Spin vs Speed

[treethacker]

wow! This is making my head hurt....

 

[colodiscgolfer]

I typed a few things... then erased them... just got back form being back stage at the yonder mountain string band concert at red rocks, and decided to keep my analytical mind shut for the time being...

 

[colodiscgolfer]

Oh boy, the room is spinning again, where's my dang revolver at? Stupid Christopher Walken, never returning my stuff.

If you'd like... I can put my "here's what you need to know" results in bold, then you can just skip over the analysis and get the cliff's notes version.

 

[Olorin]

Colo,

Thanks again for taking so much time to explain all this. I have to tread lightly because your an expert and I'm only an informed layman, but I have a few questions and comments. I hope I don't come across as trying to be a wise-guy because that's not my intention. I'm merely seeking knowledge and clarity.

1)I think we need to be clear on our axes (plural of axis). (Aaarggg… I can't figure out to imbed a picture… so please refer to this picture.) The X axis is the direction of flight… the green arrow. The Y axis is perpendicular to the X axis but on the same plane as disc. If the A axis is 12:00 and 6:00 then the Y axis is 9:00 and 3:00. The Z axis runs up and down through the center of the disc… the orange arrow in the picture.

X axis = roll axis (left or right side up)
Y axis = pitch axis (nose up or down)
Z axis = Lift axis (disc up or down)

To actually get the disc to stay flat, without any change in elevation over the high speed portion of the flight, the pitch needs to be somewhere between -2, and -9 degrees from the horizontal depending on the aerodynamic properties of the disc.

2) Would you please clarify this? Have you read Potts and Hummel articles? (I hope you understand that I'm merely asking this as an informational question to see if we have a common reference point. I'm not trying to sound arrogant.) Their work is based on Ultimate shaped frisbee discs, but I assume it's close enough to a golf disc to be valid. As I remember it, they indicate that a frisbee must have a nose up component in order to fly, and that a disc moves toward a 9 degree angle of Attack (AoA).

Basically when the disc starts falling (gravity now stronger than lift), the disc itself will act as sort of a parachute because it now has a horizontal component to the velocity vector.

3) I wonder if you meant "vertical" instead of "horizontal"? Isn't the velocity vector, on the X axis, already horizontal? Doesn't gravity act vertically on Z axis? Maybe I'm not understanding what you're saying.

… and to turn right because the lift pressure is greater on the right side of the disc than the left.

4) I wonder if you meant "left side" instead of "right side"? If you push the left side up (seen from behind) then the disc turns right.

Again, I'm not trying to be nitpicky or just pick out places where you may have made mistakes. This is a complex subject and I just want to make sure that each part is clear, so that we can all follow it. If I'm wrong about anything above that's fine. Please correct it.

Overall your explanations are excellent and make many things clear. Thanks again for contributing to a very stimulating subject. I look forward to reading more of your analyses. Please chime in on the Physics thread too when you have time.

 

[BogeyNoMore]

Inserting image

Just seeing if I could insert Olorin's image...

Guess not.

 

[wolito]

Colodiscgolfer,

I need to buy you a beer! Thanks for such insight on the topic. No doubt you have tried to explain something probably so easy to you in terms in which some of us disc golfers can understand. Thanks for your contributions to this site.

 

[YawningDrone]

Iasked my brother (not a disc golfer..but does possess a degree in physics) to take a look at this thread. Here is what he said:

If I get what everyone is asking I expect that it has to do with gyroscopic motion and fluid dynamics. Basically the disc is acting as gyroscope. This would explain why you see the effect more with a long distance disc if the weight is distributed more to the outside of the ring than evenly. The faster you spin the disc the greater the angular momentum that will be generated. Speed through the air (linearly) is probably better explained through fluid dynamics. That motion should be similar to that of an aircraft wing in flight.

The disc will be in a state of static equilibrium when thrown. The forward movement will produce energy that acts against any destabilization to the front/back. Say some minute thing makes the front of the disc tilt up slightly. That forward movement of the disc will provide energy that stabilizes the disc.

The gyroscopic elements will also stabilize the disc around the spin axis. However, as the disc moves through the air energy will be expended through friction etc.

Hope that helps. One of the guys on the board seemed like he was getting into that (colodiscer?). It is a bit difficult to explain without a board to draw on.

 

[colodiscgolfer]

Colo,

Thanks again for taking so much time to explain all this. I have to tread lightly because your an expert and I'm only an informed layman, but I have a few questions and comments. I hope I don't come across as trying to be a wise-guy because that's not my intention. I'm merely seeking knowledge and clarity.

