| Asymmetric Loading (P Factor)
As in the past, it has been explained that when an airplane is flying with a high angle of attack, the "bite" of the downward moving blade is greater than the "bite" of the upward moving blade; thus moving the center of thrust to the right of the prop disc area - causing a yawing moment toward the left around the vertical axis. That explanation is correct; however, to prove this phenomenon, it would be necessary to work wind vector problems on each blade, which gets quite involved when considering both the angle of attack of the airplane and the angle of attack of each blade. This asymmetric loading is caused by the resultant velocity which is generated by the combination of the velocity of the propeller blade in its plane of rotation and the velocity of the air passing horizontally through the propeller "disc." With the airplane being flown at positive angles of attack, the right (viewed from the rear) or downswinging blade, is passing through an area of resultant velocity which is greater than that affecting the left or upswinging blade. Since the propeller blade is an airfoil, increased velocity means increased lift. Therefore, the downswinging blade having more "lift" tends to pull (yaw) the airplane's nose to the left.
Simply stated, when the airplane is flying at a high angle of attack, the downward moving blade has a higher resultant velocity; therefore creating more lift than the upward moving blade (Fig. 17-46). This might be easier to visualize if we were to mount the propeller shaft perpendicular to the ground (like a helicopter). If there was no air movement at all, except that generated by the propeller itself, identical sections of each blade would have the same airspeed. But now, let's start air moving horizontally across this vertically mounted propeller. Now the blade proceeding forward into the flow of air will have a higher airspeed than the blade retreating with the airflow. Thus the blade proceeding into the horizontal airflow is creating more lift, or thrust, moving the center of thrust toward that blade. Now, let's visualize ROTATING the vertically mounted propeller shaft to shallower angles relative to the moving air (as on an airplane). This unbalanced thrust then will become proportionately smaller and continues getting smaller until it reaches the value of zero when the propeller shaft is exactly horizontal in relation to the moving air. Each of these four elements of torque effects vary in values with changes in flight situations. In one phase of flight, one of these elements may be more prominent than another; whereas, in another phase of flight, another element may be more prominent. The relationship of these values to each other will vary with different airplanes - depending on the AIRFRAME, ENGINE, AND PROPELLER combinations as well as other design features. To maintain positive control of the airplane in all flight
conditions, the pilot must apply the flight controls as necessary to compensate
for these varying values.
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