Explanation of the physics of flying discs (FIXED)

[Olorin]

About the right turn-- here's a very informative quote from Potts (his p.8, section Flying Disc Dynamics)

"The typical S shaped flightpath exhibited from a backhand throw is dictated by the pitching moment. The angle of attack increases over the flight duration causing the disc to move through the zero pitching moment trim condition, close to 10° AoA. For a righthanded throw, the negative pitching moment on release causes the disc to roll (gyroscopic precession) and bank right, whereas late in the flight the positive pitching moment causes the disc to bank left."

 

[sidewinding]

So by using all this information who can tell me what the best trajectory is for maximum distance with no wind? No cheating by looking at how the distance record holders do it. Science only please.

 

[Rameka]

So by using all this information who can tell me what the best trajectory is for maximum distance with no wind? No cheating by looking at how the distance record holders do it. Science only please.

I already explained this to the best of my knowledge on page 4 of this thread. However, I'm beginning to doubt myself now, after learning how complicated this really is...

 

[trifocal]

About the right turn-- here's a very informative quote from Potts (his p.8, section Flying Disc Dynamics)

"The typical S shaped flightpath exhibited from a backhand throw is dictated by the pitching moment. The angle of attack increases over the flight duration causing the disc to move through the zero pitching moment trim condition, close to 10° AoA. For a righthanded throw, the negative pitching moment on release causes the disc to roll (gyroscopic precession) and bank right, whereas late in the flight the positive pitching moment causes the disc to bank left."

I'm gonna memorize this quote and use it to explain to my partners how i just made that most recent ace. I'll just add " ......whereas late in the flight the positive pitching moment causes the disc to bank left.....and into the Basket."

 

[Dave242]

About the right turn-- here's a very informative quote from Potts (his p.8, section Flying Disc Dynamics)

"The typical S shaped flightpath exhibited from a backhand throw is dictated by the pitching moment. The angle of attack increases over the flight duration causing the disc to move through the zero pitching moment trim condition, close to 10° AoA. For a righthanded throw, the negative pitching moment on release causes the disc to roll (gyroscopic precession) and bank right, whereas late in the flight the positive pitching moment causes the disc to bank left."

I do not think that this is completely correct based on observation (assuming I understand precession and what is meant by "the negative pitching moment on release"): If one thows a very flippy disc with hyzer (and put a positive pitching moment on release), the disc will still flip (turn right).

If I am understanding "the negative pitching moment on release" right, this basically means you are torquing the disc clockwise and the palm of your hand will tend upwards in the follow-through. Positive pitching (throwing with added hyzer) will have your palm tending downwards on the follow-through.

 

[Rameka]

I do not think that this is completely correct based on observation (assuming I understand precession and what is meant by "the negative pitching moment on release"): If one thows a very flippy disc with hyzer (and put a positive pitching moment on release), the disc will still flip (turn right).

If I am understanding "the negative pitching moment on release" right, this basically means you are torquing the disc clockwise and the palm of your hand will tend upwards in the follow-through. Positive pitching (throwing with added hyzer) will have your palm tending downwards on the follow-through.

I think you're a bit confused about terms, Dave.
What you're describing is positive/negative rolling, if I'm not mistaken. Pitch refers to nose up/down.

Vocabulary refresher:
http://en.wikipedia.org/wiki/Flight_dynamics

Positive roll causes a right bank (turn), and negative roll causes a left bank (fade). For a RHBH-thrown disc, the yaw is constantly positive (because the disc is spinning clockwise). This means that a negative pitch causes a positive roll.

What Potts' quote is saying, is if you throw nose down, you'll get a right bank. He also says that the trim condition (equilibrium where the disc flies straight) is about 10 degrees AoA. I'm not sure exactly how informative this is; I might have to go do some tests in a field. Do even really understable discs bank left early if the release is nose-up? This may be the case...

Olorin, could you elaborate a bit on the implications of your quote and what you think it explains?

 

[80playedin10states]

wow!! go golfing!! a lot less stressful on the brain

 

[Dave242]

Oh...OK, thanks. I told you I was not sure I understood......
Maybe I should have looked at the definition of "pitch" first. I did read all about precession on Wikipedia, first though.

 

[garublador]

It's not directly physics related, but if you're going to do some field testing for this stuff I'd recommend this article:

https://www.dgcoursereview.com/dgr/resources/articles/angles.shtml

It talks some about the relationship between roll, pitch and trajectory of the disc from a flight manipulation standpoint.

 

[Olorin]

About the right turn-- here's a very informative quote from Potts (his p.8, section Flying Disc Dynamics)

"The typical S shaped flightpath exhibited from a backhand throw is dictated by the pitching moment. The angle of attack increases over the flight duration causing the disc to move through the zero pitching moment trim condition, close to 10° AoA. For a righthanded throw, the negative pitching moment on release causes the disc to roll (gyroscopic precession) and bank right, whereas late in the flight the positive pitching moment causes the disc to bank left."

