Explanation of the physics of flying discs (FIXED)

[Rameka]

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?

If my comment about the dominess of discs affecting flight on the first page was correct, then the same should apply here I think. I'm not so sure if that's right though; since my first post, a lot of my initial thoughts have had to be retracted and rethought. I'm planning on studying this all in the future, so maybe I'll have a more comprehensive solution for you at some point.

how did you know how complex I thought it was

I didn't mean to offend you. If you understood that the speeds of discs are on a relative scale, however, I don't really know why you'd ask about a speed 50 disc... it takes sophisticated engineering to make those small jumps in speed, and asking a question as you did made me assume that you were very in the dark about the process.

good question,

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

I've been dying to try out the Epic, but I haven't got a chance yet. I'll have to order it online...until then, I can't really take a stab at that one. Consider it beyond my realm of understanding for now.

 

[Ryan P.]

I would imagine that it's because one side of the disc goes faster than the other, and when it's the heavier side going faster, it gives it extreme forward momentum, but when the heavier side is going slower, it doesn't really slow the disc down that much because it's still moving forward, technically.

if you know the "correct" way to shoot a rubber band, it's the same idea.

i think the hammer throw in track and field is the same way.

probably the most common place you can also see it is in a plastic grocery bag. If you've got something in the bag, and you swing it around and around and then throw it in the air, the heavier side whips around when it goes higher in the air. something like that. If that doesn't communicate the idea, i'm not sure what would.

 

[willstradamus]

I didn't mean to offend you

I was joking, Im not really offended



as far as the speed 50 thing I knew it was relative but I didnt know if the disc manufacturers (or you) already knew how to make a disc faster than the Boss prior to the release of said disc.

thanks for all of the replys

 

[Olorin]

Factors that make a disc turn right

I'm still chewing on why a disc turns right. Here are some of the factors that I've thought of that make a disc turn right:
·Shallow rim. Less weight on the rim.
·Being beat in
·Headwind
·Low weight
·Lower density plastic
·High altitude. The air is thinner, so there are less air molecules with more space in between them.

Can you think of any others?

The next step is to find common characteristics in these factors. I've got some theories that I'm pondering...

 

[Rameka]

I'm still chewing on why a disc turns right. Here are some of the factors that I've thought of that make a disc turn right:
·Shallow rim. Less weight on the rim.
·Being beat in
·Headwind
·Low weight
·Lower density plastic
·High altitude. The air is thinner, so there are less air molecules with more space in between them.

Can you think of any others?

The next step is to find common characteristics in these factors. I've got some theories that I'm pondering...

Well, we know that a disc turns below roughly 9 degrees AoA. I'm still trying to figure that one out...

 

[Olorin]

Lorenz

Here's an interesting article I'm currently reading. Not very technical and written for laymen, but has good information and a good graphic of the forces on a frisbee--

Flying Saucers (pdf version) in New Scientist, June 2004

And more by Lorenz, especially his technical unpublished article.

 

[jace]

Is there anyway you can explain how and why wind affects disc flight.

It jacks it up really bad... and for no good reason

sorry couldn't resist

 

[Olorin]

It's looking like an understanding of the Kutta condition will be useful and perhaps it might even help to know more about the Kutta–Joukowski theorem too.

 

[M_Ram20]

So then my question is what would you change the engineering to make the perfect disc with respect to the physics... plastic, weight distribution, lip size, whatever?

 

[Rameka]

So then my question is what would you change the engineering to make the perfect disc with respect to the physics... plastic, weight distribution, lip size, whatever?

What is a "perfect disc" and what would it do? Think about your question...

 

[Olorin]

I'm still studying and pondering what makes a disc fly to the right. A fascinating subject.

Some more helpful reading:

Disc Qualities and Their Effects On Flight, by Blake Takkunen

PDGA Discussion, What makes a disc fast?, esp. _ DrD_, #619040 - 12/06/06 12:10 AM

 

[Rameka]

I'm still studying and pondering what makes a disc fly to the right. A fascinating subject.

Some more helpful reading:

Disc Qualities and Their Effects On Flight, by Blake Takkunen

PDGA Discussion, What makes a disc fast?, esp. _ DrD_, #619040 - 12/06/06 12:10 AM

DrD's post was really interesting. I didn't know that the CP could be pushed back behind the CG...that seems really confusing to me. If the CP started behind the CG, a disc would fight to turn.
Don't ALL discs fight to fade? This seems contradictory...

 

[Olorin]

I didn't know that the CP could be pushed back behind the CG...

Rameka,

Where did you get this idea? Did I miss something that Dr D said, or did you get it from Kenja's post? Please elaborate on where you got this concept.

I've only seen this theory put forth by Kenja but I don't know what scientific basis he has for the assertion, so I'm not convinced that it's true. I just asked him to clarify on the Physics of DG thread at dgreview.com.

