Airplane Turning Tendencies

Almost every modeler above rank beginner understands that a propeller-driven airplane has unwanted turning tendencies. Most unfortunately are only aware of torque. Torque is but one effect that causes our planes to stray due to physics of a turning prop.

Understanding all of the turning tendencies in a propeller driven airplane is certainly not a prerequisite to being a good modeler. However, I can certainly tell you from experience, being armed with that knowledge of the other aspects will make any aeromodeler better, regardless of their current skill level.

Before we examine the turning tendencies, let’s cover some basic aerodynamic terms that are necessary for this discussion. Regardless of your familiarity with these terms, let’s just make sure beginners and veterans alike are on the same page. To keep this article from being too basic we’re going to assume that everyone is familiar with and knows what “center of gravity” and “wing cord” are.

The first thing to cover is the three control axes of an aircraft. Each axis of movement is on a line of rotation which passes through the craft’s center of gravity. These axes are longitudinal, lateral and vertical.

As depicted in Figure 1, the movements around these axes are respectively called Roll, Pitch and Yaw.

Axis of rotation of an aircraft: roll - aileron control, pitch - elevator control, yaw - rudder control

The second principal to understand is Angle of Attack (AOA). AOA is often confused with the pitch attitude of an aircraft. Pitch attitude is the wing cord line relative to the ground or horizon. Whereas AOA is the wing cord relative to the direction of the air as it arrives at the aircraft/wing. In other words, it’s the relative motion between the aircraft/wing and the atmosphere for which it is moving through.

To explain this a little further, in straight and level flight (through air that is not rising or falling), the air or relative wind is arriving at the aircraft from the direction of the horizon, so AOA and pitch attitude are fairly close to the same. In straight descending flight, AOA becomes greater than pitch attitude because the relative wind is arriving at the aircraft from below the horizon as shown below in Figure 2.

airplane wing cord line in relationship to the relative wind/flight path

Now that we have covered a couple necessary terms, let’s take a look at the four turning tendencies, what causes them and how they affect an airplane

TORQUE

As the propeller is driven by the engine, a force known as Newton’s Third Law of Motion is acting on the air frame. You’ve likely heard this third law… For every action there is an equal and opposite reaction. Basically if the prop and airframe shared the exact same properties of physics the airframe would roll at the same speed as the prop is turning, but in the opposite direction. For those familiar with helicopter dynamics imagine a heli that has lost its tail rotor.

An airplane airframe has much more mass and drag than the prop, so the prop has only a slight “torque effect” on the airframe. Torque has its greatest influence when air speed is low and power is high such as during a take off. The best demonstration of torque on an airplane is when guys hang the plane on the prop in the 3-D maneuver known as hovering. During a hover the prop is spinning at a very high RPM and producing a lot of thrust. The planes airspeed is zero and therefore little aerodynamic stability is being imparted on the wings.  In this condition the airframe tendency is to roll to the left.

ASYMMETRICAL THRUST

Asymmetrical thrust is more commonly known as P-factor. P-factor is generated when a prop driven airplane is flying at a high angle of attack. The arc in which the prop travels is tilted relationship to the relative wind. In this attitude the descending propeller blade is taking a bigger bite of air than the ascending blade on the opposite side. Since the prop is rotating clockwise, this means the right side is producing more thrust than the left, there-fore causing the plane to yaw left as depicted in Figure 3.

asymmetrical thrust or P-factor of an airplane propeller

SPIRALING SLIPSTREAM

Everybody knows a spinning propeller produces a flow of air over the airframe, right? However, if we could see this air, or slipstream as it’s called, we would see that it’s actually coming off the prop rotating in a big corkscrew motion past the wings and fuselage. When it comes across the tail section or empennage, the spiraling slipstream hits the side of the vertical stabilizer and rudder. Since the prop is spinning clockwise, this resultant slipstream strikes the left side of the vertical stabilizer pushing it to the right, causing the plane to yaw left as shown in Figure 4. This effect is most noticeable when speed is low and throttle is high.

spiraling slipstream from a spinning airplane propeller and its affects on an airplane airframe

How air moves coming off a spinning propeller

GYROSCOPIC PRECESSION

A spinning propeller is essentially a gyroscope and exhibits all the characteristics of such. But how does a prop’s gyroscopic action relate to unwanted turning tendencies of an airplane?  To uncover that answer we look to the phenomena of precession. Explaining gyroscopic precession is not easy, so I looked to my trusty old private pilot text manual. 1“The reaction to a force applied to a gyro acts approximately 90º in the direction of rotation from the point of where the force is applied”.

If an airplane is moved rapidly from level flight to a nose high pitch, gyroscopicprecession will create a tendency for the nose to yaw right as illustrated in Figure 5. The opposite is true for rapid nose down pitch, which will cause a yaw left. The factors that affect the amount of gyroscopic precession are prop RPM, prop mass/weight and pitch rate of the airplane. Increase any of these forces, the stronger the precession will be.

Gyroscopic precession showing the applied force and the resultant action

Torque, P-factor and spiraling slipstream all have the greatest influence when power is high and airspeed is low such as during a take off, which is where modelers experience unwanted turning tendencies the most and tend to blame totally on torque. But we know different.

By understanding ALL of these effects, a modeler knows what to expect when an airplane is put through different attitudes and maneuvers. For example we can predict what will happen to a tail dragger while the tail wheel is still on the ground and the throttle is advanced very quickly… P-factor and spiraling slipstream want to make it go left. Knowing when and why such turning tendencies exist, a modeler can be proactive to the problem instead of reactive. Staying ahead of the plane makes anybody a better flyer.

I’m not an aerodynamicist, but I did sleep at a Holiday Inn last night. If you believe I missed something or got it wrong, feel let me know in the comments below. If any of this unclear, drop a question in the comments. Will do my best to answer or find a source on the net that explains it better than I can.

Referances

1 Cessna Manual of Flight – Jeppensen & Co., 1992 pg. 2-13

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Comments

  1. It’s good to understand the theory of various forces &I their effects on aircraft. It takes the mistery out of flying when we know what to expect when a control input is made. Unfortunately the new smart systems don’t require some to understand & develop skills to control a air craft. Safety conscious is all important.

  2. Stephen Perry says:

    Very informative although I will have to re-read it several more times to absorb it and apply it more intuitively.

    Thank you!

  3. Gabriel wall says:

    Hi this is a good write up but you should note that all the factors above will provide a left yawing tendency only if the propeller spins clockwise when viewed from behind.

    Torque effect will on takeoff cause the aircraft to want to roll left thus creating pressure onto the left wheel which means more friction than the right wheel making the aircraft yaw towards the left. In the air torque will create a roll left.

    Gyroscopic blade effect will cause a tail dragger to yaw left as the nose is raised but only with a clockwise prop when viewed from behind and p factor will do the same once the aircraft is in a climb as the into wind blade will have a higher velocity and thus more lift.

    I hope I’ve been of help

    Junior flight instructor

    • Gabriel, thank you very much for your input. Very good points. Guess we shouldn’t assume all planes will be spinning the prop clockwise, as you pointed out.

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