A couple of comments--

First, though, I want to be clear that anything I say is only my understanding at the moment. It may prove to be wrong as I learn more and I welcome any flaws being pointed out. I'm in the pursuit of truth and accurate knowledge. I'm more comfortable quoting experts. My views are subject to change because this is a complex subject. Some of my views have already changed in the last week as I've studied more and gained a greater understanding of what's really happening.

1) About "the negative pitching moment on release"-- This had me stumped for quite a while and took a fair amount of studying Potts and Hummel to figure out. I'm pretty sure I understand it now, though. Potts discusses this under "Experimental Results & Discussion" (p. 5 & 6 in the pdf). To understand what he means you've got to look at Fig 7(c) "Pitching Moment Coefficient". (BTW, 7c is incorrectly labeled 7(a) so this could cause a little confusion. The real 7(a) is at the top of the page.) You also have to read more of Potts to get the full context. Anyway, Fig 7(c) shows the pitching moment (CM --note: the "M" should be a subscript, but this DGCR forum can't do that formatting.) for various Angles of Attack (AoA). With the AoA from 0-9 the pitching moment CM is negative. For AoA above 9 it becomes positive. You can see the same thing in Hummel's thesis in Fig 2-4 on p. 13 where she uses a clearer graph of this Potts data. Potts also says that the AoA increases over flight. So he's not referring to how the disc is thrown as Dave said. Potts means that at the start of the flight when the AoA is below 9 it has a negative pitching moment, then as the AoA increases in the flight the CM gets to 0 then the CM becomes positive with AoA above 9. He refers to a right handed thrower because the clockwise spin will initially make the disc turn right. These same effects for a left hander would make this disc turn left.
This is clear in my mind, but I'm not sure that I explained it clearly enough. You've got to study Fig 7c to be able to see this.

2) What do I think this quote implies?
When I posted it I thought gave an explanation of why discs turn right. But after reading more closely Potts also said (p.6), "The effect of spin on the pitching moment (Fig. 8) is small but measurable.", so I don't really know how much affect it really has on the right turn. I need to study more.
Obviously, not every disc turns right, but this will confirm Rameka's assertion that discs have a component that makes them want to turn right.

The simple phenomenon of the flight of a frisbee and especially why a disc turns right seems to be much more complex than I once thought. My head is spinning…

 

[Olorin]

Rameka, Dave, and anyone who's interested,

Can you help me figure out how to visualize and explain the Advance ratio (AdvR) that Potts uses. (Hummel uses a different term that I can't recall at the moment.) I know what it is from his formula:

Frisbee Throw Biomechanics AdvR = ratio of spin rate to forward velocity (Frisbee Throw Biomechanics [FONT=&quot]spin rate [/FONT]x perimeter / forward velocity V)

And here's a definition I found online for aircraft:

advance ratio n (Aeronautics)
1 the ratio of wind speed along the axis of a rotor or propeller to the speed of the blade tip
2 the ratio of forward flight speed to the speed of the rotor tip of a helicopter

On a disc wing (Pott's term) is the AdvR the ratio of the spin rate at a point on the outer edge of the rim to the forward velocity?

I'm asking about the AdvR because it seems promising to help with the right turn question. I'm guessing that the spin rate and forward velocity are essential components in this. But I'm trying to visualize what the AdvR is.

While I'm at it... Potts also has the spin rate (Greek omega) in Hertz (Hz). I can't figure out what actual units these Hz express. Can you? It's "something per unit of time" ... maybe "something"/sec ? Is this "something" one rotation? Why is it expressed in Hz?

And on the AdvR equation, I'm also assuming that pi x c ( c = chord = diameter0 is a constant. Although golf disc diameters vary I think for this we can assume a constant diameter. Do you agree?

For Potts V is also a constant at 20 m/sec. (Obviously, this varies greatly in real life.)

 

[Rameka]

Olorin:

In response to your last post on page 7, I'm curious to know why it is that the trim condition is 9 degrees AoA.

In response to your latest post, the unit Hz translates to frequency or "cycles per second". This would be, as you suggested, the number of rotations per second. Thus, you'd have cycles/second x perimeter (the circumference of the disc). This would give a speed to compare to your forward velocity.

For example, say your disc was rotating 20 times in a second at a certain point after release. Say the circumference was 50 cm. 20/s x 50cm = 1000 cm/s. You'd compare this speed to your forward velocity and that would give you the ratio you brought up. There isn't really a unit for cycles, obviously; it's just a coefficient for the distance.

I'm not sure if that'll help you visualize it, but that's how I understand it. I still have yet to read Potts and Hummel, by the way, so if I restated something you already know, forgive me. I'm planning to read them soon. Once I do, I'll try to do a more in-depth analysis with you.

