Propeller Turning Tendencies & P-Factor – Video Transcript
Today, I want to talk a little bit about the propeller turning tendencies. We’ll go over each of these in some more detail, but the four that we’re going to focus on are torque, spiraling slipstream, precession, and what most people call P-factor (asymmetric thrust). Let’s dive right in and start with torque.
Let’s take a look at torque first. In the case of torque, it’s a result of the power that the propeller is ultimately exerting. If we imagine this is the airplane, we’re looking at the back of this airplane, so I drew the propeller piece in this blue dash circle. And if we’re looking at the airplane from behind, the propeller would rotate clockwise, so this rotation, this force, in order for the propeller to be rotated, would apply an equal and opposite reactionary force. That equal and opposite reactionary force is what we call a torque reaction. That torque reaction means that if you are in an airplane, usually smaller training aircraft don’t usually possess the amount of power to really feel a lot of the effects of this force, but if you were flying an airplane that had relatively higher power capabilities then you might experience this roll to the left in conditions where you were at maximum power.
Next up is spiraling slipstream, and to better understand this I’ll have an image on the screen. Really the idea here is that as the propeller rotates, it creates what we call propwash, which is just the air that it’s pushing behind the airplane. As such, that slipstream is rotating in the same direction that the propeller was rotating. Then it turns out that as that rotation occurs, the majority of that force would strike one side of the vertical stabilizer more than the other. Specifically, because our propeller is rotating clockwise, if viewed from the cockpit, means that it’ll strike the left side of the vertical stabilizer more than the right, which would force the tail to rotate to the right, which would yaw the nose of the airplane to the left. In this case, the spiraling slipstream, while it’s not a predominant force, may cause the airplane to yaw slightly to the left, especially once again, in high-powered situations where you have a lot of this propwash that’s being developed.
The next turning tendency we’re going to talk about is precession, and this is the same as the normal gyroscopic precession that we’ve talked about before. Really what this is then, is the fact that a propeller, while it’s rotating, is for all intents and purposes acting exactly like we would see out of any other gyroscope. I’m going to use this bicycle wheel here to simulate exactly how that would work. If we imagine that I’m sitting in the airplane right now, here’s the propeller rotating in front of me. Anytime I make a change to pitch, I’m going to apply a force to this propeller. For example, if I were going to pitch down, it would go down like this. If I were going to pitch up, it would come up like so. As we remember from our gyroscopic instruments though, this principle of precession means that the force will not be applied at the place where we were actually flying, it’ll be a felt 90 degrees later. For example, if I pitch the nose down, I won’t feel the force there, but instead 90 degrees in the direction of rotation causing the airplane to want to yaw to the left or the propeller to yaw to the left. Vice versa, if we pitch the nose of the airplane up like this, instead of the force being felt there, we will feel it 90 degrees later, and now we would feel this yaw to the right. Something to make clear is precession, just like any of the other turning tendencies, are a result of how much power or how fast, in this case, the propeller is rotating. The more rotational velocity we have, the more we can feel those effects. So that gives you a general idea of how we would feel precession. As a common example of when we typically talk about this being maybe the greatest effect, is often in tailwheel airplanes. In tailwheel airplanes, as we go down the runway and gain air speed to rotate, the tail of the airplane rises up. As it does, it applies this force that can oftentimes create a pretty violent left turning tendency in some tailwheel airplanes. This is one of the most popular ways of describing precession because it occurs in a position where we’re a little bit more vulnerable and have less wiggle room to make mistakes as we’re trying to take off. That is gyroscopic precession and how it applies to the turning tendencies of a propeller.
Asymmetric Thrust (P-Factor)
Last factor is what a lot of people call P-factor, or it can be also referred to as asymmetrical thrust. To get us started with understanding this, I want to reiterate first that just like any of these other factors that we’ve discussed, this assumes and can only really be applied if we are at a high-power setting. So, the same as all of these turning tendencies, if you had the power at idle, then certainly you’re not going to feel any of these effects. If we assume a high-power setting, and in addition to a high-power setting this, P-factor requires our flight path to be at a different angle than our pitch attitude. So, if we think of an example of that, we can think of something like a maneuver like slow flight. So, if we think of a maneuver like slow flight, in that slow flight condition we are typically a nose high attitude, pitched up, but flying at the same altitude. So, our flight path is level, but our pitch attitude is pitched up, and we’re generally at a relatively high-power setting. That will be a great example of when we might experience something like P-factor. Let’s dive into what we would see and how the propeller would deal with that kind of condition. In this picture, I have a cross-section of the propeller blades and then you see the spinner out of the front there. Each of these propeller blades has the chord line drawn there for both the ascending blade and the descending blade. Then in addition to that, we can start to draw then the relative wind for each of these blades. The first way we’ll start is the propeller is rotating, as it rotates, it creates its own relative wind. So that plane of rotation is the largest factor of the relative wind for each propeller blade. Now in addition to that, our aircraft’s flight path, the relative wind of the airplane itself, will also affect the relative wind then that the propeller blades feel. So, if I draw those flight path relative wind, this vector, is going to be level. Not parallel to the pitch attitude, but instead level to the earth. So, in this case, what we’ll notice is there’s going to be a slight difference in the relative wind that we experience for each blade. This effective relative wind is really what the propeller feels and therefore this effective relative wind compared to the chord line is what would give us the angle of attack for each blade. As we see from this image, the descending blade then has a relatively larger angle of attack compared to the ascending blade, because of those differences in what we can call the effective relative wind. In our example of something like a maneuver like slow flight, we would notice then that the descending blade, meaning the blade on the right side, is going down a larger angle of attack. If that’s true, then that means that blade will produce more thrust. So, we have wrapped up each of these turning tendencies now. Hopefully, this has been helpful and insightful.