Prop on Top: Rule, Guidline, or Myth?

Recently, I had the opportunity to fly a brand new 182 still in the engine break in phase. It had everything a pilot could want in it: full glass cockpit, autopilot, air conditioning, heated propeller, oxygen, ADS-B in, and was even turbocharged.

However, while reading the POH on the best procedures for engine break in, something caught my eye. Among learning to lean the mixture using the turbine inlet temperature gauge and other things, unless my memory is at fault, it actually recommened keeping the manifold pressure (MP) higher than the propeller RPM. While flying the 182 with someone more experience than I, he also recommended using the higher MP setting. While in cruise, after walking me through all the differences with the plane and best practices while flying it, he asked if I had any questions. And of course, the first thing out of my mouth was, "Why are we keeping the MP higher than the RPM? I thought keeping "prop on top" was a best practice for reducing engine wear?".

For those that have not gotten their complex endorsement, or have never flown a plane equipped with a constant speed propeller, here is what you need to know. Power in a plane with a constant speed prop is still controlled by your throttle, but it is no longer measured in RPM. Now it is measured in MP (manifold pressure). Manifold pressure is the measurement of air pressure in the intake manifold, measured in inches mercury, before it goes to the cylinders for combustion. RPM (revolutions per minute) is strictly referring to how fast the propeller is spinning, per minute. Manifold pressure can be adjusted by your throttle, and RPM can be adjusted by your blue propeller levers. As you adjust manifold pressure, whether it is an increase or decrease in power, your propeller changes pitch to obtain that RPM that was set.

For example, let's say my MP was 18", and my RPM was set at 2,000. If I increase my MP to 20", the engine is now going to create more power, wanting to spin the blades of the propeller faster. However, I have already set 2,000 RPM for the speed of my prop. What my propeller does in turn, is increases its pitch so that it is able to slow down. As it encounters more air, it is able to return to the 2,000 RPM that was originally set. Violia, the magic of the constant speed prop! Another benefit of the constant speed prop is, as I reduce the RPM as such, I can increase fuel efficiency by producing more thrust with the propeller's greater blade angle, while reducing manifold pressure.

What has been taught to me is to keep "prop on top". In other words, keep the propeller RPM higher than what manifold pressure is set at. The terms undersquare, square, and oversquare describe this best.

Undersquare (keeping prop on top): 20" MP, 2300 RPM

Square: 23" MP, 2300 RPM

Oversquare (not desirable for long periods of time): 23" MP, 2000 RPM

Now as I'm writing this article, and after reading the POH, I think back to how the POH did not mention once the adjustments of the MP in relation to the RPM. There was no talk of "undersquare" or "oversquare". To answer my question previously of why we didn't keep "prop on top" in that situation, I found this great article by Mike Busch (flight instructor and mechanic).

https://www.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-10_flying-efficiently.pdf

After a quick and interesting read, it opened my eyes to the world beyond "prop on top" and the interworkings behind the manchinery. The article is focused on getting the best bang for your buck in aircraft and engine performance, taking a deep dive into it all. It actually paints "prop on top" as the opposite of what we want to do. I found it all quite interesting, and I am really not sure as to how I didn't see the benefit of oversquaring before. As always, I will continue to do my due dilligence and research best practices from POH to POH, FAA guidlines, or seek out advice or wisdom from mechanics around the airport.