flat or domey and its effect on stability

[Tim Carter]

I would like to hear peoples ideas about the overall general effect of disc flatness on stability. It seems to me, and I have discussed w some people, that for certain discs, everything else being equal, flatter discs are less stable than domey ones (e.g. destroyers) while for other discs domier is less stable (e.g. buzzz).

So is there a master theory to address the effect of dome on stability? Thoughts/ experiences?

[Alan MacLean]

Calling on Karl...

My guess is that it would only have an effect on stability if the change in top plate when cooling created a force on the edge profile (pushing it down or pulling it up) as the edge profile is what I think determines the stability. I'd assume that with two discs with the exact same edge profile (shape AND at the same angle) but with different top plates, the "domey" one would have more glide as the pressure differential over the longer-path domey plate would create more lift vs. the flat one... while the additional drag of a domey plate is probably negligible.

I really think edge profile consistency between runs is what has the greatest effect. I've had two identical discs in the same plastic purchased at the same store at the same time. One had a convex bottom rim, one had a concave. Same disc! No way did they fly the same which leads me to believe this is the greatest factor.

BUT... I think we should allow Karl some time to disprove everything I just said...

[Tim Carter]

Thanks Alan. Edge profile definitely is noticeably different in flat vs domey destroyers, less so in my buzzzes. Others say parting line is the key factor (tho that is the same or similar to edge profile right?)

I too await karls wisdom as well as others ideas.

Would soooo rather be throwing than thinking about throwing. Tomorrow!

[Dave Hickson]

Dave Howey were are you?

[Karl Molitoris]

as the edge profile is what I think determines the stability.

It certainly has a LOT to do with it!

I'd assume that with two discs with the exact same edge profile (shape AND at the same angle) but with different top plates, the "domey" one would have more glide as the pressure differential over the longer-path domey plate would create more lift vs. the flat one... while the additional drag of a domey plate is probably negligible.

So did I until 2005 when I got 2 first run Spirits...and the more domey of the 2 (although both had a large dome) was quite a bit more over stable . This was a shock as the Epics and Flicks I had were flat-topped (and over stable) and the Orcs, et al were domier and less over stable. So I was confused....

BUT... I think we should allow Karl some time to disprove everything I just said...

Nope, can't...because most of what you stated I agree with !

And so getting back to Mr. Carter's initial post question...

So is there a master theory to address the effect of dome on stability?

...not sure !

I've had the opportunity to go to CA and "learn" from one of the most intelligent men in the world about fluid dynamics (of which air is one...just not too viscous a one). I've taken 4 years of calculus and my knowledge ends where his begins! It's hard to follow (stupid, wicked technical some times) but I try my best and consider it an honor just to be able to work a little with him a few times a year.
When it comes to stability, there's a lot involved. He and I have done a LOT of testing out there and have talked an awful lot about this. And we're not done. I'll be going out there again in a couple of months.
Having said this, I'll try to list - not necessarily in order of importance - a few of the factors (understanding that with the additive properties of multiple factors comes potential non-linear correlations)...or these things are mostly true IF all other factors are "equal" (on the 2 discs being compared):

a. Concave wings create more over stableness than do convex wings
b. Bigger winged discs will always be "squirrelier" than smaller winged discs (think Boss vs. Buzzz) and thus the 'stability' of bigger-winged disc is accentuated (either way).
c. The more disc surface (where air can 'hit it') there is below what I believe you're calling the "parting line" proportionate to that above that line, the more over stable the disc (think beaded Avair vs. Avair, Wizard vs. Voodoo, or ANY new disc vs. one that's "beat in" - where the edge is knurled over...making the air molecules "see" less of the disc's bottom).

And as a side note...the reason that most drivers "fade" at the end is NOT what most people think! It's because, as the disc slows down (at the end of its flight), it "drops" in altitude at a greater proportional rate. NOW the disc acts like it was thrown "nose up" (the air is now not hitting it at its leading edge but is hitting it lower. And what happens when we throw a driver "nose up"? They hyzer out quicker! Same thing.

A fascinating topic (disc flight), and one that even the most learned people are continuing to learn new stuff about!
Practically (and knowing - as you've pointed out - that there are examples of "both" situations occurring (big domes = more over stable and big domes = more under stable)) the best way is to try one of your buddies discs before YOU buy one !

Hope I helped...well, helped more than confused at least .

Karl

Ps: Glad you didn't ask about "glide". That's an even greater mystery .

