Is it possible that the yaw instability is caused by a phenomenon called vortex shedding? As air flows along the fuselage it will due to the pressure differential want to flow into the low-pressure area created by the flat plate form on the fuselage. Once it fills the void it will break away and form a vortex behind the helicopter and then it will happen again where air from the other side of the fuselage will want to fill the void and then it is shed forming a vortex behind the helicopter. When the air flows in from the other side the opposite side has a higher pressure on the fuselage surface and the pressure difference can cause the fuselage to move in the direction of the lower pressure. This pressure differential theory is what makes the NOTAR system work.
But then again it is only a theory.
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The Cat