Why do overhand throws turn backwards?

[knettles]

Aren't there 2 sorts of thumber grips?

1. Hook thumb (most common in today's game)
and
2. Overhand wrist flip grip turned upside-down

Interesting. So would that essentially be the same grip as a grenade but instead of being thrown at an extreme hyzer it's an extreme anhyzer?

 

[armiller]

I think in today's vernacular just the first is commonly called a thumber. But I vaguely remember some more experienced players talking about a thumber meaning something completely different when the word was first used...

 

[sidewinder22]

We essentially said the same thing, just different frame of reference. I was referring to the orientation to the ground, hence super over stable, and spinning the same direction as a FH, while you were referring to the top of the disc, so you used opposite terminology.

Mmm, I don't think we said the same thing, it's not acting overstable. A thumber is always rolling over in the understable direction from its spin direction until it loses enough speed or angle of attack. A FH throw turns over(understable) the opposite barrel roll direction of a thumber. A FH will fade out(overstable) in the same direction of the thumber turning over(understable). A thumber barrel rolls clockwise, while a FH barrel rolls counterclockwise.

 

[DG_player]

Mmm, I don't think we said the same thing, it's not acting overstable. A thumber is always rolling over in the understable direction from its spin direction until it loses enough speed or angle of attack. A FH throw turns over(understable) the opposite barrel roll direction of a thumber. A FH will fade out(overstable) in the same direction of the thumber turning over(understable). A thumber barrel rolls clockwise, while a FH barrel rolls counterclockwise.

The spin direction depends on your frame of reference: If you look from the context of lift pointing up and gravity pointing down, the disc is spinning counter-clockwise (same as a forehand). If you look from the top of the disc, it is spinning clockwise (same as a backhand).

 

[sidewinder22]

The spin direction depends on your frame of reference: If you look from the context of lift pointing up and gravity pointing down, the disc is spinning counter-clockwise (same as a forehand). If you look from the top of the disc, it is spinning clockwise (same as a backhand).

That doesn't change the direction of precession.

 

[curmudgeonDwindle]

Interesting. So would that essentially be the same grip as a grenade but instead of being thrown at an extreme hyzer it's an extreme anhyzer?

My understanding of the 'grenade' is that it is essentially a backhand delivery w/ the disc upside down. The disc tombstones on a spot and is a great specialty shot for 'up-and-over' situations, very controllable. Sam Ferrans is the first person that I'm aware of to 'popularize' that particular delivery.

#2 thumbeR in my comment (for golf) is essentially the same delivery as a forehand roller - with the same effect, surer grip and shorter range, however you can also deliver it like a forehand - flat. There were several players who used this delivery for GUTS burns back in the day, notably https://sites.google.com/site/okpdga/oklahoma-hall-of-fame/jeffrey-homburg His throws were nigh uncatchable in his prime and he's a real nice fellow as well. Many players have used this delivery for technical rollers...

If you are really adventurous, you can bring #2 thumbeR closer over your head like a tomahawk, with roughly the same result. Some players have used that delivery for DDC dumps and burns.

All of these deliveries, with the exception of the grenade, can be very tough on the shoulder joint. Take care...

 

[DG_player]

That doesn't change the direction of precession.

It changes whether you call it over stable or under stable.

 

[sidewinder22]

It changes whether you call it over stable or under stable.

How so?

 

[DG_player]

How so?

If I had a symmetric disc with no defined top or bottom, and threw this disc with a forehand motion, and said disc rolled to the right, would you say the disc is acting under stable or over stable?

 

[sidewinder22]

If I had a symmetric disc with no defined top or bottom, and threw this disc with a forehand motion, and said disc rolled to the right, would you say the disc is acting under stable or over stable?

Symmetric top and bottom would be the definition of stable and have two aerodynamic centers.

 

[DG_player]

Symmetric top and bottom would be the definition of stable and have two aerodynamic centers.

That doesn't really answer my question.

 

[ThrowBot]

I'd be interested to hear how the Aerodynamic Center applies to this conversation. As far as I can tell, the Center of Pressure is a more relevant point to consider.

Also, it's key to realize that the force acting at the Center of Pressure is a vector. The direction of that vector matters. "Lift" is always oriented perpendicular to the oncoming airflow (by definition), but that doesn't mean that it's always pointing up. For example, I agree with armiller's assertion that the lift is oriented to the right when a RHT throw is released near vertical.

The idea that lift is pointing up (opposite gravity) is based on an assumption that the disc is oriented close to level, with the flight plate on top. That assumption is wrong for much of the thumber flight. From what I can tell, the "lift is always up" fallacy is responsible for some red herring theories in this thread.

 

[DG_player]

I'd be interested to hear how the Aerodynamic Center applies to this conversation. As far as I can tell, the Center of Pressure is a more relevant point to consider.

