Disc Physics...???

[Doofenshmirtz]

A negative pitching moment wants to push the nose down; the lift is behind the CG.

I think that this can also happen when the disc is inadvertently released with a nose-down angle of attack that is below the zero lift AOA. In fact, I strongly suspect that is what is happening when people roll their wrist over, and cause an immediate turn to the right (likely in addition to some unintended anhyzer), and blame it on "OAT." In such a case, it isn't lift centered behind the spin axis, but negative lift centered forward of the spin axis that causes the early roll.

More speculation of course.

 

[ToddL]

Did some reading on the Magnus Effect. Two questions:

1. The Magnus Effect is not dependent on friction, is it? Like with the baseball analogy, it would still work without seams (and with a perfectly smooth covering)?

2. It seems that the amount of air affected by the Magnus Effect is incredibly small, in terms of the total air effects on the disc. Is it enough to cause turn?

1. I believe Magnus is entirely friction-driven. It's a result of the interaction between the spinning surface and the moving air. The only interaction is friction.

2. Almost certainly not. The Magnus force on a disc (especially a golf disc) is minuscule compared to the other aerodynamic forces.

 

[ToddL]

I think that this can also happen when the disc is inadvertently released with a nose-down angle of attack that is below the zero lift AOA. In fact, I strongly suspect that is what is happening when people roll their wrist over, and cause an immediate turn to the right (likely in addition to some unintended anhyzer), and blame it on "OAT." In such a case, it isn't lift centered behind the spin axis, but negative lift centered forward of the spin axis that causes the early roll.

More speculation of course.

It sounds reasonable, but that's a tricky call. Negative lift would be awfully difficult to visualize on a golf disc with its non-continuous bottom surface.
IF there's negative lift occurring in front of the CG, then it would precess to the right side and act as a downward force and cause a roll to the right. Admittedly, I have no idea where the lift would be acting, in this case.

 

[Doofenshmirtz]

It sounds reasonable, but that's a tricky call. Negative lift would be awfully difficult to visualize on a golf disc with its non-continuous bottom surface.

Well, think of it as Newtonian flow redirection if that helps, but in any event, if the nose points down enough, there should be a center of "lift" acting downward on the disc, forward of the spin axis.

 

[bluTDI09]

It sounds reasonable, but that's a tricky call. Negative lift would be awfully difficult to visualize on a golf disc with its non-continuous bottom surface.

Lift is just a force. A "negative" lift vector is effectively a positive net force on the top surface of the disc (higher pressure above than below).

Throwing a disc truly "nose down," meaning the angle of attack is less than the point with zero lift, would result in a throw that goes pretty much straight into the ground.

 

[ToddL]

Oh, no, I can visualize negative lift just fine. (BS/MS AE degrees, pretty good at aerodynamics and flow visualization.) I just can't say for sure where the CP would occur on the disc. Before or after the CG? I'd have to see some good CFD to say for sure.

But like bluTDI90 says, true negative lift would slam your disc straight into the ground really quickly. I doubt you'd be able to tell whether it was rolling to the right or not in the amount of time the disc is in the air.

On an airfoil you get these nice, predictable flow patterns:

On a disc, the bottom surface is pretty weird.

 

[Doofenshmirtz]

Throwing a disc truly "nose down," meaning the angle of attack is less than the point with zero lift, would result in a throw that goes pretty much straight into the ground.

It depends on launch angle. Ignoring precession and turn/fade for the purpose of describing only the verticality of the flight path, if the disc is thrown upwards such that the nose-down AOA is actually level with the ground, the path of the disc will soon become level to the ground. The spin of the disc will keep the disc, more or less, level to the ground and you will get level flight - at least until the disc slows to the point where lift can't overcome gravity. Then it will move downward with the AOA becoming nose up until it lands.

If you throw it with a level launch angle and an overly nose-down AOA, it probably would go into the ground fairly quickly.

 

[bluTDI09]

It depends on launch angle. Ignoring precession and turn/fade for the purpose of describing only the verticality of the flight path, if the disc is thrown upwards such that the nose-down AOA is actually level with the ground, the path of the disc will soon become level to the ground. The spin of the disc will keep the disc, more or less, level to the ground and you will get level flight - at least until the disc slows to the point where lift can't overcome gravity. Then it will move downward with the AOA becoming nose up until it lands.

If you throw it with a level launch angle and an overly nose-down AOA, it probably would go into the ground fairly quickly.

That is true. I was assuming an initial velocity close to parallel to the ground.

 

[bluTDI09]

Oh, no, I can visualize negative lift just fine. (BS/MS AE degrees, pretty good at aerodynamics and flow visualization.) I just can't say for sure where the CP would occur on the disc. Before or after the CG? I'd have to see some good CFD to say for sure.

But like bluTDI90 says, true negative lift would slam your disc straight into the ground really quickly. I doubt you'd be able to tell whether it was rolling to the right or not in the amount of time the disc is in the air.

...

On a disc, the bottom surface is pretty weird.

Yeah I can tell by your previous posts that you understand the concept. I wasn't trying to be condescending or anything, just trying to clarify for others reading the thread.

 

[ChrisWoj]

As no one seems to have mentioned it yet, some of the best/only actual research on the aerodynamics of disc golf was conducted by Jonathan Potts (PhD) of the British company Discwing (makers of the Quarter K). It appears the company domain is defunct now, but their site is preserved here:

https://reddwarf.1st4domains.co.uk/discwing.com/research/aerodynamics.html

The first article there (Frisbee Aerodynamics), for example, suggests that the 'pitching moment' is responsible for both 'turn' and 'fade'. I admit to not fully understanding the physics he's talking about, however, and will defer to anyone better able to understand/interpret that research than I.

