Disc Physics...???

[ToddL]

I look at it as the difference between the center of gravity (COG) and the center of lift. Wing shape and speed determine how much and where the lift is, so as spin and forward speed decrease that center of lift moves in relation to the center of gravity. Lift in front of the COG keeps the disc in the air (glide), and lift to either side of the COG will tilt the disc into turn or fade.

First two sentences, yes.
Last sentence, No.
Doorfenschmirtz went over that in his talk about precession.
Watch this video: https://www.youtube.com/watch?v=eTjGTxSevHE

Lift in front of the CG will precess 90 degrees in the direction of rotation and force a tilt to the left.
Lift to the right of the CG will precess 90 degrees and force a nose down.
Lift to the rear of the CG will precess 90 degrees and force a tilt to the right.
Lift to the left of the CG will precess 90 degrees and force a nose up.

 

[Doofenshmirtz]

OK, bold area 1. So you're saying that, even though the relative air speed is higher on the left side of the disc than the right side, that that won't accentuate the lift the disc's cross section causes on one side or the other? Because if forward air speed causes a pressure differential between the air passing over the disc vs. under, it would seem to follow that varying the relative airs speed from one side to the other would also cause the lift to vary.

Although Bernouilli's Principle can be a difficult principle to grasp, keep in mind that, at its base, it describes the difference in pressure between bodies of air that are moving in relation to each other, not in relation to objects. So a somewhat simplistic explanation is that the air that flows over a flying disc has to move faster than the air that flows under a disc, because the path over the disc is longer. Because the air above is flowing faster than the air below, it has lower pressure. The disc moves toward the area of lower pressure, or upwards as long as the pressure differential is enough to overcome gravity. Spinning the disc doesn't change the speed of the two air bodies in relation to each other, so there is no effect on lift.

and 2. If this is 100% right, then it blows the left side vs. right side theory out of the water anyway, so never mind 1.

Funny how those two things seem to go together . . . It cannot be consistently claimed that precession causes fade at the end of the flight when the spin has slowed AND that precession does not exist earlier in the flight when the spin rate is fastest.

 

[Doofenshmirtz]

However, when a disc is flying, it's acting like a wing. More air speed will give more lift.

This is correct.

On the left side of the disc, if you look at all of the linear vectors from the spin you'll see they all have a forward component. On the right side they all have a backwards component. So on the left side there's more lift and on the right side there's less lift.

This is not correct.

Lift is caused by the difference in the speed of the air above the disc ONLY in relation to the speed of the air below the disc. The speed of the air in relation to the disc's surface does not matter. So the fact that the airspeed in relation to the disc's surface is different on the right than left does not create more lift on the right than left.

AND to go completely off into looneyland . . .

IF the argument about vectors and such means that there is more "airspeed on the left than right" how could that create lift when that is true equally for the top and bottom of the disc?

. . .

IF it mattered (and it really doesn't) that there is more comparative airspeed (disc surface vs. air) on the left than right, then the result would not be more lift on the left than right. The result would be that the disc would move toward the left in a flat turn, because the differential is left side to right side and not top to bottom on any part of the disc!

 

[ToddL]

Although Bernouilli's Principle can be a difficult principle to grasp, keep in mind that, at its base, it describes the difference in pressure between bodies of air that are moving in relation to each other, not in relation to objects. So a somewhat simplistic explanation is that the air that flows over a flying disc has to move faster than the air that flows under a disc, because the path over the disc is longer. Because the air above is flowing faster than the air below, it has lower pressure. The disc moves toward the area of lower pressure, or upwards as long as the pressure differential is enough to overcome gravity. Spinning the disc doesn't change the speed of the two air bodies in relation to each other, so there is no effect on lift.

You've been correct in every statement you've made in this thread except the bolded one. The "longer transit" hypothesis is incorrect when used as an explanation for why the air is moving faster on top of the wing. It's useful in that it generally shows which side will experience faster air, but it's not the reason that the air is faster.
Just a little nit-picking here.

Lift is caused by the difference in the speed of the air above the disc ONLY in relation to the speed of the air below the disc. The speed of the air in relation to the disc's surface does not matter. So the fact that the airspeed in relation to the disc's surface is different on the right than left does not create more lift on the right than left.

AND to go completely off into looneyland . . .

IF the argument about vectors and such means that there is more "airspeed on the left than right" how could that create lift when that is true equally for the top and bottom of the disc?

