Expert: Paul Soderman - 10/17/2000

Question
I have a physics problem that has been bothering me.  When you throw a flat (symetrical) disk why does it curve?  
I had long thought a certain thing which predicts the opposite curve as is seen experimentaly (more drag on the bottom of the foreward turning side than any other portion). Is it a precession due a torque from the air due to the upward angle of attack?
To save you from throwing anything: the curve is CCW from the thrower`s perspective if it has a downward (RH) angular momentum.
Please mention why this effect is not seen on frisbees
Thank you  

Answer
OK - this can be tricky, but let's go step by step.

First, a spinning disc has angular momentum that wants to stay constant and will until acted on by external forces.  I.e., the disc wants to go straight, but aerodynamic forces will cause it to curve.

Consider a baseball.  A ball spinning clockwise as seen from above will curve to the right as seen from above.  I remember that from high school.  The reason is that because of viscosity, the local velocity on the ball is greater on the right side (airspeed + surface speed) and less on the left side (airspeed - surface speed).  So, the air pressure is lower on the right side and the resultant side force causes the ball to move to the right.

A perfectly thrown disc that is thrown horizontally (CW) with no angle of attack will curve slightly to the right like the baseball.  However, the shape does not lend itself to a strong side force like the baseball does.  

Now consider a slight angle of attack, which is very hard to avoid.  A lift force will be created that is much stronger than the side force described above, and because the local velocities are greater on the right side, the upper surface pressure will be lower (just like an airfoil - high speed over the top = low pressure) and the resultant lift on that side will be greater than on the left side.  Hence, the disc will lean to the left and curve left (CCW as seen by the thrower).

You raise a good point about gyroscopic reaction.  The drag force coupled with the angular momentum will also tend to force the disc to the left.  Without going through the numbers, my initial thought is that this effect is small because the drag is small.

A frisbee thrown CW should act like the disc. However, I can get a frisbee to curve right or left depending on the release.  With the left side up, the angular momentum is pointed to the right and the frisbee will curve to the right because of lift.  With the left side down, the opposite happens.

So, the trajectory of a spinning disc is complicated but can be predicted if everything is known about the start condition, angular momentum, and aerodynamic forces.

(You might get some more ideas at http://wings.ucdavis.edu/Book/instructor.html)