Why dutch roll increases with altitude and increasing mach no.?

[extricate]

Hi there,

Any advice on this?

Thanks

[Goldenrivett]

Very simply and with no maths:

ALT.
Think of it due to increased Kinetic Energy (due bigger TAS) in the system at high ALT with the same IAS. The fin and rudder effectiveness depends on IAS.

Mach Number.
Consider the effect of wing sweep with yaw. The wing which is presented to the apparent airflow with less sweep develops more lift (AC now rolls) but the induced drag is much greater at high Mach No. (due critical Mach No. is less on that wing) hence the yaw rate now increases in the opposite direction.