Disc Shape, Lift Aerodynamics and Stability

[JHern]

I've noticed, as many of you probably have, that the most over-stable discs tend to have a curved-domed upper surface with little flatness (e.g., look at the profile of a Wraith), while the most under-stable discs have a larger flat center region on the upper surface (e.g., look at the profile of a Stratus). As a physicist familiar with gyros and stuff, I know that the more over-stable discs must have more lift on the front of the disc than on the tail end of the disc at high speeds relative to the more under-stable discs...this is what causes the disc to precess/turn in different ways in flight. This knowledge, combined with the flatness observation, means that stability must have a lot to do with the extent of the flat region on the top of the disc and the aerodynamics of lift at the front vs. back of the disc in flight.

My question is this...what is it, exactly, about the size of the flat area at the top surface of the disc that makes the tail lift relatively larger than the nose lift, and hence make it fly in a more under-stable way? Or vice versa, why does a more domed/curved upper surface without much of a flat area make the nose lift relatively larger, and make the disc fly in a more over-stable way?

I'm sure there must be a fairly simple explanation in terms of the aerodynamics, but I haven't stumbled on a simple reason myself.

 

[Rogue Skunk]

My guess: Devil Magic

 

[adamn]

The flat top area may have something to do with it, but I think the Lip or rim of the disc is what makes them unnderstable or overstable. I notice most of the overstable discs have a shallow lip or flatter lip on the bottom while the understable discs have a bulkier, deeper lip. Just a novices observation

 

[Three Putt]

I have no idea what I'm talking about, but I thought lift had to do with the air above the disc needing to move faster than the air under the disc, and the amount of dome would effect that airspeed and move something called the "center of lift." As the center of lift moves closer to the nose, it effects the stability of the disc. Or something. Like I said, I have no idea what I'm talking about.

There are a lot of variables other than the dome that effect stability. A blunt nose will make a disc more stable. A notch on the wing or bead on the rim will make a disc more stable. A convex wing will make a disc more stable. A longer wing will make a disc more stable. All of these variables makes it hard to generalize that any one factor will defiantly lead to an overstable or understable disc.

 

[ryankilgore]

From what I've read and experienced, the dominess/flatness of the disc isn't as important as the shape of the wing. As an example of this, compare a firebird (highly overstable), to an archangel (highly understable). The firebird is flat on top, but has a wing edge that is high, meaning that the very edge of the wing is up close to the top of the disc. On an archangel, the wing edge is lower toward the bottom of the disc, and it has a lot of dome.

Check out this article: http://www.aerobie.com/Products/Details/EpicTuningArticle.htm.

-Ryan

 

[Three Putt]

Yeah, it's one of the mysteries of disc golf. A flat Roc is more stable than a domey Roc, but a domey Flash is more stable than a flat Flash. Confused? I sure am. The answer has to be in the wing.

 

[Goon13]

Rocket Scientist

From a rocket scientist's point of view

The flight of a disc is a balance between aerodynamic and gyroscopic forces, that is true. But lift is not the only aerodynamic force/moment acting on the disc. Magnus moment and force also act on the disc. Magnus is basically the change in pressure over a symmetric body that is caused the the object spinning (golf balls, baseballs, and discs).

Just to make life more difficult the disc acts as a gyroscope. So any torque applied to the disc (lift forces not acting on center of disc or magnus moments) causes a torque on the disc 90 degrees apart from the original torque. If there were no magnus then a simple lift on the front of the disc would turn it right or left (depending on RHFH or RHBH)

So now the wonderful magic of disc design..... To get that long nice gliding flight, the designer must balance the aerodynamic forces (lift and magnus) and gyroscopic precession so there is no net torque. And to make the life of a disc designer so much fun, the aerodynamic tourque is all dependent on speed, RPM, disc deflection (beat or new), HF/Annie angle, and Mung (?) angle, while gyroscopic forces are dependent on mass properties and RPM.