. . .

IF it mattered (and it really doesn't) that there is more comparative airspeed (disc surface vs. air) on the left than right, then the result would not be more lift on the left than right. The result would be that the disc would move toward the left in a flat turn, because the differential is left side to right side and not top to bottom on any part of the disc!

I contend that the left side of the disc would actually have a tiny tiny bit less lift than the right due to additional friction from the advancing surface slowing the air down a little bit. If anything, the center of lift would move to the right side with faster rotation.

And your last paragraph gets into the so called Magnus effect (which is really the same as the Bernoulli principle being driven by friction). The left side of the rim (the actual vertical face, more prevalent on an ultimate disc) would slow down the air on the left, while the right side would speed up the air, resulting in a sideways force to the right.

 

[Dan Ensor]

I still have a question that's been addressed variously in multiple threads, like:

From http://www.dgcoursereview.com/forums/showthread.php?t=2250&highlight=physics

"Because the velocity on the port (left) wing of the disc is higher, the air must flow faster over its dorsal surface on that side, causing its pressure to decrease. On the flipside, air is travelling much slower over the dorsal side of the starboard (right) wing, meaning the pressure is much higher. This pressure gradient causes the disc to tilt to the right, which is what we know as turn. Because turn is velocity-based, it is easy to see why it is dominant only during the initial part of a disc's flight."

The flaw of this argument is that it is based on air pressure, which is caused by air's movement (not the disc's). They would like you to think that a point on the disc moving relatively faster or slower equates to air moving faster or slower. It does not. The wind is displaced equally across the disc by the disc's shape and speed. The air doesn't have to move any farther over the top of the disc just because the disc is spinning one direction or another.


If this has already been said, sorry. tl;dr


Also, spin creates precession and stability. Furthermore, depending on whether the precession is caused by torque or torque free spin the precession will be in one direction or another (I believe the difference is 90*).

I need to do some reading up on the Magnus effect.

 

[bikinjack]

Do you trust MIT more?

You left off the first part of that quote that says "In general". This implies that the center of lift is usually, but not always in front of the center of rotation.

It occurs to me that we may be discussing two different types of flight. You may be thinking about the slow drifting rightward flight (RHBH) that many discs display, while what I'm thinking of is more of a disc that rolls progressively to the right as it flies. I would be pretty much on board with the Magnus effect on the first type of flight as being a major part of what is going on.

From the Stanford study you posted earlier:

It is also worth noting that the viscous no-slip condition at the boundary of the spinning disc causes the disc to generate some degree of vorticity. The circulation about the disc and the free-stream flow of air past the disc causes a force in the direction of the cross product of V with the angular momentum of the disc. This is attributed to the Magnus Effect, which is caused by one side of the disc percieving a higher free-stream velocity than the other, causing a pressure gradient. This will cause a flying disc thrown clockwise to veer to the left, which is particularly noticable as the viscous effects become more pronounced at the end of the flight. It is this same effect that causes a ping pong ball to travel along a curved path when a skilled player puts spin on it with the paddle.

The emboldened part is attributing the fade at the end of flight to the Magnus effect, at least the way I read it. This seems completely wrong to me.

 

[prototypicalDave]

One of my favorite shots is a hyzer flip to flat that flies straight for most of its flight before beginning to turn for the last third or so of its flight. If I throw it high enough I can almost throw question mark shape flight paths as they fade back. This shot works with comets, pa4's and beat aviars especially well. Every once in a while I'll pull it off with a driver.
So far my current theory is that it has something to do with the deceleration of the spin. I believe that high enough rpm will tend to counteract almost any aerodynamic forces, and as that changes the other forces come into play, but I really have no idea. What do you guys think?

 

[ToddL]

You guys realize the MIT and Stanford papers you keep referencing are just class papers written by undergrads, right? There's nothing that guarantees that they're correct. For all we know, those kids got F's on those papers.

 

[ToddL]

From the Stanford study you posted earlier:

It is also worth noting that the viscous no-slip condition at the boundary of the spinning disc causes the disc to generate some degree of vorticity. The circulation about the disc and the free-stream flow of air past the disc causes a force in the direction of the cross product of V with the angular momentum of the disc. This is attributed to the Magnus Effect, which is caused by one side of the disc percieving a higher free-stream velocity than the other, causing a pressure gradient. This will cause a flying disc thrown clockwise to veer to the left, which is particularly noticable as the viscous effects become more pronounced at the end of the flight. It is this same effect that causes a ping pong ball to travel along a curved path when a skilled player puts spin on it with the paddle.