Happy to see someone else link it. I was just CTRL+Fing through for a reference to them. I used their info for a Physics of Everyday Things presentation back in undergrad.

 

[ChrisWoj]

Oh, no, I can visualize negative lift just fine. (BS/MS AE degrees, pretty good at aerodynamics and flow visualization.) I just can't say for sure where the CP would occur on the disc. Before or after the CG? I'd have to see some good CFD to say for sure.

But like bluTDI90 says, true negative lift would slam your disc straight into the ground really quickly. I doubt you'd be able to tell whether it was rolling to the right or not in the amount of time the disc is in the air.

On an airfoil you get these nice, predictable flow patterns:

On a disc, the bottom surface is pretty weird.

Does this help? On the right hand side is a disc, brighter areas are areas of higher air pressure (I believe, it has been a few years since I looked at these) in the one, and areas in red are higher pressure in the other. The negative image is illustrating the bottom of a disc.

 

[drmoody]

It seems that gyroscopic precession is also responsible for skip shots. The ground applies force to left side of disc which lifts the nose up. Conversely, it also explains why a disc striking the ground on an anhyzer angle tends to "dig in". Thanks science.

 

[jedijon]

If you're watching the disc fly from above, you'll see your sharpie point speed up on the left side and slow down on the right (for a RHBH throw).

Imagine the path it would trace as it travels. It will not be a straight zig-zag back and forth. It will be a curve where it covers far more distance on the left side than it does on the right.

Agh to both of you. Speed - relative to AIR not to the disc [olorin] or to the ground [bradharris]. It's aerodynamics we're talking about, right?

Friction and moment of inertia do differ on opposite sides of a spinning object that's traveling through a substance even of limited (air) density.

 

[jedijon]

For a visual representation:
https://www.youtube.com/watch?v=eTjGTxSevHE

Any roll the disc experiences is not due to the center of pressure moving left or right (as you might guess would be the case if the advancing and retreating side were generating different amounts of lift), but is instead due to the center of pressure moving forward and backward, which could be due to airspeed, angle of attack, flow separation, etc.

And I think this is so obviously unassailably right. The whole helicopter thing were getting wrapped up in is interesting - in its differences not similarities.

Yes, one side of the discs airspeed is marginally different. And yes, there's friction. BUT more importantly - while a helicopter's rotors are separated by air...a disc is...round. We don't fly our airplanes by spinning them in a circle around the exit row seats...we do it by shoving them through the air.

 

[jedijon]

I look at it as the difference between the center of gravity (COG) and the center of lift. Wing shape and speed determine how much and where the lift is, so as spin and forward speed decrease that center of lift moves in relation to the center of gravity. Lift in front of the COG keeps the disc in the air (glide), and lift to either side of the COG will tilt the disc into turn or fade.

And also yup. Plus, in the post that proceeds this, there's some more good helicopter talk about the axis of lift.

Consider that your disc turns MORE when you the off-axis torque is visibly obvious. A disc with more lift moves the COL further from the COG, and this is predictable. So you like the disc. And it turns. But when you give it OAT, it turns more. So too throwing it into a headwind...

A case can be made for blunt discs (lids/putters) to exhibit these characteristics due to higher air resistance. Think about which axis they put the tail rotor on that helicopter...

 

[jedijon]

One more question:

I may be incorrect in this but when I throw a disc with a poor release and it is fluttering, it turns more (goes right with a RHBH throw). If I'm not crazy and this is what's happening, how does that fit into what's been discussed so far.

I was originally in the party of "well different air speeds relative to disc surface so more lift on left side" but have been completely turned off that with the realization that lift is indeed dependent on the difference in pressure above and below disc so any effects from rotational speed are equal on top and on bottom.

I'm not so sure this is a 'just one question' kinda thing, but to address your first bit about OAT - we're on a similar wavelength. My allusion to this above is pretty incomplete, so allow a little expansion. Wobble increases friction which reduces speed, more importantly it dramatically decreases lift and all this is visible because that wobble shows up as the SUM of two vectors-precession+the off-axis torque. It's a great question.

As for your understanding of forward vs rotational effects on lift, keep in mind that even ToddL admits the undersurface of the disc is "pretty weird" which is to say that it IS kind like 1 giant golfball dimple when it comes to the generation of aerodynamic turbulence. While not the greatest effect, certainly nothing to write off. And bigger than a differential speed friction effects like Magnus.

Wouldn't it be neat to toss a disc w/a smooth underside. I guess we'd need some pretty grippy rubber gloves to throw it...

 

[Curtis_Valk]

Wouldn't it be neat to toss a disc w/a smooth underside. I guess we'd need some pretty grippy rubber gloves to throw it...

We could drill holes like a bowling ball in the bottom edge! :clap:

 

[BogeyNoMore]

We could drill holes like a bowling ball in the bottom edge! :clap:

... then it wouldn't be aerodynamically smooth, would it? :\

 

[Curtis_Valk]

... then it wouldn't be aerodynamically smooth, would it? :\

..........no, but it might whistle! :thmbup:

 

[Shapers]

If I've understod this correctly: the Magnus effect is caused by the friction between the air and the disc. This causes a force which is 90* (I don't know how to make the degrees-thing..) to the spin-axis. On a RHBH throw the disc is spinning clock-wise, causing the force to be to the right, if seen from behind.

This is also explaining why a flat disc is more OS. There is less friction between the "top" of the disc than on a domey disc. However, a domey disc will have a greater difference in air pressure, causing more lift = more glide.

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..