Now the point - if you have the same disc type (plastic, weight, wear) but with different domes - we can make an educated guess at what will happen. But with most disc comparisons, there are a few variables that change and it's a challenge to know why a disc flies the way it does.

I have bought more discs than I can count based upon my impression of the dome or wing and they have flown nothing like I thought they would.

I have tried to attach an image with the pressures on a Buzzz generated by a CFD code - I can run that, but I can't figure out how to post in this forum. You can see how pressure of the disc varies both front to back and left to right.

Hoped this helped - just another science geek who likes disc golf

 

[Dave Walters]

http://www.skyirondiscs.com/disc-flight.html

 

[Ryan P.]

From what I've read and experienced, the dominess/flatness of the disc isn't as important as the shape of the wing. As an example of this, compare a firebird (highly overstable), to an archangel (highly understable). The firebird is flat on top, but has a wing edge that is high, meaning that the very edge of the wing is up close to the top of the disc. On an archangel, the wing edge is lower toward the bottom of the disc, and it has a lot of dome.

Check out this article: http://www.aerobie.com/Products/Details/EpicTuningArticle.htm.

-Ryan

Ryan, first of all, great name. Secondly, great first post.

And to the OP, if you want to know about disc flight, the first thread you should read is this one. http://www.dgcoursereview.com/forums/showthread.php?t=2056&highlight=physics+disc+flight

 

[ManU]

I think JHern's comprehension is beyond most

he isn't asking this in a noob type manner

 

[yawpstang64]

He was also asking this over 3 years ago.. so I doubt he cares anymore.

Please people, just a quick glance at the date!

*I enjoyed that skyiron article, but what I want to see are those diagrams applied to a vulcan vs an ape.

 

[dantheman0721]

This is an interesting thread. I have a buddy who is a physics major and he goes discing with me sometimes. He's terrible and he's alays trying to point out some BS math or logistics as to why the disc is doing what it is. I wish I could remember some of the things he said, you guys would laugh at it.

Fact is, there is a lot of math and science behind the flights of these discs we thow. I myself would also like to understand, but I don't think my brain is up for it.

As I like to say - Grip it and Rip it!

 

[Ryan P.]

Hah, my bad, I posted the wrong thread. This is the thread I was looking for: http://www.dgcoursereview.com/forums/showthread.php?t=2250&highlight=physics

This is a pretty in-depth explanation of disc flight, and at the end of that thread is a link to this site, which links you to anything you'd want to know about the physics of discs. http://sites.google.com/site/dgresources2/Home/aerodynamics/aero-resources

 

[optidiscic]

Food for thought...my experience with destroyers with a domier top

They do have more glide
They also dont cut through the air as fast as flatter destroyers
Therefore they dont move as fast
therefore they never reach the required speed to meet their flight characteristics
therefore they behave in a more overstable manner
so you are left with a glidy fade out
this is how you get a beefy disc with domey top
I hope that makes some sense

 

[optidiscic]

Food for thought...my experience with destroyers with a domier top

They do have more glide
They also dont cut through the air as fast as flatter destroyers
Therefore they dont move as fast
therefore they never reach the required speed to meet their flight characteristics
therefore they behave in a more overstable manner
so you are left with a glidy fade out
this is how you get a beefy disc with domey top
I hope that makes some sense

also explains why some stable discs in flatter forms behave more understable than typical.....they get up to and exceed their flight ratings and become flippy....think of some flat top versions of ROCS etc

 

[Three Putt]

I think JHern's comprehension is beyond most

he isn't asking this in a noob type manner

Yeah, the problem is that we don't know. We throw stuff and guess at what causes what. We have anecdotal evidence based off our experience. That's not really an answer to the question. To get a scientific answer, I'd talk to a scientist. If I was a scientist, I'd set up some testing.

I'm not sure that understanding all the science behind why a disc flies like it does would really help me all that much as a player, though. It would help me annoy the crap out of the people I play with, though. :thmbup: