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Thread: Max Engine RPM

  1. #11
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    Default Re: Max Engine RPM

    Chuck,

    1. Could you give me the formula for mach speed versus RPM for given prop lengths?
    Warp has 2 pretty good calculators here: http://www.warpdriveprops.com/propspd2.html

    They are generic, not specific to Warp propellers and a "rule of thumb" only. I do not know of anyone that publishes specific calculators for any specific prop.

    2.Are the performance percentages a percentage of the mach one speed?
    No, but there is certainly a correlation, 65% of tip speed does not equal 65% efficient. That is specific to each prop design, hp/torque/rpm and pitch and can only be determined by testing the configuration.

    You can get some pretty good ideas though by changing the diameter, pitch and/or rpm in the calculators shown to see the theoretical change in performance.

    As others have mentioned, the 1st thing I would verify is rpm readout accuracy. I would also check to see that you are getting full range on your throttle cable at the carb and go from there.

    Greg

  2. #12

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    Default Re: Max Engine RPM

    I went over to the Warp Drive Web site and checked out their calculator. It looks like if I pitch to gain 5000 rpm at T/O, I'll really take it on the chin for cruise speed. I'll suppose a compromise is in order.
    Chuck Gruby
    Petal, Kit Fox III Flying

  3. #13
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    Default Re: Max Engine RPM

    Yes Chuck, it will always be a compromise unless you have an in flight adjustable pitch prop!

    I feel compelled to add though as you've mentioned 5000 rpm. You are totally in uncharted territory. Reasoning:

    There were at least 4 companies doing Subaru conversions with various redrives, none are currently in business. None of them and to my knowledge - no one (including Subaru) has ever published a horsepower/torque chart for that engine.

    Torque is the true measure of the real ability (grunt) to turn the propeller. No one knows where that (what rpm) occurs on this engine or if it can effectively do it with longevity at high rpm and within an (unknown) max MOI (moment of inertia).

    MOI is most critical to the redrive - basically it is the measure of stress put on the shaft and bearings based on where the mass weight of the propeller occurs.

    We don't know what redrive you have (other than 1.80/1), or if that manufacturer ever specified a max rpm and max MOI.

    Further, Ivo does not publish MOI's for their props - they simply say they are "flexible" - that's fine for the prop, but says nothing about the stress put on the redrive. You could calculate the MOI from here: http://www.ul.props.fr/MOI/rotax_MOI.pdf it is from Rotax, but is the same for all props. Useless info though unless you know the limits of the engine and redrive combo.

    I know there are plenty of Subaru installations out there, some also in Avids and Zeniths primarily - you might try some of those forums too. Maybe someone with an installation similar could chime in.

    Good luck, but be careful at high rpm given all the unknowns here.

    Just curious: After the previous owners prop strike, was the engine and redrive completely torn down and inspected?

    Greg

  4. #14

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    Default Re: Max Engine RPM

    OK here’s what we’ve got: According to Warp Drive’s calculation:
    5000 RPM yields 563 mph tip speed (73% of mach 1) (speed of sound 767 mph).
    The prop was at 16 degrees, I adjusted it to 14 degrees. We’ll see what the next flight brings.
    Chuck Gruby
    Petal, Kit Fox III Flying

  5. #15

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    Default Re: Max Engine RPM

    Today I went out to fly and damaged my rudder getting the plane off the trailer, so I'm out for a while. Grrrrrrrrrrrrr!!!!
    Chuck Gruby
    Petal, Kit Fox III Flying

  6. #16

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    Default Re: Max Engine RPM

    I got the rudder repaired, and it turns out it was due. The tubing had several rust holes in it and evidence water had stood at times 2 inches deep. I cut out the old and spliced in new, recovered and repainted. This time I put in a couple of drain grommets.



    My original problem was that my engine only turned 4,000 RPM on T/O. Feeling I should be turning 5,000, I checked the prop pitch and found it to be 16 degrees, so I turned the adjustment 1 turn ccw and came up with 14 degrees. I went out and flew today and now see 4,400 RPM on T/O. I couldn't tell from a pilot's standpoint that there was any change in performance, so I probably won't adjust it any more than that.


    Thanks everybody,
    Chuck Gruby
    Petal, Kit Fox III Flying

  7. #17
    Senior Member kmach's Avatar
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    Default Re: Max Engine RPM

    Ivoprops are easy to adjust ,so why not try some different settings. There will be a sweet spot to your liking at a certain setting.
    Kevin,

    Kitfox Outback
    912 ULS
    Airmaster AP332CTFH-WWR70W
    Summit Aircraft Wheel Skis
    C-FOXW

  8. #18

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    Default Re: Max Engine RPM

    Thanks Kevin, good advice.
    Chuck Gruby
    Petal, Kit Fox III Flying

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