Disc Physics...???

[Toro71]

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."

And from http://www.dgcoursereview.com/forums/showthread.php?t=99010&highlight=moment+lift

which links to

https://www.dgcoursereview.com/dgr/forums/viewtopic.php?f=17&t=14825

which seems to agree with "sarah":

"Turn is defined here as the disc's natural tendency to decrease its hyzer angle while in flight through precession (right for RHBH). Turn is caused by a center-of-pressure/lift that is behind the center of the disc. I.e., lifting the trailing edge of the disc more than the leading edge causes the disc to precess in a manner that makes it turn."

So, basically, there seems to be two theories on "turn," one looks at the disc as analogous to both wings on a plane (more or less,) the other looks at it as one wing (more or less.) Interesting to me, is that both theories seem to consider spin a factor in turn, and not just as a stablizing effect.

Do we have a consensus here? Both ideas have merits from a layman's POV, just wondering if any of the supernerds here can shed light?

 

[bradharris]

Turn is generally caused by the Magnus Effect

Fade is the result of precession.

 

[sandman365]

I've always thought more spin=more turn, but could be wrong.

 

[Toro71]

Well, spin stabilizes an object generally (think a top) but there seems to be some differing ideas about how that factors in with forward velocity.

@ bradharris: so like a curveball...?

 

[Olorin]

"Because the velocity on the port (left) wing of the disc is higher,

I think that this fundamental premise is incorrect, therefore the conclusions are flawed. It would probably take too much verbiage to explain why I think this, but I will make a brief attempt. This idea is very widely held myth that has reached the status of becoming "true" merely through constant repetition.

Here's what I think:
1) Start by using a sharpie to mark a point on the outer rim of a disc. We will call this point "1".
2) IF there was a point on the left side of highest velocity, call this "A".
3) Then IF there is also a point somewhere of lower, and thus lowest, velocity, somewhere on the right side call this "B".
4) How can point 1 be faster at point A than at point B? That would mean that by some unknown force point 1 speeds up until it reaches A then it slows back down when it reaches B then it speeds back up to get to A... This speeding up and slowing down makes no sense; it is illogical, therefore the assertion is false.

Does that make sense to you? Do you agree?

I think this idea started by looking at pressure differntials on fixed wing aircraft and working backward to postulate that the left side must be slower. But a rotating disc cannot be compared to an airplane.

So I think that the shifted Center of Pressure explanation is much more plausible.

The rotation of a disc makes it incredibly complex, and I have not seen any information on DGR, DGCR, or the internet that satisfactorily explain all of the observed phenomena. But it is still fun to discuss and learn about!

 

[jrawk]

So, basically, there seems to be two theories on "turn," one looks at the disc as analogous to both wings on a plane (more or less,) the other looks at it as one wing (more or less.) Interesting to me, is that both theories seem to consider spin a factor in turn, and not just as a stablizing effect.

Do we have a consensus here? Both ideas have merits from a layman's POV, just wondering if any of the supernerds here can shed light?

A disc is an unstable wing which uses spin/gyroscopic effect for it act as a stable wing. The rate of rotation is a constant variable, which makes the flight direction of disc a constant variable as well. Any slight variance in rate of rotation will cause the disc to pitch/yaw in a different direction.

I've always thought more spin=more turn, but could be wrong.

in a vacuum constant spin would have zero effect on flight direction. However outside a vacuum, the friction of the disc turning through air will push it toward the direction of it's rotation. i.e. clockwise rotation will push it right, where counter clockwise rotation will push it left.

Just my thoughts.

Edit: and I totally agree with Olorin

 

[Toro71]

I think that this fundamental premise is incorrect, therefore the conclusions are flawed. It would probably take too much verbiage to explain why I think this, but I will make a brief attempt. This idea is very widely held myth that has reached the status of becoming "true" merely through constant repetition.

Here's what I think:
1) Start by using a sharpie to mark a point on the outer rim of a disc. We will call this point "1".
2) IF there was a point on the left side of highest velocity, call this "A".
3) Then IF there is also a point somewhere of lower, and thus lowest, velocity, somewhere on the right side call this "B".
4) How can point 1 be faster at point A than at point B? That would mean that by some unknown force point 1 speeds up until it reaches A then it slows back down when it reaches B then it speeds back up to get to A... This speeding up and slowing down makes no sense; it is illogical, therefore the assertion is false.

Does that make sense to you? Do you agree?

I think this idea started by looking at pressure differntials on fixed wing aircraft and working backward to postulate that the left side must be slower. But a rotating disc cannot be compared to an airplane.

So I think that the shifted Center of Pressure explanation is much more plausible.

The rotation of a disc makes it incredibly complex, and I have not seen any information on DGR, DGCR, or the internet that satisfactorily explain all of the observed phenomena. But it is still fun to discuss and learn about!

I'm not sure. In a vaccuum, I see where you're coming from. Rotational velocity can't be different on one side of the disc or another. But I can't seem to make up my mind if that spin causes differences in relative air speed between the side of the disc spinning into the direction of travel vs. the side spinning away...in other words making separate moments of lift, one "greater" than the other.

bradharris' point about the "curveball" effect would seem to make sense in conjunction with the single moment of lift forward/behind the axis of spin.

 

[Toro71]

....except that "turn" doesn't necessarly = moving to the right off line for a rhbh. Think hyzerflip into a shallow hyzer. Or, am I just trying to make something more complicated than it is?

 

[bradharris]

I think that this fundamental premise is incorrect, therefore the conclusions are flawed. It would probably take too much verbiage to explain why I think this, but I will make a brief attempt. This idea is very widely held myth that has reached the status of becoming "true" merely through constant repetition.

Here's what I think:
1) Start by using a sharpie to mark a point on the outer rim of a disc. We will call this point "1".
2) IF there was a point on the left side of highest velocity, call this "A".
3) Then IF there is also a point somewhere of lower, and thus lowest, velocity, somewhere on the right side call this "B".
4) How can point 1 be faster at point A than at point B? That would mean that by some unknown force point 1 speeds up until it reaches A then it slows back down when it reaches B then it speeds back up to get to A... This speeding up and slowing down makes no sense; it is illogical, therefore the assertion is false.

Does that make sense to you? Do you agree?

I think this idea started by looking at pressure differntials on fixed wing aircraft and working backward to postulate that the left side must be slower. But a rotating disc cannot be compared to an airplane.

So I think that the shifted Center of Pressure explanation is much more plausible.

The rotation of a disc makes it incredibly complex, and I have not seen any information on DGR, DGCR, or the internet that satisfactorily explain all of the observed phenomena. But it is still fun to discuss and learn about!

If you're watching the disc fly from above, you'll see your sharpie point speed up on the left side and slow down on the right (for a RHBH throw).

Imagine the path it would trace as it travels. It will not be a straight zig-zag back and forth. It will be a curve where it covers far more distance on the left side than it does on the right.

 

[Ranger Rick]

Stop all this math mumbo-jumbo, you are sucking all the fun out of it

 

[Aim For The Chains]

If you're watching the disc fly from above, you'll see your sharpie point speed up on the left side and slow down on the right (for a RHBH throw).

Imagine the path it would trace as it travels. It will not be a straight zig-zag back and forth. It will be a curve where it covers far more distance on the left side than it does on the right.

MVP Disc Sports Photographer's Note: This photo was captured with a long shutter exposure and throwing a Tangent with lights attached to create this stellar flight path.

 

[Mocheez]

I think that this fundamental premise is incorrect, therefore the conclusions are flawed. It would probably take too much verbiage to explain why I think this, but I will make a brief attempt. This idea is very widely held myth that has reached the status of becoming "true" merely through constant repetition.

Here's what I think:
1) Start by using a sharpie to mark a point on the outer rim of a disc. We will call this point "1".
2) IF there was a point on the left side of highest velocity, call this "A".
3) Then IF there is also a point somewhere of lower, and thus lowest, velocity, somewhere on the right side call this "B".
4) How can point 1 be faster at point A than at point B? That would mean that by some unknown force point 1 speeds up until it reaches A then it slows back down when it reaches B then it speeds back up to get to A... This speeding up and slowing down makes no sense; it is illogical, therefore the assertion is false.

Does that make sense to you? Do you agree?

I think this idea started by looking at pressure differntials on fixed wing aircraft and working backward to postulate that the left side must be slower. But a rotating disc cannot be compared to an airplane.

So I think that the shifted Center of Pressure explanation is much more plausible.

