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Setting propller RPM in high altitudes

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How should the propeller RPM be set in cessna (182RG for that matter) when I fly in high altitudes (in order to fly efficiently)?Chen

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>How should the propeller RPM be set in cessna (182RG for >that matter) when I fly in high altitudes (in order to fly >efficiently)? >>Chen Hi Chen,do you just want to know the best cruise speed. You can calculate the miles per gallon at different speeds. You can lean the mixture for efficiency. Or do you know the model is wrong and want to change it?That woulb be a problem which needs editing of the .air file and there is a board to support that at: http://www.avhistory.org/cgi-bin/boards/Cf...ebbbs_config.plas I don't have the answer off the top of my head. There are prop tables in there with:Rec 508 sets indicated torque vs. RPM; Rec 509 sets friction torque vs. RPM;Rec 511 sets prop efficiency vs. blade angle and advance ratio;Rec 521 sets power coef. vs. blade angle and advance ratio;HTH Ian

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To set the proper power setting for a given altitude you would need the aircraft performance charts. Since MS doesn't give us those, I just use 23" manifold pressure and 2300RPM. At high altutudes you may not be able to get 23" MP, so use full throttle and 2300RPM.

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Prop control is kinda funny 'cause you *directly* set the RPM which *indirectly* sets the prop pitch. Generally, you want a finer pitch (high RPM) during takeoff and climb and a course pitch (low RPM) during cruise.The propellor governor adjusts the prop to maintain the RPM you've set. So even if you never touch the prop control the propellor pitch adjusts to "effeciently" use that RPM at the current airspeed (and altitude!)."Go Fast" mode is the highest RPM and the highest manifold pressure. So if "efficiency" is getting there fast, leave everything at the top of the green.If you're not running in "Go Fast" mode, then you want the lowest RPM (prop setting) that gives you the desired power setting.In my Skylane, I usually run 24 inches of manifold pressure and 2200 RPM @ 6000'-7000'. I get the same power output around 23/2300 and 22/2400 (I don't have the book in front of me, so these may not be exact!).For a good article on props and prop operations see http://www.avweb.com/news/columns/182082-1.html

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""Go Fast" mode is the highest RPM and the highest manifold pressure. So if "efficiency" is getting there fast, leave everything at the top of the green.If you're not running in "Go Fast" mode, then you want the lowest RPM (prop setting) that gives you the desired power setting."Isn't that backwards? :-)High rpm, fine pitch, high power, --> takeoff Low rpm, coarse pitch, high speed, --> cruiseI haven't had my coffee yet...so I'm having trouble keeping up... :-lolDannyCYVR

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No coffee needed. You're pretty much right. The higher you go, the less dense the air, therefore you need to increase the blade angle to take a bigger "bite" of air to compensate for the lower density. If you leave the prop at max RPM, you're gonna be wasting alot of power as well as go slower.

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Ok. Thank's all.Indeed I meant fast mode (I'm not paying for the fuel...).I'm not much of an expert: what is manifold pressure and how do I adjust it in the simulator?I tried reducing RPM when @ 10000 feet and the engine sounded louder (a higher pitch??) but it seemed to give me more speed. Not sure though. Does this make sense?Chen

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Manifold pressure is the the pressure measured at the air intake of the cylinder heads. The 182 has a gauge labeled MAP, that's it."I tried reducing RPM when @ 10000 feet and the engine sounded louder (a higher pitch??) but it seemed to give me more speed. Not sure though. Does this make sense?"You reduced RPM, that means you pulled the lever. The RPM dropped because the propeller pitch increases and there's some more resistance. Now since you have a higher pitch it really makes sense the thing accelerated. And I am although even not sure about the sound and why it's sounding like it's turning faster.Etienne :-wave

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A loose analogy given to me many years ago by a flight instructor is to compare high RPM to first gear on an automobile and the lowest usable RPM to overdrive.Does that help at all (assuming we all know about stick shift cars, then auto trannies)?The aircraft performance tables are needed to accomodate different density altitudes (which in part are corrected for in reading IAS as opposed to TAS). Like an auto supposedly peaked at standard temperature now to be most efficient at 55 mph, the engine and prop are designed/tuned for best efficiency and performance but not necessarily achieving both at the same combinations.

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RiddlePilot says ...If you leave the prop at max RPM, you're gonna be wasting alot of power as well as go slower.Actually not ... From the 182 POH ...2400 + 23" @ 6,000' = 72% power, 134 KTAS, 12.1 gph2300 + 23" @ 6,000' = 69% power, 131 KTAS, 11.6 gph(23" is max throttle at 6,000')Throttle + prop at the top of the green will yield the highest power setting and the fastest speed. However, if you're looking for anything other that maximum power you have some choices ...2400 + 21" @ 6,000' = 63% power, 126 KTAS, 10.8 gph2100 + 23" @ 6,000' = 63% power, 126 KTAS, 10.8 gphNotice that effective power, airspeed and fuel burn are identical. The slower prop RPM requires greater throttle input ... the engine is trying to go faster, so the blades will move into a coarser pitch to counteract the engine. So the prop is producing more thrust for a each revolution, but the engine has to work harder on each stroke to maintain the RPM.Although the manifold pressure is higher, the lower RPM is easier on the engine and the ears.Now if you climb, you'll get to wide open throttle and the only way to maintain 63% power is to run max power and max RPM ...2300 + 21" @ 8,000' = 63% power, 127 KTAS, 10.7 gph2400 + 20" @ 10,000' = 63% power, 130 KTAS, 10.7 gphNotice, though, that true airspeed increases. So the airplane is more efficient at higher altitude, but it requires a faster turning prop to compensate for the reduced power output of the engine.Are we having fun yet :)

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So for maximum (true) speed in 10,000 feet I need 2400+20"? Is this the maximum according to the POH?

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So for maximum (true) speed in 10,000 feet I need 2400+20"? Is this the maximum according to the POH?Maximum continuous power is 80%, but you'll only be able to get that at lower altitudes in a normally aspirated engine. Top true airspeed at max weight is around 135 KTAS, so ...2400 + 26" @ 2,000' = 80% power, 135 KTAS, 13.4 gph2400 + 25" @ 4,000' = 78% power, 136 KTAS, 13.1 gph2400 + 23" @ 6,000' = 72% power, 134 KTAS, 12.1 gphYou can generate a higher percentage power down low (without turbo charging), but operate more efficiently at lower power settings up high.I usually fly my Skylane between 5000' and 7000' at 65% - 70%. For me, that seems to give the best compromise of speed vs. power vs. fuel burn.

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Skylanepilot, could you type up or post all of the cruise performance charts for your 182?

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Ouch! That's a lot of charts to type :)I'll try to scan them in sometime this weekend and I'll post back on this thread when they're available.Happy flyin'

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