Disc flight for noobs: fact check?

[Phil Esra]

Hey, here's an email I sent to some noob friends. I'm a noob myself--did I get anything seriously wrong here? Your thoughts please!

Regarding the question of why discs turn more and fade less as they get "beaten in": the basic mechanism for "lift" on a wing is differential air pressure top vs. bottom. The faster the wing is moving through the air, the more lift is generated. Easy enough. The disc is a spinning wing. So (simplifying), one side of the disc is moving forward and one is moving backward, relatively. The side moving forward sees a greater air speed and therefore greater lift. I don't know if that causes a change of direction, or just a change of tilt which then causes a change of direction, but that's the basic dynamic for turn.

Based on this, then, you end up with these:
For lefties, turn is to the left, and vice versa (for backhand throws).
The harder you throw it, the faster it goes, and the more it turns.
The lighter it is, the harder/faster you can throw it, so the more it turns.
Because the disc's "wing" is only being supported at the top (by the "flight plate"), when the disc hits something, the bottom edge is weaker and tends to fold inward. Equals more convex. Equals greater lift, equals more pronounced lift differential, equals more turn.
More turn = "understable." More fade = "overstable."

Also:
When the disc is being made, it pops out of the mold hot, and during cooling it becomes more convex. (To varying degrees depending on the different types of plastics and disc shapes, and even the ambient air temperature.)
In my experience, all else equal, lighter discs beat in faster. I picture the less-dense plastic being less strong (they use various fillers to change the plastic's density). But it might just be because I can throw them harder/faster, resulting in harder impacts with trees and stuff.

The "fade" is caused by "gyroscopic precession"--a force is generated at a 90-degree angle to the axis of rotation, pushing away from direction of rotation. This force is always there, BUT as the disc slows down, the aerodynamics become a relatively less important factor (air resistance varies with the square of velocity, so the change relative to disc speed is pretty pronounced), and this gyro force starts having a relatively more important effect on the disc's flight. Therefore, as the disc slows down at the end of the flight, it fades. Also, the gentler/slower the throw, the relatively larger role of fade--so you end up with a different flight if you throw it slower than you normally do.

Disc manufacturers tinker with wing size and shape and overall disc profile to tweak flight characteristics. The type of plastic also plays a big role in initial disc shape (post-mold, post-cooling, before throwing) and in how the disc flies as it beats in. Using Innova's plastics as an example, the spectrum of durability is DX -- Pro -- Star -- Champion. (There are sub-flavors too, which I don't know much about. G-Star is tweaked for cold weather grip, for example. People seem to dislike it for mild CA-type weather.) Star is more expensive than Champion mostly just because the market allows it, I think.

For DRIVERS (especially) AND MIDRANGES: I find Champ pretty damn unforgiving and "overstable"--lots of fade. It is VERY slow to change. Star is supposedly a mix of "mostly" Champ plastic plus some Pro plastic (~80/20). I think it's the sweet spot--throwable but durable.

For PUTTERS: Almost everyone prefers base (cheap) plastic for putters. More forgiving/softer, and putters fly slow (even when used as driver), so they don't change fast.

 

[xare]

There's too much being said here for me to offer commentary on everything in one shot. I'll just bring up one thing... why the obsession with "convex"? Which side up do you think they cool the discs on (this impacts the disc too)? What about tuning discs? The reason most people prefer base plastic for putting is for the grip and better glide.

The rest of it I don't really want to comment on since trying to explain physics makes my head hurt. But the general premise is thrown right-handed backhand, the disc is spinning in a clockwise motion, but faster on the leading side of the rotation (left side in this case). That's why it fades in that direction.

 

[wolfmandragon]

No, look up gyroscopic procession, that is what causes turn and fade. The turn is always 90 degrees from where the lift is being generated. There are threads, nay even scientific papers, dedicated to this.

 

[ToddL]

A disc is not a helicopter. The advancing side does not generate more lift than the retreating side.

 

[PWaggoner]

Here we go again...

 

[roggenb3]

Here we go again...

Yay physics! :hfive:

 

[tomkat]

.. that's kinda/sorta like what I do with my noob friends:

1) this is a disc, not to be confused with a frisbee, we throw it a bit different and be kinda careful, it hurts when it hits you

2) watch me and then you throw yours kinda like I do towards that metal thingy out there ... don't worry, if it doesn't go real good I will pick you out one that flies better for you, I have plenty ...

