Table 1.

Shapes and turning directions of ellipse depending on reference vibration frequency.

Reference frequencies	Shapes	directions
0 < ω2 < 1		Forward
ω2 ≈ 1		Forward
1 < ω2 <(1 + r2)		Forward
ω2 ≈ 1 + r2		·
(1 + r2) < ω2 < (1 + 2r2)		Backward
ω2 ≈ 1 + 2r2		Backward
ω2 > 1 + 2r2		Backward