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Thread: Constant speed prop question

  1. #1
    Todd copeland's Avatar
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    Constant speed prop question

    I just changed the prop on my glastar from a fixed pitch sensenich prop pitched more on the climb side for a hartzell constant speed prop. I have an io320b1a 160hp. My question surrounds the climb performance. While I have a minor adjustment to make on the governor to up the toms from 2610 to hit 2700, the climb performance of my plane is way down. I regularly saw 9-1400 fpm and now the most I can manage is 5-600fpm. The cruise is improved but I could have improved the cruise and sacrificed the climb by have my fixed pitch changed for a cruise pitch. Could that 90rpm adjustment make that much difference? What am I missing, this is really disappointing.Todd

  2. #2
    Chad Jensen's Avatar
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    Were the conditions the same for comparison? Really hot? 90rpm won't make a huge difference, but may be noticeable...though it won't make a 4-500fpm difference. Where do you pull your rpm indication from? Did something change there? Just thinking out loud here...
    Chad Jensen
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  3. #3

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    Did you happen to get the manifold pressure being produced by your engine while you had the fixed prop? How does it compare to the MP now? If the engine's putting out the same power, then I suspect that you're not getting the fine pitch you should be getting.

    One more thought: have you used a strobe-type tachometer on the prop during a static run-up to verify the aircraft's tach?
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  4. #4
    FlyingRon's Avatar
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    Also, that Hartzell prop weights a lot more than your fixed pitch. If your prop was pitched for climb before, all things being equal, you'd expect to see some decline. Also, if you did nothing else to fix the balance, you're using a lot more elevator down force (and hence drag) to counter hanging that additional weight pretty far out on the arm. When we hang big engines on Navions, we either move the battery back farther in the tail or add a few pounds to the very aft of the aircraft to counter.

  5. #5
    Jim Hann's Avatar
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    Another out loud thought, what RPM were you pulling in a climb with the fixed pitch prop? Was it greater than 2700?

    Jim
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  6. #6
    Todd copeland's Avatar
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    Thanks for the thoughts, the rpms were lower with the fixed pitch prop. We are investigating the cg further. While we haven't added that much to the nose, it is all at the nose! It also feels nose heavy, and if I can feel it it must be a factor. Reading the manual it states the glaster is very sensitive to the forward cg. I weighed the airplane today and I am working it out.Thanks,Todd

  7. #7
    Chad Jensen's Avatar
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    I'm curious as to the actual weights between the two props, including the governor for the CS...just for an actual data point to be out there.
    Chad Jensen
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  8. #8

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    Things I don't think it is

    1. Weight. C'mon - 40 pounds makes that much difference? I don't think so.
    2. CG - yes, with more weight on the nose there must be more downforce on the elevators, but that sounds insufficient to account for the difference. Gut check, no math.

    Let's look at the reported data:
    Improved cruise, worse climb. If a CS prop does that when compared to a FP, you have to look at the area and CL of the blades. However, a CS should be able to adjust enough to overcome that. So, is anyone looking at the governor itself?

    I took my "transition training" in an RV-6 with CS prop. The takeoffs were dramatic. My 7A with FP prop set up for cruise has a decent climb rate - around 1500 fpm, but that's at least 500 and maybe 1000 fpm less than the -6 on the same 180 HP. These reported results are the exact opposite of what anyone would expect.

    And I do agree that you have to compare performance on a similar day. Hot air can make a huge difference. That said, the hot air won't make the cruise any better.

  9. #9
    Eric Page's Avatar
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    More out loud thoughts...

    - I'm not familiar with how CS prop blades are mounted in the hub, but is it possible that one or both blades could be mounted at an improper angle of incidence? is there any vibration that might suggest an asymmetry?
    - Was any other work done at the same time that might effect engine performance? I'm thinking of things like carburetor work or throttle/mixture cable adjustments.
    - Are you certain that the prop control cable isn't binding? Even if the governor is working properly, a bound cable might prevent it achieving full fine pitch.
    - I presume the cowling was off for the installation. Could there be a loose piece of baffling in the induction airflow path? A rag left behind?

    Good luck! Let us know what you find.
    Eric Page
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  10. #10
    Todd copeland's Avatar
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    CG seems to be the issue. Did a new weight and balance and with the new prop I am at the absolute forward acceptable point in the CG. For a confirmation of results I flew with 50pounds in the baggage compartment to move the CG aft and I climbed at 3-500 fpm better. Also got another 5knots in cruise. Now I will agg some weight as far aft as possible to get a more acceptable range for the airplane. So there you have it, forward CG WILL hurt performance. I should have done a new weight and balance before flying it and shame on me for not doing it. I knew it would shift the weight forward but not that much and I wasn't afraid of a forward CG like an aft one.Todd

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