3) then we gab and tell each other jokes on our way to pick it up and throw it at the thingy again ...

4) the first one to put theirs in, is the champion of the entire free world (until the next hole)... and then we get to do that about 8 more times, telling stories and having a ball the entire way ...

... as soon as we are done, they usually ask (with a grin from ear to ear) when can we do it again?!?!

... I send them home with a disc, and tell them to check YouTube to see how others are doing it ...

.. if I did it right, they are hooked .. just like I was

 

[Phil Esra]

... why the obsession with "convex"?

Sorry, I mean "domey" : )

Which side up do you think they cool the discs on (this impacts the disc too)?

No idea. Do you know? Do they vary it? Is the magic of FAF due to upside down cooling?

The reason most people prefer base plastic for putting is for the grip and better glide.

Oh yeah, that sounds familiar. I don't totally get the grip thing, since putting is so much less "rippy" than driving, but I've never tried to putt in subfreezing weather.

A disc is not a helicopter. The advancing side does not generate more lift than the retreating side.

Damn! I read *most* of two gigantic old physics threads a few months ago, but apparently I misremembered the conclusions. As I recall, you had some invaluable insights on one or both threads. So, uh, why does a disc turn?

.. that's kinda/sorta like what I do with my noob friends:

1) this is a disc, not to be confused with a frisbee, we throw it a bit different and be kinda careful, it hurts when it hits you...

Heh, well, that was us a few months ago for sure. But now everybody wants to know why their DX Leopard doesn't fly the same any more (and I'm the only one with a surplus of free time)...

 

[wolfmandragon]

Gyroscopic procession. Most discs create lift on the nose, not all, but most.
Also, lift is not the exact same thing a glide.
I guess this argument goes on until I have enough money to build a wind tunnel.

 

[Phil Esra]

Ah, OK, I reread some of the later posts in one of those two threads (http://www.dgcoursereview.com/forums/showthread.php?t=2250). I see that both ToddL and Wolfman chimed in at different points. To my layperson brain, based on that previous thread, the simplistic answer to "why does a disc turn and fade?" is, "a changing imbalance in center of gravity vs. center of pressure tries to turn the nose of the disc up or down, but gyroscopic precession turns this into a left or right tilt." Fair enough? This is largely based on the summaries reproduced below from rybob and Clark.

The related question is "why does a disc's flight change as it beats in?" There seem to be two schools of thought--surface damage (golf ball dimples, more or less) vs. altered PLH (in my head, = "more convex"). IMHO, the altered PLH hypothesis is waaay more compelling. Comparing a DX disc and Star disc that have both beaten in a similar amount, the DX disc is a ragged mess, while the Star disc just looks lightly scuffed. This seems totally incompatible with the dimple hypothesis.

...It'd be helpful to look at this image when reading: http://picasaweb.google.com/orcsroc/DiscAerodynamics#5319157673425650690

1. Why does a disc turn?

The center of pressure (ie, where the lift force is centered; CP in the image) is ahead of the center of gravity (CG in the image). That means the lift generated by the disc tends to make it rotate (ie, a torque will be applied) in such a way that the nose goes up or down (Nose Up in the image). So if the disc was thrown without spin, this would cause a rapid change in pitch: basically the disc would flip end-over-end and not go anywhere. But because the disc is spinning, it behaves like a gyroscope. Torque applied to a gyroscope along its axis of spin cause rotation at right angles to the axis (called gyroscopic precession). What this means for the disc is the torque that would change the pitch actually cause it to roll. Or in terms of the image, the spin transfers "Nose Up" torque into "Bank Left" torque.

2. Why does a disc turn right at high speeds?

The disc turns right at first because at angles of attack near zero (ie, nose flat), the flight properties of the disc actually makes it pitch nose down. So the precession or spin transfers nose down torque (aka negative "Nose Up") to bank right torque (aka negative "Bank Left").

This happens more at high speeds because lift is proportional to the square of velocity. So if you throw twice as fast, the disc will generate four times the lift. Lift is the force ultimately causes the roll, so more speed means more lift means more roll.

