Disc Physics...???

[bradharris]

I'm still not getting this. I'm not saying your wrong, I just don't see how it is possible. Unless the disc flexes wider on the left side I don't see how a circle spinning would be traveling faster on one side.

The rotational speed on either side is the same. The speed relative to the ground (or the air it's traveling through) does change though.

 

[bikinjack]

I'll chime in here with my theory on turn vs. fade and all things disc flight. A disc "turns" in flight for the same reason that it fades. It is due to precession. It is well understood that precession causes a disc to fade at the end of its flight due to the center of lift being forward of the center of rotation with regard to the direction of flight. Turn is the result of the center of lift being behind the center of rotation with regard to the direction of flight, causing the disc to "turn" to the right (for RHBH or LHFH). Both cases are due to the conservation of angular momentum of a rotating mass.

 

[Mocheez]

Any free, rotating body always rotates about its center of gravity.
When you're throwing, the disc rotates about your finger. When you release it, it immediately begins rotating about its center of gravity.

That is only true if the disc is released cleanly from one fixed point.

Read Section 3.1 in the link below. The author states "The moment of inertia is greatest when the axis of rotation is the axis of symmetry."

If, by your assumption, the axis of rotation is always the axis of symmetry, then why would he make this statement?

http://people.csail.mit.edu/jrennie/discgolf/physics.pdf

 

[bradharris]

I'll chime in here with my theory on turn vs. fade and all things disc flight. A disc "turns" in flight for the same reason that it fades. It is due to precession. It is well understood that precession causes a disc to fade at the end of its flight due to the center of lift being forward of the center of rotation with regard to the direction of flight. Turn is the result of the center of lift being behind the center of rotation with regard to the direction of flight, causing the disc to "turn" to the right (for RHBH or LHFH). Both cases are due to the conservation of angular momentum of a rotating mass.

The center of lift is never behind the center of rotation.

 

[bikinjack]

The center of lift is never behind the center of rotation.

This is incorrect. There is no other way a really understable disc could truly turn over.

 

[bradharris]

This is incorrect. There is no other way a really understable disc could truly turn over.

Lift differential between the left side and right side is what causes the turn.

From a paper written at Stanford.

For a wing with an elliptical cross-section, the center of pressure due to lift is offset ahead of the center of gravity. Therefore, if one were to simply throw a flying disc, the lift would also cause a moment on the disc and cause it to flip over backwards

 

[Doofenshmirtz]

The rotational speed on either side is the same. The speed relative to the ground (or the air it's traveling through) does change though.

But, importantly, the speed of the disc relative to the air or the ground is not what creates aerodynamic lift. Lift is caused by the differential in the speed of the air above the disc versus the speed of the air below the disc (Bernouilli's Principle). This is not affected by the disc's spin, but instead by it's forward movement through the air. If lift were caused by the speed of air moving over the disc, then the disc should rise when spinning, even if it is not moving forward.

Another point that is seemingly lost on people who equate pecession with fade. Fade is caused by precession as the angle of attack changes as lift is lost. This change in angle of attack is caused by the loss of lift due to loss of airspeed of the disk, i.e., the disc while remaining more or less horizontal, starts moving more downward. The center of lift moves forward and is converted, by precession, into a roll that we call "fade."

However, precession cannot be ignored earlier in the flight, when the disc is spinning faster. Lift on one side of the disc will also be converted, by precession, into tilt, not roll. I.e., more lift on one side of the disc does not cause turnover. If there were more lift on the left or port side of a disc thrown RHBH, than on the right, it would cause the disc to tilt upward, not roll to the right.

 

[bikinjack]

Brad, I think we're gonna just have to agree to disagree on this one. I've read that before and disagreed with it before. The relative location of the center of lift changes with the forward speed of flight.

 

[Mocheez]

That is only true if the disc is released cleanly from one fixed point.

Read Section 3.1 in the link below. The author states "The moment of inertia is greatest when the axis of rotation is the axis of symmetry."

If, by your assumption, the axis of rotation is always the axis of symmetry, then why would he make this statement?

http://people.csail.mit.edu/jrennie/discgolf/physics.pdf

"Torque-free precession occurs when the axis of rotation differs slightly from an axis about which the object can rotate stably: a maximum or minimum principal axis. Poinsot's construction is an elegant geometrical method for visualizing the torque-free motion of a rotating rigid body. For example, when a plate is thrown, the plate may have some rotation around an axis that is not its axis of symmetry. This occurs because the angular momentum (L) is constant in absence of torques. Therefore it will have to be constant in the external reference frame, but the moment of inertia tensor (I) is non-constant in this frame because of the lack of symmetry. Therefore the spin angular velocity vector (ωs) about the spin axis will have to evolve in time so that the matrix product L = Iωs remains constant." - http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Precession.html

 

[bradharris]

Brad, I think we're gonna just have to agree to disagree on this one. I've read that before and disagreed with it before. The relative location of the center of lift changes with the forward speed of flight.