The rotation of a disc makes it incredibly complex, and I have not seen any information on DGR, DGCR, or the internet that satisfactorily explain all of the observed phenomena. But it is still fun to discuss and learn about!

The only exception to this would be if the axis of rotation is outside the center of the disc. In which case the rotational velocity of opposing points on the outside edge of the disc would indeed have differing rotational velocities (the side that is closer to the center of rotation would be rotating slower than the side which is further from the center of rotation).

 

[Toro71]

If you're watching the disc fly from above, you'll see your sharpie point speed up on the left side and slow down on the right (for a RHBH throw).

Imagine the path it would trace as it travels. It will not be a straight zig-zag back and forth. It will be a curve where it covers far more distance on the left side than it does on the right.

 

[46YearOldSlinger]

Whoaaaaaaaa! Flashback!!!!!!!

 

[Olorin]

The only exception to this would be if the axis of rotation is outside the center of the disc.

The axis of rotation is always through the center of the disc; it MUST be. Even if this axis (the Z axis) is tilted (producing wobble) it must always be through the center of a balanced disc golf disc. See here. The Center of Pressure may be elsewhere, though. See this Center of Pressure picture. The CoP can be in front (nose up), behind (nose down), right (right to left turn) or left or any combination thereof.

 

[Olorin]

It will be a curve where it covers far more distance on the left side than it does on the right.

Impossible. One point on a rotating circle cannot travel farther than another point on the same circle.

If you were looking straight down on that cool black picture the arcs would be identical.

Try an experiment- On a quarter mark the L on "Liberty". Slowly rotate it one time while pushing it forward. Does the L travel farther than the number 1 or 2 on the date below? No way! That is simply not possible!

 

[bradharris]

Impossible. One point on a rotating circle cannot travel farther than another point on the same circle.

If you were looking straight down on that cool black picture the arcs would be identical.

Try an experiment- On a quarter mark the L on "Liberty". Slowly rotate it one time while pushing it forward. Does the L travel farther than the number 1 or 2 on the date below? No way! That is simply not possible!

I think I misunderstood your statement. All points on the disc do travel the same distance (within twice the disc's diameter). However, at any moment in time, the point that is on the left side has a higher forward velocity than the point on the right side. This is what creates the differential lift which actually causes the left side to elevate.

 

[ToddL]

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."

That's the right thought process with utterly wrong conclusions.

RHBH spin, no forward velocity, looking from the top, clockwise spin: The left side of the disc is travelling forward while the right side of the disc is travelling backward. Both points see the same rotational speed, but they experience different velocities because they are in different directions.

RHBH throw, yes forward velocity, looking from top, clockwise spin: The left side of the disc is advancing toward the oncoming air while the right side of the disc is retreating away from the oncoming air.

In a helicopter, you have spin combined with forward velocity, but the conclusions are different than a disc. I think this is where Rameka messed up. In a helicopter, the advancing blade (left side in our example) sees the speed of the blade (W) added to the speed of the helicopter: U + W. The retreating blade (right side) sees the speed of the blade subtracted from the speed of the helicopter: U - W. This makes the airfoils on the left side see a higher speed and therefore create more lift.

This is where I get into my own speculation.
In a disc, though, you don't have blades. I don't think the conclusions are the same as a helicopter. The left side of the disc will see the oncoming air as faster (U + W), but that won't actually affect the lift. The only aerodynamic effect the added rotational speed will have is that it will have added friction with the air. The air on the left side of the disc will be experience more friction than the air on the right side, meaning the air on the left side will be travelling slower. This means the left side will generate less lift than the right side, so the center of pressure would actually slide to the left.

Now this seems to contradict what we all experience in practice, so there's got to be something else at work. And I don't really know what that is.

 

[Olorin]

Toro71, have you ever read Hummel's frisbee thesis? Fascinating and informative reading.

I wonder if using an ultimate lid makes is slightly different from a golf disc, though. For example, with ultimate lids turn opposite of DG discs. That is, long RHBH throws turn right and RHFH throws turn left.

 

[Olorin]

However, at any moment in time, the point that is on the left side has a higher forward velocity than the point on the right side. This is what creates the differential lift which actually causes the left side to elevate.

Why do you believe this? What is the source of this statement? Would you please provide evidence to prove this?