So, how does a frisbee fly?

Matt Whiting wrote:
Robert M. Gary wrote:
I suspect the same is true of a golf ball (I'm a golfer, but not a
terribly good one). I've watched a number of balls hit by amatuers and
pros and I've never seen one rise above the launch trajectory. The
backspin will certainly make the trajectory much flatter than a
ballistic trajectory, but I don't think the ball will rise above a
tangent line to the path leaving the club face.

Golf balls clearly have lift and Cl is one of the criteria used when
evaluating ball standards. Here is a pdf describing some of the
formuals used and how the coefficient of lift for different balls
effects flight.
http://www.usga.org/equipment/techni...ublication.pdf

"It has been shown (Bearman, Harvey, 1976) that the two aerodynamic
coefficients, CD
and CL, are related to the dimensionless Reynolds number (Re) and spin
ratio (W)."

Spin is one component that determins the lift produced by the ball.

Here is an article from Cislunar Aerospace, Inc
http://wings.avkids.com/Book/Sports/...d/golf-01.html

"How a Golf Ball produces Lift

Lift is another aerodynamic force which affects the flight of a golf
ball. This idea might sound a little odd, but given the proper spin a
golf ball can produce lift. At first, golfers thought all spin was
detrimental (not good). However, in 1877, British scientist P.G. Tait
learned that a ball, driven with a "backspin" (the top of the ball
turning back toward the golfer) actually produces lift.

The dimples also increase lift. Remember, dimples help keep the flow
attached to the sphere. The dimples also cause the flow to be "focused"
into the flow of the wake. In this figure, the smoke shows the flow
pattern around a spinning golf ball. The flow is moving from left to
right and the ball is spinning in a counter-clockwise direction. The
wake is being forced downwards. This downward movement of the wake
means that a lifting force is being applied to the golf ball.
"

-Robert

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

"Friedrich Ostertag" wrote in message
...
I don't quite fully understand what you are stating. Probably my English
is not good enough.

Probably. Nothing in my post states that the primary determinant for the
path the frisbee takes is ballistics, and in fact I specifically point out
that aerodynamic lift is what prevents a ballistic flight path.

AES wrote:
Just got our dog a new frisbee (he goes thru 'em pretty fast if we're
careless and leave them within reach after a session).

This one has a large outer wire rim, small inner ring, and stretched
radially between these 8 or 10 tapered spandex wedges (i.e., these
wedges get wider as they go radially outward). There are gaps of
comparable width between each of these blades, and each wedge is twisted
by 180 degrees between inner and outer rings with most of the twist
occurring close to the inner ring, so that these wedges give a
reasonably convincing imitation of a multi-bladed propellor.

This thing seems to fly just about as well, however, whether you fly it
right side up or upside down, and whether you flip it to spin CW or CCW.
(Hurts my left arm when I try to spin it CCW, however.) In other words,
if there's any helicopter effect here, it's pretty weak.

So, how does a frisbee fly, anyway? Another of his favorites is just a
10" diameter weighted outer rim filled in with a slightly saggy "cloth
drumhead", which visibly bulges upward 1" or so at the center into a
fair imitation of an airfoil when you throw it. I've always figured the
heavy rim kept the thing spinning and thus semi-rigid, and the forward
motion of the airfoil shape gave the lift.

I'm not 100% sure and I'm not aerodynamicist, but I think two things
help a frisbee fly, or any disk even one lacking an airfoil shape.

1. The gyroscopic stabilization provided by the spinning imparted when
thrown. Just try to throw a frisbee or any disk without spinning it.
It won't go anywhere.

2. A small AOA imparted also when thrown. This is the essential part
and (1) above is really only important to the extent that it allows the
angle of attack to be maintained during flight.

Almost any object will create lift if it has some AOA. You can verify
this by extending your hand out of the car window when driving, which
most of us did as kids. If you rotate your hand from horizontal to say
30 degrees relative to the wind, you can create a fair bit of lift. And
most of our hands aren't exactly an ideal airfoil.


Matt

Matt Whiting wrote:
Almost any object will create lift if it has some AOA. You can verify
this by extending your hand out of the car window when driving, which
most of us did as kids. If you rotate your hand from horizontal to say
30 degrees relative to the wind, you can create a fair bit of lift. And
most of our hands aren't exactly an ideal airfoil.