1)I think we need to be clear on our axes (plural of axis). (Aaarggg… I can't figure out to imbed a picture… so please refer to this picture.) The X axis is the direction of flight… the green arrow. The Y axis is perpendicular to the X axis but on the same plane as disc. If the A axis is 12:00 and 6:00 then the Y axis is 9:00 and 3:00. The Z axis runs up and down through the center of the disc… the orange arrow in the picture.

X axis = roll axis (left or right side up)
Y axis = pitch axis (nose up or down)
Z axis = Lift axis (disc up or down)


2) Would you please clarify this? Have you read Potts and Hummel articles? (I hope you understand that I'm merely asking this as an informational question to see if we have a common reference point. I'm not trying to sound arrogant.) Their work is based on Ultimate shaped frisbee discs, but I assume it's close enough to a golf disc to be valid. As I remember it, they indicate that a frisbee must have a nose up component in order to fly, and that a disc moves toward a 9 degree angle of Attack (AoA).

3) I wonder if you meant "vertical" instead of "horizontal"? Isn't the velocity vector, on the X axis, already horizontal? Doesn't gravity act vertically on Z axis? Maybe I'm not understanding what you're saying.

4) I wonder if you meant "left side" instead of "right side"? If you push the left side up (seen from behind) then the disc turns right.

Again, I'm not trying to be nitpicky or just pick out places where you may have made mistakes. This is a complex subject and I just want to make sure that each part is clear, so that we can all follow it. If I'm wrong about anything above that's fine. Please correct it.

Overall your explanations are excellent and make many things clear. Thanks again for contributing to a very stimulating subject. I look forward to reading more of your analyses. Please chime in on the Physics thread too when you have time.[/QUOT

you have found some chinks in may armor, yes, you are correct on many of these things. Meant vertical, said horizontal. These are not something I have taught to others before, and learning as I go. IF you ever want to learn about why a sailboat can get lift, or a wind turbine can get power... I'll chew your ear off. Discs are a new thought to me, and an interesting one.

 

[bikinjack]

Could AoA have something to do with a disc turning to the right, and then fading to the left at the end of flight? The AoA does change throughout flight, initially negative, then positive at the end, at least on a throw that turns to the right.

 

[A.Mutt]

Aaarggg… I can't figure out to imbed a picture

here ya go:

 

[sloppydisc]

I'm just a sales guy. I think my brain just exploded. Then again, we have had a bunch of Ivy League Grads and Ph D's in Gov't for years and see where that is getting us. Just because it works in a book or on a chalkboard doesn't mean an actual person can make it work. Otherwise we'd be flying like Luke Skywalker or moving like Keanu in the Matrix.

Just go throw!

 

[BogeyNoMore]

When I posted my first thread on the forum, I wanted a simple explanation of how velocity and spin affect a disc. There was some discussion, but I never really got what I felt was a good explanation. I finally think I got it.

Many factors affect disc flight, but let's try to keep it to these (to the extent possible) by assuming we hold all other factors constant:
• Velocity: the initial forward velocity of a disc when it leaves your hand.
• Spin: the angular velocity of a disc when it leaves your hand.
• Speed/Speed Rating: a somewhat arbitrary number intended to provide an idea of what initial velocity a disc must be have to obtain the intended flight pattern relative to other discs by that manufacturer.
• Stability: a disc's tendency to turn (or resist turning) during the high speed portion of the flight – perhaps a bit mangled at times, but please work with me here.
Just so we're clear: Unstable ≠ Understable

The following are my opinions based on my knowledge of physics (fairly solid, but by no means comprehensive), personal observation, a bit of research, and to some degree, dissection of the English language. I'd to get a dialogue going so that everyone can understand how these factors affect disc flight. What do you agree with, disagree with, want to elaborate on, or have questions about? Sorry for the uber long post, but it wouldn't fit in a Tweet. Please keep it simple where possible.

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Spin gives a disc stability. If you "push" (i.e. no spin at all) a disc off a platform with an initial velocity, it will; flounder as it goes through the air, won't develop much lift or glide, won't go very far, and won't have a predictable, easily repeatable flight path. With absolutely no spin, it's completely unstable (i.e. it has absolutely no stability – specifically not talking about a stability rating of 0.0 on anyone's scale).

Launch the same disc at the same initial velocity, but this time with spin. It won't flounder as it travels through the air, will have significantly more lift/glide, will travel much farther, and have a much more predictable (i.e. repeatable) flight path.

