Disc Physics...???

[Royalhghnss]

And finally: this explains why a beat up disc will turn more. The beat up disc will cause more friction between the disc and the air, thus creating a bigger force by the Magnus effect.

The Magnus force: https://www.youtube.com/watch?v=23f1jvGUWJs

Please tell me that I've got this figured out now?

PS: I'm sorry for my bad english, I'm from Norway..

Pretty sure the chief reason a disc acts more US as it ages is the nose being bent down, not the roughing up of the surface. It might have some effect, but nothing compared to the wing changing shape.

 

[Doofenshmirtz]

If I've understod this correctly . . .

You most clearly have not. Although, to be fair, not everyone apparently agreed with all of the discussion in this thread.

The Magnus Effect is NOT what makes a disc turn over. If the Magnus Effect affected discs, it would cause them to change direction without rolling left or right. Magnus Effect probably has little to no affect on a thrown golf disc.

The two principles that your should try to grasp are: 1) aerodynamic lift, especially the concept that the center of that lift moves forward and rearward in relation to the axis of spin depending on angle of attack and disc speed; and, 2) gyroscopic precession and it's conversion of that off-center lift to roll.

 

[HyzerUniBomber]

Alright, while I've got you physics masters here:

Does a disc lose any noticeable spin over the course of it's flight or will it be spinning very near it's original rotations per (unit of time)? I've always thought of it as a gyroscope that would not have sustained enough friction from air to slow it down much at all.

Does having a higher rate of spin effect how long a disc will take to end up nose up or is that more a product of the disc forward velocity?

Sorry if I am using the wrong physics words... I am but a simple cave man.

 

[BogeyNoMore]

^ my educated guess is that it certainly loses some spin (angular velocity) due to friction, however, I'm pretty sure that friction (air resistance) has a far greater effect on the disc's translational velocity (i.e forward velocity) than it does on angular velocity. You can often see the effect the spin has on the disc as it hits the ground. If it were spinning slowly, that energy would easily be absorbed by the ground, but clearly there's enough to affect its motion after it makes contact. Unfortunately, rate of spin is much harder to measure in practice (and often harder to observe) than translational velocity.

 

[bikinjack]

Alright, while I've got you physics masters here:

Does a disc lose any noticeable spin over the course of it's flight or will it be spinning very near it's original rotations per (unit of time)? I've always thought of it as a gyroscope that would not have sustained enough friction from air to slow it down much at all.

Does having a higher rate of spin effect how long a disc will take to end up nose up or is that more a product of the disc forward velocity?

Sorry if I am using the wrong physics words... I am but a simple cave man.

I'm of the opinion that a disc loses very little of its spin during flight. This opinion comes mainly from observing a disc land flat in snow or wet grass and watching it spin for a bit while it isn't moving otherwise. Also, just play catch with a lid and see how much spin the disc has near the end of it's flight when it isn't moving forward very much.

 

[Doofenshmirtz]

Does having a higher rate of spin effect how long a disc will take to end up nose up or is that more a product of the disc forward velocity?

Sorry if I am using the wrong physics words... I am but a simple cave man.

If not for lift, a disc would travel forward more like a baseball or bullet. It's flight path would be closer to a simple arc. Lift elongates and flattens that arc by allowing the disc to travel part of its path in level flight. At the end of the flight as lift wanes, the disc begins moving downward. If thrown level to level ground, its spin will keep it oriented level in relation to the ground even as it moves downward. So the result is a nose up "angle of attack," i.e., the disc becomes nose up in relation to the downward path that it is traveling, not nose up in relation to the ground.

However, as the disc moves downward and the angle of attack becomes more nose up, the center of lift moves forward. This lift is converted by precession into a roll and the disc "fades." It also changes direction, just as an airplane that is banking would. So instead of going "nose up" at the end of the flight, it is going "wing up." Because the side that is rolling upward is away from the thrower, it may look nose up to the thrower.

Incidentally, I too am of the opinion that the rate of spin probably slows very little throughout the disc's flight unless it is raining or the disc hits something. But you'd have to somehow measure it to be certain of the rate of slowing.

In any event, I think that the answer to this part of your question is that timing of a particular disc's fade is dependent primarily upon the speed of travel of that disc. A disc that is spinning more slowly will probably fade a little more quickly than if it were spinning faster. But this is probably: 1) affected by the initial rate of spin more than any loss of spin rate during the flight; and, 2) a much less significant factor than disc speed. However, this is just my guess and not based on much of anything really.

