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Why don't engine sounds change when I move the throttle?
The sound of the engine is determined by the engine rotation speed and with the constant speed propellers of the P38 that rotation speed is kept constant by changing the angle of the propeller blades. If the rotation speed is too high the governor adjusts the blade angle to a more coarse setting which increases the blade angle and the blades bite into the air more strongly. Then if the speed decreases the blade pitch is reduced to reduce the load on the engine. The blades are adjusted so quickly that the rotation speed is pretty much constant throughout the entire flight, unless you fly very slow and with low power. This means the propellers spin at a constant speed all the time and therefor the sound is very monotonous, just like in the real aircraft.