Definition
A vibration whose frequency is a whole-number multiple of a fundamental (base) frequency. The fundamental itself is the first harmonic; twice that frequency is the second harmonic, three times is the third, and so on.
Plain English
A vibration that happens at a frequency which is an exact multiple of some original vibration. If something vibrates 100 times a second, then 200, 300, and 400 times a second are all harmonics of it.
Context Anchor
Seen in discussions of radio signals, electrical noise, engine or propeller vibration, and sound.
Derivation
From the Greek harmonikos, meaning 'musical' or 'in tune.' In music, a string vibrates at a base note plus exact multiples of that note, which is why those multiples are called harmonics. Aviation uses the same idea for any vibration or signal, not just sound.
Why Pilots Care
Uncontrolled harmonics can produce structural fatigue, component damage, or sudden failure in flight-critical parts.
Grounding Statement
If the main vibration happens 100 times per second, a vibration at 200, 300, or 400 times per second is a harmonic of it.
Intuition Check
Harmonic does not just mean a pleasant musical sound. In aviation and technical use, it means a frequency that is an exact multiple of a main frequency.
Example Sentence 1
The placard restricts continuous operation between 2000 and 2250 RPM because a harmonic in that range can cause excessive propeller vibration.
Example Sentence 2
New dampers were installed to reduce the harmonic that had been shaking the engine mount.