What sustained Np should we avoid to prevent damage from ground resonance?

To prevent damage from ground resonance in the T-6A, it is essential to avoid operating within the sustained Np range of 62 to 80%. This specific range can induce harmonic frequencies in the rotor system, which may lead to destructive vibrations and potential structural damage. Ground resonance occurs when the natural frequency of the rotor system aligns with the frequency of the ground vibrations, and sustained operation within this Np range can exacerbate those vibrations.

Choosing to operate outside of this range, particularly below 62% or above 80%, helps to mitigate the risk of resonance and promotes a safer operational environment for the aircraft. Understanding these parameters helps pilots maintain the integrity of the helicopter structure while ensuring safe ground operations.