Do you trust MIT more?

the lift on the front half of
the disc is slightly larger than the lift on the back half

 

[ToddL]

But, importantly, the speed of the disc relative to the air or the ground is not what creates aerodynamic lift. Lift is caused by the differential in the speed of the air above the disc versus the speed of the air below the disc (Bernouilli's Principle). This is not affected by the disc's spin, but instead by it's forward movement through the air. If lift were caused by the speed of air moving over the disc, then the disc should rise when spinning, even if it is not moving forward.

Another point that is seemingly lost on people who equate pecession with fade. Fade is caused by precession as the angle of attack changes as lift is lost. This change in angle of attack is caused by the loss of lift due to loss of airspeed of the disk, i.e., the disc while remaining more or less horizontal, starts moving more downward. The center of lift moves forward and is converted, by precession, into a roll that we call "fade."

However, precession cannot be ignored earlier in the flight, when the disc is spinning faster. Lift on one side of the disc will also be converted, by precession, into tilt, not roll. I.e., more lift on one side of the disc does not cause turnover. If there were more lift on the left or port side of a disc thrown RHBH, than on the right, it would cause the disc to tilt upward, not roll to the right.

Holy crap you're my hero.

 

[Tfire25]

Holy crap you're my hero.

Only if he says "darn you perry the platypus!"

I watch to much stuff with my kids.

 

[ToddL]

"Torque-free precession occurs when the axis of rotation differs slightly from an axis about which the object can rotate stably: a maximum or minimum principal axis. Poinsot's construction is an elegant geometrical method for visualizing the torque-free motion of a rotating rigid body. For example, when a plate is thrown, the plate may have some rotation around an axis that is not its axis of symmetry. This occurs because the angular momentum (L) is constant in absence of torques. Therefore it will have to be constant in the external reference frame, but the moment of inertia tensor (I) is non-constant in this frame because of the lack of symmetry. Therefore the spin angular velocity vector (ωs) about the spin axis will have to evolve in time so that the matrix product L = Iωs remains constant." - http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Precession.html

I believe they're talking about an angular difference between the axis of symmetry and the axis of rotation.
From the Princeton link:

The torque-free precession rate of an object with an axis of symmetry, such as a disk, spinning about an axis not aligned with that axis of symmetry can be calculated as follows: [equation goes here]
where omega_p is the precession rate, omega_s is the spin rate about the axis of symmetry, alpha is the angle between the axis of symmetry and the axis about which it precesses, I_s is the moment of inertia about the axis of symmetry, and I_p is moment of inertia about either of the other two perpendicular principal axes.

They mention nothing about a distance between the axis of symmetry and the axis of rotation -- only an angle.

On a disc, the axis of symmetry would be a line through the center of the disc (also the center of gravity), perpendicular to the flight plate.
The disc is more than happy to rotate around an axis that's not perpendicular to the plate (wobble or flutter). It's not gonna rotate around a point that's not the CG.

 

[ToddL]

double

 

[ToddL]

But, importantly, the speed of the disc relative to the air or the ground is not what creates aerodynamic lift. Lift is caused by the differential in the speed of the air above the disc versus the speed of the air below the disc (Bernouilli's Principle). This is not affected by the disc's spin, but instead by it's forward movement through the air. If lift were caused by the speed of air moving over the disc, then the disc should rise when spinning, even if it is not moving forward.

Another point that is seemingly lost on people who equate pecession with fade. Fade is caused by precession as the angle of attack changes as lift is lost. This change in angle of attack is caused by the loss of lift due to loss of airspeed of the disk, i.e., the disc while remaining more or less horizontal, starts moving more downward. The center of lift moves forward and is converted, by precession, into a roll that we call "fade."

However, precession cannot be ignored earlier in the flight, when the disc is spinning faster. Lift on one side of the disc will also be converted, by precession, into tilt, not roll. I.e., more lift on one side of the disc does not cause turnover. If there were more lift on the left or port side of a disc thrown RHBH, than on the right, it would cause the disc to tilt upward, not roll to the right.

