"Left turning tendencies", "Torque Effects", etc refer to the aerodynamic coupling effects of an airplane's powerplant and propeller on yaw and roll moments. In conventionally-designed propeller-driven airplanes, increasing power setting, increasing angle of attack, and decreasing speed induce **left yawing and rolling moments**. For the purposes of this page, "conventional-design" refers to a tractor-configuration (front mounted powerplant and propeller), with clockwise rotation when viewed from behind (ie the pilot's perspective). # Torque "Torque" refers to Newton's third law of motion, applied to the powerplant/propeller and airplane: for every action (powerplant/propeller rotating clockwise), there is an equal and opposite reaction - a **left rolling moment**. The greater the mass and moment of inertia of the rotating powerplant components and propeller, and the greater the rate of rotation (RPM), the greater the induced left rolling moment will be. ![[A4NA torque modified.png]] During ground operations (ie takeoff roll), a **left yawing moment** is also induced; the left rolling moment forces the left wheel into the ground and increases friction, resulting in a tendency to turn left. # Slipstream "Spiraling slipstream", "slipstream swirl" or "corkscrew effect" refer to the effects of induced rotational flow to the air downstream of the propeller. Airflow follows a spiral or "corkscrew" pattern longitudinally about the airplane's fuselage and tail, imparting a side force on the vertical tail surfaces. This produces a **left yawing moment**, and a **right rolling moment**; the right rolling moment works in opposition to the left rolling moment induced by torque in flight. As power is increased or as airspeed is decreased, the spiral flow becomes more compact, and more effective in creating these yawing and rolling moments. ![[A4NA slipstream.png]] ![[A4NA slipstream 2 modified 2.png]] # Asymmetric Loading Asymmetric loading or "P-Factor" refers to unequal generation of thrust by the propeller blades. The down-going or descending right propeller blade is at a higher angle of attack and higher local airspeed than the up-going or ascending left propeller blade. The angle of attack and speed differential produces a "lift" (thrust) differential; the propeller's thrust vector is effectively shifted to the right, resulting in a **left yawing moment**. ![[A4NA P factor top.png]] ![[A4NA propeller AoA modified.png]] The relative wind of a propeller blade is determined by the rotation of the propeller ("rotational flow component") and the forward movement of the airplane ("axial flow component"). The sum of these vector components forms the third leg of a triangle that describes the **direction and magnitude** of the relative wind to the propeller blade. If the propeller disc is inclined to the airplane's relative wind (ie the airplane is flown at a positive angle of attack), the angular relationship between the axial and rotational flow components will change throughout the rotational arc of the propeller. On the descending right blade, the angle becomes obtuse: the angle of attack increases and magnitude (airspeed) increases. Conversely, on the ascending left blade, the angle becomes acute: the angle of attack decreases and magnitude (airspeed) decreases. ![[propeller blade angle of attack breakdown 3.png]] Another way to visualize the change in local airspeed of the propeller blades is to consider the distance traveled by the descending and ascending blades. The propeller blade travelling through the right half of the propeller arc (from top to bottom) must cover a greater distance than the propeller blade travelling through the left half of the propeller arc (from bottom to top), when the propeller disc is inclined to the airplane's relative wind. ![[A4NA P factor modified.png]] # Gyroscopic Precession The propeller is subject to effects of gyroscopic precession; when a force is applied to a gyroscope, the resultant force takes effect 90° later in the rotation. For the propeller, this means that **pitching forces produce yawing moments**, and **yawing forces produce pitching moments**. Most notably, raising the tail of a tailwheel airplane during the takeoff roll produces a left yawing moment. ![[PHAK precession.png]] # Additional Coupling Effects In addition to the left rolling moment produced by torque, if the airplane is allowed to yaw left (ie the pilot applies insufficient right rudder), [[Yaw-Induced Roll]] will result in an additional left rolling moment. While increasing power setting directly increases the effects of left turning tendencies, [[Pitch and Power Coupling]] also produces a nose-up pitching moment, and tends to increase angle of attack and decrease airspeed, further increasing the effects of left turning tendencies. # Design Considerations The ability to manage left turning tendencies presents a critical requirement for rudder authority in airplane design. As such, designers may employ a number of design features intended to reduce the adverse effects of the phenomena described above: - increasing the angle of incidence of the left wing, to overcome the left rolling moment induced by torque; note that this would also increase the induced drag of the left wing, and create an **additional left yawing moment** - mounting the engine angled slightly to the right, to oppose left yawing moments - setting a slight left angle of incidence to the vertical stabilizer, to oppose left yawing moments - installing a ground adjustable rudder trim tab ![[Bonanza engine cant and vertical stabilizer angle.png]] For example, the Beechcraft Bonanza series utilizes engine mounts canted slightly right, and a vertical stabilizer canted slightly left. Whatever features the designers elect to use, the airplane's left turning tendencies will be exactly compensated for **only** under a specific set of circumstances; normally, cruise flight. Any factors that **increase or decrease** the magnitude of the left turning tendencies will require the flight control pressures to be adjusted accordingly. If left turning tendencies are very minimal (ie low power, high-speed descent), the pilot may need **left rudder** to compensate for these same design features. ## Unconventional Airplane Designs Airplane designs that deviate from the "conventional" clockwise-rotating tractor-configuration may experience **right turning tendencies**. To reduce manufacturing costs, many pusher-configuration designs use commonly available reciprocating engines, mounted in reverse; the propeller spins counter-clockwise when viewed from behind. In addition, some vintage foreign designs utilize a tractor-configuration with an engine that turns in the opposite (counter-clockwise) direction. # Factors Affecting Left Turning Tendencies For a particular model of airplane, left turning tendencies are greatest at high power settings, high angles of attack, and low airspeeds. ### Aircraft Design - Less directional and lateral stability will resist left turning tendencies less - Powerplant/propeller selection, and low speed flight characteristics will determine the extent to which left turning tendencies can develop - [[Left Turning Tendencies#Design Considerations]] may exist to aid the pilot in overcoming left turning tendencies ### Power Setting - Increasing power setting and propeller RPM increases the magnitude of all left turning tendencies ### Angle of Attack - Increasing angle of attack increases asymmetric loading ### Airspeed - Decreasing airspeed decreases directional and lateral stability - Decreasing airspeed increases the effects of slipstream - Decreasing airspeed reduces control effectiveness and requires greater control deflection - Lower airspeeds are associated with higher angles of attack # Additional Information For additional information, see *Torque and P-Factor* (pgs. 5-30 to 5-33) in the *Pilot's Handbook of Aeronautical Knowledge*, *Torque* (pgs. 30 to 37) in *The Compleat Taildragger Pilot*, and *Slipstream Rotation* (pgs. 293 to 294) in *Aerodynamics for Naval Aviators*. # References - [[faa.gov/regulations_policies/handbooks_manuals/aviation/faa-h-8083-25c.pdf](https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/faa-h-8083-25c.pdf)]() - [[The Compleat Taildragger Pilot - Harvey S. Plourde - Google Books](https://books.google.com/books/about/The_Compleat_Taildragger_Pilot.html?id=-NCMswEACAAJ)]() - [[00-80T-80.pdf](https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/00-80T-80.pdf)]() - [[beech_v35b_f33a_f33c_a36_a36tc_b36tc_g36_mm_v2005.pdf](http://www.aeroelectric.com/Reference_Docs/Beech/beech_v35b_f33a_f33c_a36_a36tc_b36tc_g36_mm_v2005.pdf)]() ### By Kevin Sakson