Definition
A vibration in a rotating component, such as a helicopter rotor system, that occurs once per revolution of the rotor. It is typically caused by one blade being out of track or out of balance relative to the others, producing a single pulse of vibration each time the rotor turns through one full cycle.
Plain English
A shake that happens exactly once for every full turn of the rotor. If something is off on just one blade, you feel one bump per spin.
Context Anchor
Used in helicopter flight-test, maintenance, and troubleshooting discussions when describing the rate or pattern of a vibration.
Derivation
The name describes the ratio directly: one vibration pulse for every one revolution of the rotor. The 'one-to-one' refers to that 1:1 relationship between vibrations and turns.
Why Pilots Care
Uncorrected one-to-one vibrations can lead to structural stress, reduced component life, and uncomfortable or unsafe flight conditions.
Analogy
Think of a washing machine with one heavy item bunched on one side of the drum. You feel a thump once every rotation, not a constant buzz. That single thump per spin is the same idea as a one-to-one vibration.
Intuition Check
“One-to-one” does not mean personal instruction or a simple comparison here. It means one shake for one turn of the rotor.
Example Sentence 1
After landing, the pilot reported a noticeable one-to-one vibration, so the mechanic scheduled a rotor track and balance check.
Example Sentence 2
Pilots reported a one-to-one vibration that increased with rotor RPM during the preflight check.