[Tim Carter]

Thanks Karl. Lots of factors affect disc flight, definitely. I would just like to understand one thing. When will domier discs be more stable, and when will they be less stable. That being the only variable (while recognizing that in practice many inconsistencies exist between even the same mold of discs.)

[Mike Cormier]

I have never really gone out and bought a multiples of the same disc at the same time so to this point I have tried to just treat each disc as I buy it as an individual BUT, by the way it sounds in your description domey equals an over exaggeration of what the disc was meant to be. Overstable becomes even more overstable and less the stable becomes more understable.

[Karl Molitoris]

Mike,

I'm not sure I'm saying that about "domey", but I am saying that about "wider-winged discs".

TC,
Your...

When will domier discs be more stable, and when will they be less stable

...I can't answer. Yet!
We're getting close but I can't be quite sure without a bunch more "figer'in out" to do.

Karl
Ps: Understand that, at the speeds and - more importantly - the rpms WE (humans) throw discs at, a wide-rimmed disc (say, an Epic or a Katana) will NEVER be able to hold the same "straight line" (think tunnel shot) as a Teebird / Striker / etc., because the wider-rimmed discs wide rim causes lift. Lift which both - at separate times during the flight - 'turns it over' and hyzers it out. Middies don't have that wing and so the air can't exert that lift force on it (and it'll pretty much go on the plane you first launched it on).
IFFFF we could increase our rpms NATURALLY, we'd find that ANY disc could 'hold a line'. That's the concept of a gyroscope!

[Shawn Mullen]

http://www.discgolfreview.com/forums/vi ... =5&t=15165

It's all in the parting line height of the disc and how fast/slow it cooled.

[Karl Molitoris]

It's all in the parting line height of the disc and how fast/slow it cooled

Not sure if "It's all", but it has "some" to do with it! This is akin to my c. point in a previous post. And to make things even more complicated, I'd say - even though the parting line usually denotes where the air molecules have to "make a decision" - do I go over or under the disc as I flow by - this parting line may NOT be the actual point where this occurs.
Example: You could make a PoleCat (or any rather-to-very-flat edged disc) with a parting line near the flight plate or near the rim's edge...and it wouldn't make nary a difference.

As for how fast / slow it cooled, I don't know (never been to a factory).
Perhaps a faster/slower cooling mold imparts a higher/lower parting line, maybe not. But be careful of using JUST the parting line as the "end all". As I mentioned before, it's a combination of quite a few facets (of the disc).

Karl

[Alan MacLean]

Karl, I too took 4 years of calc, a year of differential equations, fluid dynamics, and aerodynamics... that's why I found this topic so interesting, but also knew you'd also be a go-to for info since all my theories were just that... theories, whereas you probably had facts and test data to back it all up.

I never thought of the fading of a disc due to the disc falling as it loses its speed/lift and relatively speaking, acting like a nose-up throw. Makes sense. I always thought it faded left (RHBH) because of the two characteristics we could quantify (speed and spin) that the speed decreased greater than the spin. Because the air resistance is exponentially proportional to speed, discs slow down quickly while the forces to slow the spin are far less. During the speed decrease, spin remains high, so you have essentially two halves of the disc - the left and right (from thrower's POV). The left half is rotating INTO the wind, while the right half is rotating WITH the wind. Therefore as the disc slowed and air resistance to speed decreased, the effect of the spin (assuming rotational velocity has remained somewhat constant) has a greater effect and causes the fade to the left.

A bit of a drift now, but I think the disc golf community has figured out (at least the serious players) that the wider the wing, the more squirrely the flight. Probably because there'e more mass at the inside of the rim (towards the center) than at the edge which probably screws around with its gyroscopic properties... unlike a Buzz which has all that edge mass pushed right to the edge. If you are going to CA to work on disc development, perhaps these ideas (if PDGA friendly) can be considered to make a more friendly wide-rim flyer...

1) Like the Ion putter, make a hybrid disc of two plastics, with the heavier one at the edge of the rim.
2) Or if allowed, incorporate a heavier material, maybe a metal, in a dense ring fully embeded within the outer portion of the rim moving the center of mass (looking at the x-sectional view) towards the edge. No exposed metal though, only plastic.
3) If #2 is the worst idea ever, do just the opposite. Incorporate a "ring of air" inside the rim inside the fatter part (towards the center). This would make the plastic at the outer edge have a greater gyro effect... at least I think. Plus you could make big distance discs in lighter weights... like a Groove... but one people would throw... besides me