Which way the propeller turns
Everything in this lesson assumes a propeller that turns clockwise as seen from the pilot's seat. PHAK's discussion of these effects assumes that rotation, and notes that most United States built engines turn that way.
NOTE: Most United States built aircraft engines rotate the propeller clockwise, as viewed from the pilot’s seat. The discussion here is with reference to those engines.
PREDICT FIRST
Your propeller turns clockwise as seen from the pilot's seat. You open the throttle to full power for a slow climb. Which way does the nose want to go?
Four causes, one name
To the pilot, “torque” (the left turning tendency of the airplane) is made up of four elements that cause or produce a twisting or rotating motion around at least one of the airplane’s three axes.
Torque reaction
The engine turns the propeller one way, so the aircraft is pushed the other way. In flight PHAK describes this as a force tending to roll the aircraft, to the left with this propeller.
As applied to the aircraft, this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the aircraft in the opposite direction.
When the aircraft is airborne, this force is acting around the longitudinal axis, tending to make the aircraft roll.
Torque reaction yaws the nose too. AFH names torque with P-factor as a cause of left yaw that needs right rudder. PHAK also notes that the slipstream's corkscrew flow gives a rolling moment to the right.
In addition, torque that acts opposite to the direction of propeller rotation causes the airplane to roll to the left. Under these conditions, torque and P-factor cause the airplane to roll and yaw to the left. To counteract this, right rudder and aileron flight control pressures should be used.
Note that this rolling moment caused by the corkscrew flow of the slipstream is to the right, while the yawing moment caused by torque reaction is to the left
On the take-off roll the left wheel carries more weight and more friction, which also yaws the nose left.
As the left side of the aircraft is being forced down by torque reaction, more weight is being placed on the left main landing gear. This results in more ground friction, or drag, on the left tire than on the right, causing a further turning moment to the left.
Spiralling slipstream
The propeller sends the air behind it back in a spiral, and the spiral strikes the fin. At high propeller speed and low forward speed the spiral is tight and pushes hard. As forward speed rises it stretches out and weakens.
When this spiraling slipstream strikes the vertical fin, it causes a yawing moment about the aircraft’s vertical axis. The more compact the spiral, the more prominent this force is. As the forward speed increases, however, the spiral elongates and becomes less effective.
PHAK's text does not say which way the spiral pushes the tail. AFH lists the slipstream effect with torque and P-factor, which together may produce a strong left yaw. The simulation does the same and shows the three as one combined yaw.
Gyroscopic precession
When a force tilts a spinning rotor, the result appears 90° further round, in the direction of rotation. A spinning propeller is a rotor like that.
Any time a force is applied to deflect the propeller out of its plane of rotation, the resulting force is 90° ahead of and in the direction of rotation and in the direction of application, causing a pitching moment, a yawing moment, or a combination of the two depending upon the point at which the force was applied.
PREDICT FIRST
You lower the nose sharply, as when the tail comes up on a tailwheel take-off. What does gyroscopic precession do to the nose?
Precession acts only while the attitude is changing, and a faster change gives a bigger yaw. Raising the nose works the same way and yaws the nose right. PHAK says the effect is considered more prominent in tailwheel aircraft, and most often occurs as the tail is raised on the take-off roll.
It is important to note that nose-down pitch movement produces left yaw, the result of gyroscopic precession created by the propeller. The amount of force created by this precession is directly related to the rate the propeller axis is tilted when the tail is raised, so it is best to avoid an abrupt pitch change.
P-factor
At a high angle of attack the descending blade, on the right as seen from the cockpit, takes a bigger bite of air than the ascending blade and makes more thrust. The centre of thrust moves right and the nose yaws left. As the propeller shaft lines up with the airflow, the difference shrinks to nothing.
When an aircraft is flying with a high AOA, the “bite” of the downward moving blade is greater than the “bite” of the upward moving blade. This moves the center of thrust to the right of the prop disc area, causing a yawing moment toward the left around the vertical axis.
This unbalanced thrust then becomes proportionately smaller and continues getting smaller until it reaches the value of zero when the propeller shaft is exactly horizontal in relation to the moving air.
Built in for cruise
Aircraft are designed to compensate for some of these effects. PHAK says the compensation for torque reaction is generally set for cruising speed, where most flying time is spent. AFH says the design also compensates for P-factor and slipstream in powered flight. The handbooks do not say at which speed the yaw compensation balances. The example aircraft in the simulation is balanced at its cruise setting.
Generally, the compensating factors are permanently set so that they compensate for this force at cruising speed, since most of the aircraft’s operating time is at that speed.
PREDICT FIRST
You close the throttle to glide. Which rudder will you probably need to stay straight?