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Thread: Sport Cub POH Power Setting Table

  1. #11
    Member Steve Hamblin's Avatar
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    Default Re: Sport Cub POH Power Setting Table

    One interesting note on the O-200D vs O-200A, the D model is 8.5:1 compression verses 7:1 on the A....

  2. #12
    Member reileyr's Avatar
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    Default Re: Sport Cub POH Power Setting Table

    Thanks for the catch. The attached power curve is for the old 0-200A. The 0-200D produces more power. At 2750 RPM, the 0-200A will make about 98 horsepower. At 2750 RPM, the 0-200D will make about 105 horsepower because it is a higher compression engine.
    Last edited by reileyr; 02-04-2010 at 02:04 AM.

  3. #13
    Senior Member Pilawt's Avatar
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    Default Re: Sport Cub POH Power Setting Table

    The only way to reliably calculate cruise power setting is with the "Altitude Performance" chart in the O-200 manual (Fig. 10). It accounts for all the variables in reileyr's post, and produces a valid BHP value.

    Unfortunately there are two problems associated with this chart. (1) Manifold pressure is a part of the calculation and most of us don't have MP gauges in our Sport Cubs; and (2) even if one had a MP gauge the format of this chart is unworkable for use in the cockpit. At cruise I'm interested in finding a power setting that will provide the preferred combination of speed and range without undue stress or wear on the power plant. For me that's usually 65% - 75% power, which is about 5-6 gph in an O-200-A properly leaned for cruise. Thus a handier, if less precise, cockpit guide is helpful.

    The TCM Altitude Performance chart indicates that the Sport Cub POH cruise power setting table is not accurate. In the example I mentioned before, the POH says 1950 rpm @ 2000' = 63% power. According to the TCM table, manifold pressure would have to be almost 27" to produce 63% at that rpm and altitude. I'm sure there's no engine-prop combination on a Sport Cub that would produce a 27"/1950 rpm condition, nor would one want to. So the true power at 1950 rpm @ 2000' has to be much less than published.
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    Jeff Jacobs
    Vancouver WA / KVUO
    C-172N-180


  4. #14
    Member reileyr's Avatar
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    Default Re: Sport Cub POH Power Setting Table

    "The only way to reliably calculate cruise power setting is with the "Altitude Performance" chart..."

    A simple approximation is often more practical than an exact calculation. Fuel burn in a well tuned engine is a pretty good estimate of engine power since there are no free lunches. It is easy to get a feel for different power settings by keeping a record in the pilot log book of fuel burned per hour every time the tanks are topped off.

  5. #15
    Member reileyr's Avatar
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    Default Re: Sport Cub POH Power Setting Table

    Further to this discussion of percentage power in cruise, I have attached a propeller load curve for the 0-200A. This curve is for a propeller that will allow the engine to reach max 2750 rpm at full throttle. In cruise flight, advancing the engine throttle full open will generate this power curve and fuel burn for a propeller that reached but does not exceed 2750 rpm.

    In climb with wide open throttle (WOT), power is calculated using the Altitude Performance Curves that Jeff Jacobs provided in his post.

    The operating information in the Sport Cub Pilot Operating Handbook is not wrong, but information is out of context and leads pilots to the wrong conclusion about the amount of power being produced by the engine. As Jeff correctly stated, for the 0-200A to generate 63% power at 2000' and 1950 rpm as shown in the POH, the propeller pitch would need to be coarse enough to stabilize at 1950 rpm in level flight with full throttle. Even thought the percentage power information in the POH is technically correct, it is for very different load conditions than our engines are subject too. Advancing the throttle in our Sport Cubs immediately pushes the propeller through 1950 rpm so there is no way the engine/propeller combination installed in Sport Cubs can ever meet the conditions published in the POH.

    The attached propeller load curve is accurate for percentage power produced in a Sport Cub, but only for a propeller that stabilizes at 2750 rpm with WOT. A flatter pitch propeller that exceeds the 2750 rpm limit will require the throttle to be partially closed to prevent exceeding redline, thus reducing engine power output. Constant speed propeller achieve ideal operating conditions by letting the engine produce maximum power without exceeding redline. With fixed pitched propellers, about the best we can manage is 2550 to 2625 rpm in climb. A propeller pitched to operate at rated engine rpm in climb is just too easy to inadvertently exceed redline if the airspeed is allowed to increase. The compromise we accept for the simplicity of a fixed pitch propeller is that the engine is not going to make its full rated horsepower in climb.

    Personally, I normally run a fairly flat pitch cartridge allowing the engine to run at a higher rpm (make more power) during climb. As soon as I begin to level out, I must immediately reduce power to prevent the engine from exceeding redline. I can still cruise the high speed, but at a higher engine rpm. If I'm going on a long XC flight, I change to a coarse pitch cartridge so I can run WOT at altitude without exceeding redline. I love the performance and versatility of my Sensenich Ground Adjustable propeller. The performance improvement over the standard wood prop is transformational, especially with a 46 pitch cartridge.

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    Last edited by reileyr; 02-06-2010 at 09:24 AM.

  6. #16
    Senior Member Pilawt's Avatar
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    Default Re: Sport Cub POH Power Setting Table

    Richard, thank you for the excellent explanation. Bottom line, the numbers in the Sport Cub POH cruise performance chart do not reflect the prop load conditions experienced in level-flight cruise with less than WOT, and thus cannot be relied upon to set a desired % of power and fuel consumption for cruise in a Sport Cub. For that purpose I'll continue to use the C-150G cruise chart (attached, with irrelevant speed and range numbers redacted) as a rough guide, and fine-tune with actual experience in my Sport Cub with this prop.
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    Last edited by Pilawt; 02-06-2010 at 07:49 PM.
    Jeff Jacobs
    Vancouver WA / KVUO
    C-172N-180


  7. #17
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    Default Re: Sport Cub POH Power Setting Table

    At the risk of offending someone, this is getting a little silly. For 15 years I flew behind an O-540 with a constant speed prop and the POH told me exactly what to do if I wanted 55%, 65%, 75%, or 85% power. I understand that temperatures other than standard change things, but the POH gave me that information too. Why can't I just turn to page #? in the POH and find accurate information that will tell me that if I'm at 4,000' and I want 75% power I should use Xrpm? If CC wants to throw in the an adjustment for temperature, that would be great. Frankly, we shouldn't have to figure this stuff out on our own. Like I said, this is becoming silly.

    Larry
    Last edited by Larry@FOK; 02-07-2010 at 04:34 PM.

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