Running props over square?

Roger said:
IIRC his CHT gauges are in degrees Celsius. According to Google, 180° C comes out to about 360° Fahrenheit. Another oddity of the Commander...
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Yeah it's in Celsius. The commander is a weird plane, but it flys real good.
 
Here's something for the "prop on top" people to munch on:

When the engine is not running the MP is 29-30 and the RPMs are 0. You never hear anybody complain about that power setting.
 
spbrian said:
untrue statement, at low power settings you have less pressure and at full throttle you have ambient pressure (whatever the pressure is at the altitude you are at)
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Still incorrect. This would imply that you have a 100% efficent engine(I get your point though, sorry to be nit-picky). Most NA engines are around 85-90% efficent.
 
Old wives tales are fun! ;)

As milleR already noted, our airplanes (C402C with TSIO-520s) are run constantly oversquare. Absolutely nothing wrong with it. Consult your AFM, run the airplane within the operating limitations described therein, and you won't have a problem. Food for thought: the 402C, per the AFM, has no limitation other than temperature on running the engines at 39" MP, 2700 RPM all day long.
 
mshunter said:
Still incorrect. This would imply that you have a 100% efficent engine(I get your point though, sorry to be nit-picky). Most NA engines are around 85-90% efficent.
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I notice about an inch less than atmospheric on the takeoff roll, anything significantly less will make me nervous.
 
trafficinsight said:
There is more than one type of efficiency, so you're both right ;)
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The only other "efficiency" I'm familiar with is volumetric efficiency, and I don't gather that the loss of static pressure in the intake manifold is a measure of it. According to Wikipedia, there are some normally aspirated techniques that can get volumetric efficiencies well over 100%.
 
You have thermal efficiency, which is around 20 to 30%, as you say.

You have volumetric efficiency, which varies depending on filtration, intake design, turbo or supercharging, a butterfly flapping its wings in China, and, of course, throttle position.

And you have mechanical efficiency, which is how much horsepower is lost due to friction, which is around 80 to 90% usually, but also depends on engine condition. Your average R-2800 puts out 2000 horsepower... if it's 80% mechanically efficient that means it takes 500 horsepower just to turn the engine.

 
kiloalpha said:
Out company for years and years have told us to run at 2300 rpms and 23" on the MP. Usually trues us out at 170-172kts and burns about 30-32 gph.

Now we are being told to run at 2200 rpms and 24" of MP. We are told this has no negative effect on the engine as is better for the props since they work more efficiently now. Is this true? Are we going pop cylinders and crack cases now? or is this actually going to work?

We have Lycoming IO-540s that make 290hp. Company research shows a fuel savings of 10% with a 1-2% drop in airspeed.
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No at 24 inches you can go to 1800 according to the engines chart here: http://www.lycoming.textron.com/support/tips-advice/key-reprints/pdfs/Key Operations.pdf on page 45. This shows the continuous operation safety levels for MP vs RPM difference for the IO-540 engine. I would advise going even further, utilize the limitations of the engine to maximize performance.

For instance 26" 2000 is within engine limitations and the cut of 200 rpms should drop you another couple gallons an hour. This is a better way to operate the engine, it is completely safe for the engine, it reduces wear, increases TBO (not actually but since tach is what deems TBO if you operate 200 rpm less all the time...), quiets the cabin, keeps the engine running cooler, and most importantly saves ALOT of gas.

This chart is available for every engine and IMO should be referenced before flying a constant speed aircraft. Then you can use the airplane like an airplane and maximize its utility. If Vfe is 150 do you wait to 140 to extend the flaps to be safe?
 
mshunter said:
Still incorrect. This would imply that you have a 100% efficent engine(I get your point though, sorry to be nit-picky). Most NA engines are around 85-90% efficent.
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I think you need to read this: The engine can now get all the air it wants, with the only restrictions being the filter, the small
area of the wide-open throttle plate (edge-on), and the turns in the ducting on the normal engine.
Usually, those factors will cost you about an inch of MP, or a bit less.

From this article: http://www.advancedpilot.com/downloads/prep.pdf

Manifold pressure can be best thought of as a spectrum where the center point would be ambient pressure, any lower values would be incurred from suction of air into the intake lowering pressure and any increased values would be from air being forced in such as a turbo.

When you full throttle the engine you have basically nothing restricting the engine from sucking air inside except for the filters and such described above. If you took out these filters and had an open intake with no filters bends and a straight shot into the engine you would always see MP matching ambient pressure at full power.

Just so it is said, we typically cause this suction by closing the throttle and forcing the engine to suck in harder on the intake which lowers the pressure.
 
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