Lift

 

Typically a disc is designed with the Center of Lift (CP) just forward for the Center of Mass.  The reason behind this is to give the disc a natural angle of attack, or tilt,  into the air flow. 

 

A down side to this design is that the disc experiences an unstable pitching motion to either side.  This can easily be seen if you toss a Frisbee without any spin.  The disc will pitch to one side or another.
   

 

This can be resolved by introducing a spin to the Frisbee.  This spin is called Angular Velocity.  By introducing angular velocity into the system, the unstable pitching motion transforms into a precession roll rate.

 

 

 

You can visualize this by imagining a string with a continuous wave running down the length of the string.  Now imagine attaching the ends of the string together, making a ring with a wave running around it.  If you were to place a pencil on the ring, having both ends of the pencil resting on the ring, then if you were to move the pencil along the ring it would rotate and move up and down.  This would be an example of a precession rate.

(Please excuse the drawing, physicist != artist)

 

As the air flow travels across the surface of the disc, many interesting things happen to it.  One of these being the Kutta Condition.  This condition states that a fluid ( in this case air) will tend to travel along the contour of a curved surface.  This effect actually contributes to the lift force.

 

It is important here to discuss something called the Bernoulli Effect.  Simply stated, areas of high velocity (speed) over a surface in a fluid experiences a low pressure area, where as an area of low velocity experiences high pressure.  The Frisbee can be generalized as an airplane wing.  Air flow over the top of the disc travels at a higher velocity than the lower flow, so an area of low pressure is created, so the disc rises.

(Note: This is a very generalized explanation) (6)

 

 

 

As descent as this explanation is, it is not entirely correct.  The Bernoulli Effect, which works great in general, disregards the vorticity of a fluid.  In a best case scenario we can use it as an approximation.

There is also another component which aids in the lift force, but this will be covered later.

 

 

 

To the left is the equation for Lift.  The "little p" stands for density, v is for velocity, A is the area, and C is the drag coefficient.

(5)

 

 

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