from page 25 and 26
Initially the velocity vector is pointed slightly above horizontal and the lift is sufficiently large to overcome the weight causing the Frisbee to accelerate upward. Because drag and gravity slows the disc down, by the end of the flight the velocity has decreased to nearly 1/3 its original speed to less than 4 m/sec.
Decreasing speed means the lift will no longer exceed weight and the Frisbee will start to fall. However, as the downward component of velocity increases, the angle of attack increases. Lift increases with increased angle of attack; therefore the Frisbee does not fall as fast as it would if the angle of attack remained constant.
Because the pitch angle does not change much, less than 2° (Fig. 2-9b), angle of attack will continue to increase as long as the Frisbee is slowing down and falling. Also, once the angle of attack exceeds 9°, the pitching moment becomes positive (the COP moves forward of the COM ) which results in a negative roll rate and the right side of the Frisbee (viewed from the back) to rotate up, causing the common "left curve" of the disc at the end of a flight.
This is what I would describe as the normal descent flight path of a stable to overstable disc (and is also common for understable discs as well under many throwing conditions) but there is a definite difference physically between this type of descent and the type of descent I quoted in my previous post.
In the "overstable descent" flight path the author says that as the disc begins to fall its angle of attack becomes more nose up and this increases lift, thus giving the floaty descent. The author does not describe, unless I am mistaken, that lift increases in the "understable descent" flight path (or "Downward Steady Glide" phase). This does not seem to fit with my theory that the force of gravity drives rotation as the disc descends and creates lift.
At the beginning of the paper on page 30
Mitchell (1999) also measured Frisbee lift and drag using wind tunnel tests. He confirmed the observation of Stilley that spin affects lift and drag only little.
..among other quotes about how previous studies have shown that spin does not create lift (and autorotation, as I claimed, doesn't happen).
However, I wonder if what we think of as
GLIDE is actually two different things. I definitely don't know the flight numbers of a "frisbee" as it is being discussed here but lets consider the ultrastar, the objectively superior, end game content of the frisbee evolutionary tree. I don't think an ultrastar actually would have that high glide numbers. I haven't thrown a rattler, but people seem to describe the discraft rattler as flying like an ultrastar (besides also being shaped like a mini one) and the rattler
has a glide of 3.
Maybe "ultrastar glide" is really a function of a disc being slowwwwwww and having a deep dish, wide rim that when the nose lifts as the disc begins to descend maximally plows air downwards for lift.
Most of the discs rated for high glide (4/5 and up) are understable fairway to distance drivers and maybe this is just how people talk about putters/midranges vs drivers (glide meaning DISTANCE with drivers especially) but I actually think there is something to this. Going back to the part of the paper where the author describes "Downward Steady Gliding" (I labelled it "understable descent") the author doesn't mention the role of spin here at all, but when you throw a comet, roadrunner, a teebird, river, saint etc... those faster discs that people describe as having an extra bit of glide, I think spin plays a major role in that glide.
I armchair hypothesized that this glide was from autorotation. (As a reminder autorotation is a phenomena of descent where the flow of air upwards past a helicopter blade begins to drive the rotation of the helicopter blade and create lift). Perhaps the opposite is happening.
Perhaps when a disc golf disc enters the "Downward Steady Gliding" flight path, gravity begins to help counteract a loss of velocity from drag but not a loss of spin from drag. Thus the spin-to-speed ratio goes down so the disc progressively acts more flippy as it slows down and this counteracts the opposing tendency of fade. Perhaps this does not induce lift, but with an understable, glidey disc it might keep the flight path of the disc projecting forwards when fade would have won out long ago on a more overstable disc and caused it to dump out on a hyzer.
In other words when you throw a comet/teebird/saint/river whatever cleanly and it enters a downward steady glide after the apex the relationship between spin in terms of speed and stability (which the graph of might look quite different for different discs) keeps the disc cradled at a nice horizontal angle that maximizes the lift capability of the disc (consider that when a plane enters a banking turn, the lift of its wings decreases because the wings are no longer horizontal and the lift force is no longer directly counteracting gravity).
I do believe I observe this tendency in zephyr/polecat type discs too, but I am now thinking these may be unusually slow speed examples of discs that exhibit this phenomena of glide except they also don't have the low speed fade.
From the author's review of the literature it doesn't seem like disc golf discs have been studied tooo deeply, and even if they have I doubt researchers have walked up to a comet and asked "Why is the comet objectively the most beautiful midrange disc to watch fly when it is thrown well?" so I doubt I can find a direct answer to this. It probably takes some pretty subtle, specific design a disc so that when it enters the "downward steady gliding" phase tends to counteract its fade with its increasing flippiness from loss of spin vs speed.