That sentence bears repeating: Spin is what gives a disc stability. No spin = no stability.
A disc that requires a lot of spin to maintain a stable flight at its intended velocity, is said to be "understable."
A disc that requires relatively little spin to maintain a stable flight at its intended velocity, is said to be "overstable."
A disc that requires a moderate amount of spin to maintain a stable flight at its intended velocity, is said to be "stable."
Presumably, every disc is stable at some velocity with a given rate of spin.

Example 1: Say you have a disc that you can throw quite smooth and straight at its intended velocity. That disc is stable for you given your unique (or not so unique) body mechanics, throw, and style. Another way to put it is that disc is stable at that specific combination of velocity and rate of spin (i.e. velocity:spin ratio).

Example 2: Same disc, same amount of spin, but significantly less initial velocity. We all know what will happen: it will "newb hyzer" …because its velocity:spin ratio is too low. It behaves "overstable" because you didn't get the disc "up to speed."

Example 3: Same disc, same spin, but significantly higher initial velocity. Again, no secret here: it will turn over …because we've exceeded the velocity:spin ratio it was designed for. It behaves "understable" because it was thrown at a greater velocity than the spin it was thrown with could completely stabilize given the design of that disc.

The key here is the ratio between velocity and spin. Most of us can easily control the velocity with which we throw a disc, yet we have little to no ability to accurately control how much spin we impart to it. If we did, we could adjust the amount of spin to compensate for how fast we throw the disc.

In example 2, if you could decrease the spin by the right amount (i.e. to match the velocity:spin ratio at which the disc was stable in Example 1), you would achieve a straight and stable flight. But since the disc has less initial velocity (i.e. less kinetic energy) to begin with, it won't go as far as it did in Example 1.

Likewise, if you could increase the spin in example 3 by the right amount, (again, matching the disc's stable velocity:spin ratio), you will also achieve a straight and stable flight, but since the disc had a higher initial velocity, it will travel further.

This means each disc actually has a "continuum" of different velocities and rates of spin for which it is stable. Once you find the "sweet spot" in example 1, you should be able to throw it faster or slower and still keep it stable, as long as you vary the spin to maintain the velocity:spin ratio for which that disc is stable (i.e. more spin to keep the disc stable at a higher velocity, less spin to keep it stable at a lower velocity).

But since we can't easily control the amount of spin we impart to the disc, we change the stability of the disc instead. Manufacturers make discs of different stabilities to accommodate a range of different velocity:spin ratios in order to achieve a stable flight.

Assuming you throw with the same velocity and spin every time, say you throw a disc and it follows the flight described in example 2. Assuming you want a straighter flight, you could:
a) Throw a disc of similar stability, but with a lower speed rating (i.e. designed to be thrown at a lower velocity than the disc thrown in example 1) at the same velocity. This would help you achieve the velocity:spin ratio the disc needs to achieve stable flight. Or…
b) Throw a disc of similar speed rating that has less stability to arrive at the velocity:spin ratio necessary to achieve a straight, stable, flight. Some will say this isn't the "right thing to do," but that's a different discussion, and IMO not relevant to understanding this aspect of disc physics.

The same holds true trying to straighten out shots with discs you always seem to turn over. You can:
a) Throw a disc of similar stability, but a higher speed rating - so that it's being thrown near to the velocity:spin ratio required for its stable flight.
b) Throw a disc with of similar speed rating, but greater stability, such that the velocity and spin you throw it with is close to the velocity:spin ratio that disc requires for stable flight.

Once you achieve a stable flight with a disc, here's what varying any single aspect of velocity or spin will do:
• Increase velocity: disc will turn over - behaves "understable."
• Decrease velocity: disc will not turn over, but will fade earlier in its flight – behaves "overstable."
• Increase spin: as I've said spin creates stability. Increase spin, and disc will fade earlier, and behave overstable.
• Decrease spin: with less stability (as a result of less spin), the disc is more likely to turn over (or won't fade as early if it isn't getting up to speed in the first place), and behave "understable."

Two people can throw the same disc with about the same power and get very different results if their mechanics impart substantially different amounts spin to the disc. Likewise, they may be able to throw seemingly different discs at about the same power and get quite similar results, because they impart very different amounts of spin to the disc.

Since want we don't always want the disc to go dead straight, we can vary stabilities (for discs of similar speed ratings) and pretty much throw the same shot to get them to hook hard left, hook mildly left, go straight, turn over and come back, or turn over and hold that line. We can also change speed ratings to achieve somewhat similar results.

I could go on, but it's already too long.
Time for discussion (if anyone bothered to read it all).

 

[Halcón]

Holy wall of text Batman!!

 

[BogeyNoMore]

post deleted.

 

[BogeyNoMore]

Holy wall of text Batman!!

LOL! I got carried away... and a lot more.