This subject is so complex that there's tons of bad information on discussion boards about the aerodynamics of frisbees. There's also not enough scientific research so even very knowledgeable people have to resort to conjecture. To be honest, my default mode is that I don't trust assertions unless they can point to trustworthy outside sources. On the PDGA board DrD is the person I trust most because he has a PhD and knowledge of aerodynamics. Unless it matches what I've read in Potts, Hummel, or Lorenz I usually don't put much faith in it without sources cited.

BTW, according to Potts, the AoA increases over the course of a disc's flight so the disc becomes more nose up which makes it naturally die left due to precession.

 

[Olorin]

Here's a cool picture that I got from a DrD post on the PDGA thread that has a very clear description of the forces on a golf disc. All credit for the picture belongs to DrD. Check these (they're the same picture):

Forces on a golf disc
For logged in DGCR users: Forces on a golf disc

 

[Rameka]

Rameka,

Where did you get this idea?

In DrD's post, he said:

Why do some discs naturally turn right instead of left? That is because the pitching moment is negative or that the center of pressure is behind the center of gravity.
...
As a disc gets 'beat up' its surface geometry changes - these subtle changes can be enough to move the CP behind the CG and cause the disc to be 'understable'.

If what he says is true, AoA would not increase over time. At least I can't think of any way it would. It would do the opposite, and become a worm burner before it got 50 feet.

 

[Olorin]

There's one more thing I want to try explain, all the way back from Figure 2. This is partially hypothetical because I have not seen this in any research, but am merely extrapolating from my knowledge of other objects in flight. This is about why a disc becomes more understable as it "beats in". A golf ball, as we all know, has dimples. Dimples cause air turbulence around the sides of the ball, which causes the air to adhere to the sides more because of the increased surface area. In a similar way, when you beat up a disc, it gets nicks and scratches all around the outer circumference. These nicks and scratches act like dimples, and cause the points of air separation to come later, meaning that the disc has a smaller wake. This smaller wake means the ratio of pressure from the front and the back of the disc is less, causing the disc to stay fast for a longer time, just like the dimples in golf balls.

That's just my guess. If anyone else could confirm or deny this, I'd be delighted.

Rameka,

After the studying I've done lately I'll agree with your assessment. I'm still working on a unified theory of what makes a RHBH disc turn right but there's not enough solid research on this question to verify much objectively and scientifically. See the page "What makes a RHBH disc turn right?" for my list of factors. My hope is that by finding common traits among the factors we can better understand the aerodynamics. I'm not really sure how far this can go until we just run into unverified conjecture, though.

I attempted to answer the question about why a beat in disc turns right, and my conclusions were the same as yours above. Go to "3. Being beat in" to check out my reasoning. I'm pursuing scientific knowledge so if I'm wrong I'll be glad to know it. All I ask is that any claims be backed up by reputable sources. There's so much opinion, hearsay, and legend out there that I place very little faith in someone just stating an idea as fact without proof to back it up.

I'll be glad to hear any comments you have on this topic.

 

[Olorin]

Here's my answer that the site above links to. I think that Hummel's work on the "rings of Headrick" is a better analogy to the behavior of a beat in disc than a golf ball, although the aerodynamic principles are the same.

--------------------------------------------------------------------
3. Being beat in

Short answer: When a disc gets beat in with cuts, nicks, and irregularities this reduces overall drag by changing the boundary layer air flow. For some reason, needing further research, this reduced drag then makes a RHBH thrown disc turn right.

Longer explanation: The irregularities creates turbulence at the boundary layer on the disc so that overall drag is reduced.
Consider the effect of the concentric rings on the top of a frisbee (the "rings of Headrick"). Why do the concentric rings reduce drag? See Hummel, 2.2.7 Concentric Rings (p. 15). "The Frisbee design includes an approximately 9.5 cm inner radius to 11.5 cm outer radius region of concentric surface roughness rings on the top sloping surface (not on the top flat plane of the Frisbee). The purpose of these rings is to manipulate the boundary layer and ultimately reduce drag. The boundary layer is a region of relative motion between adjacent layers of fluid particles. There is a no-slip condition between the air and the Frisbee at its surface, but moving away from the Frisbee the velocity profile changes until the velocity of the airflow in the boundary layer matches the airflow unaffected by the motion of the Frisbee. Theoretically, as the air moves across the concentric rings, flow in the boundary layer transitions from laminar to turbulent flow. By virtue of the increased energy in the boundary layer, the air flow stays attached to the surface of the Frisbee longer, delaying separation more than if the flow were laminar. Drag is minimized by keeping the airflow connected to the surface as long as possible."[1]

One must keep in mind that Hummel's work was based on an ultimate frisbee with a much thicker lip than the more aerodynamic shape of disc golf driver. The air flow and drag are certainly different for these two shaped objects.

