Disc Physics

[JHern]

Yeah, that would be a big turnover. What kind of parameters led to this flight? 275' of lateral movement at a decreasing speed and spin? Was there a tornado nearby? Just kidding, and curious.
KP

No wind here, just a big arm. I don't recall the parameters.

Its more like 150' of right-left-right, when measured off the line from release to landing.

[JHern]

OK, so I've got this code to simulate disc flights. I'll put it online where everyone can play with it, sooner or later (when I find a server that I can get all the necessary stuff installed on...). It would be fun to put it on discgolfreview.com if that were possible.

In any case, I need some data, for anyone who is willing to pitch in on this effort. I need to calibrate the model for some standard molds, to get the correct lift and drag force coefficients. This could be done with a video camera, a tape measure, and a stop watch. I need to know how the speed at release, total distance travelled over flat ground until touch down (no wind), and total duration of flight in the air, are related for several stable molds. I would propose using Innova molds: Aviar, Roc, Teebird, Wraith. It would be best if the flights were as straight as possible, but a statistical sampling of a bunch of throws that are almost straight is also OK. Speed at release can be measured by filming from a direction orthogonal to the release point and direction of flight, putting a grid (on a painted board, or something) on the other side of the thrower (the board should be parallel to the throw direction) with measured increments clearly displayed. Then the speed can be calculated from the video (this requires good resolution at high speeds).

If anyone else has better ideas to obtain this data, I'm definitely all ears...

[JR]

And didn't the grid need height information as well and shouldn't it be spread out to a large distance? That has intimidated me from trying this and the fact, that the field i was planning to try this on is uneven and the surface changes. It is artificial turf with ground rubber pieces between the plastic fake grass slivers. The field is brushed with a tractor pulling the brush and the surface is always changing and uneven. So i can' get accurate elevation data there. After thinking of this and checking out several sand fields in the town i've noticed, that all of them have so large diameter stones in the mix, that the surface is uneven. I'm not taking cameras out in below freezing conditions on ice and ice is uneven too on many occasions. So i too await info on how this test could be performed with real world constraints.

My high speed camera does not have good resolution, 446x384 or something like that at 210 FPS.

[JHern]

And didn't the grid need height information as well and shouldn't it be spread out to a large distance?

No, this is a simpler scheme than before, the grid needs only be beside the thrower, and using this only to get the speed at release. If you have a radar gun, that could also work.

My high speed camera does not have good resolution, 446x384 or something like that at 210 FPS.

The high frame rate is more important than pixels, as it allows a small grid to be used. The only thing that is really important, is to film as far away as possible, and zoom in to the throw as much as possible, to compress depth and avoid geometric parallax or optical distortion. Ideally, the poster board will be exactly orthogonal to the direction you point the camera, which will in turn be orthogonal to the throw direction.

If you could also figure out a way to get the height at apex, and the nose angle (relative to level ground), it would be even better. Maybe even rough estimates would work.

[JR]

Apex height could be quesstimated by an object of known height. If one were to trust the bubble scale on the tripod the camera would be level. I would not trust my system. Exit speed can be calculated from the disc diameter, because my camera has fast enough shutter speed to eliminate visible blurr from the disc. Of course a 21.2 cm disc leaves measurement inaccuracies. Could you write a program to detect the nose angle of the disc if such image recognitions software isn't available? I would think that image editing softwares would have an angle meter. Paging graphics guys help please for suggestions on software, that would do this.

Could you give specs for the grid? I can't easily haul around large objects. The larger the grid, the larger the cost. There is an accuracy throwing goal on one course around here. Too bad it's aligned so that one would have to throw across two fairways. And the ground is a bog after all the rain. The ground won't carry and will be less slippery only after it freezes. That means that indoors throwing is needed and lighting is gonna induce flicker to the pictures and force too long shutter speeds, that can easily lead to blurred disc images. Indoors halls may very well have indoor soccer goals so measuring one could help in creating the grid.

[JHern]

Apex height could be quesstimated by an object of known height. If one were to trust the bubble scale on the tripod the camera would be level. I would not trust my system. Exit speed can be calculated from the disc diameter, because my camera has fast enough shutter speed to eliminate visible blurr from the disc. Of course a 21.2 cm disc leaves measurement inaccuracies. Could you write a program to detect the nose angle of the disc if such image recognitions software isn't available? I would think that image editing softwares would have an angle meter. Paging graphics guys help please for suggestions on software, that would do this.

Could you give specs for the grid? I can't easily haul around large objects. The larger the grid, the larger the cost. There is an accuracy throwing goal on one course around here. Too bad it's aligned so that one would have to throw across two fairways. And the ground is a bog after all the rain. The ground won't carry and will be less slippery only after it freezes. That means that indoors throwing is needed and lighting is gonna induce flicker to the pictures and force too long shutter speeds, that can easily lead to blurred disc images. Indoors halls may very well have indoor soccer goals so measuring one could help in creating the grid.

If you can calibrate distance by the disc diameter, then that would work great, no grid needed. Or just put something in the scene with known length, that is easier to measure. The nose angle just needs a level reference, and like I said, even just a quick and dirty measurement would be great.

[JR]

No promises, but i'll see, what is possible. My club has an indoor hour per week through the winter and it might not be possible to get the cameras far away, but i've not been there. And no idea about the lighting, so there might be a lot of flicker in the video. The flattest local soccer field is closed, because they are prepping it for ice skaters. Meaning tankers are spewing water onto the field for days in a row now The next best field might be ok, but it's getting colder and wetter all the time and i have two days of throwing behind me and i'm not so fresh. So no promises, but if i'm up to it, i might have a quick and dirty version ready tomorrow, if i'm not dead from previous ripping. Thursday will be rainy so no go and i've got other stuff for the rest of the week.

[JHern]

Hey JR, sounds great. Anything would be awesome. When Avery Jenkins comes back home, I might see about getting him and a film crew out to a field to make some measurements at the more extreme end of the spectrum, just to check that everything scales properly...this is important for validating the model parameterization. This kind of exercise should allow me to fine tune the lift and drag coefficients. Once I can put them together for several different molds, then I can begin to assign physical numbers to Blake's flight chart by mapping, and begin to simulate every disc that exists, even before I get around to wind tunnel testing (the ultimate test).

[jubuttib]

Damnit, I have a slomo camera (210 fps, even the 420 fps could be usable), but don't have any means of making the reference grid.

[JR]

I think i must do this today even though i'm sore and stiff and sleep is an issue. Since Avery might help i'll use Innova only. I have to go through my stacks of discs to get discs, that are in new conditions. For limiting fade Roc and Leo are given. I have at least one lighter Champ Beast, that should be in pristine order. Unfortunately I'm outta luck with Innova putters in new condition. Leo needs to be hyzer flipped and the Beast needs a lot of power and even lower lines to avoid the onset of fade. I've just woken up and off the top of my head can't think of other common discs from Innova, that have no fade options other than on very low lines. i think my Star Dart would be ok, but throwing it low at high power is new to me, so i'll have trouble finding the correct initial hyzer angle for a flip to flat.

I think i'll buy a small water scale for level reference at the apex. I'll take a tape measure with me to the field i'll go to and will get reference height there, but i'll take a small plank with me as well. It's more accurate, than just using a disc. Lately i've had no luck at all with file sharing sites so i may need to post the results on DVD. That is if i can throw well enough. Off to rummage piles of plastic.