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Why does the speed of a plane decrease when pitch increases?
When a plane is increasing its pitch, this means it is attempting to climb, or to get higher in the sky. This means it is fighting against both drag and gravity - more so then when it was flying straight and steady - and needs to work harder. Just like when you try run up a hill, you slow down, because you need more power to climb up.
When increasing the pitch, the amount of lift produced is also slightly lowered, because of the changed angle of attack, and the plane is also going to experience more drag, because the streamlined shape is not perfectly stright on with the oncoming air.