What makes the thud sound on helicopter?

The thud sound on a helicopter is usually caused by the main rotor blades passing through the helicopter's tail rotor wash. This happens when the helicopter is hovering or flying slowly, and the tail rotor is creating a strong vortex of air. The main rotor blades then pass through this vortex, causing them to vibrate and produce a thud sound.

The thud sound can also be caused by other factors, such as:

* A loose or damaged tail rotor assembly. This can cause the tail rotor to wobble, which can in turn cause the main rotor blades to vibrate.

* A misaligned tail rotor. This can also cause the tail rotor to wobble, leading to the thud sound.

* A high-speed tail rotor. Some helicopters have high-speed tail rotors that can produce a thud sound if they are not properly maintained.

In most cases, the thud sound is not a cause for concern. However, if the sound is accompanied by other problems, such as vibration or instability, it is important to have the helicopter inspected by a qualified mechanic.