled with the hanger door closed. Easy to watch the different scale readings bushing here lifting there... Yes, outside with even a puff of wind would vary the readings a lot.

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