Aerodynamics of Flight

Effective Resultant force 90° force Yaw 1. Intake Applied force Figure 5-49. Gyroscopic precession. A p p lie d Re f s o u rc lt e a nt fo rc e 5-32 Y a w E eff of cc r e vit e Asymmetric Loading (P-Factor) When an aircraft is flying with a high AOA, the “bite” of the downward moving blade is greater than the “bite” of the upward moving blade. This moves the center of thrust to the right of the prop disc area, causing a yawing moment toward the left around the vertical axis. Proving this explanation is complex because it would be necessary to work wind vector problems on each blade while considering both the AOA of the aircraft and the AOA of each blade. This asymmetric loading is caused by the resultant velocity, gyroscope and thus has similar properties. Any time a force which is generated by the combination of the velocity of the is applied to deflect the propeller out of its plane of rotation, propeller blade in its plane of rotation and the velocity of the the resulting force is 90° ahead of and in the direction of air passing horizontally through the propeller disc. With the rotation and in the direction of application, causing a pitching aircraft being flown at positive AOAs, the right (viewed from moment, a yawing moment, or a combination of the two the rear) or downswinging blade, is passing through an area of depending upon the point at which the force was applied. resultant velocity, which is greater than that affecting the left or upswinging blade. Since the propeller blade is an airfoil, This element of torque effect has always been associated with increased velocity means increased lift. The downswinging and considered more prominent in tailwheel-type aircraft blade has more lift and tends to pull (yaw) the aircraft’s nose and most often occurs when the tail is being raised during to the left. the takeoff roll. [Figure 5-50] This change in pitch attitude has the same effect as applying a force to the top of the When the aircraft is flying at a high AOA, the downward propeller’s plane of rotation. The resultant force acting 90° moving blade has a higher resultant velocity, creating more ahead causes a yawing moment to the left around the vertical lift than the upward moving blade. [Figure 5-51] This might axis. The magnitude of this moment depends on several be easier to visualize if the propeller shaft was mounted variables, one of which is the abruptness with which the tail perpendicular to the ground (like a helicopter). If there is raised (amount of force applied). However, precession, were no air movement at all, except that generated by the or gyroscopic action, occurs when a force is applied to any propeller itself, identical sections of each blade would have point on the rim of the propeller’s plane of rotation; the the same airspeed. With air moving horizontally across this resultant force will still be 90° from the point of application vertically mounted propeller, the blade proceeding forward in the direction of rotation. Depending on where the force is into the flow of air has a higher airspeed than the blade applied, the airplane is caused to yaw left or right, to pitch retreating with the airflow. Thus, the blade proceeding into up or down, or a combination of pitching and yawing. the horizontal airflow is creating more lift, or thrust, moving the center of thrust toward that blade. Visualize rotating the It can be said that, as a result of gyroscopic action, any yawing vertically mounted propeller shaft to shallower angles relative around the vertical axis results in a pitching moment, and any to the moving air (as on an aircraft). This unbalanced thrust pitching around the lateral axis results in a yawing moment. then becomes proportionately smaller and continues getting To correct for the effect of gyroscopic action, it is necessary smaller until it reaches the value of zero when the propeller for the pilot to properly use elevator and rudder to prevent shaft is exactly horizontal in relation to the moving air. undesired pitching and yawing. The effects of each of these four elements of torque vary in value with changes in flight situations. In one phase of flight, one of these elements may be more prominent than another. In another phase of flight, another element may be more prominent. The relationship of these values to each other varies with different aircraft depending on the airframe, engine, and propeller combinations, as well as other design features. To maintain positive control of the aircraft in all flight conditions, the pilot must apply the flight controls as necessary to compensate for these varying values. Figure 5-50. Raising tail produces gyroscopic precession.