Slip ball and yaw string disagree

[ldang]

Let me know if I understand it completely wrong about the relationship between yaw string and slip ball, but AFAIK in level flight (or in general) they should agree, since if the yaw string is centered, then the aircraft experience no sideways force, and thus the ball should be centered?

In this example I found the yaw string point directly at me (copilot seat) while the ball is centered. The disk is leveled and I am in a slight climb. The yaw string seem to agree more with doppler rather than the ball, which I think is not the correct behavior?

Edited June 12, 2023 by ldang

[Ramsay]

On 6/12/2023 at 10:46 PM, ldang said:

Let me know if I understand it completely wrong about the relationship between yaw string and slip ball, but AFAIK in level flight (or in general) they should agree, since if the yaw string is centered, then the aircraft experience no sideways force, and thus the ball should be centered?

TL;DR:

Helicopter's don't trim like fixed wing aircraft and crab slightly in forward flight

Detail:

• centered ball = in aero-dynamic trim
• centered yaw string / doppler needle = in nose to tail trim

Being in nose to tail trim is useful as the nose (and hence gun, rockets, etc.) is facing the direction of flight.
Aero-dynamic trim is the most efficient/least draggy and preferable  most of the time.

Casmo explains it well in this Apache video @21:34

 

Edited June 14, 2023 by Ramsay