Hi, I''ve just joined GameDev.net having only recently come across this excellent site...anyhow... Does anyone have any idea on where i could find some solid information on the physics of frisbees or disc flight, possibly with some computer simulations or source code i could get a look at? Thanks for your time

i would try google if you haven''t already, but other than that i don''t know.

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shurcool
wwdev

Frisbees generate lift just like a regular airfoil (pressure imbalance between top and bottom surfaces... or view it as potential vorticity), although because of the rotation about an axis perpendicular to the direction of motion, the resultant lift force does not act through the centre of mass. Our illustrious forum moderator, Graham, is particularly knowledgeable on these matters... perhaps he might share some references with you... although both he and I would support shurcool''s suggestion of using google.

Cheers,

Timkin

Actually, I wrote a 6-degree-of-freedom frisbee flight simulator for Worldbook a few years ago, and it can be found in the Worldbook Multimedia Encyclopedia on CD-ROM (1998 and later), . And our simulator was actually quite accurate while a computer game called "Frisbee Golf" from Headgames was horribly bad and wrong.

Its actually hard to find the right references using google to answer this question (though I do strongly advocate google for searches in general!).

I don''t have many references, but I will say that the "How Things Work" reference is basically useless. Its the short answer for kids. There used to be a book dedicated to the physics of frisbees a while back, but its out of print and I forgot the title. The most useful information I''ve ever seen was a technical paper out of Russia from the 1960''s, wild stuff. There is one document on the web that might prove useful (though its incomplete):

www.people.virginia.edu/~cjm9x/school/documents/phys105_frisbees.doc

This link may also prove useful:

www.afda.com/skills/physics.htm

I''ll offer a few comments.

It isn''t because of spin the the center of pressure is not through the center of mass. The center of pressure would not be through the center of mass even if the frisbee were not spinning. Just chalk that up to the weirdness and asymmetry of aerodynamics, adverse and favorable pressure gradients, and stuff like that.

The spin does cause the aerodynamic load to be asymmetric left-to-right. So if the frisbee is moving forward, the side moving into the same direction as the frisbee''s motion will have more lift than the side moving away from the direction of motion. This causes the frisbee to rotate about the axis through its center of mass and in the direction of forward motion, with the higher-lift side moving up and the lower-lift side moving down. (Wish I had a way to host images here, but if you have access to the encyclopedia there are some images in the simulation that are helpful.) Once the frisbee starts to flip/turn onto its side, the lift force will be partially acting to the side, causing the frisbee to actually turn towards the direction the frisbee is spinning (i.e., if it is spinning clockwise and you throw it flat and level, it will eventually rotate and start turning towards the right). (If the frisbee was thrown at an angle and does not begin flying flat and horizontal, then it could be facing the other way and in this case it might start turning away from the spin.)

It would take some discussion to provide quick-and-dirty formulas to approximate the lift force, and I don''t really have time to do this. But here''s a clue. For a well-thrown frisbee, the total lift force will be approximately the weight of the frisbee, plus or minus a few percent depending on how the frisbee is angled into the wind. Assume that the center of lift is halfway between the center and the rim, in the direction of forward flight. That''s not exactly right, but its close enough for a game. To calculate the moment caused by asymmetric lift, assume that lift is caused by two vectors, offset left-to-right from the axis of flight by half the radius. And make them different in magnitude by some moderate amount that is a function of the spin rate.

Gyroscopic effects are also incredibly important when it comes to simulating frisbee flight. The gyrosopic effects interact with the asymmetric lift to cause the unique turning motion of the disc. While you will get a disc that turns to the side without gyroscopic effects (due only to asymmetric lift), it won''t look quite right. You won''t get a correct-looking simulation if you don''t deal with gyroscopic effects.

I hope this information helps. I realize it may raise more questions than answers, .

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

quote:

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Original post by grhodes_at_work
It isn''t because of spin the the center of pressure is not through the center of mass.

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I didn''t suggest that it was. 8)

quote:

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Original post by grhodes_at_work
The spin does cause the aerodynamic load to be asymmetric left-to-right. So if the frisbee is moving forward, the side moving into the same direction as the frisbee''s motion will have more lift than the side moving away from the direction of motion.

