This started as my rambling notes on how these things must fly. The short answer is that it is a work-in-progress. The longer answer is that I'm right about a lot of things, but I need to put it all together into a good treatise on the matter. I also need to weed out the errors and match theory to practice.
Flying Disc stability comes from a relationship of spin, angular momentum and speed of flight. Faster spin & more weight creates more angular momentum. The higher AM creates gyroscopic forces which offset the aerodynamic forces acting on the disc, and create a disc which will fly straight, or turn. I'll just call them gyroscopic forces.
Most terms refer to a thrown disc with a clock-wise (CW) rotation. That is from a typical RHBH (right-hand back-hand) throw -- or from a LHFH (left-hand fore-hand) throw. If you throw RHFH (right-hand fore-hand) or LHBH, the direction of rotation will be reversed (CCW), and the effects will reverse too. In other words, you'll need to change left->right and right->left in all the following discussion.
Discs have an initial high-speed portion of their flight, and as they slow down at the end they will reach the later low-speed portion of their flight. The stability needed for one flight regime will different from the stability needed for another flight speed ... and so the disc will turn instead of going straight. I like the Aerobie, I can tune them so they go straight the entire distance; however, what if you want them to hook L/R at the end of flight for a twisted course, or curve L/R during the high speed portion for a curved fairway? The curves have a reason, and a straight-flying disc will need more throws on a given course.
One thing to remember is that all discs will turn left as they slow down at the end of their flight. The speed is slow, but the spin is about the same, and the gyro forces over-power the aero forces.
Why does a flying disc fly really well, say compared to a disc that doesn't rotate when it flies? Well, the real answer is that physics in the name of lift, angular momentum, and gyroscopic precession become involved.
In a wonderful cacophony they cause the darn thing to fly!
What forces are going on here?
One thing to note is that the disc rotation speed is important (most starting throwers can't put enough spin on the disc to make it flight correctly). That's probably one rease I couldn't throw a frisbie well! The reason the aerobie works for me is that I can spin it just fine -- all the weight is in the rim, and that is the only part that needs to spin!
Since disc rotation speed is constant, which means forces due to angular momentum are constant during the flight.
The other thing to note is that aerodynamic forces will be proportional to the speed of the aerodynamic object. Faster speed through air -> more aero force .. slower, less force.
Look at the disc like a wing section, similar to a helicopter rotor blade. The advancing left part of the disc sees a higher relative wind-speed on the top of the disc, which creates more bernoulli lift (higher speed lower pressure). More airspeed, more lift to the advancing (left) side.
The center section of the disc (unlike a pitched rotor blade) will see a lift due to angle-of-attack. This lift is uniformly upward, doesn't create a pitching moment, and just keeps the disc flying (like a funky wing). Well, actually all wings create a pitching moment, it depends on the CP an the CL. Typically this causes a pitch-up action during flight.
The retreating side of the disc has a lower relative airspeed over the top of the disc, and gets less bernoulli lift (and perhaps even a stall).
The end result is that the disc wants to flop over to the to the retreating side -- right in the case of the CW thrown disc.
Why doesn't the disc just turn over and tumble in flight?
That's due to the gyroscopic effects (angular momentum), which act 90 degrees ahead in the direction of rotation. That means, that the lift forces can't raise the left edge of the disc -- instead, due to gyroscopic precession, the nose of the disc will want to lift instead. This pitch-up will increase the angle of attack of the disc, which will slow it down a bit, reducing the right-turning gyroscopic forces, and also keep the disk demanding more lift (and level flight) as airspeed decreases.
Well, say the disc speeds up? For example, throwing into a wind (a head-wind), or thrown with a higher speed? This increases the speed over the advancing part of the aerfoil, and generates more lift and flipping moment. This extra flipping moment is then moved 90 degrees forward, and it causes the disc to pitch up at the front ... which slows it down. This is why a disc thrown hard or into the wind climbs! The stability mechanism is trying to keep it speed stable by pitching it up.
What if the disc slows down (say in the middle of flight)? The lift differential L/R will decrease and the upforce on the left will decrease. This is the same as the disc wanting to tip to the left (right up). Due to gyroscopic precession, this pitches the back of the disc up, or the front of the disc down. This will decrease the AOA of the aerfoil, and increase speed. Voila, the disc descends slowly, but keeps flying at the level speed.
XXX this is bogus... what am I missing?
At the end of the flight, the disc starts getting really slow. Slow enough that the overall lift force decreases (due to slow airspeeds) But, the angu angular momentum forces remain about the same, since the disc speed is relativeley constant. Disc slows -- gyro forces cause it to roll