For instance, in a small plane you have to use the rudder when you start rolling for take off.
I must admit I didn't even see this the first time around.
There are a few reasons why you need rudder (usually right) with the application of power on the takeoff roll for light aircraft.
Firstly the airflow from the prop corkscrews around the fuselage and strikes the fin at an angle of attack on the port side inducing a yaw to the left - remedy = right rudder.
Secondly the torque reaction to the engine tries to spin the airframe / fuselage in the opposite direction, increasing the friction on the left wheel, again a yaw to the left - remedy = right rudder.
Thirdly theres the P-factor. The downgoing blade strikes the airflow at a greater angle of attack -> more thrust -> thrust displaced to the starboard side -> yaw to the left -> and you guessed it right rudder!
Lastly, in taildraggers, as you lift the tail theres the gyroscopic effect precession (force acting 90 degrees and ahead on the spinning mass ie: prop) which again yaw to the left - remedy = right rudder.
(note this is all for a conventional clockwise rotating as view from the cockpit engine).
And as to how this applies to turbofans
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