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Too Much MP LOP


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I'm a bit confused about high MP low RPM LOP cruise. Let's say you want to cruise at 55% power at low altiude and have high MP available. It seems a bit out of line to be 2200RPM and 29" MP? What is all that excess air doing when fuel is being limited to the necesary required lower fuel flow? Do you guys EVER find the need to throttle back while LOP in normally aspirated airplanes? What else do I need to know about making use of available excess MP when LOP?

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Sometimes when the density altitude is low (winter)  or the baro is high, I occasionally have to limit MP to 27" in order to get it to run smoothly below 8-9 GPH down low (below 2000 DA).  The excess air not used for combustion goes for cooling, it passes through the engine. 55% power is 7.3 GPH.     FWIW my standard low altitude (<3K) cruise is WOT, 2500 RPM, and 10 GPH.  Slower is 2300 RPM and ~7.5-8.0 GPH.  Loiter is 15", 2200, and 4.7 GPH (peak).

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Quote: 201er

I'm a bit confused about high MP low RPM LOP cruise. Let's say you want to cruise at 55% power at low altiude and have high MP available. It seems a bit out of line to be 2200RPM and 29" MP? What is all that excess air doing when fuel is being limited to the necesary required lower fuel flow? Do you guys EVER find the need to throttle back while LOP in normally aspirated airplanes? What else do I need to know about making use of available excess MP when LOP?

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Mike, about 2-3 KTAS per 100 RPM reduction.  FF goes down, so does speed. However, it does improve range slightly  and it is quieter. But your fixed costs still roll on by the clock hour, so the cost per mile really doesnt change much.  Decide how fast you want to go. Then pull RPM and FF to get that speed. If still too fast, then consider reducing MP.

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My 2nd question is in regards to efficient high speed cruise rather than for going slow. How do you choose an RPM for maximum efficiency while LOP? You could do less RPM or more RPM and less on the mixture to achieve equal fuel flow. How do you choose?

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You're after the prop efficiency, which varies by propeller of course.  With our normally aspirated Mooneys, we're primarily limited by MP at efficient cruise altitudes (7-9k feet).  You can run whatever RPM you wish, and at typical cruise altitude any RPM less than 2700 will reduce power since we're already WOT.  


To figure out your most efficient RPM, I'd try a lower altitude (say 5,000') and set up a LOP cruise at 2600 RPM and whatever LOP fuel flow setting you wish (perhaps 9.0 GPH).  Let it stabilize for several minutes in smooth air and record RPM and airspeed.  Reduce to 2550 and re-lean to 9.0 GPH, wait several minutes, record the data.  Repeat for 2500, 2450, 2400, etc.  That data will produce a nice curve for that altitude, and it will vary a bit at different altitudes.  Plot it and you'll find your most efficient RPM for a 9.0 GPH setting at 5,000'.  


I don't know what it is for my 201 (with an MT prop) but plan to do this test soon.

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here is the info for an early M20J with 20 degrees of timing, 30 LOP, 6500' (8800 DA)', and 20C. 


Changes since then are 25 degeees of timing, better gear door and controls rigging, and SOP of more like 15 LOP. Add 8-10 MPH IAS to these to find the airspeed in our buggy.


Notice that between 2500 and 2400 RPM, you lose 1 KTAS. Then every 100 RPM costs you 2-3 KTAS.  MPG goes up a half NMPG per 100 RPM, but fixed hourly costs (28$ an hour) eat most of that.  So, if you got time to kill, you can save a few bucks an hour.  The real value in flying at peak efficiency is the increased range.  Especially in a 54 gallon bird.  Skip a fuel stop and your day goes much faster. 

post-6498-13468141043051_thumb.jpg

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