Precession – Is It Really A Left Turning Tendency?

Flight instructors teach their students about the left-turning tendencies an airplane encounters on takeoff.  Unfortunately, some flight instructors may not fully understand the dynamics of takeoff and might pass a misconception or two on to the next generation of new pilots. 

It is not their fault, as they are victims of the “old days,” when most training airplanes had tailwheels and more than one novice pilot experienced an intense lesson in left turns on takeoff.  This is not the case with today’s trainers outfitted with nosewheels.

Why an airplane turns left on takeoff is one of the more misunderstood concepts among inexperienced pilots and experienced pilots alike.  Specifically, the most misunderstood concept regards the role precession plays in the takeoff scenario.

“Back in the old days . . .”

When training airplanes were taildraggers, pilots had to be ready with the right rudder.  Today, pilots must still use right rudder on takeoff, but it is not as critical as it was in the days of conventional landing gear.  The difference between taildraggers and the modern trainers of today is that precession actually aids the pilot in keeping the airplane tracking straight on takeoff.

Most pilots know about the four left-turning tendencies: torque, spiraling slipstream, P-factor, and precession.  The first three are left-turning tendencies.  The last, however, comes with a large dose of “depends on what you are doing with the airplane.”  If you look at each turning tendency alone, it is easy to understand how each turns the airplane to the left. 

Torque comes from Newton’s third law that for every action, there is an opposite and equal reaction.  As the engine spins the propeller to the right, there is a lot of force involved in that action.  Consequently, the opposite force tries to make the airplane roll to the left which, combined with the lift vector of the banked wing, introduces a left-turning tendency. 

During World War II, the leading cause of non-combat related death among P-51 Mustang pilots was the go-around on short final.  The Mustang, powered by a very high horsepower (1,200 to 2,000 hp, depending on model) swinging a large, heavy, four-bladed propeller, had a tendency to flip inverted if the pilot applied too much power all at once.  Disoriented, many young pilots pulled the stick back to gain altitude for safety, which resulted in an inverted collision with the ground.

The next left-turning tendency, spiraling slipstream, results when the air disturbed by the propeller swirls around the fuselage striking the vertical fin on the left side.  This creates a yawing moment forcing the tail of the airplane to the right, forcing the nose to the left.  Of course, the aircraft will follow the nose to the left so the pilot must counter this action with right rudder. This is more applicable to low airspeed situations.

Another left turning force is P-factor.  In high-angle-of-attack-flight, the descending propeller blade has a greater angle-of-attack than the ascending blade.  Essentially, the propeller blade “grips” more air on the right side of the engine cowling than on the left causing the plane to yaw left.  This is especially true at high power settings.  An airplane in slow flight at idle requires little right rudder, while one at full throttle might require all the right rudder available. 

The fourth left-turning tendency is the most misunderstood – gyroscopic precession.  The reason it is so misunderstood is that sometimes, it is not a left-turning tendency, but rather a right-turning tendency.  As mentioned previously, it depends on what the pilot is doing with the airplane.

The principle of gyroscopic precession maintains that a force applied to a moving gyroscope results in that force being exerted 90 degrees in the plane of rotation.  Where this becomes tricky, is when a pilot taking off moves the nose up or down.  In the case of a nosewheel airplane, this movement is up.  For taildraggers, the initial movement of the nose is down.  If the nose attitude is constant, there is no force exerted, therefore, there is no turning tendency in either direction.

In a taildragger, right rudder on the takeoff is of critical concern.  This is particularly so when the airplane is taken off in a left crosswind.  A pilot facing this equation:

T + Pf + SS + Pr + LXW

which translates into Torque + P-factor + Spiraling slipstream + Precession + a left crosswind, can quickly find him or herself sitting on the left side of the runway eating lunch in the middle of a ball of tangled airplane parts.

For taildraggers, T+Pf+SS+Pr+LXW can equal an accident in very short order.  This revelation usually occurs before the pilot realizes the tail they see flashing by the right side of their cockpit is their own.  The only way to avoid the accident is with proper use power and flight controls.  Flying taildraggers is not mysterious and only requires proper technique.

First, let the tail slowly fly itself off the ground.  The pilot who forces the tail up causes a tremendous left turning tendency with precession.  Coupled with the other left-turning tendencies, this can be disastrous.  By allowing the tail to lift on its own, the nose slowly rotates down with a slower pitching moment, allowing better rudder authority to keep the airplane straight.

Another technique is to allow the airplane to “fly-off” in a three-point attitude.  By limiting the pitch down force, the pilot thus limits the left-turning force of precession.  He or she still must deal with the other left-turning tendencies, but the there is a reduction in the ferocity of the left turn.

In airplanes equipped with a nosewheel, precession is actually a right-turning tendency during takeoff.  Many flight instructors emphasize the left turning tendencies to the exclusion of precession.  When the nose pitches up, the tipping force applied to the plane of the rotating propeller is at the bottom of the propeller arc.  This force translates 90 degrees in the plane of propeller rotation, which means the twist on the spin axis forces the nose to the right.  All the other left-turning forces combined, however, counter this right-turning force. 

In the cockpit, the pilot still notices the ball fall out of the inclinometer to the right.  This requires right rudder pressure to keep the airplane tracking true in a straight line.  Sometimes, a student or two who later becomes a flight instructor might misinterpret what gyroscopic precession is all about and pass on the misinformation to others. 

The trick is to keep learning until you fully understand all the quirks of flying.  Then when you become a flight instructor, pass the proper knowledge on to your students.

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©2011 J. Clark

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