Airplane Left-Turning Tendencies

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It may seem odd to discuss a left turn in itself, but many airplanes turn left of their own accord during some phases of flight. The reasons for this stem from a clockwise-turning propeller (when viewed from the cockpit). The propeller exerts some natural turning forces on the aircraft, among which are torque, gyroscopic precession, spiral slipstream effects, and P-factor. The sum of all of these forces results in a left-turning tendency that must be countered by the pilot. If the propeller turns in the opposite direction, as is common with some European engines, and power plants employing a speed reduction mechanism, the natural tendency would be a turn to the right, for the same reasons.

Torque force is present in all propeller-driven aircraft. Simply stated, the airplane reacts to the engine torque on the propeller by rolling in the opposite direction. The amount of torque felt depends entirely upon the amount of power applied to the propeller. The amount of pilot effort required to combat this roll depends upon the effectiveness of the flight controls, and indirectly on the speed of the airplane.

Gyroscopic precession results from the spinning mass of the propeller itself. The propeller may be rather heavy and is a wonderful gyroscope. That is to say, it exhibits some resistance to movement, such as an aircraft turn, climb, or descent. This resistance is felt by the pilot as a turning tendency that is prevalent mostly when attitude adjustments are made.

Naturally, the spinning propeller creates a great deal of spinning airflow behind it. This is the notorious spiral slipstream, which engenders visions of the propeller blast resembling a tornado that shrouds the airplane. A clockwise-turning propeller creates a clockwise-turning airflow, which spirals back around the airplane and strikes the rudder from the left side, causing a left turn. The strength of the turn is also related to propeller thrust and aircraft speed. The problem could be solved by placing the rudder on the bottom of the fuselage, but that would complicate the landing gear system.

P-factor tends to yaw the aircraft to the left most powerfully in a climbing attitude. P-factor yaws the aircraft due to unequal thrust across the propeller disc. As a two-bladed propeller rotates in straight-and-level flight, both blades get an equal “bite” of air, which produces an equal amount of thrust from each blade. However, in a climbing attitude, the descending blade (the one on the right when viewed from the cockpit) gets a much larger bite of air and creates more thrust than the ascending blade. Because of this unequal thrust, the aircraft is yawed to the left. The proper corrective measure to offset the effects of both torque and P-factor is the application of right rudder as needed to maintain heading.

The airplane must resist powerful torque effects from the rotating propeller. Left alone, the engines’ effort at the propeller would be mirrored when the airplane attempts to roll the opposite direction. Since most aircraft have clockwise-turning propellers, when viewed from the cockpit, the natural response of the airplane to increased power (torque) is a roll to the left.

Thrust created through the spinning propeller is imparted a spin of its own —like a tornado spiraling back around the fuselage or nacelles of the airplane. This spiral motion of the slipstream causes the air to strike the rudder at an angle that produces a natural yaw. Air rotating clockwise around the fuselage would strike the vertical stabilizer from the left and cause a left turn.

Gyroscopic effect
The propeller itself develops rather powerful gyroscopic effects. Indeed, at high power settings the propeller may offer powerful resistance to changes in aircraft attitude. This is mostly unfelt at the flight controls, except for a dynamic response that also pries the airplane into a left turn. Other effects are felt through the engine bearings and are responsible for high wear in the crankshaft of many aerobatic airplanes.

The propeller thrust may be unbalanced by the effects of angle of attack. As the angle of attack increases, the thrust becomes more powerful on the right side of the propeller arc, which pulls the airplane to the left. The unbalancing of thrust results from the propeller blade on the downward side of its arc (right side, when viewed from the cockpit) getting a more efficient bite of air —greater angle of attack— than the opposite side. This unbalancing of thrust increases as the airplane’s angle of attack is increased.

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