2.6. Power and Torque Effects

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2.6.1. Slipstream Effect (Figure 2.3). The thrust generated by the rotation of the propeller induces a phenomenon called corkscrew slipstream effect. Specifically, the rotating prop produces a helical (or corkscrew) shaped airstream about the longitudinal axis. This slipstream strikes the wing root, fuselage, and tail surfaces with a constant high-energy force proportional to power setting and airspeed. The addition of power increases airflow over the tail surfaces and makes them more effective at slow speeds. In the T-6, the corkscrew slipstream induces a slightly higher angle of attack (AOA) on the left wing root and left tail surfaces, and slightly lowers AOA on the right wing root and right tail surfaces. This causes the aircraft to yaw to the left when power is increased, and requires right rudder input to counter the yaw and maintain coordinated flight. As the power is increased by moving the PCL forward with the left hand, the right foot/rudder pedal must move forward to counter the yaw that is induced to the left. The amount of rudder movement is proportional to the amount and rate of PCL movement. The amount and rate of rudder movement can be determined by looking out the front of the aircraft and using the rudder to keep the nose from swinging either left (too little rudder application) or right (too much rudder application). A power reduction has the opposite effect by requiring left rudder to maintain coordinated flight.

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2.6.2. P-factor (Figure 2.4). P-factor is another effect of the propeller. It is caused by AOA being higher on the downward-moving propeller blade than on the upward-moving propeller blade. This occurs when the aircraft’s thrust line is above the free airstream relative wind or at low speeds and high AOAs with power on. This moves the aerodynamic center of the propeller to the right of the shaft on a clockwise-rotating propeller causing the aircraft to yaw left as AOA or power is increased. This is why increasing right rudder is required to maintain coordinated flight as AOA is increased on the aircraft, such as in a pull-up for an over-the-top aerobatic maneuver. As the airspeed decreases and the AOA increases, the aerodynamic center of the propeller shifts to the right and right rudder is required to keep the aircraft in coordinated flight. The opposite is true when the thrust line is below the free airstream relative wind. The upward-moving propeller blade then has a higher angle of attack than the downward-moving blade. This moves the aerodynamic center of the propeller to the left of the shaft on a clockwise-rotating propeller, causing the aircraft to yaw to the right and requires left rudder to maintain coordinated flight. A right yawing situation seldom occurs since pushing over to the point of shifting the thrust line below the free airstream relative wind is rarely warranted.

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2.6.3. Torque (Figure 2.5). Torque reaction in a propeller-driven aircraft acts opposite the direction of propeller rotation. The aircraft tends to roll to the left as a result of torque when power is increased, and the aircraft tends to roll right when power is reduced. This effect is most discernable at low airspeeds and higher torque settings. Rudder and the TAD are the primary means for compensating for engine torque.

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2.6.4. Gyroscopic Effects (Figure 2.6). Gyroscopic reactions are called gyroscopic precession. This occurs when a force is applied to displace a spinning mass such as the propeller or, in the case of a spin, the entire aircraft. Gyroscopic precession causes an applied force to act in a plane 90 degrees from that in which it was applied (applied in the same direction as the rotation). The effect of gyroscopic precession depends on the rate of movement about the pitch or yaw axis. Increased rotation rates tend to increase the effect. The relatively large propeller on the T-6 and high revolutions per minute (rpm) result in more precession effect than an aircraft with a lighter, smaller propeller turning at a slower rpm. Typical reactions from a clockwise-turning propeller (as viewed from the pilot’s seat) include:

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2.6.4.1. If the nose is yawed to the left, the nose tends to pitch up.

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2.6.4.2. If the nose is yawed to the right, the nose tends to pitch down.

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2.6.4.3. If the nose is pitched down, a left yaw tends to develop.

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2.6.4.4. If the nose is pitched up, a right yaw tends to develop.

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