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Which means that when you look at the resultant (average) lift force taken over the surface of the disk, its not acting through the centre of mass... which is what I was trying to suggest without all of the extra explanation. Obviously I should have either a) put in the explanation; or, b) left it to you! 8^)

Thanks for the extra info too!

Cheers,

Timkin

Try "Build Your Own Flight Sim in C++"
here or here (Used). The graphics part is outdated (DOS), but the physics should be fine. Full source code is on the CD.

I bought Christopher Lampton''s book on flight sims a few years ago (when DOS was still in power). Although I did learn a few things from the book, the code was awful and it didn''t really cover the physics of flight very well. Tonight when I go home, I''ll look through it again to be sure but either way, I''m sure there are newer books that would have more to offer. This being a DOS-based book with bad coding style (then again, aren''t all authors sloppy coders?) I really wouldn''t recommend it.

...ummm, lemme put it this way: imagine Andre LaMothe''s code but only a thousand times worse.

Now I haven''t read this book but I have a lot of faith in O''Reilly books so I''m sure it''s worth adding to the collection. Hmmm, actually, I''m getting my credit card out now. This sounds killer...

Topics covered: Mathematical formulas and sample C/C++ code for physics for simulations and games, basic concepts in physics, Newton''s Laws of Motion, coordinate systems and vectors; mass, center of mass and moment of inertia; kinematics (velocity and acceleration), constant and nonconstant acceleration, 2-D and 3-D particle kinematics, rigid body kinematics, angular velocity and acceleration, force (force fields and friction, fluid dynamic drag, buoyancy, springs and dampers, torque), 2-D, 3-D, and rigid body kinetics; collisions (impulse-momentum, impact, linear, and angular impulse), projectiles (simple trajectories, drag, the Magnus Effect, variable mass), simulating aircraft (geometry, lift and drag, controls), simulating ships (flotation, volume, resistance, and virtual mass), simulating hovercraft and cars (including stopping distance and banking during turns), basic real-time simulations (integrating equations of motion, including Euler''s Method), 2-D rigid body simulator, implementing collision response (including angular effects), rigid body rotation (rotation matrices and quaternions), 3-D rigid body simulator for an airplane (including flight controls and rendering), multiple bodies in 3-D (including implementing collisions), particle systems, reference tutorials for vector, matrix, and quaternion mathematical operations

Okay, I''m sold.



- Jay

"Strictly speaking, there is no need to teach the student, because the student himself is Buddha, even though he may not be aware of it." - Shunryu Suzuki

Get Tranced!

quote:

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Original post by Timkin
The spin does cause the aerodynamic load to be asymmetric left-to-right. So if the frisbee is moving forward, the side moving into the same direction as the frisbee''s motion will have more lift than the side moving away from the direction of motion.

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Yeah, I kind of figured out what you were trying to say after I posted my reply, . As a studied aerodynamicist, I tend to think about center-of-lift in the sense of leading-edge to trailing-edge, aka front-to-back, and not left-to-right. And my comment on location of center of lift with no spin was in that sense. Although it is the spin that largely causes the lack of symmetry left-to-right.

coderx75, I agree with your comment about Chrisopher Lampton''s book. I remember seeing that in Bookstar half a decade ago and deciding not to buy it because it looked like it lacked any real meat. David Bourg''s book (your amazon.com link) is better, but is quite a mixed bag. Some things are good, and some bad. The flight physics section seems to go into too much detail about some things (the concept of boundary layers), but is a good sort of introductory book. I''m looking forward to David Eberly''s "Game Physics" book, due out by the end of the year. Although it too will probably to cover a little bit about everything and will be a mixed bag as well.

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

Thank you very much for all the advice and information, i did do google for information but it was hard to find specific and pertinent information.

Thanks again.

quote:

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Original post by grhodes_at_work
As a studied aerodynamicist, I tend to think about center-of-lift in the sense of leading-edge to trailing-edge, aka front-to-back, and not left-to-right.

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That surprises me somewhat, since wings and most other real world objects have very interesting asymmetric pressure fields and I know you know this, given your former work and studies. Perhaps it''s too much staring at two-dimensional images of airfoil sections in books!

On the frisbee front, I''m particularly interested in the gyroscopic effects you mentioned, since these don''t come up in most other aeronautics situations and I therefore haven''t come across them in that context before. Would you have the time to elaborate a little more on the effects these have on lift/flight dynamics please?

Thanks,

Timkin