A letter received here at the factory asked a question we have heard quite often:
"Is it a fact, or is it fiction, that engines with constant speed props should not use power settings where inches of mercury exceed RPM in hundreds? I am referring of course to non-turbocharged engines in general ."
The answer to this question is easily found in cruise power charts of the airframe Pilotís Operating Handbook. Whatever the combinations of RPM and MP listed in the charts ó they have been flight tested and approved by the airframe and powerplant engineers. Therefore, if there are power settings such as 2100 RPM and 24" MP in the power chart, they are approved for use.
The confusion over so-called "squared" power settings (i. e. 2400 x 24" MP), appears to have been a carry-over from some models of the old radial engines which were vulnerable to excessive bearing wear where a MP higher than "squared" was used. More pressure on the bearings with the higher than "squared" MP was the cause of their problem. However, changes in design, metals, and lubricants permit changes in operation in the more modern flat opposed powerplants.
Letís look at the power charts in a couple of the Pilotís Operating Handbooks of two different aircraft manufacturers, but where both are using the four cylinder 200 HP Lycoming engine.
Cessnaís Model 177 RG, using the Lycoming IO-360-A1B6D, in the cruise range at 6,000 feet, lists a cruise power setting range at that altitude of anywhere from 2100 RPM to 2500 RPM with variations all the way from 18" MP to 24" MP. They list a recommended Power setting for 66% power at 2100 RPM at 24" MP.
The Piper Arrow, powered by the Lycoming IO-360-C series engine, lists the following cruise power settings at 6,000 feet in their chart at 65% power at full throttle (about 23" MP) x 2100 RPM.
The complete chart for 65% power is shown as follows:
In addition to the quieter and smoother consideration, lower RPM means lower friction hp. This reduced loss of horsepower due to friction also translates to slightly improved fuel economy.
The Pilotís Operating Handbook is the basic reference for the pilot as this subject illustrates.