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Cruise RPM

bob888

Well Known Member
At a constant %power/constant fuel flow I get higher TAS at higher RPM, which seems counterintuitive. For example, at 10,000 ft and 60% power/11 gph my TAS is 172 kt at 2200 RPM and 175 kt at 2500 RPM. Similar result at 5,000 ft. I conclude that one should cruise at high RPM...have others found similar results?
 
How exactly did you do this? At 11 gal/hr and 2200 rpm, if you increase rpm to 2500, the fuel flow would increase. Did you bring this back to 11 with the throttle, or mixture?
 
RPM = $'s

Also, just for weighing on on the discussion, remember that RPM = $'s.

For every hour of engine, this goes to TBO of the engine. 300 RPM adds to the cost of the engine as it reduces it's life.

Just some food for thought.
 
At a constant %power/constant fuel flow I get higher TAS at higher RPM, which seems counterintuitive. For example, at 10,000 ft and 60% power/11 gph my TAS is 172 kt at 2200 RPM and 175 kt at 2500 RPM. Similar result at 5,000 ft. I conclude that one should cruise at high RPM...have others found similar results?

Yup, I get around 178 kts at 2500, but only about 172 at 2400.

-Marc
 
To answer Bob Turner, fuel flow was kept constant with mixture; this was all done LOP.

I think this is the wrong procedure. I think you should have increased rpm, then decreased MP, until fuel flow was back to 11 gal/hr. The MP/RPM settings should match the IO-540 power chart so that both the higher and lower rpm settings were producing the same power. Then any airspeed change would be due to prop efficiency changing. I suspect you were a little rich of peak with the mixture. You need to be well lean of peak for power to be linear with fuel flow.

For the comment about TBO: True tach time runs faster at higher rpm. But here in the US TBO times are not mandatory, and many factors contribute to when an overhaul is really needed.

As to Marc’s comment: Your numbers are borderline unbelieveable, just from a basic physics perspective (power required proportional to cube of airspeed, etc.) Something else is going on.
 
R/M

Ive always been of the opinion that high rpm increases engine wear compared to lower rpm.
The relatively low utilization of typical EAB aircraft makes any wear from slightly higher r/m irrelavent.
For those who fly 50 hours a year it will take 44 years to reach TBO. In many case the engine will sustain significant damage from rust long before reaching TBO.
 
Ive always been of the opinion that high rpm increases engine wear compared to lower rpm.

May be, but it's not that obvious. Remember, we're talking about keeping the power constant, so at higher RPM the internal combustion pressures will be lower. Less force on the connecting rod to crankshaft bearing, less force on the piston ring to cylinder wall, etc.
 
What RPM is peak torque?

High RPM would have the greatest piston ring wear.

But I would think peak torque would put the highest stress on the engine.
 
The relatively low utilization of typical EAB aircraft makes any wear from slightly higher r/m irrelavent.
For those who fly 50 hours a year it will take 44 years to reach TBO. In many case the engine will sustain significant damage from rust long before reaching TBO.


Kind of. Don?t forget TBO is based on hours or years, whichever comes first.
 
Bob, I've pondered the same question, here is what I have:



The graph shows mixture sweeps at 2000 to 2400 RPM. DA about 10,000 ft., dead stock setup. I was interested in efficiency...nmpg, so I plotted the isopleths for reference. If you don't care about airspeed, the highest efficiency is at low RPM. Keeping in mind that fuel flow isn't quite the same as power - air fuel ratio may be changing too - the graph also shows that on the rich end of the spectra, speed is increasing with RPM, constant fuel flow. It doesn't hold up though as mixture is leaned. At the very lean end there is a suggestion that speed increases as RPM declines.
 
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As to Marc?s comment: Your numbers are borderline unbelieveable, just from a basic physics perspective (power required proportional to cube of airspeed, etc.) Something else is going on.

I know, huh. I was surprised too! I was at 10500', throttle fire-walled which was (IIRC) about 22 inches. I leveled off and started to lean to my normal ROP setting, saw the speed was faster than my normal 172 kts, then brought the RPM down to 2400 from my 2500 climb RPM. I slowed down! I went back to 2500 and saw the speed go (slowly) back up to 178. I'll play with it again when we do another XC in a couple of weeks.

-Marc
 
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