My conclusions: When a golf disc gets beat in the irregularities create more turbulent flow at the boundary layer. This turbulent flow has better adhesion and is less prone to separation, so that the eddies that flow in the gap behind the trailing edge of the golf disc are smaller. The net effect is a reduction of the overall drag.

This still doesn't explain why a beat in disc turns right, though. I'll conjecture that this reduced drag causes either a pitching moment or a change in the Advance ratio, and that turns the disc right. More research would be required to validate this theory.

[FONT=&quot][1][/FONT] http://mae.ucdavis.edu/~biosport/frisbee/HummelThesis.pdf

 

[harr0140]

This is darn hilarious. I know you guys know what you are talking about and I am sure you enjoy it, but I have got to sit down with a printout of all these comments and figure it all out for myself. I do find it very funny that this topic has been going so long, but I appreciate the effort on your part to analyze the disc dynamics. I know I will enjoy reading it all if I get the chance.

 

[Olorin]

From the Spin vs Speed thread:

here's a quick and dirty analysis of the situation presented. I may not get through it all because I'm trying to catch a round with my buddy and he'll be here in a few minutes, so there may be a "to be continued"....

Case 1) same spin, different speed high speed stability analysis.
assumptions:
RHBH throw
disc 1 initial speed = 40
disc 2 initial speed = 50
initial spin speed measured at farthest point of disc from rotational axis= 10

case 1 disc1)

lift calculations for high speed stability:
left side of disc relative speed to air = 40+10 = 50
right side of disc relative speed to air = 40-10 = 30
ratio of left side speed to right side speed = 50/30 = 1.6 (the higher this number the more high speed turn due to lift factors)

rotational stability will also be a factor that is dependent on the polar moment of inertia about the rotational axis of the disc. If you've ever done the bicycle wheel suspended buy a string experiment, it is similar to this. The faster the disc is spinning, the more it wants to stay straight so the more force it takes to move it from it's initial placement. This will be greatly different for different discs because drivers have most of their mass located at a far distance from the rotational axis, and putter have a fairly uniform distribution of mass, thus drivers won't be as sensitive to these lift factors as a putter will. For this case it is not as important as it will be in case 2 where the rotational stability will different for the different shots.


case 1 disc 2 high speed stability analysis:
ift calculations for high speed stability:
left side of disc relative speed to air = 50+10 = 60
right side of disc relative speed to air = 50-10 = 40
ratio of left side speed to right side speed = 60/40 = 1.5 (the higher this number the more high speed turn due to lift factors)

This means at a fixed spin rate the slower flying disc (1) will have more high speed turn than the faster disc (2) mainly because the ratio of rotational speed to disc speed is higher in a slower disc.

Colo,

Great answer! If it's OK with y'all I put this quote here so that we can continue the discussion on this thread. I'm just an amateur who finds this interesting, so I'd like to ask a few questions in order to understand it better.

You said, "initial spin speed measured at farthest point of disc from rotational axis= 10"

1) The farthest point is just a point. Since the rim is round a point one cm clockwise or counter clockwise away will have a different vector to the airspeed. It's only that one tiny point that is directly parallel to the air speed. Also, looking at one side, say the right side, the rotational speed increases from the Z axis of spin from something less to 10. At some place closer to the Z axis the rotational speed is only 9. Does this cumulative effect on the line of this radius make any difference? Plus this rotational speed is only on one thin line of the Y axis.

2) I know you're just using numbers for illustration and the ratios you presented made your point quite well. I've wondered what an average rotational speed is, though. What units is it measured in? My hunch is that the rotational speed must be some order of magnitude higher than the air speed, but I can't find any numbers in the articles I referenced earlier in this thread. And how do you translate rotational speed into units the same as the airspeed? If the rotational speed is way higher than the air speed then maybe the change on the left and right sides is negligible.

I'm not sure if what I've written makes sense to anyone else because I find it hard to describe in words what I'm thinking.

Colo, I'd really like your input on this, though.

3) Oh, and while you're at it, would you please give any feedback you can on this post? There's still lots I don't understand about the Advance Ratio, but I think it's important to these discussions.

 

[Ryan P.]

I'm still chewing on why a disc turns right. Here are some of the factors that I've thought of that make a disc turn right:
·Shallow rim. Less weight on the rim.
·Being beat in
·Headwind
·Low weight
·Lower density plastic
·High altitude. The air is thinner, so there are less air molecules with more space in between them.

Can you think of any others?

The next step is to find common characteristics in these factors. I've got some theories that I'm pondering...

Hey Olorin, from personal experience high altitude doesn't make a disc turn to the right any more. I'm not exactly sure why though. I've thrown a pretty understable disc out in the Rockies and I've gotten hardly any turn on it, much less than it will actually turn, because of the elevation. I would guess it has something to do with the lower air pressure. Obviously the speed of the disc has something to do with turn, but I'm thinking that it could be more related to how many molecules of air the disc moves through. But again, i don't know for sure.