The emboldened part is attributing the fade at the end of flight to the Magnus effect, at least the way I read it. This seems completely wrong to me.

Visualize the disc as a squatty cylinder being thrown RHBH. Make it 8 inches in diameter and 4 inches tall. As it's spinning clockwise and traveling forward, the left side of the cylinder wall is advancing into the flow and the right side is retreating with the flow. The friction the air experiences on the left side slows down the air on that side of the disc. The friction the air experiences on the right side speeds up the air on that side.

Slower air on left compared to faster air on right will result in higher pressure on the left, and a force pushing the disc to the right. Seems to me like captain Stanford over there got it completely backwards.

And for a golf disc, this effect is absolutely minuscule.

 

[bikinjack]

Visualize the disc as a squatty cylinder being thrown RHBH. Make it 8 inches in diameter and 4 inches tall. As it's spinning clockwise and traveling forward, the left side of the cylinder wall is advancing into the flow and the right side is retreating with the flow. The friction the air experiences on the left side slows down the air on that side of the disc. The friction the air experiences on the right side speeds up the air on that side.

Slower air on left compared to faster air on right will result in higher pressure on the left, and a force pushing the disc to the right. Seems to me like captain Stanford over there got it completely backwards.

And for a golf disc, this effect is absolutely minuscule.

I got the picture long, long ago, and yes Captain Stanford got it completely backwards.

 

[Olorin]

A better example.

Put the red dot at the 12:00 position and the blue dot at 6:00. Now slide your quarter forward but only rotate 180 degrees. Which one went farther?

Now rotate the quarter another 180 degrees while sliding it forward. Now which dot has traveled farther?

 

[Olorin]

There are several flaws with using this picture...

To overcome these issues just try this:
-On a quarter use a sharpie to mark a blue dot then mark a red dot 180 degrees on the other side.
-Use a ruler to draw a straight line on a piece of paper.
-Put the center of the quarter on the line
-Push the quarter straight forward while rotating it. Make sure that the center (rotation point) stays on the line.
Result: The red and the blue dot always travel exactly the same distance.

Incorrect interpretation of the results.
Don't look at the total distance of the red dot vs the blue dot.
Look at the distance either dot travels on the left side of the centerline vs the distance the dot travels on the right side of the centerline.

When a disc goes through one revolution every point on it has traveled an equal distance (the circumference of the disc).
Using a quarter, for simplicity lets estimate the circumference as 3 in.
Put the blue dot at 12:00 and the red dot at 6:00.
Push the quarter forward on the line 1 inch while making it rotate 1 time.
Every point on the rim of the quarter has now traveled 1 inch forward + 3 inches in a circle.
(OK, if you want to be picky, at the very end of a disc's flight, in the last revolution some points on the rim will have rotated a distance slightly more than others.)

All of this talk about distance is misguided, though. The real issue is ground speed vs. air speed.

 

[ToddL]

When a disc goes through one revolution every point on it has traveled an equal distance (the circumference of the disc).
Using a quarter, for simplicity lets estimate the circumference as 3 in.
Put the blue dot at 12:00 and the red dot at 6:00.
Push the quarter forward on the line 1 inch while making it rotate 1 time.
Every point on the rim of the quarter has now traveled 1 inch forward + 3 inches in a circle.
(OK, if you want to be picky, at the very end of a disc's flight, in the last revolution some points on the rim will have rotated a distance slightly more than others.)

All of this talk about distance is misguided, though. The real issue is ground speed vs. air speed.

Again, you're missing the point.

Once the red dot has traveled from 6:00 to 12:00 in a clockwise fashion (along the left, advancing side of the disc), it ceases being the dot on the left side of the disc. It's now the dot on the right side of the disc.

Yes, two dots drawn on the disc will travel the same distance during the throw. But the distance covered by the dots when they're on the left side of the disc is greater than the distance covered by the dots when they're on the right side of the disc.

Not that it matters.

