Disc too over/understable? There's a simple solution...

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Re: Disc too over/understable? There's a simple solution...

Postby JHern » Tue Jan 08, 2013 11:50 am

Peter172 wrote:I was reading in another article that domier discs fly slower and that causes them to seem more overstable, since the slower a disc flies, the more HSS it seems to have. The flater topped version of the same disc flys faster (I'm thinking domey topped vs. "Vulcan" topped destroyers) is less HSS, since it's moving faster. Sounds reasonable to me...


Why does that sound reasonable? Sure, a domey disc has greater cross-section projected along the air stream (but only when thrown flat, at angle of attack of several degrees the difference becomes negligible) but since the dome is a smooth feature (hence encouraging smooth air flow over the top) it has much less affect on drag than you'd think.

Stability has everything to do with how interaction with the air lifts the nose up or presses it down. Bend the PLH up, and you get more downward deflection of air at the leading edge of the disc...balancing forces you can see that air deflected down=nose lifted up. Nose lifting causes the disc to precess over-stably. Nose depressing causes the disc to precess under-stably. Of course, it is all ultimately a balance between the air flow over the top and the air deflected under the bottom, such that the optimal perfectly stable PLH will change depending on the top you put on the disc. But as far as modulating stability, you don't realize a great deal of nose lifting or depressing by messing with the top, the action at the leading edge of the disc (the nose) is much more influential.

The one thing that a larger dome should do, and the physics is pretty straightforward, is to increase the aerodynamic lift (glide). This is why airplanes have wings that deform during take-off and landing, so that they have greater lift at slower speeds. The air stream flowing over the top of a disc will move faster when it is domey than when it is flat, because it must travel a slightly longer distance. Lift is proportional to the air speed squared, so even a slight change will have a larger than linear effect.

But the lift feeds back into stability, by amplifying the sensitivity. The stronger you are lifting it, the more torque you will exert when the center of lift is offset by a given amount, thus the greater the turn.

Man, I really need to get my disc aerodynamical measurements going...only 8 months left until we move to Tokyo, and no longer have a car to mount the rig!

Much of the rest has to do with correlations between PLH and domey-ness inherited from the disc manufacturing process. We've discussed some of the physical reasons behind correlations in domey vs flat and PLH in another thread regarding disc cooling and contraction after plastic injection molding...that story is fairly straightforward: A weak flight plate (think FR Blizzards) domes up because it doesn't resist the contraction of the rim, but this leaves the PLH high because the flight plate doesn't exert much force on the rim. A stronger flight plate, on the other hand, torques the PLH down on cooling because it exerts a resistive force on the top of the rim, so as it shrinks it droops the wing. This is the essential reason Blizzard discs can be molded up very light, but still fly stably...it would be a mistake to think that it is all about the dome. In fact, many people I know flattened the dome on these early blizzard discs (with the bubbles in the flight plate) using hot water, and it didn't affect the stability so much as it affected the sensitivity to nose angle (i.e., amplification of torque by lift, as discussed above).
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Re: Disc too over/understable? There's a simple solution...

Postby jubuttib » Wed May 29, 2013 4:57 pm

JHern wrote:
Peter172 wrote:I was reading in another article that domier discs fly slower and that causes them to seem more overstable, since the slower a disc flies, the more HSS it seems to have. The flater topped version of the same disc flys faster (I'm thinking domey topped vs. "Vulcan" topped destroyers) is less HSS, since it's moving faster. Sounds reasonable to me...


Why does that sound reasonable? Sure, a domey disc has greater cross-section projected along the air stream (but only when thrown flat, at angle of attack of several degrees the difference becomes negligible) but since the dome is a smooth feature (hence encouraging smooth air flow over the top) it has much less affect on drag than you'd think.
Been thinking about the same thing lately, struggling with my domey C-MD2s (I don't want them to be useless).
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