Reader's Digest version:

Once you achieve a stable flight with a disc, here's what varying any single aspect of velocity or spin will do:

• Increase velocity: disc will turn over - behaves "understable."
• Decrease velocity: disc will not turn over, but will fade earlier in its flight – behaves "overstable."
• Increase spin: as I've said spin creates stability. Increase spin, and disc will fade earlier, and behave overstable.
• Decrease spin: with less stability (as a result of less spin), the disc is more likely to turn over (or won't fade as early if it isn't getting up to speed in the first place), and behave "understable."

 

[sidewinder22]

Stability is determined by the aerodynamic design and weight distribution. Spin maintains the disc's orientation/nose angle, not its stability.

 

[BogeyNoMore]

Stability is determined by the aerodynamic design and weight distribution. Spin maintains the disc's orientation/nose angle, not its stability.

Agreed that the design of the disc determines its stability as a dynamic attribute. What I'm saying is that spin is a major contributing factor. Without spin, a disc has no stability - it just has a rating.

For any disc of a given stability rating, increasing spin will make it behave more overstable. Decreasing spin will make it behave more understable.

I'm not trying to imply that spin alone determines a discs stability, but that a disc's observable stability (i.e. in flight) is a function of velocity:spin ratio as well as it's design.

If a disc is turning over and you don't want it to, throwing with more spin (not that it can really be done) will also increase the discs HSS during flight. But since you can't really control spin that well, they make discs of differing stabilities.

I'm just trying to isolate what happens to a given disc when velocity and spin are fiddled with.

 

[sidewinder22]

For any disc of a given stability rating, increasing spin will make it behave more overstable. Decreasing spin will make it behave more understable.

Statement is not true. Overstable disc's want to go to the ground, more spin doesn't do that, it's not acting overstable. Decreasing spin makes disc fade to the ground as it loses its attack angle, that's not acting understable.

 

[BogeyNoMore]

So spin most directly affect the disc's ability to maintain its attitude (not a typo) in flight?
I also need to look into exactly what attack angle refers to - my first inclination is that of pitch (nose up/down), but I'm not sure.

 

[turkey67]

Ok, I've run this over a few times and basically it comes down to the height at which the disc is at when the gravitational forces begin to take over the lift forces generated by the discs forward motion. A lot of assumptions need to be made to get a clear answer on the same spin different speed case. The original question poses the situation where the disc is thrown perfectly flat. This really isn't the best way to get the disc to go straight because of the misalignment of the center of lift pressure, and the center of mass. When folks say 'nose down', there's a reason. To actually get the disc to stay flat, without any change in elevation over the high speed portion of the flight, the pitch needs to be somewhere between -2, and -9 degrees from the horizontal depending on the aerodynamic properties of the disc.

I'll run two scenarios here, one where both discs are thrown at exactly the same height (actually impossible with the same spin and different speeds), and another where the faster disc is higher (more likely scenario) when the lift forces due to the speed are overcome by gravitational forces. Basically when the disc starts falling (gravity now stronger than lift), the disc itself will act as sort of a parachute because it now has a horizontal component to the velocity vector. The spin of the disc is still creating a lift pressure that is centered somewhere directly ahead of the disc's center of rotation, in the direction of travel. The amount of lift decreases in a clockwise direction around the disc when viewed from above (because of the RHBH spin direction). To visualize the lift vectors, picture the threads of a screw that has been cut in to a piece where there's one full revolution of the center post in the threads. The highest thread is at the front of the disc. (I know this isn't exact, but a good idea of the general picture we're looking at) This will cause the disc to turn more nose up because there is a lift force greater on the front of the disc than on the back, and to turn right because the lift pressure is greater on the right side of the disc than the left.

If we assume they are beginning to drop from the same height:
The higher speed disc will have lower spin velocity because it has traveled further, and thus more damping has occurred to the spin velocity. This will translate to a lower lift ratio both front to back and right to left than the slower thrown disc. This means that if it is falling from the same height, the higher initial speed disc will fade less at the end of it's flight than the slower speed disc.

If we assume the faster disc has gained more elevation than the slower disc:
It all comes down to how much the difference in height is. The faster disc will have further to fall, but the angle of turn is also decreased as in the first case. They may land in the exact same distance off to the left of the original line of throw, but the slower disc will turn harder, and doesn't have as far to fall. In this case there's too many other factors that come into play to really give a 100% dead sure answer unless we make a ton of other assumptions. Basically... the answer is 'it depends'. The big thing to take form this is that the faster disc will have a smaller degree of fade from the original direction, but will carry further in this fade.

Case 2 will be continued at a later time...

Mind blown.