3. Why does a disc fade left at low speeds?

The disc fades left late because the angle of attack increases over the flight of the disc (see #5). At angles of attack over 10 degrees or so (depending on the disc), the flight properties of the disc make it pitch nose up - relatively hard, too, as the angle of attack increases past 10 degrees. Again the precession transfers this "Nose Up" torque into a roll to the left.

4. Why do some discs turn or fade more than others?

The disc shape and mold changes the flight properties of the disc to have varying centers of pressure, lift and pitch moments at different angles of attack. In the Potts and Crowther papers Olorin linked to, they tested three shapes: a throw-and-catch frisbee, a flat plate, and something in between those two. They each had very different flight properties, so there is a lot of play in disc design to give discs different flight properties. For example, I suspect a disc that holds its line (like the Buzzz) may have its center of pressure closer to the center of gravity, making less pitch torque which results in less roll. That's just speculation, but you can imagine the effect varying some of these properties would have on the flight of a disc.

5. Why does a disc's angle of attack increase during flight?

I don't know. I haven't seen an explanation for it, only a statement that it occurs. One idea is that there is a measurable roll moment when spinning, according to Potts and Crowther, which might pitch the disc because of precession.

rybob's post is followed immediately by this one:

...Points 1-4 above are spot on!

The angle of attack is defined as the angle between the plate of the disc and the flight path. If you throw flat, the angle of attack is zero. If you spin the disc and drop it straight to the ground, the angle of attack would be close to 90 degrees.

You will notice that the disc fades back to the left only after it has reached its maximum height. This is because the angle of attack is increasing past that 9-10 degrees point that marks the zero-pitching-moment angle. Due to the spin on the disc, it doesn't nose up or down (for the most part) during its flight.

Assume your disc is still spinning flat as it drops from its apex. The air going past the disc is rushing up at it. If you can imagine that air going straight from right to left, then the disc would be angled up compared to it. This is a positive angle of attack. Your point #3 takes over from here and you get fade on the disc.

 

[Phil Esra]

I guess a more basic question is, why does tilting a disc make it turn in that direction? (The stuff above just takes that behavior as a given.) It seems like the answer must be that g.p. isn't an all-or-nothing on-off switch, so you get some combination of nose-up/tilt-left or nose-down/tilt-right, which results in an arcing/banking path instead of a disc just going in a straight line doing a barrel roll along the axis it's traveling on.

 

[ru4por]

I guess a more basic question is, why does tilting a disc make it turn in that direction? (The stuff above just takes that behavior as a given.) It seems like the answer must be that g.p. isn't an all-or-nothing on-off switch, so you get some combination of nose-up/tilt-left or nose-down/tilt-right, which results in an arcing/banking path instead of a disc just going in a straight line doing a barrel roll along the axis it's traveling on.

Why ask why?

 

[wolfmandragon]

I guess a more basic question is, why does tilting a disc make it turn in that direction? (The stuff above just takes that behavior as a given.) It seems like the answer must be that g.p. isn't an all-or-nothing on-off switch, so you get some combination of nose-up/tilt-left or nose-down/tilt-right, which results in an arcing/banking path instead of a disc just going in a straight line doing a barrel roll along the axis it's traveling on.

Gravity. The nose angle is the direction of the greatest wind speed in respect to the downward pull of gravity. Part of the lift is no longer 180 degrees off from the pull of gravity. Lift is still pushing the disc in the direction of the flight plate. There still is turn and fade on the disc even on hyzer and anhyzer shots.
This is why understable discs can be hyzer flipped and overstable discs can be flexed.

 

[wolfmandragon]

Gravity. The nose angle is the direction of the greatest wind speed in respect to the downward pull of gravity. Part of the lift is no longer 180 degrees off from the pull of gravity. Lift is still pushing the disc in the direction of the flight plate. There still is turn and fade on the disc even on hyzer and anhyzer shots.
This is why understable discs can be hyzer flipped and overstable discs can be flexed.

Excuse me, nose angle is it the angle of tilt vs the vector... The above statement is true if the disc is released perfectly flat though. I tried to over simplify the answer

 

[dontBLAMEtheDISC]

wrong thread

 

[Phil Esra]

Why ask why?

