It seems to me that you're concentrating an awful lot on prop RPM, when that is but one component of power. Manifold Pressure is even more important - And if you don't truly understand what MP is (and I wouldn't blame you, since the FAA materials on this absolutely suck), go directly to this article and read it (do not pass go, do not collect $200 unless you blow it at the FBO):
Manifold Pressure Sucks!
When operating rich of peak EGT, which you probably are in the carbureted Skylane, power is limited by the amount of airflow into the engine. 100% power would be 29.92" of manifold pressure (sea level, standard day) and 2600 RPM. At 10,500 MSL, if you're at full throttle and 2450 RPM, you should be getting maybe 19" MP, so (19/30)*(2450/2600) = approximately 60% power.
Now, if you operate at 19" MP and 2450 RPM at 3000 feet, you'll also be developing roughly 60% power, but you won't be at full throttle if you're at 19" MP at that altitude. But, if the power setting was truly 60% at each of those altitudes (3,000 and 10,500), you could expect to have:
- A faster True airspeed at 10,500, since you have roughly the same thrust (thanks to the constant speed prop and the same power setting), but lower drag thanks to the thinner air.
- A similar indicated airspeed. Thrust and drag will always seek an equilibrium, so with the thrust being roughly equal, the amount of air hitting the plane (and the pitot tube) will also be roughly equal.
- A higher prop pitch: In the thinner air, with the same amount of power, there would be less resistance at the same pitch. The governor will thus have increased the pitch of your prop blades to compensate.
Now, we don't normally operate at 19" MP down low - For example, I usually run 24" MP and 2300 RPM in my plane. Looking at the various altitudes:
- Below 6,000 feet, the throttle will be farther out the lower I am because of the denser air.
- At 6,000 feet, I'll be at full throttle to get my 24" MP.
- Above 6,000 feet, I'll still be at full throttle, but MP will decrease with altitude.
I would dispute that. The air is thicker down low, so the prop develops the *same* thrust using a *lower* pitch, assuming the engine is developing the same amount of power. It's the same as your wing: If you keep the same true airspeed (prop RPM), you'll need a higher angle of attack (pitch) to maintain the same amount of lift at a higher altitude because the air is thinner.
As you go higher, the prop will increase pitch to maintain the same RPM.
Correct.
Again, though, I feel like you're focusing really heavily on the prop here, when manifold pressure contributes at least as much to the power output as RPM does.
________
If you want to know more, read on - Otherwise, go absorb what I said above first. This is just fluff below:
You may have noticed I emphasized "roughly" the same power setting above. That's because, at the exact same MP, you'll get slightly *more* power up high than you will down low. Why? Pumping losses. This will make lots more sense once you read that article linked above, but using that 19" MP as an example, up high you'll have the throttle wide open, while down low you'll need to close it to keep the MP down at 19". When you close the throttle, you're restricting the airflow past it and "sucking" the air past the throttle, essentially turning your engine into somewhat of a vacuum pump, using the intake strokes of the cylinders to suck that air past the throttle to reduce the ambient pressure (29.92 on a standard day, or whatever the altimeter setting is) down to the lower (19") pressure in the intake manifold.
This is part of why, if you see two power settings for the same % power, the one with the higher manifold pressure and lower RPM is the more efficient one. For example, at 9,000 feet I could develop 65.77% power at full throttle (21.39" MP on a standard day, in theory) and 2300 RPM, *or* I could theoretically develop the same amount of power (same airflow) at 19.68" MP and 2500 RPM. However, I'll actually get more power at full throttle and 2300 RPM for a few reasons:
- I won't have the pumping losses from sucking air past the throttle. OK, again, I say "in theory" because even a wide-open throttle still has the plates there and parallel to the airflow so there's a small restriction there still. I should point out that there are also still pumping losses from pulling air through the intake air filter as well, generally about a 1" decrease in MP from that alone. These are the reason why, even at full throttle and sea level, you will not see your MP as high as it is when the engine isn't turning.
- Less friction inside the engine. Piston rings on cylinder walls, connecting rods rotating at both ends, crankshaft riding in its bearings - All of these will be happening slower and thus we'll be losing less energy to friction.
- Less parasitic drag on the prop, since it's spinning at a lower RPM. Big props can be especially susceptible to this, since prop efficiency drops dramatically above a tip speed of 0.85 mach. I should say, at that point they become more efficient at generating noise than thrust.
Anyway... Have fun learning and flying!
