10.2 Left-Turning Tendencies

ACS takeoff, climb, and stall knowledge all assume you can name why the nose wants to go left when a U.S.-certificated single is at high power and low airspeed. PHAK Chapter 5 lists four left-turning tendencies: torque reaction, gyroscopic precession, spiraling (corkscrew) slipstream, and P-factor (asymmetric propeller loading). American trainers rotate the propeller clockwise as seen from the cockpit (looking forward). Reverse that rotation and every sign flips — but that is not the PAR default.

Language first, because the test writes it carefully. These are moments about an axis — a roll about the longitudinal axis, or a yaw about the vertical axis — produced by the propeller. They are not a sideways aerodynamic “force” shoving the airplane to the left of the runway centerline the way a crosswind does. If a stem says “left-turning force,” look for the answer that renames it as a tendency or yaw/roll moment. The correction is still right rudder (and, for torque in flight, a hint of right aileron). The vocabulary is the point.

Torque reaction

Newton’s third law: the engine-propeller combination exerts a clockwise torque on the air; the airframe receives an equal counterclockwise torque. On the takeoff roll that unloads the right main and loads the left main. Extra rolling friction on the left tire yaws the nose left. In flight the same moment is a left roll. Torque is strongest at high power and, on the roll, at low speed before the rudder has much dynamic pressure. It does not require a high angle of attack.

Corkscrew / spiraling slipstream

The propeller throws a helical slipstream back around the fuselage. That corkscrewing flow typically strikes the left side of the vertical fin, yawing the nose left. (A secondary, smaller effect can change the local angle of attack at the tailplane.) Slipstream yaw is strongest when the propeller is moving a lot of air and the airplane is not — full power, low airspeed. That is the takeoff roll and the first chunk of the climb. As airspeed rises the helix stretches out and the fin sees a more axial flow, so the yaw fades.

Gyroscopic precession

A spinning propeller is a gyroscope. Apply a force at the rim and the resultant appears 90° ahead in the direction of rotation, in the same sense as the applied force. For a clockwise-from-behind propeller:

• Pitch the nose down (raise the tail of a tailwheel airplane) = a forward force at the top of the disc → felt 90° later at the right side → right side goes forward → yaw left.
• Pitch the nose up produces the opposite yaw.
• A yaw produces a pitch moment — the same 90° rule, swapped axes.

Gyroscopic precession is therefore the tailwheel liftoff specialty. You raise the tail to get the airplane flying, and the nose tries to swing left just as the rudder is becoming effective. A nosewheel airplane does not make that pitch change on the roll; precession is quiet until you rotate or make an abrupt pitch change in flight. Do not invent a big gyroscopic story for a Cessna 172’s ground roll.

P-factor (asymmetric blade effect)

At a high airplane angle of attack the propeller disc is tilted. The descending blade (the right blade on a U.S. single) meets the relative wind at a higher blade angle of attack than the ascending left blade. The right blade takes a bigger bite — more thrust on the right half of the disc — and the airplane yaws left. PHAK also calls this asymmetric loading.

P-factor requires that tilt. On a tricycle-gear takeoff roll the thrust line is nearly aligned with the relative wind; descending and ascending blades see almost the same angle of attack, so P-factor is small. It becomes the dominant extra yaw once you rotate into a climb attitude or hold a power-on stall attitude. That is the highest-rate PAR trap in this family: “P-factor is why you need right rudder on the takeoff roll.” For a nosewheel trainer, torque and slipstream are the roll story; P-factor is the climb story.

Why a climb wants more right rudder

A Vx or Vy climb stacks the three that do not need a tailwheel: high power (torque + a tight slipstream helix) and high propeller AOA (P-factor). Airspeed is low, so the rudder has less authority just when the moments are largest. That is why the ball slides right and the nose walks left if you climb feet-on-the-floor. Cruise at the same RPM with the nose down is a different airplane: less P-factor, a stretched slipstream, and a more effective fin. More right rudder in a climb than in cruise is the oral-test sentence.

A go-around is the same stack, closer to the ground, often with flaps still out. Add power, hold the pitch, and the left-turning moments arrive before you have thought about rudder. Uncoordinated plus a stall is the spin entry in 10.3.

What this is not

Adverse yaw is a different left-or-right story: the rising wing makes more induced drag when you deflect aileron. You cancel it with rudder into the turn. Do not file adverse yaw under “left-turning tendencies.”

Multiengine asymmetric thrust (the live engine yaws you toward the dead one) is a later class rating. PAR ASEL does not require you to work VMC numbers. One line is enough: a twin can yaw from unequal thrust even with the propeller discs level; that is not P-factor.

Scenario: Luis on the 172 roll versus the Cub tail-raise

Luis lines up a tricycle-gear trainer, firewalled. During the roll the nose eases left. The examiner asks which tendency. Torque (left-main loading) and spiraling slipstream (fin) are the honest pair. P-factor is the wrong hero until he rotates and the disc tilts. After liftoff, still at Vx, the ball needs a heavier right foot — now P-factor has joined the party.

The next flight is a tailwheel. Same clockwise propeller. When Luis raises the tail, the nose swings left even though the airplane is not yet at a climb AOA. That swing is gyroscopic precession. Right rudder before the tail comes up is how tailwheel pilots keep the centerline. Same four names, different moment winning at each phase.