What is meant by the resonant frequency of an aircraft structure?

• A. The frequency that produces the greatest amplitude of vibration in the structure.
• B. The frequency at which the structure yields maximum mechanical advantage.
• C. The frequency of the electrical power supply.
• D. The frequency at which the structure resonates with ambient wind.

Answer: (A)

Resonant frequency is the natural rate at which a structure tends to vibrate when disturbed. It depends on the structure’s mass and stiffness: a stiffer or lighter structure has a higher natural frequency, while a heavier or more flexible one has a lower natural frequency. When the excitation matches this natural frequency, the input energy is added in step with the structure’s motion, causing the vibration amplitude to become very large. That peak response is what we mean by the resonant frequency. In real systems damping limits the growth, but the maximum vibration still occurs near this natural frequency. The other ideas don’t describe what resonance is: it isn’t about mechanical advantage, nor about the electrical power supply, and while wind can excite a structure, resonance is defined by the structure’s own natural frequency rather than the wind frequency itself.