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Old 03-09-2009, 08:20 PM
Rameka Rameka is offline
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Default Explanation of the physics of flying discs (FIXED)

I messed up the images on the other one. Sorry for any inconvenience. This is the fixed version...please just let the other one die as it is confusing and problematic without the right diagrams.

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For those of you who are curious, this is my attempt to explain why discs do what they do. I've done a bit of research and, although the information doesn't seem obviously available, after a bit of looking I was able to piece together this model. I'm only a lower level physics student, so if any of this is off, forgive me (and correct me, if you know more than I do!).

Please note that I am not an exceptionally strong player, merely a curious mind. So, here we go.

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As most of you should know, all discs (yes, all, overstable and understable alike) follow the same pattern in terms of physics. That is, they have a tendency to fight to go to the right on a RHBH throw (turn) and, as they slow down, they fight to come back to the left (fade). Discs that resist turn are still affected by turn, and discs that hold a turn throughout their path still fight to fade back over. Given enough height, all discs would fade back over to the left, regardless of their high speed turn rating. The Innova diagrams, for example, cut off at a certain point in the flight because they assume that the disc will hit the ground at some average point. The math-proficient among us may have noticed that these flight diagrams mostly resemble x graphs in nature.


Figure 1: Paths of overstable and understable discs, generalized.

Okay, that's easy enough. All discs have an S shaped flight path (like an x graph)...in some cases, the faster or slower section is diminished or augmented, but this is just a manipulation of the natural S flight path which all discs share. Even in cases like the Whippet or the Viper from Innova, the discs still fight to turn in their high-speed phase...the shape of the disc has merely been manipulated to vastly compensate for this.

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Now, for a more in-depth explanation of why discs turn and fade.

Discs, at least on the outer rim, act a lot like golf balls. Now, obviously they aren't spherical, so there are differences (for now, accept this simple explanation: different shape, different physics...see the diagram below for a bit of a better explanation). However, the outer surface area reacts in a similar way. Due to air friction, there is a lot of pressure on the nose of the disc as it flies through the air. However, due to "separation" of air, there isn't as much pressure on the tail of a disc (we are not speaking about the underside or the topside or the rims at this point). This is just like a golf ball, and is the guiding principle behind why things slow down in the air and come to a stop, in the x-coordinate (IE parallel to the ground...this has nothing to do with gravity).

Here's how the outer rim acts like a golf ball (will be more related in a moment, read on):

Figure 2: Dorsal view of disc, disc is flying up.

Here's how the disc does not act like a golf ball:

Figure 3: 3/4 side views of disc, disc is flying to the left.

Glide: In the latter diagram, the vertical cross-sectional airflow is shown. This is what causes glide, which only needs a brief description. However, it is very important for explaining turn and fade. The main thing to note here is that for air, speed is inversely proportionate to pressure. The speed over the dorsal surface of the disc is higher, because the air needs to catch up with the air on the ventral surface. Thus, it has less pressure. Inversely, the air on the ventral surface is slower, because it doesn't need to travel as far (all the way over a dome), but the pressure is higher. Because the pressure on the ventral surface is higher than the pressure on the dorsal surface, we get lift, or glide. This counteracts gravity and allows discs to actually fly for as long as they do. On a side note, yes, domier discs are more understable because of this!

Turn: When a disc is fired off, it carries a lot of speed in its initial path. This causes the disc to act like it ignores the friction of air for a short period of time. Because we can count air friction as having a negligible effect during this period, all that factors into the tendency of the disc is torque. The direction of torque on a RHBH throw is clockwise (if seen from above). This means that the disc has a speed about its center of gravity which is different from its speed at its port and starboard wing.


Figure 4: Dorsal view of disc, disc is flying up.

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 other part is dominated by...

Fade: Fade is not as easy to explain. Fade happens because of a phenomenon called precession. Did any of you play with gyroscope toys when you were kids? Or even things like tops? Precession is the change in the direction of the axis in rotating objects. Precession is not off-axis torque, or in any way associated with it. Precession happens because of a pressure gradient, just like turn. You see, pressure that builds up in the underside of the disc due to slower air flow isn't radially symmetric. There's a point (which differs for different discs, obviously) called the center of lift. To reiterate, the center of lift is generally in a different spot from the center of gravity. This creates an unbalance on the horizontal plane of a disc as seen from behind, just like turn.


Figure 5: 3/4 side views of disc, disc is flying to the left. Air flow is shown in blue again.

Since air slows down in the pocket of the underside of the disc, pressure builds up in this initial area of the pocket, where air is slowest of all. In other words, the leading half of the disc experiences the most pressure; this should seem intuitive. This is the center of lift. Since the disc is spinning clockwise (looking down from above the disc), this pressure's force acts, and then continues to act, for a large portion of the spin, shown above in pink. This causes the starboard wing of the disc to lift up, and, consequently, the port side to dip down (also shown in pink). This causes the disc to fade.



Different flight paths are merely the result of different manipulations of the shape of the disc (the convex of the outer under-rim, for example) to compensate in one way or another. As many others have pointed out, throwing a disc with more power than it was intended will cause it to turn over quicker and not fade back as easily...this is due to the massive pressure gradient created by differing velocities. Throwing a disc with less power than it was intended for will do the opposite; it will allow precession to take over the flight pattern sooner and cause the disc to fade hard earlier on. Obviously release angles are a lot simpler, but the principles above apply to differing release angles in just the same way. For example, releasing an understable disc hard at a hyzer angle will cause a "hyzer flip", where the disc pops up to stable from a hyzer. This is due, again, to the pressure gradient caused by differing velocities.

