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"More right rudder!" It's something you've probably heard from your flight instructor. And they most likely said (or shouted) it during takeoff, as you were careening toward left edge of the runway.
There's a reason your plane was veering left. Actually, there are 4 of them, and they're called left-turning tendencies. Here's a breakdown of each one.
The first left-turning tendency is torque, and the idea behind it comes from a pretty famous guy named Sir Isaac Newton. Newton's third law states that "for every action, there is an equal and opposite reaction".
Most western aircraft have engines that rotate clockwise when viewed from the cockpit. This is where torque starts coming into play. As you throttle up your engine for takeoff, the right-turning direction of the engine and propeller forces the left side of the airplane down toward the runway. When the left side of the airplane is forced down onto the runway, the left tire has more friction with the ground than the right tire, making the aircraft want to veer to the left.
P-Factor, also called 'asymmetric propeller loading', happens when the downward moving prop blade is taking a bigger 'bite' of air than the upward moving blade.
This happens in two scenarios: 1) your plane is flying at a high angle of attack, and 2) you're taking off in a tailwheel airplane.
In both of these scenarios, the downward sweeping blade is at a much higher angle of attack than the upward sweeping blade. And with a higher AOA, the downward sweeping blade creates much more lift (or thrust) as well, making the airplane want to yaw to the left.
A spinning propeller is essentially a gyroscope (a spinning disc). That means it has the two properties of a gyroscope: rigidity in space and precession. But don't worry, we're not going to make this next part a physics lesson. We're just going to quickly (and painlessly) explain the precession part.
Precession happens when you apply force to a spinning disc. Here's how it works: you apply a force to part of the disc, and the effect of that force (the resultant force) is felt 90 degrees in the direction of rotation of the disc.
This, for the most part, only applies to tailwheel airplanes when they lift their tail off the runway during takeoff. As the tail comes up, a force is applied to the top of the propeller. And since the propeller is spinning clockwise, that force is felt 90 degrees to the right. That forward moving force, on the right side of the propeller, creates a yawing motion to the left.
The fourth and final left turning tendency is spiraling slipstream, which happens when your prop is moving fast and your plane is moving slow. Takeoff is a great example of this scenario.
During takeoff, the air accelerated behind the prop, known as the 'slipstream', follows a corkscrew pattern. As it wraps itself around the fuselage, it hits the left side of your aircraft's tail, creating a yawing motion, and making the aircraft turn left.
Spiraling slipstream is highly dependent on an aircraft's design, as well as your phase of flight, so it's hard to quantify how much effect it really has on your plane. But here are a couple awesome pictures that help visualize it.
So there you have it. Next time you're rolling down the runway, you'll know exactly why you're stepping so hard on the right rudder to stay on centerline.
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at email@example.com.