Re: [Phys-l] football orientation in flight

[John Denker <jsd@av8n.com>]

On 05/22/2008 05:00 AM, Jeffrey Schnick wrote:
> Does this work?  (Discussed as viewed by the thrower.)  The ball is
> spinning clockwise meaning it has forward directed angular momentum.  

Or, equivalently, we could just say the angular momentum bivector 
is clockwise as viewed by the thrower ... so we don't need to 
mess with angular momentum vectors or the right-hand rule.

> As
> it arrives at the top of its flight it has a positive angle of attack.

OK.

> This results in some circulation 

Does that mean circulation around the roll axis (i.e. spin axis) 
or circulation around the pitch axis?  I'm assuming the latter.

> which means the air acquires some
> rightward angular momentum. 

Probably not.  Most fluid dynamics treats the air as irrotational,
which means vortices can only be created in pairs.  When airfoil
starts to produce circulation it sheds a _starting vortex_ equal
and opposite to the bound vortex.
  http://www.av8n.com/how/htm/airfoils.html#fig-trailing

As a separate argument leading to the same conclusion, if you throw 
(or kick) a football /without/ any spin-stabilization but with an
angle of attack, little or no torque is observed.  If the torque
depends on angle of attack, the same lack of torque would occur
in the spinning case.  (This is a problem because you would need 
a lot of torque in order to change the spin axis in the spinning
case.)

>  This means the ball gains some leftward
> angular moment which, with its big roughly forward angular momentum,
> means it yaws left.  This results in some leftward lift and its
> associated circulation which means the air acquires some upward angular
> momentum.  This means the ball gains some downward angular momentum
> which, with its big roughly forward angular momentum means it pitches
> downward.

Even if we hypothetically accept the air-acquires-angular-momentum
part of the argument, this part says it pitches down only after it
has yawed left.  This is a recipe for precession or wobble of the 
axis _around_ the forward direction, without bringing the axis any 
closer _to_ the forward direction.