Left Turning Tendencies

One last topic today, triggered by a reader question

Q: The things I’ve seen indicate that rotating to climb (applying an upward force on the clockwise-spinning propeller) causes the airplane to yaw left.  But the right-hand rule says
Angular momentum (forward) X force (upward) = torque (to the right)
i.e., this results in a yaw to the right.  What have I got wrong? 

A: There are several forces acting to turn the airplane to the left or right. The FAA  (and hence the written test, CFIs, and DPEs) likes pilots to be aware of 4 forces. In all of these, we’ll assume a propeller in front of the plane (tractor) that turn clockwise, as seen from the pilot’s view – this is the most common configuration in the US. Other configurations will cause different effects. Overall, I recommend reading the excellent section on propeller effects in the Pilot’s Handbook of Aeronautical Knowledge.

P-factor or asymmetric loading

This is the force that seems to dominate during climbs. It is a result of the fact that your climb path is lower than your pitch angle. Because of this difference in angles, the right side of the propeller disc (the blade as it comes down) is biting the air at a higher angle of attack than the left side (the rising blade). This causes the right side of the propeller arc or disc to generate more thrust, which causes the airplane to want to yaw to the left. In level cruise flight, this effect will negligible, but much greater in slow flight at high power settings (like climbs or slow flight)

Spiraling slipstream or corkscrew effect

For the most part, airplanes are symmetric in design. The vertical stabilizer/rudder is one design feature that isn’t. The rotation of air behind the propeller will impinge on the left side of the vertical tail surfaces, trying to push the tail to the right and trying to yaw the nose to the left. This effect is always happening, but changes with speed/power.

Torque reaction

We’ve always hear “for every action, there is an equal and opposite reaction”. Same with the spinning propeller and engine – those parts turning clockwise try to turn the rest of the airplane counterclockwise, creating a slight left bank (and thus turning) tendency.

Gyroscopic precession

Any rotating mass has certain properties, similar to a gyroscope or a spinning top. One of those properties is precession. Any force applied will act 90 degrees forward in rotation of the applied force. In the example of an airplane “rotating” (pitching the nose up) for takeoff, you can think of this as a push at the bottom (6 o’clock position) of the propeller; the force effectively pushes from the 9 o’clock position of the propeller, trying to yaw the airplane to the right. For tailwheel airplanes (aka tail draggers), you first lift the tail off the ground (push the nose down) on take off – this causes a left turning tendency, but once you rotate, you are back to a right turning impact. This effect is only seen as you change the angle. Once established in a climb, not a factor

So…?

In reality, from a practical standpoint, we need to know how to anticipate our airplane’s reaction to certain inputs. Add power to accelerate down the runway – you’ll need right rudder. Pitch the nose up to climb, you’ll need right rudder. Point the nose down for a descent? – you may need left rudder! That’s because all the features that have been designed in to counteract left turning tendency may now be causing a right turning tendency. I am generally less worried about understanding the theory behind, as long as you know when to expect it and how to counteract…

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