 

[Olorin]

I'm curious to know why it is that the trim condition is 9 degrees AoA.

I think we need to be a little more accurate with our terms here to avoid any confusion. It would be better to say, "The pitching moment coefficient Frisbee Throw Biomechanics [FONT=&quot]CM[/FONT] equals 0 at ah AoA of 9 degrees." Fig 7(c) was empirically derived by plotting the data from Pott's wind tunnel tests. I think the answer about why is found in this elucidating paragraph in THE FLOW OVER A ROTATING DISC-WING by Jonathan R. Potts & William J. Crowther. (This paper is easier to read, and offers some help, but not quite as much as his other.) Here's the whole thing with the most relevant part highlighted by me:

"In its simplest form a disc-wing can be described as an axi-symmetric wing. The disc considered in this study has an approximate elliptical cross-section and hollowed out underside cavity. The centre of pressure of this configuration is ahead of the centre of the disc and hence the centre of gravity also. This results in a destabilising nose up pitching moment at typical flight angles of attack. Due to the symmetric shape of the disc, the rolling moment at this condition is approximately zero. If the disc is rotating, gyroscopic effects dictate that a pitching moment results in a precessional rolling motion of the disc. This provides pitch stabilisation at the expense of roll stability. For a typical disc rotating in the direction of positive yaw, using the conventional body fixed axes definition (Fig 1, arrows point in the positive direction and rotation), then a positive pitching moment will induce a negative roll motion."

A disc is typically thrown with 0 or very small AoA but the AoA increases over time because the COP is in front of the COM.

In response to your latest post, the unit Hz translates to frequency or "cycles per second". This would be, as you suggested, the number of rotations per second.

Thanks for confirming and clarifying this. I think the rotations needs to have a symbol, though. Lets call it R (even though Potts uses R too for sth else too.) Then...
AdvR = (Spin rate x pi x diameter (in m) / V) = (Rm/sec divided by m/sec) = units of R because the m/sec on the top and bottom cancel each other. And in Potts' experiment pi, the diameter, and V are all constant the only variable is rotation speed.

 

[willstradamus]

Thanks again Rameka, the info on the reason for te friver being faster than the putter was perfect

I have 2 more questions

1)do you have an answer as to why the Boss for example is faster than the Wraith?
2)is there any reason why they dont just skip a speed 14 disc and make a speed 50?

my hypothesis on #2 is to make more money, but could they do it if they wanted too?

 

[willstradamus]

alright my first ever par

 

[Geoffro]

Congrats, Will.

 

[Rameka]

Thanks again Rameka, the info on the reason for te friver being faster than the putter was perfect

I have 2 more questions

1)do you have an answer as to why the Boss for example is faster than the Wraith?
2)is there any reason why they dont just skip a speed 14 disc and make a speed 50?

my hypothesis on #2 is to make more money, but could they do it if they wanted too?

I haven't used the Boss or the Wraith, but assuming the general trend holds, the Boss will be faster because its outside rim is sharper than the Wraith's. This means that the Boss has to deflect less air. The faster a disc, however, the less lift, as less pressure will build up in a lower profile disc.

As for your second question, the speed ratings on Innova's website are relative. There's no such thing as an objective speed 13 disc, this is just where on the current scale it lies. If what you mean by a speed 50 disc is just a disc that is almost 5 times faster than the current drivers, then I think the same applies. Is what you mean by that, that the disc would have the potential to go so many times further? In this case, you should understand that speed isn't necessarily proportional to distance. There are a lot of variables to consider. The fastest disc I can imagine is a thin circular sheet of plastic with no profile at all. Would this hypothetical disc fly far? Probably not...it has no lift. Disc engineering is a process that must incorporate all variables in order to produce a successful model.

It's a lot more complex than you think

 

[Dave242]

The fastest disc I can imagine is a thin circular sheet of plastic with no profile at all. Would this hypothetical disc fly far? Probably not...it has no lift.

Think Aerobie. I know they not a technically a disc, but it is very low profile and goes a long way and is very fast. With the flick of the wrist it takes to toss a driver 150 ft, it is easy to toss one of those 300 feet.

Rameka, talking about Aerobies, can you do a physics analysis of why bending one upwards before throwing makes it turn one way....and bending it downwards, the other?

 

[willstradamus]

It's a lot more complex than you think

how did you know how complex I thought it was

 

[willstradamus]

Think Aerobie. I know they not a technically a disc, but it is very low profile and goes a long way and is very fast. With the flick of the wrist it takes to toss a driver 150 ft, it is easy to toss one of those 300 feet.

Rameka, talking about Aerobies, can you do a physics analysis of why bending one upwards before throwing makes it turn one way....and bending it downwards, the other?

good question,

also why does the uneven rim of the epic add distance to overhand throws?