 

[ToddL]

Couple of little illustrations I made that should help with the matter.

These are just estimations of what the flow field looks like and where the Center of Lift actually occurs. There are a ton of factors that go into the flow and its separation point. Surface curvature, surface smoothness, air velocity, etc. This is just a vague illustration.

Low Angle of Attack, side view:

Flow is smooth and attached across most of the airfoil upper surface. Flow separation doesn't occur until late on the upper surface, if at all. Center of lift is (presumably) behind the center of gravity.

Low Angle of Attack, top view:

CL is behind CG. The lift force precesses 90deg in the direction of rotation and creates a force at the point X on the drawing. This results in a right roll and a left-to-right flight path.


High Angle of Attack, side view:

Flow is smooth for 50-75% along the upper surface before separating into a turbulent wake. The wake sucks at creating lift, so all the lift is generated near the forward half of the disc. Center of lift is ahead of center of gravity.

High Angle of Attack, top view:

CL is ahead of CG. This force precesses 90deg in the direction of rotation and ends up acting on the right side of the disc. This results in a roll left and a right-to-left flight path.

 

[HyzerUniBomber]

okay, total thread high-jack here - but since you guys are clearly physicists - I want to pick your brain.

We've been discussing over in the Form/Analysis section this idea for a while and I wrote the following:

Thanks guys, I'm in the same boat as quite a few of you... I wanted to tap into that special thing that was completely illusive.

I'd watch my buddy throw, and it was happening so slow and then fast that I just couldn't understand it. Then I'd ask him to slow it down and of course he didn't really know HOW he was doing it... so I would go back to the field and work through the various components (hips, feet, blah blah blah).

I'd end up with a few huge putter shots and think that I'd found that special magic... but what was probably happening was that I was getting lucky and hitting the timing a little better, added with some bigger forces from those hips, etc, but I was still missing on the magic.

I think in my next blog post, I really want to figure out a way to plain-english explain that magic mechanism, which I don't really have a good idea what to call...

The best way I've thought of yet, seems to be that you have a dedicated end-point where you are releasing the disc (the release point). It's at a physical location out in front of you and I'm going to call it Release Station!

You gotta get off the train there at EXACTLY 1:00pm, because that's when your hand will be there.

The single most efficient means of creating hand speed, is to say "how far can I keep my hand away from Release Station, knowing that my hand WILL end up at the release point at 1:00?"

The longer you stay down the tracks a ways... let's say you're still out at "Hand on the Outside Bend" which is 10" from the station until 12:59:59.80 - that means the train is going to have to do about 100mph in that last 2 tenths of a second to get to the station on time.

That's why pulling up to "Hand on the Outside Bend" already doing 80mph, doesn't make any sense... you might blow past the bend and end up 8" from the station, which means that the train doesn't have to accelerate to 100mph to get there on time.

It would only have to go 80mph to make it the shorter distance, which would suck, because we could have just rolled up the bend at 20mph and known we'd waited as long as possible and then exploded to the full 100mph to get there on time.

That's also why hand strength/grip is really important. If you manage to start the acceleration at a nice 20mph and you're right at the last 3" before hitting the station, but you let go because the disc slips out - you might have only accelerated to 80mph because your train blew off the tracks.

Holding on with all that hand speed, multiplied by an opening wrist, means that disc is coming out like a rocket at the station - so find the balance to keep it on the tracks all the way until your release point.

That's the best explanation I've thought of yet.

And just to be clear, the time I used had nothing to do with a clock face on the disc.

I know we often refer to the 9:00 to 3:00 position when talking about this stuff and I don't want to confuse the two things.

Any thoughts on if that makes sense or terms that would work to call that mechanism? I'm thinking its something about accelerating around an arc with a continuous(?) force and maybe on a hinged lever?

 

[sidewinder22]

Coupling? I think that's what Brad Walker called it at least when talking about the multiplier effect. Blake also used a term like golden spiral or something to describe the endless arcing.

 

[ToddL]

There's a certain amount of double pendulum effect in the physics of your arm. From googling around, there are a ton of double pendulum articles relating to tennis and golf. Those are a decent parallel to disc golf. The only real caveat is that we don't get as much of an advantage from the second pendulum since the lever arm is so much shorter (wrist to club head in golf vs wrist to center of disc in dg).
Hell, add the shoulder pivot in and make it a triple pendulum.