 

[Toro71]

Although Bernouilli's Principle can be a difficult principle to grasp, keep in mind that, at its base, it describes the difference in pressure between bodies of air that are moving in relation to each other, not in relation to objects. So a somewhat simplistic explanation is that the air that flows over a flying disc has to move faster than the air that flows under a disc, because the path over the disc is longer. Because the air above is flowing faster than the air below, it has lower pressure. The disc moves toward the area of lower pressure, or upwards as long as the pressure differential is enough to overcome gravity. Spinning the disc doesn't change the speed of the two air bodies in relation to each other, so there is no effect on lift.



Funny how those two things seem to go together . . . It cannot be consistently claimed that precession causes fade at the end of the flight when the spin has slowed AND that precession does not exist earlier in the flight when the spin rate is fastest.

I understand Bernouilli's Principle pretty well. I think what was giving this left/right pressure differential more credibility (and making me ask was it l/r lift differential, or fore/aft) for me was that I didn't fully have my head wrapped around precession. If it really is about 90 degrees like the posters are saying, it pretty much ipso facto voids the idea that right tilt ('turn') is caused by a 'two wing' or 'helicopter blades' effect.

It makes it kind of neat, because it means spin does matter, since I'm guessing rotational velocity effects the 'degree' (if that's the right word) of precession, which means that depending on the amount of spin, you can get different orientation on the 'tilt' into anhyzer. I.e. the way some turnover shots aren't just tilted right, but also nose down (tilted forward.)

Man, I was worried at first, but I'm glad I started this thread.

 

[Olorin]

This may be helpful... research resources for the aerodynamics of flying discs at DG Resources.

 

[Toro71]

I'm going to try to sum up what I think I'm taking away from this thread...bear with me if I use the terminology wrong.

The disc is more like a single wing. Like a plane's wing, the shape of the disc, as it moves forward, causes an air pressure differential between the air moving over the top of the disc vs. air moving over the bottom, generally creating 'lift.' The speed and shape of the disc will effect the lift, not only in it's degree, but where it's "centered" on the disc, along the center line drawn in the direction of flight. So, some discs with enough speed will have the center of lift ("moment?") occur behind the center of the disc, meaing the lift is 'centered' there.

Now, what I think I was missing is, it's easy to assume this would make the back of the disc lift more than the front, causing a nose-down tilt. But with a spinning object, there's 'precession,' which for lack of terminology, we can say causes that force of lift to essentially transfer along the disc in the direction of rotation. So, instead of 'lifting' the back of the disc, it actually causes (for rhbh i.e. clockwise spin) the lifting to occur somewhere on the left side of the disc, tilting it right, into a turn.

That about where we're at, or some of us?

 

[Toro71]

^looking up precession on wikipedia, I'm not so sure I'm putting it right, or even getting it.

 

[Rockwell]

Could someone explain, in fine detailed physics, the flight of a Thumber?

 

[ToddL]

I'm going to try to sum up what I think I'm taking away from this thread...bear with me if I use the terminology wrong.

The disc is more like a single wing. Like a plane's wing, the shape of the disc, as it moves forward, causes an air pressure differential between the air moving over the top of the disc vs. air moving over the bottom, generally creating 'lift.' The speed and shape of the disc will effect the lift, not only in it's degree, but where it's "centered" on the disc, along the center line drawn in the direction of flight. So, some discs with enough speed will have the center of lift ("moment?") occur behind the center of the disc, meaing the lift is 'centered' there.

Now, what I think I was missing is, it's easy to assume this would make the back of the disc lift more than the front, causing a nose-down tilt. But with a spinning object, there's 'precession,' which for lack of terminology, we can say causes that force of lift to essentially transfer along the disc in the direction of rotation. So, instead of 'lifting' the back of the disc, it actually causes (for rhbh i.e. clockwise spin) the lifting to occur somewhere on the left side of the disc, tilting it right, into a turn.

That about where we're at, or some of us?

Yup. That's pretty much it.

^looking up precession on wikipedia, I'm not so sure I'm putting it right, or even getting it.

Reading physics or math on Wikipedia is generally not very useful.
Have you watched the video I linked earlier? https://www.youtube.com/watch?v=eTjGTxSevHE

 

[Rockwell]

It occurs to me that we may be discussing two different types of flight. You may be thinking about the slow drifting rightward flight (RHBH) that many discs display, while what I'm thinking of is more of a disc that rolls progressively to the right as it flies. I would be pretty much on board with the Magnus effect on the first type of flight as being a major part of what is going on.

If the magnus effect is what is causing the turn, it makes sense that we see the effect heightened dramatically when throwing into a headwind, and lessened when throwing into a tailwind.