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There's one more thing I want to try explain, all the way back from Figure 2. This is partially hypothetical because I have not seen this in any research, but am merely extrapolating from my knowledge of other objects in flight. This is about why a disc becomes more understable as it "beats in". A golf ball, as we all know, has dimples. Dimples cause air turbulence around the sides of the ball, which causes the air to adhere to the sides more because of the increased surface area. In a similar way, when you beat up a disc, it gets nicks and scratches all around the outer circumference. These nicks and scratches act like dimples, and cause the points of air separation to come later, meaning that the disc has a smaller wake. This smaller wake means the ratio of pressure from the front and the back of the disc is less, causing the disc to stay fast for a longer time, just like the dimples in golf balls.

That's just my guess. If anyone else could confirm or deny this, I'd be delighted.

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Thanks for reading. I hope this was at all informative for some of you. All the images and text is original, so I'd appreciate if you asked before quoting this or using any of the images. I'll always say yes, but I'd just like to know first. Cheers!



Sources:
www.aerospaceweb.org
www.wikipedia.org
www.odgc.ca
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Old 03-09-2009, 08:47 PM
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BrotherDave BrotherDave is offline
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*puts .45 to head, pulls trigger* I really tried to read this but I never did understand physics, or math for that matter, being a history major and all. I prefer to think that the disc works by magic and that the invisible hand of Jesus causes it to turn and fade. Thanks anyway though, I'd be way too lazy to research this.
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Old 03-09-2009, 09:20 PM
Rameka Rameka is offline
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Quote:
Originally Posted by BrotherDave View Post
*puts .45 to head, pulls trigger* I really tried to read this but I never did understand physics, or math for that matter, being a history major and all. I prefer to think that the disc works by magic and that the invisible hand of Jesus causes it to turn and fade. Thanks anyway though, I'd be way too lazy to research this.
That's cool...I know this'll just go over some people's heads. I had fun piecing it together though, so if even if no one gets anything out of it but me, it was worth it
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Old 03-09-2009, 09:44 PM
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swellerdiscgolf swellerdiscgolf is offline
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I understood some of it, but man thats a lot of info to read at 9:45pm. Are you studing to be a rocket scientist? This is a lot of interesting info, keep it up.

On a side note, how do the dimples help or hinder the "DT" discs from Quest?
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Old 03-09-2009, 10:01 PM
Rameka Rameka is offline
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Quote:
Originally Posted by swellerdiscgolf View Post
I understood some of it, but man thats a lot of info to read at 9:45pm. Are you studing to be a rocket scientist? This is a lot of interesting info, keep it up.

On a side note, how do the dimples help or hinder the "DT" discs from Quest?
You mean like the Defender? The dimples on the outer part of the dorsal section of the flight plate for that disc would cause the turbulent air there to "stick" to the disc longer, allowing for a delay in the formation of the wake, as new, fast air currents rush in. This leads to a smaller wake, which means the disc slows down...slower. A disc that stays fast for longer is more understable, because the impact of precession is not enough to make a disc going that fast fade.

So the Defender (and discs like it) should have a more extreme high-speed-turn because of the dimples...and it is, according to Joe's Flight Chart.


Addendum: No, I am not studying to be a rocket scientist...I'm a second year B.Sc student who hasn't even chosen his major yet

Last edited by Rameka; 03-09-2009 at 10:04 PM.
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Old 03-09-2009, 10:26 PM
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Nice work, Rameka. I haven't read a more understandable and complete description of the physics of disc flight. I appreciate your time and effort, pretty cool analysis.
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Old 03-09-2009, 10:36 PM
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bjreagh bjreagh is offline
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Wow! That is good work. Cool diagrams.

Is there anyway you can explain how and why wind affects disc flight. (headwind, tailwind, left to right, and right to left.) I think I understand based on the above info, but just want to make sure.

Thanks.
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Old 03-09-2009, 10:48 PM
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Wow very well explained! I understood most of it.

I do have one question. I have heard some people say to pull down on the disc as if to aim towardas the ground from your full reached back position. Is this because you will get lift on the front of the disc after it travels some distance in nose down position? Does the nose down position have less drag thus increasing distance?
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Old 03-09-2009, 10:54 PM
Rameka Rameka is offline
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Quote:
Originally Posted by bjreagh View Post
Wow! That is good work. Cool diagrams.

Is there anyway you can explain how and why wind affects disc flight. (headwind, tailwind, left to right, and right to left.) I think I understand based on the above info, but just want to make sure.

Thanks.
As you probably know, it is recommended that a fairly overstable disc be used for headwinds. As far as I can tell, this is because increasing the airspeed towards the disc causes the difference between the velocities of the port and starboard wings to increase. This means there is even less pressure, in the initial part of the flight, on the port wing, and far more on the starboard wing. This causes the disc to bank to the right (for RHBH) for a more sustained length of the flight. An overstable disc is used to compensate for this tendency to turn right. Without the right amount of speed, however, this can have a reverse effect, as the pressure difference from the nose to the tail of the disc is much more extreme, meaning the distance is compromised either way. Keep this in mind.

For tailwinds, the opposite is true. The difference between the velocities of the port and starboard wings gets smaller, and may even level out. This causes the turn to be very subdominant, and the fade takes over earlier. So, in tailwinds, most people use straight or understable drivers as opposed to overstable ones. Again, though, keep in mind that if thrown with too much power, the disc will hold a turn for a long time in a tailwind, because the higher pressure on the tail means the disc won't slow down as fast.
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Old 03-09-2009, 11:00 PM
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Try some of these theories out on your physics advisor. You may have an interesting topic for a seminar here (not to mention a good excuse for throwing some plastic on school time!)
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