 

[Toro71]

^^try looking up stuff on a "Trebuchet," a type of medieval catapult:



I believe that much of the BH throw mechanics are reflected in the physics of this contraption, particularly the "arc stacking" or what have you, and have been thoroughly studied by weapons nerds.


BTW, anyone used to watch "Northern Exposure?" Remember Maggie had Chris throw her mother's piano into the lake with his homemade trebuchet? Called it 'the fling?'

 

[HyzerUniBomber]

http://video.mit.edu/watch/double-pendulum-6392/

That's a pretty good representation of a shoulder -> elbow and how staying loose can create insane whippy distances.

I believe a trebuchet is much more built around the concept of using a shorter arm on a rotational arc on one side of the pivot, with a very heavy dropping weight... which forces the longer arm on the other side to have to accelerate much faster at the end of the longer arm.

 

[zenbot]

BTW, anyone used to watch "Northern Exposure?" Remember Maggie had Chris throw her mother's piano into the lake with his homemade trebuchet? Called it 'the fling?'

I loved that show.

 

[Toro71]

I loved that show.

Totally. That's one that could use a Netflix revival.

 

[ToddL]

http://twu.tennis-warehouse.com/learning_center/doublependulum.php

This graphic shows the motion of an arm and tennis racquet at 0.05s intervals. At the beginning of the swing, the arm and racquet both make very small movements in 0.05s. At the end of the swing, though, the arm moves some distance X, but the racquet moves much more than that, call it twice the distance, 2X (because the racquet gets the base-level motion of the arm plus the added speed from the wrist).

I think this graphic is insanely great.
For the first 0.2s of the swing, the arm and the racquet are moving at the same rotational velocity. However, right at the hit point, the arm effectively stops moving and "transfers" all of its rotation into the wrist. This is the "coupling" effect and gives the racquet a wickedly fast rotational velocity. The racquet goes from 200rpm at 0.20s to 400rpm at 0.25s. That's essentially the same thing as the hit in disc golf; that last split second before the release when the wrist uncurls and adds that extra oomph to the arm speed.
If your wrist stays completely locked with your forearm, the disc will be moving pretty quick, but you could get a nice bonus if you add a little wrist action.
(Again, it won't be as much of a difference as you'd see in tennis because the moment arm from wrist to disc is much less than from wrist to racquet)

 

[Toro71]

http://video.mit.edu/watch/double-pendulum-6392/

That's a pretty good representation of a shoulder -> elbow and how staying loose can create insane whippy distances.

I believe a trebuchet is much more built around the concept of using a shorter arm on a rotational arc on one side of the pivot, with a very heavy dropping weight... which forces the longer arm on the other side to have to accelerate much faster at the end of the longer arm.

Yeah, but the rope attached to the longer side is like the forearm, and it has it's own rotational arc, "stacked on," sort of what I was thinking.

 

[ToddL]

http://video.mit.edu/watch/double-pendulum-6392/

That's a pretty good representation of a shoulder -> elbow and how staying loose can create insane whippy distances.

I believe a trebuchet is much more built around the concept of using a shorter arm on a rotational arc on one side of the pivot, with a very heavy dropping weight... which forces the longer arm on the other side to have to accelerate much faster at the end of the longer arm.

The magic of the trebuchet comes from the double pendulum. Otherwise it's just a differently setup catapult. Heavy weight on small arm ==> high speed on long arm. No biggie there; that's just a catapult. Add a second hinge on the long arm to get the double pendulum, whipping motion and you'll get massive distance.

Good video from the MIT lab. The very first motion of the pendulum is effectively what your arm is doing. After that, the double pendulum breaks down into chaos (which is interesting stuff in its own right).

 

[HyzerUniBomber]

Yeah, but the rope attached to the longer side is like the forearm, and it has it's own rotational arc, "stacked on," sort of what I was thinking.

Ahh yeah, the rope! Forgot about the rope.

Damn guys, my head is hurting from all this learning! I have limited space up here, so if I forget my mom's birthday now, I'm blaming you guys.

 

[Toro71]

http://www.discovery.com/tv-shows/other-shows/videos/time-warp-counterweight-trebuchet.htm

 

[HyzerUniBomber]

I see that there's something similar, but I think the arm is different for this key reason: we're not allowing the arm/rope to equalize on purpose and we're actually rotating the entire trebuchet mechanism.

We're saying that we can hold the trebuchet's projectile back longer (delaying the acceleration out at right pec), THEN with the added force of the rotation of the tebuchet/shoulders - the arm/rope pulls the projectile/disc around the arc with all that added acceleration.