From a strictly evolutionary-psychology perspective, there is no good answer to that question.

Excuse me, nose angle is it the angle of tilt vs the vector... The above statement is true if the disc is released perfectly flat though. I tried to over simplify the answer

Mmm, nope, can't quite wrap my little brain around that--I will have to chew on it some more. But it seems possible to me--maybe even likely--that flying discs have been flying incorrectly this whole time.

 

[discRabbit]

From a strictly evolutionary-psychology perspective, there is no good answer to that question.

Evo-psych doesn't have good (or at least compelling) answers to any question
My thinking is for my doing, not sure about everybody else's

 

[wolfmandragon]

wrong thread

There are no threads, everything is off topic.
This is DGCR!

 

[SonicGuy]

I am always enraged at the BS pushed by public schools. Everyone was taught that faster relative air speed = lower pressure and thus lift. Nope, not how wings work, a complete fabrication. But generations of Americans have been made to sound like idiots via public education physics class. Not to turn this political, but if they can't get hard sciences correct, how do you think they do on history and politics?

At any rate, the speed of the surface area spinning through the air does not cause lift. The angle of attack does.

 

[ToddL]

Your quote from rybob is correct.

High speed and/or low angle of attack ==> Center of lift moves backward
Low speed and/or high angle of attack ==> Center of lift moves forward

When the center of lift is behind the center of gravity (same thing as geometric center of the disc), the lift force will precess in the direction of rotation and produce a roll. For CW spin (RHBH or LHFH), the gyroscopic precession will lift the left side of the disc and produce an understable roll.

When the center of lift is in front of the center of gravity, gyroscopic precession will lift the right side of the disc into an overstable roll (fade).

The disc will see a high angle of attack as it starts to fall back toward the ground. Even if the disc is still flat relative to the ground, if it's descending at a 10deg angle, it sees the air hit it at 10deg. Therefore it's flying nose up (and will also be at a lower speed) and the center of lift will be pulled forward.

Depending on the exact geometry of the disc, the center of lift might be extremely sensitive or not-so-sensitive to these factors. An Aviar flies fairly straight whether it's high or low speed or high or low angle of attack. The wing is shaped just correctly so that the center of lift doesn't stray very far away from the center of gravity. A Katana flies understable at high speed/low angle of attack but overstable at low speed/high angle of attack. That's just a result of how the wing is shaped. The location of the center of lift is very sensitive to the speed/AOA.

I guess a more basic question is, why does tilting a disc make it turn in that direction? (The stuff above just takes that behavior as a given.) It seems like the answer must be that g.p. isn't an all-or-nothing on-off switch, so you get some combination of nose-up/tilt-left or nose-down/tilt-right, which results in an arcing/banking path instead of a disc just going in a straight line doing a barrel roll along the axis it's traveling on.

Even when it rolls, it's still producing lift.
Lift is produced perpendicular to the flight plate, not the ground. When the disc is flat, lift will pull it straight up. When the disc is tilted 45deg to the right, lift will pull it partly up, partly to the right. When the disc is upside down, lift will pull it downward toward the ground.

I am always enraged at the BS pushed by public schools. Everyone was taught that faster relative air speed = lower pressure and thus lift. Nope, not how wings work, a complete fabrication. But generations of Americans have been made to sound like idiots via public education physics class. Not to turn this political, but if they can't get hard sciences correct, how do you think they do on history and politics?

At any rate, the speed of the surface area spinning through the air does not cause lift. The angle of attack does.

Bernoulli still holds, despite the popular backlash against it. On a streamline (or within a flowfield with constant initial conditions), if the air speeds up, its static pressure will decrease. This is an absolute 100% fact.
On an airfoil, the air over the top is moving faster than the air over the bottom. If you can calculate the velocity field, you can use Bernoulli's equation to determine the pressure field, and from that you can calculate the lift. This is again a fact.
A Newtonian approach also works. If you can calculate the amount of downwash off the trailing edge, you can perform a momentum balance to calculate the upward force on the airfoil.

Bernoulli and Newton are both legitimate methods to calculating how much lift is created. Neither of them really explain the why question, and it shouldn't be taught that they do. The answer to the why question is worth a few years of grad school.