Peter Duniho wrote:
"cjcampbell" wrote in message
oups.com...
That, and a positive angle of attack. The spin keeps it
gyroscopically stable. As the forward motion decreases the Frisbee
begins to settle, increasing the angle of attack until becomes a
kind of parachute.

True, but the increase in angle of attack is strictly a result of the
change in relative wind. The frisbee remains in basically the same
attitude throughout. It has no means of trimming for constant lift
or anything like that.

But
not always. Throwing the Frisbee up will give it a positive angle of
attack as it climbs.

The vertical path is primarily a result of one throwing the frisbee
in that direction.
The path would curve down ballistically except
for the basic 1G of lift that the relatively modest angle of attack,
basically identical to the AOA in straight and level flight, provides.

I don't quite fully understand what you are stating. Probably my English is
not good enough. Do you think the lift (created by AOA and possibly
bernulli-shaped foil) does create a significant amount of lift or not? You
say that the (vertical) path is primarily a result of throwing, hence
determined by ballistic effects. Yet you state 1G of lift. If the lift is in
the order of magnitude of 1G, then it's certainly significant. Most
airplanes create lift in that range.. That holds also with my experience,
you don't need to give a frisbee much initial rate of climb (or any at all)
to make it go a long way without loosing much altitude. Although I have
never measured this :-) I even believe you can make it climb after releasing
it with a horizontal trajectory.

I think I have an explanation, why it doesn't work so well when throwing
CDs: As has been stated, CDs will quickly turn sideways when released in an
horizontal attitude. But a frisbee will do just the same, only slower! If we
look at the frisbee (or CD) as an airfoil creating lift (whether newtonian
or benoullian is not really important), the center of the aerodynamic force
will alwas be forward of the middle line of the plane (as with all aircraft
wings). With the cg in the middle of the disk, the aerodynamic force
supporting it, will at the same time try to pitch it up! The gyroscopic
force from the rotation translates this torque round the horizontal axis
into one round the longitudinal axis, thus a rolling torque. In fact, when
throwing a frisbee, at the point of release the disk should be slightly
rolling in the opposite direction (and/or in a rolled attitude), to keep it
roughly horizontal as long as possible.

Only due to the much lower inertia this doesn't work as well with CDs, as
the roll is so much faster!

Once the forward motion stops the angle of attack
can become negative,

Negative. As in, not true. The frisbee still has positive angle of
attack, and descends back along roughly the same path it took upward.
It's a bit lazy-eight-ish and, as you know, you don't need negative
lift to do those.

As to the mechanics of making a frisbee return to the thrower, I don't
understand the explanation given by CJ, especially the assumption of
negative lift. However, it can not be explained purely by ballistics, unless
it's thrown vertical. There needs to be some aerodynamic force towards the
thrower beyond drag, drag stops the moment the forward motion stops.

If, at the point of zero horizontal movement, the disk maintains it's
attitude towards the horizon (that is, slightly pitched up in the direction
of the original movement) it is now also pitched down in the direction
towards the thrower. While moving towards terra firma, it will gain speed
towards him, just like an aircraft will gain speed at the expense of
altitude when pitched down.

regards,
Friedrich