For a visual representation:
https://www.youtube.com/watch?v=eTjGTxSevHE

Any roll the disc experiences is not due to the center of pressure moving left or right (as you might guess would be the case if the advancing and retreating side were generating different amounts of lift), but is instead due to the center of pressure moving forward and backward, which could be due to airspeed, angle of attack, flow separation, etc.

 

[Toro71]

But, importantly, the speed of the disc relative to the air or the ground is not what creates aerodynamic lift. Lift is caused by the differential in the speed of the air above the disc versus the speed of the air below the disc (Bernouilli's Principle). This is not affected by the disc's spin, but instead by it's forward movement through the air. If lift were caused by the speed of air moving over the disc, then the disc should rise when spinning, even if it is not moving forward.

Another point that is seemingly lost on people who equate pecession with fade. Fade is caused by precession as the angle of attack changes as lift is lost. This change in angle of attack is caused by the loss of lift due to loss of airspeed of the disk, i.e., the disc while remaining more or less horizontal, starts moving more downward. The center of lift moves forward and is converted, by precession, into a roll that we call "fade."

However, precession cannot be ignored earlier in the flight, when the disc is spinning faster. Lift on one side of the disc will also be converted, by precession, into tilt, not roll. I.e., more lift on one side of the disc does not cause turnover. If there were more lift on the left or port side of a disc thrown RHBH, than on the right, it would cause the disc to tilt upward, not roll to the right.

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.

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.

 

[garublador]

I'm still not getting this. I'm not saying your wrong, I just don't see how it is possible. Unless the disc flexes wider on the left side I don't see how a circle spinning would be traveling faster on one side.

It helps if you think of the rotation (spin) in terms of linear vectors. When a disc is spinning you can represent the velocity at any point on the edge of the disc as a vector tangential to that point. So, using the coin illustration, if you put a quarter on a piece of paper and write clock numbers around it then imagine it spinning clockwise, the linear velocity of the coin at 9 o'clock is a vector pointing straight "up." At 3 o'clock, it's pointing straight "down." At 12 it's to the right and at 6 it's to the left. Because all of those vectors add up to zero, the disc doesn't go anywhere.

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

Honestly, I don't know how correct that is (I suppose it makes sense to me, but I haven't looked into it that closely), but that's the claim put in my words in case it helps clear anything up.

Here's another analogy I came up with. Pretend you're on a circular, moving walkway that looks like a bit disc with a treadmill going around the outside of it, but on top of the flight plate. Pretend that disc is on wheels and it motorized so it can move linearly, like a car. Pretend that the track is moving clockwise at 1rpm and you're standing on it. The whole disc is moving forward at 1mph. So you're going forward at 1mph relative to the ground (in one hour you'll be one mile away from where you started) and around the disc at 1rpm.

Now pretend someone is walking next to this contraption at the 9 o'clock position and walking 1mph. He'll always be at the 9 o'clock position. When you get to the 9 o'clock position you'll be moving forward faster than him. Otherwise how would you ever get to 10, 11 or 12, all of which are in front of him? When you're at 3 o'clock you're moving forward slower than him. Otherwise how would you get to 4, 5 and 6, which are behind him?

 

[ToddL]

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.

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.

1. The relative airspeed is NOT higher on the left side. See my post on the bottom of page 2. The advancing side of the disc is going faster relative to the wind than the retreating side, but this will not affect the lift of the disc. The disc is not a helicopter. The airspeed relative to the surface of the disc doesn't matter. The only thing that matters is the airspeed on the top of the disc relative to the airspeed on the bottom of the disc.

2. Yea. If there was a significant difference in left vs right side lift, it would actually result in a nose-up or nose-down effect.

 

[jimbosprint]

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.

 

[ToddL]

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

Honestly, I don't know how correct that is (I suppose it makes sense to me, but I haven't looked into it that closely), but that's the claim put in my words in case it helps clear anything up.

Your understanding is correct for a helicopter. The leading edge of the advancing blade sees the air coming toward it at U + W (flight speed + rotational speed). The leading edge of the retreating blade sees the air coming toward it at U - W. The retreating blade is actually cutting into the air backwards (backwards relative to the orientation of the helicopter).

However, for a disc, there's only one leading edge. It sees the air coming toward it at U (flight speed of the disc). The advancing and retreating sides see the air coming at them the same way. The retreating side is not cutting into the air backwards like a helicopter blade does.