The magic question though is that B or N? I found as a kid (I guess I
was bored) that curving my hand resulting in better "lift", which would
seem to implicate B.

-Robert

Robert M. Gary wrote:

Matt Whiting wrote:

Almost any object will create lift if it has some AOA. You can verify
this by extending your hand out of the car window when driving, which
most of us did as kids. If you rotate your hand from horizontal to say
30 degrees relative to the wind, you can create a fair bit of lift. And
most of our hands aren't exactly an ideal airfoil.


The magic question though is that B or N? I found as a kid (I guess I
was bored) that curving my hand resulting in better "lift", which would
seem to implicate B.

Neither, it is C.

Matt

AES wrote:
So, how does a frisbee fly, anyway?
Its all done with invisable strings!
Rocky

Looks like someone figured it out. A frisbee produces lift by traveling
through the air just like an airplane wing. The spinning of the frisbee
does nothing to increase the lift but it does produce the stability to
keep it properly oriented to produce lift.

http://www.physlink.com/Education/AskExperts/ae16.cfm

"A frisbee has the same shape, in side view, as an airplane wing: the
curved uppper surface causes the air to move faster accross the top
frisbee than it does across the 'flat' bottom, when the frisbee is
thrown. The 'lift' is then produced by the pressure difference between
the bottom and top sides of the frisbee - the lower pressure is on top
and the higher on the bottom - therefore the net force on the frisbee
will be up - producing the needed lift.

The turning of the frisbee mostly produces 'stability', that is the
spinning keeps the frisbee level in flight due to the 'gyroscopic'
effect, i.e. any spinning disc likes to stay spinning at the same
angle.

You can see for yourself that the spinning has nothing to do with the
actual 'flying' of the frisbee by noting that you can spin a frisbee in
place, on a stick or whatever, and it will not 'lift up'. You also can
see that the frisbee is still spinning at nearly 'full speed' when it
finally hits the ground, so you have another piece of evidence that
shows that the spinning doesn't lift the frisbee."

In article .com,
"Robert M. Gary" wrote:


The turning of the frisbee mostly produces 'stability', that is the
spinning keeps the frisbee level in flight due to the 'gyroscopic'
effect, i.e. any spinning disc likes to stay spinning at the same
angle.

You can see for yourself that the spinning has nothing to do with the
actual 'flying' of the frisbee by noting that you can spin a frisbee in
place, on a stick or whatever, and it will not 'lift up'. You also can
see that the frisbee is still spinning at nearly 'full speed' when it
finally hits the ground, so you have another piece of evidence that
shows that the spinning doesn't lift the frisbee."

And, the "soft frisbees" that are sold in pet stores, and that fly quite
well, consist of a thick and quite heavy but soft tubular ring around
the outer perimeter (it's the size of your thumb or larger, and almost
feels as if it had sand in it), and then just a thin piece of plastic-y
cloth like a saggy drumhead across it.

Because of the weight of the outer ring, you can put a good spin on it
when you launch it, at which point the "drumhead" takes up an upward
dome shape, and it maintains that shape and spin throughout its entire
flight.

As another data point, a sudden gust of wind coming head-on at it can
make it suddenly "jump" vertically upward by a sizable amount -- and a
gust from behind can make it suddenly drop, even "crash-land".

So, the physics of this seems to hold up . . .

"AES" wrote in message ...
Just got our dog a new frisbee (he goes thru 'em pretty fast if we're
careless and leave them within reach after a session).

....
So, how does a frisbee fly, anyway? Another of his favorites is just a
10" diameter weighted outer rim filled in with a slightly saggy "cloth
drumhead", which visibly bulges upward 1" or so at the center into a
fair imitation of an airfoil when you throw it. I've always figured the
heavy rim kept the thing spinning and thus semi-rigid, and the forward
motion of the airfoil shape gave the lift.

It is magic! Just like flying an airplane...

These folks are trying to explain away the magic he http://mae.ucdavis.edu/~biosport/frisbee/frisbee.html

Really, it's just magic...