i ahve one guys saying i changes prop picth you saying its just fuel thing
The statements are all part of a whole. Let's see if we can pull it all together here.
For constant-speed propellers another way of describing manifold pressure and RPM control is this: manifold pressure controls the amount of power that is "available" to spin the prop while RPM/prop pitch controls the amount of power "required" to spin the prop. The throttle (MAP) controls the amount of air that can be sucked in by the pistons and thus the amount of air available to the engine. RPM controls the speed at which the crankshaft can spin and thus how fast the pistons are allowed to pump. Along with fuel mixture (which is automatic in AH) these three controls determine the power output of the engine.
For a prop in constant speed operation there isn't a direct RPM control for the actual engine shaft. RPM's are controlled via the propeller governor that senses how fast the shaft is spinning and then adjusts the propeller pitch that either creates more force or less force for the engine to oppose. The force created by the prop varies with the amount of "lift" (thrust) that it is producing. The higher the aoa of the propeller blade, the more lift created. The more the lift, the greater the induced drag thus the more force needed to spin the prop. The opposite is also true. Using these forces the prop governor then maintains the engine RPM at the set speed by adjusting blade aoa/pitch as the shaft overspeeds or underspeeds the desired RPM.
What does have to do with aircraft performance? It's related to the amount of thrust and power available to propel the airplane. In flight, the efficiency of the propeller to convert engine power into usable power will vary by the propeller advance ratio, J.
J = airspeed / prop_rotational_speed * prop_diameter
For a fixed pitch propeller max efficiency of a propeller occurs at a specific value of J (specific airspeed & rpm). Outside of these values the efficiency drops. With constant speed propellers we're trying to maximize propeller efficiency over a range of airspeeds thus the maximum amount of power converted from the engine to flight. The way this is done is setting the RPM constant and then automatically varying the propeller aoa with varying airspeed to maintain a J value that maximizes propeller efficiency.
How all this is related to fuel consumption is that the fuel consumption of the engine can be maximized by adjusting MAP (power available) and RPM (power required) so that power output of the engine is at a setting most economic for fuel consumption.
For a fixed pitch propeller, RPM is completely tied to MAP/throttle setting of the engine. All AH aircraft are constant-speed props which means adjusting the RPM means adjustment of the blade pitch (aoa).
Hope that enlightens the topic!
Tango
