I flew a CS prop airplane several hours before fully understanding what was going on. Once I got it sorted out it was easy. One pilot friend of mine has a CS prop Piper and he really doesn't understand what he is doing, or rather why...but anyway...


As was mentioned, the prop is always turning the same speed - RPM - as the engine. (for a fixed pitch prop and for a CS prop) Think of the prop as basically just an extension off the engine crankshaft. (there are some new diesel General Aviation engines which use a gear box between the engine and prop but it is not common to most GA planes)


In the airplane there is a throttle lever and a prop lever. I see your confusion on the engine power and fuel amounts etc. A concept to understand is that an engine can develop differing amounts of power at the same exact RPM. More or less fuel and air can be put into the engine at the same speed which will make it develop different power amounts. Normally we associate power with RPM but in a CS prop engine combo an engine can be set at differing amounts of power at the same exact RPM.

Power is set by the throttle which reads manifold pressure, that is the air/fuel going into the engine - some airplanes have fuel injection instead of a carb.

For a normally aspirated engine (not turbo charged) a cruise setting is commonly 22 inches of manifold pressure and 2400 RPM, but a lower cruise setting is 18 inches manifold pressure at the same 2400 RPM. The airplane will go faster at 22 inches than at 18 inches. At the 18 inch setting the engine is making less power but it is still turning the same RPM as at a higher power setting. The way the engine can turn at the same speed and have higher power is that the prop will actually be at a coarser pitch angle as compared to lower throttle setting. So the prop absorbs the higher power and converts that higher power into thrust by using a more coarse angle than at lower power setting.

Picture this: I am in cruise flight at 17 (or whatever) inches manifold pressure and cruising 110 knots. The prop is set at 2400 RPM (remember prop RPM = engine RPM) Now I increase the throttle to 22 inches and the speed increases to 130 knots. Now what happens is the engine with more fuel and air actually wants to increase RPM but the prop will prevent that by taking a larger bite of air (coarser pitch) to keep the RPM at the selected speed. So when the power is increased the prop will put a greater 'load' on the engine to keep the RPM at the selected amount. So the prop changes angle as necessary to convert more or less power which yields different airspeeds.


The most power the engine can develop is at full throttle and highest engine/prop RPM; that will be fine pitch which lets the engine turn fastest, usually up to the 2700 RPM redline in most GA piston airplanes. The most power possible will be at the highest RPM (which will be the highest fuel flow the engine can accept). So for takeoff the prop control is full forward along with the throttle which will give the finest pitch, this lets the engine spin up to full RPM to make the most power. But in cruise the prop control is brought back to slow the engine/prop to say, 2300, 2400, 2500 RPM or whatever. A lower prop setting (coarser pitch or bigger 'bite') is more aerodynamically efficient.

Pilots say "set the prop" at whatever RPM, but that is just the way of saying it, it really means setting the engine speed too because the engine and prop speed are one and the same. But a pilot may say "what do you cruise your prop at?" because the prop control is the primary RPM control in cruise as opposed to the throttle. Of course on a fixed pitch prop airplane there is only a throttle lever so it is used to change power and RPM together but the two cannot be controlled seperately from each other. (after getting used to flying a CS prop airplane going back to a fixed pitch prop airplane feels sort of primative and restrictive in operation. once you get used to a CS prop and fully appreciate the benefits and flexibility of engine speed and power management fixed pitch seems very simple)

In a CS prop airplane the throttle lever is more like a power or thrust control, and the prop control becomes the RPM control. (of course at low throttle the RPM will decrease no matter how fine/flat the prop pitch goes but that is at low power settings like used in the pattern)

So in short, the engine can make differing amounts of power at the same RPM and the prop angle (which is controlled by oil pressure) will change to convert the differing power into the air.

One more quick example to see what goes on. I am flying in level cruise at 22 inches and 2400 RPM at 130 knots indicated. Now I pitch the nose up into a climb without touching the throttle or prop control. The airplane now slows to 100 knots in the climb. What the prop has now done is go to a finer pitch angle because there is a a lower airspeed and climb load as compared to cruise. If the RPM stays exactly the same, that means the prop HAS to change angle between the airplane going 130 knots to 100 knots. All the while the pilot will not notice anything because you can't see the prop change angle. Now if I level back off the airplane will accelerate back to 130 and the prop will increase the pitch to a coarser angle - actually the prop going to a coarser angle is what makes the airplane accelerate back to 130 knots. {normally the throttle and the prop/engine speed in increased by the pilot in a climb but by not adjusting the throttle or prop in the previous example one can understand what the prop does}

Here is a column from Flying Magazine. Most of it is about diesel engines for GA but there is some explanation of CS props; click "2" bottom left, it is 2 pages:

http://www.flyingmag.com/article.asp...&page_number=1