Aerodynamics - Propellers

Read more about this topic in the Pilot’s Handbook of Aeronautical Knowledge Chapter 5: Aerodynamics of Flight (Pages 28-33)

	Basic Propeller Principles
		The aircraft propeller consists of two or more blades and a central hub to which the blades are attached. Each blade of an aircraft propeller is essentially a rotating wing. As a result of their construction, the propeller blades are like airfoils and produce forces that create the thrust to pull, or push, the aircraft through the air. The engine furnishes the power needed to rotate the propeller blades through the air at high speeds, and the propeller transforms the rotary power of the engine into forward thrust.
		Left Turning Tendencies
		1) Torque
			(1) Newton's 3rd -> Roll (2) On ground roll - pressure on left tire Þ yaw to left · The rolling tendency (torque) is transferred into a turning moment; The rolling tendency forces the left main gear downward; The result is more ground friction on the left wheel than the right; The net effect is the airplane yawing to its left about the vertical axis. (3) Design - left wing w/ more AOI; Cant engine right; Aileron trim (4) Factors Affecting Torque - HP; prop size vs. aircraft; surface · Amount of horsepower in the engine. · Size of propeller & Size of the airplane · Ground surface conditions · Speed of the engine (most pronounced during the beginning of the takeoff roll.) · Corrective Actions · Torque effect is greatest at low Airspeeds, high power settings, and high angles of attack so be aware of it
		2) Asymmetrical Thrust (P-Factor)
			· P-Factor, also known as asymmetric propeller loading, is the left yawing moment created while flying at a high angle of attack and is the direct result of one side of the propeller providing more thrust than the other does · The downward moving blade has a higher angle of attack. · Amount of thrust a propeller produces is dependent on how fast the blades are moving through the air. · When an airplane is at a high angle of attack the downward moving blade has a higher resultant velocity since it is moving forward into the relative wind. · This means it produces more thrust than the upward moving blade, which has a lower resultant velocity, since it is moving away from the relative wind. · The unbalanced condition causes the airplane to yaw to the left. · P-Factor is most pronounced during conditions of high power and high angles of attack. · The pilot directs for P-Factor by applying rudder.
		3) Spiraling Slipstream
			· As the propeller rotates in a clockwise direction, as viewed from the rear: The propeller forces the air rearward in a spiraling clockwise direction of flow around the fuselage Þ A portion of this spiraling slipstream strikes the left side of the vertical stabilizer forcing the airplanes tail to the right and the nose to the left Þ causes aircraft to rotate around its vertical axis
		4) Gyroscopic Precession (mainly tail dragger)
			· Precession is one of the properties of a gyroscope where the resultant action of a force applied to a gyroscope will be 90 degrees ahead of the applied force in the direction of rotation · With the spinning propeller taking on the properties of a gyroscope the resultant force of any applied force to the plane will occur 90 degrees ahead of it in the direction of rotation and in the direction of the applied force. · Any time a pilot changes pitch or yaws an aircraft, a resultant force due to precession tries to yaw or pitch the aircraft respectively. · Best illustration - the tail wheel airplane raising its tail on takeoff; The applied force as seen by the prop is forward to the top of the propeller disc; The resultant force, 90 degrees clockwise from the top of the disc, is the right side of the disc causing the aircraft to yaw left about the vertical axis. · Precession in an aircraft can be summed up as such: · Pitching up results in yawing right. · Pitching down results in yawing left. · Yawing right results in pitching down. · Yawing left results in pitching up. · Precession only happens while a force is applied. · Once the applied force is removed the resultant also is removed.
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