precession

Definition

The tendency of a spinning rotor (such as a gyroscope) to respond to an applied force as if that force had been applied at a point 90 degrees later in the direction of rotation. In aviation gyroscopic instruments, precession also refers to the gradual drift of the gyro away from its set reference, requiring periodic correction.

Plain English

When you push on a spinning wheel, it doesn't tilt where you pushed it — it tilts at a point a quarter-turn further around in the direction it's spinning. In gyro instruments, precession is also the slow wandering of the indication over time, which is why pilots have to reset them.

Context Anchor

Seen in discussions of gyroscopic instruments and propeller effects, especially during takeoff in tailwheel airplanes.

Derivation

From Latin praecedere, 'to go before.' The spinning rotor's reaction appears 'ahead' of where the force was applied — the effect shows up further along in the direction of spin rather than at the point of contact.

Why Pilots Care

Explains why a propeller airplane yaws left on takeoff and why attitude indicators show a slight lag or error when the airplane is turned quickly.

Analogy

A fast-spinning bicycle wheel can feel like it resists a simple push and responds by twisting in a different direction. Precession is that same kind of spinning-body behavior, applied to aircraft parts such as propellers and gyro rotors.

Grounding Statement

Picture a fast-spinning propeller: when the airplane’s nose is moved, the propeller does not react like a still object; its strongest reaction appears farther around the spin.

Intuition Check

Do not assume the force acts only at the exact place it is applied. With precession, the main turning effect appears about 90 degrees later in the direction the object is spinning.

Example Sentence 1

After fifteen minutes of cruise, the pilot noticed precession had caused the heading indicator to drift several degrees off the magnetic compass, so she realigned it.

Example Sentence 2

During a rapid turn, the heading indicator showed a temporary error caused by gyroscopic precession.

Definition

The characteristic of a spinning rotor (such as a gyroscope) by which an applied force is felt not at the point where it was applied, but at a point ninety degrees around the rotor in the direction of rotation. This causes the rotor's axis to tilt or move in a predictable, offset manner rather than yielding directly to the force.

Plain English

When you push on a spinning wheel, it doesn't tilt where you pushed it -- it tilts ninety degrees later, in the direction it's spinning. That offset reaction is called precession.

Context Anchor

Seen in discussions of gyroscopic flight instruments, propeller effects, and airplane handling during pitch changes.

Derivation

From the Latin praecedere, meaning 'to go before' or 'move ahead.' The idea is that the reaction appears to 'move ahead' of the force around the rotor, showing up at a different point than where the push was applied.

Why Pilots Care

Pilots must understand and compensate for precession errors to maintain accurate readings in heading and attitude instruments.

Analogy