Its "peak RPM" for that prop knob setting.
What you do is this. You climb up, now you level out. You use your throttle to select manifold pressure. You use your prop knob to select your rpm. Now you pull the mixture out and watch the tachometer. You will see a slight rpm drop when it goes "too lean". Then you richen slightly. You are now running as lean as possible, which is ok if you are at 65% power or below.
The reason you're seeing a slight drop could be for one of two reasons:
1) Your prop governor is lagging and needs to be overhauled.
2) You're leaning so far that you're dropping out of the governing range momentarily, meaning you're so lean that you're losing a LOT of power.
Best watch the EGT gauge down lower. You leaned it right through peak and so far into the other side it was below the power level needed to spin a very flat prop.
Bingo.
It's NOT because the prop is full flat. A pilot can prove that by having the prop knob out a bit when leaning.
There is NO WAY for the pilot to definitively know whether or not the prop is at the stops other than to drop the power low enough (via either throttle or mixture) that RPM drops, in which case you're at the flat stop.
You can have the prop knob full forward and be off the stops, or you can pull the prop back a little bit and be on the stops. Here are some examples:
1) You're on the runway, ready for takeoff but at idle so you're on the stop. You increase the throttle to full with the brakes on so you don't move, you'll develop just slightly less than full rated RPM in most instances. You're still on the stop. You let go of the brakes, and as the plane starts to move, the RPM will increase to the full value. *As soon as it hits full RPM*, if you keep accelerating, the prop will begin to increase pitch to maintain that full RPM. Your prop knob is still full forward, but the prop is no longer on the stop.
2) Do the same thing as above, except before brake release, pull the prop knob back just far enough that the RPM goes down. You're now off the stops. Pull the throttle back until the RPM drops more. Your prop is now at the flat stop even though your prop knob is not fully in.
(Note: This is a thought experiment... You don't have to actually do this, as you still don't have a "prop pitch gauge" or any other way to see whether you're actually on the stop except for knowing the systems. I take no responsibility for anything you do to your engine.)
What I described is what the propeller does. The valve that makes that happen is called the "pilot valve". If that didn't happen, we wouldnt be able to lean via rpm drop. And pilots can lean to rpm drop, thanks to the "pilot valve". Why do you think they call it the pilot valve?
What you described is your misunderstanding of the constant speed prop system. You can't lean to peak power via RPM drop in a plane with a constant speed prop unless the prop is at the stop, which you only know to be true if the plane is standing still on the ground, the prop knob is full forward, and you are developing less than full rated RPM.
Back on topic, if we can disregard the "Pilot Valve" or "Pilot's Value" part of the discussion for a bit ... is there something to leaning for best power by having the prop out just slightly and watching for the RPM drop?
No.
The truth is, when you reduce power, you will see a small rpm drop and then the rpms will try and climb back to where they were. It is least noticeable at high power and most noticeable at lower powers. You often hear the argument that the rpms are dropping because it is "at the stops", but it will do it when not at the stops also.
If you notice the prop dropping and returning to its previous RPM, either you have a problem with your prop governor, or your engine. The only time I have ever seen that behavior is when an engine started making a lot of metal, to the extent that there wasn't sufficient oil pressure for the prop governor to do its job as quickly as it should.
You can prove that it is not at the stops by turning the prop knob out a bit first to make sure you are NOT at the stops.
As explained above, just because the knob isn't full forward, doesn't mean you're not at the stops.
Notice I said that this rpm drop occurs when you reduce POWER. It will do it when you back off the throttle also (do it at less than full power so you can notice it). Leaning reduces power also. So it does it when you lean also.
And in both cases, you either have a problem with your prop governor (if it returns to the selected RPM without you increasing power or richening mixture), or you're dropping power so low that you're below the governing range (if the RPM stays low when you don't increase power/mixture).
Notice that when you lean to reduce power, you do NOT see a drop in manifold pressure.
Nor should you expect to. That doesn't mean that you're developing the same amount of power.
ALL THREE controls affect the amount of power, as do a number of other things, like mag timing. Reducing MP via the throttle reduces the amount of air coming into the engine, thus limiting power. The mixture does the same thing with the fuel side of the equation. To a much lesser extent, the prop control affects engine power as well via changing airflow and limiting the number of power strokes per minute.
You can't look at a single gauge to determine the amount of power you're making (although some fancy engine monitors show a calculated % power approximation). When you're rich of peak EGT, power is limited by airflow. When you're lean of peak EGT, power is limited by fuel flow. When you're near peak EGT, you may have some cylinders ROP and some LOP.
The pilot valve puts this "lag" in rpm in the system so the pilot can lean the engine properly. Like all mechanical control systems the rpms hunt, lag and maybe overshoot etc. and that can be controlled to some extent. Good thing it does that so we pilots can lean to an rpm drop, then richen slightly so we know we are running at or very near max lean.
The only thing that is true about that paragraph is that if you run just slightly rich of where you see an RPM drop, you are definitely "running at or very near max lean." You're also leaving a lot of potential power on the table, as you're running at a similar power setting to if you pulled the throttle back until the RPM dropped and then pushed it in just slightly. That's significantly less than 50% power on my plane, and I'd prefer to be closer to 65%.
I'm only partway through it and feel like I'm learning all over again.
