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Frisbees normally generate lift because their "wing" is moving through air, so what would happen if I just get the frisbee spinning without pushing its center of mass forward? Assume there is no gravity so there is no falling "parachute effect".

Let's define a frisbee as a 2D cross section which is rotated about the y axis, so that the object has complete rotational symmetry about the y axis. The spin for this experiment is then applied about the y axis and lift is the y component of acceleration. (If there were no rotational symmetry, one could obviously generate lift by using a "helicopter blade", or by simply adding some 45-degree "elevator tabs" to the bottom of a normal frisbee.)

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  • $\begingroup$ By assuming a frisbee as a 2D cross section, do we need to neglect the curvature near the circumference? $\endgroup$ – Guru Vishnu May 9 at 14:48
  • $\begingroup$ No - That curvature is allowed. As I tried to write in the question, a frisbee is a 3D object with the same 2D cross-section for any rotation angle from which you view it. $\endgroup$ – bobuhito May 9 at 14:55
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A frisbee simply spinning with its center of mass at rest will generate no lift. This is easy to see simply by considering that it's not pushing any air downwards. It's the horizontal motion through the air which generates lift. Spin is only needed to stabilize the disc, so that it doesn't wobble or flip over.

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  • $\begingroup$ It could be pushing a little air downwards, how do you know? $\endgroup$ – bobuhito May 9 at 16:00
  • $\begingroup$ @bobuhito if it pushed a little downwards, it would also push a little upwards, so there'd be no net effect. $\endgroup$ – Ruslan May 9 at 16:01
  • $\begingroup$ I'd say it's not symmetric (top of frisbee and bottom of frisbee are different), so can't agree so easily. $\endgroup$ – bobuhito May 9 at 16:05
  • $\begingroup$ @bobuhito if you consider the disc frictionless, then, to the air molecules hitting the disc, it's as good as fully resting: they don't notice that the surface is in motion because there's no force that makes the molecules change direction—other than the normal force, which simply reflects the molecules in a way independent of angular speed. $\endgroup$ – Ruslan May 9 at 16:10
  • $\begingroup$ Consider the streamlines over a spinning but otherwise stationary frisbee. They are all planar circles. There is no lift-generating component to the flow. $\endgroup$ – Guy Inchbald May 9 at 16:26
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Simple. A frisbee without rotation just drops to the ground. But a @Ruskan said, so does a rotating one. It takes rotation for stability and motion relative to the air for a frisbee to generate lift.

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  • $\begingroup$ A frisbee with rotation also just drops to the ground. $\endgroup$ – Ruslan May 9 at 15:55

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