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Old 06-07-2010, 01:20 PM
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SirRaph SirRaph is offline
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Disc Stability Changing - In Terms of an Airplane Wing

I've read some of the threads on this forum regarding the different theories as to why discs become less stable as they're used.

I was thinking about the theory that impacts cause the tip of the wing, or Parting Line Height of a disc to drop lower and lower, which causes the disc to be less stable. But I'm not sure I've seen it elaborated as to why a lower PLH (Parting Line Height) might cause the disc to become less stable. So I thought I might post what I've been thinking:

What if impacts don't just point the wing tip down, but sort of warp the disc's wing to a more downward pointing shape. That is, by bending the nose down, it's stretching to top surface of the wing slightly, and "smashing" the bottom surface of the wing inward, and by effect, shrinking it. So each impact changes the ratio of air friction - from mostly being exerted on the bottom of the wing, to more and more being exerted on the top.

Thinking of it in terms of an airplane wing vs. an F1 race car spoiler:

With tip of the wing pointed up (Overstable) , air would have to travel farther along the (already larger) bottom of the wing than it would over the (already smaller) top. This would cause sort of a "reverse lift" effect, similar to the upside-down wing design in F1 race spoilers. Right-hand back-hand, the disc spinning clockwise, the left edge would get the most wind speed/friction, and so would cause the left side to be pulled down more easily.

With tip of the wing pointed down (after repeated impacts) , air would have to travel farther along the (now elongated) top of the wing than it would over the (now shrunken) bottom. This would cause a lift effect similar an airplane wing. Right-hand back-hand, the disc spinning clockwise, the left edge would get the most wind speed/friction, and so would cause the left side to be pulled up (and potentially over) more easily.

Does that make any sense?

Last edited by SirRaph; 06-07-2010 at 01:25 PM.
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  #2  
Old 06-07-2010, 02:09 PM
jtencer jtencer is offline
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The change in local velocity due to disc rotation and its effect on lift actually has a negligible effect on disc turn and fade. The major cause of this behavior has to do with the conservation of angular momentum.

Personally, I would explain the effect of disc wear as consisting mostly of 2 components.

1. As your disc wears it becomes lighter. This decreases the moment of inertia and makes it "flippier." This is magnified because most of the weight is lost near the rim where it matters most for the moment of inertia. This also increases the glide of your disc since it decreases the effect of gravity relative to the lift generated.

2. You disc also becomes less aerodynamic. This decreases the cruising speed of your disc and makes it easier for you to overpower your disc and turn it over.
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Old 06-07-2010, 02:12 PM
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VictorB VictorB is offline
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Multiple impacts will indeed bend the wing of a disc downward...which makes nose down angles more drastic in addition to what was already mentioned, increasing a worn discs's propensity to fly more understable.

Quote:
Originally Posted by jtencer View Post

1. As your disc wears it becomes lighter. This decreases the moment of inertia and makes it "flippier." This is magnified because most of the weight is lost near the rim where it matters most for the moment of inertia. This also increases the glide of your disc since it decreases the effect of gravity relative to the lift generated.
weight doesn't have very much effect on this, as the moment of inertia difference is negligible in terms of human application of force when sorting out a few grams.
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Old 06-07-2010, 02:20 PM
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Seems to have merit. I personally don't have enough knowledge about aerodynamics so this has little meaning in terms of whether it's right or wrong. But, seems to make a bit of sense to me with what little knowledge I do know about it.
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Old 06-07-2010, 02:33 PM
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SirRaph SirRaph is offline
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Quote:
Originally Posted by jtencer View Post
The change in local velocity due to disc rotation and its effect on lift actually has a negligible effect on disc turn and fade. The major cause of this behavior has to do with the conservation of angular momentum.
Is there any formal write-up that's been done on this? I'm not questioning it, I'm just hoping there's some research we can all pull from.



Quote:
Originally Posted by jtencer View Post
1. As your disc wears it becomes lighter. This decreases the moment of inertia and makes it "flippier." This is magnified because most of the weight is lost near the rim where it matters most for the moment of inertia. This also increases the glide of your disc since it decreases the effect of gravity relative to the lift generated.
I guess I can see how a disc could lose some mass as it chips. But it seems the disc would have to lose A LOT of mass for it to have more than a very slight effect on stability.

Quote:
Originally Posted by jtencer View Post
2. You disc also becomes less aerodynamic. This decreases the cruising speed of your disc and makes it easier for you to overpower your disc and turn it over.
This does make sense to me. I could see how surface wear/chips on the leading edge of a disc could create enough added friction to alter the discs flight. It also makes sense that as a disc warps, a less uniform spin would lessen the stability as well.

Personally, the idea that impacts create a more uniformly down-pointing wing seems sort of implausible. I would imagine that in the case of some molds, with a wing that's pointed slightly up, impacts would bend the wing UP more, which would make the disc MORE stable. But that never seems to be the case.

I can't imagine the larger manufacturers haven't done some research on this...
I'm curious as to why they wouldn't publish their findings.
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Old 06-07-2010, 02:48 PM
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VictorB VictorB is offline
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A wing that's 'slightly pointed up' translates to a mold with a high parting line height.

Even the molds with the highest parting lines will become more understable the more impact they experience. Flexion of a disc folds away from the weakest area (ie a flight plate will bend up when it hits a tree) and cause the wing to slightly bend downward. It may seem implausible, but it is wing based aerodynamics. As the wing is bent downward, the dome also bows up more, forcing air over the flight plate at a higher velocity, and creating more lift - which is interpreted as understability by a golf disc.
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  #7  
Old 06-07-2010, 02:50 PM
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Quote:
Originally Posted by VictorB View Post
Flexion of a disc folds away from the weakest area (ie a flight plate will bend up when it hits a tree) and cause the wing to slightly bend downward.
That makes sense as well.
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