Hi Rod,
Here's a question that I can't seem to find the answer to (no matter who I ask):
Thanks,
Douglas Fink
Greetings Douglas:
Engine vibration is one reason pilots like to have choices between RPM and manifold pressure (MP) settings. Some engines just don't run smooth at high RPMs, while others run rough at lower RPM settings. Vibration depends on many things: the type of engine, the type of propeller, the condition of the propeller as well as the airframe, engine mounts, etc. With variable RPM and MP settings available to you, choose the combination that provides the desired power and smoothness of engine operation.
Additionally, the propellers on some airplanes produce a lot of noise at high RPMs. Excessive noise is not only an annoyance, it's also associated with a decrease in propeller efficiency. Select the RPM-MP combination that reduces the noise to acceptable levels.
A common question asked by pilots is whether it's best to operate at high MP and low RPM or low RPM and high MP in cruise flight. Looking at the accompanying power chart excerpt, you'll notice that either of these combinations can produce the same engine power output. (This should dispel the notion that operating "over-square" is harmful to the engine. It's not. As long as you remain within the limits provided by the engine and airplane manufacturer you shouldn't cause the engine any harm. In fact, the accompanying chart shows that 2,300 RPM and 25 inches of MP is an acceptable cruise setting for this airplane.)
In cruise flight, constant speed propellers operate more efficiently in the high angle of attack or low RPM configuration. This explains why a fixed-pitch cruise propeller is built to have a slightly larger angle of attack than a fixed-pitch climb propeller.
During climb, however, you want maximum power production. Since engines produce maximum power at their highest allowable RPM, the constant speed propeller is operated at its smallest angle of attack. This explains why a fixed-pitch climb propeller is built to have a small angle of attack.
Therefore, all other things being equal, you want to operate at lower RPM values in cruise flight and higher RPM values while climbing. Fortunately, most cruise performance charts identify engine and propeller operating efficiency in terms of fuel flow and estimated true airspeed. If your chart doesn't provide these values, then follow the lead mentioned above.
Although not too common, it's possible that a specific RPM-MP combination might produce unacceptable levels of engine heat. Varying these settings might reduce the heat to less critical levels.
There doesn't seem to be any significant evidence suggesting that operating at lower or higher cruise RPM values increases engine life.
I hope this helps.
Best,
Rod
For more information on this subject, see "Power Management" and "Engine Care 101."
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