The aerodynamic center is not really relevant when it comes to discs. It's used with aircraft because you have multiple torques, not just the torque generated by the wing. You have tail pushing down against the wing pushing up, the center of gravity of the entire aircraft pushing down in front of the wing, as well as the torque generated by the wing. By calculating the wing torque from the aerodynamic center you can eliminate a variable because the pitch moment on the wing remains near constant over various angles of attack.

When it comes to a disc, you're really only worried about the torque as calculated from the center of mass.

Also, it's key to realize that the force acting at the Center of Pressure is a vector. The direction of that vector matters. "Lift" is always oriented perpendicular to the oncoming airflow (by definition), but that doesn't mean that it's always pointing up. For example, I agree with armiller's assertion that the lift is oriented to the right when a RHT throw is released near vertical.

The idea that lift is pointing up (opposite gravity) is based on an assumption that the disc is oriented close to level, with the flight plate on top. That assumption is wrong for much of the thumber flight. From what I can tell, the "lift is always up" fallacy is responsible for some red herring theories in this thread.

That's correct that lift is only pointing up when the disc is flying flat. It is at all times as you mentioned pointing perpendicular to the direction of flight. The question becomes what is the lift on a vertical disc? It can be pointing in one of two direction, either to the right, assuming the disc is generating positive lift, or to the left, assuming the disc is generating negative lift. More than likely there is very little or no lift vector, since most vertical throws are probably released at near the zero lift (and minimum drag) angle of attack. Even if they were generating lift one way
or the other, the trajectory will quickly change to the minimum drag angle of attack, since the gravity vector is perpendicular to the lift vector for a vertical disc. At this point the question becomes, why does the disc pan out if there is no lift pointing in either direction? In spite of the fact that there is no lift, there is still a torque being generated. A cambered wing generates a pitch down torque when at the zero lift angle of attack (as well as at a negative lift angle of attack). You can think of it this way: There are a whole lot of various pressures pushing in different directions on different parts of the disc. If you sum all these forces in the lift direction vector, they add up to zero. However, they are still generating a torque on the disc, because there's more up pushing pressure on the back of the disc and down pushing pressure on the front. The nose down torque is going to cause the disc to precess in the "turn over" direction so to speak. So a thumber will pan right and a tomahawk pan left. As soon as it starts to tilt over, the gravity vector is no longer going to be perfectly perpendicular to the lift vector. At this point the disc is going to start generating negative lift (since it's upside down), continuing to generate a pitch down torque, and continue to precess in the "turn over" direction.

 

[sidewinder22]

I'd be interested to hear how the Aerodynamic Center applies to this conversation. As far as I can tell, the Center of Pressure is a more relevant point to consider.

Also, it's key to realize that the force acting at the Center of Pressure is a vector. The direction of that vector matters. "Lift" is always oriented perpendicular to the oncoming airflow (by definition), but that doesn't mean that it's always pointing up. For example, I agree with armiller's assertion that the lift is oriented to the right when a RHT throw is released near vertical.

The idea that lift is pointing up (opposite gravity) is based on an assumption that the disc is oriented close to level, with the flight plate on top. That assumption is wrong for much of the thumber flight. From what I can tell, the "lift is always up" fallacy is responsible for some red herring theories in this thread.

The aerodynamic center is not really relevant when it comes to discs. It's used with aircraft because you have multiple torques, not just the torque generated by the wing. You have tail pushing down against the wing pushing up, the center of gravity of the entire aircraft pushing down in front of the wing, as well as the torque generated by the wing. By calculating the wing torque from the aerodynamic center you can eliminate a variable because the pitch moment on the wing remains near constant over various angles of attack.

When it comes to a disc, you're really only worried about the torque as calculated from the center of mass.

Talking about normal golf discs here again(not DG_player's theoretical symmetrical disc)... in a RH thumber the lift vector is pointed to the Left initially - negative lift relative to the normal wing orientation as the nose/AoA is down to the relative airflow. Lift can be zero, or pointed up or down when the disc is flying flat-ish depending on the AoA or camber orientation/inverted wing.




Now as for DG_player's theoretical symmetric top/bottom un-cambered disc, which would suck at flying(as a golf disc) because the aerodynamic moment about the aerodynamic center is zero for all angles of attack. The AC also happens to be the center of mass.

A straight vertical release on this symmetrical disc will not roll or precess in any direction, it will stay completely vertical all the way from release to the ground, and lift would be pointed straight up the entire flight, and the force would be parallel to the spin axis so there is no torque.

All golf discs are technically understable because of the asymmetric cambered design which allows discs to fly straighter in certain airspeeds and AoAs. Stability in discs is just a measure of the a disc's resistance to turning over. Even your most overstable discs will be understable in category 5 airspeeds or enough negative AoA.

As an object moves through a fluid, the velocity of the fluid varies around the surface of the object. The variation of velocity produces a variation of pressure on the surface of the object. Integrating the pressure times the surface area around the body determines the aerodynamic force on the object. We can consider this force to act through the average location of the pressure on the surface of the object. We call the average location of the pressure variation the center of pressure in the same way that we call the average location of the weight of an object the center of gravity. In general, the pressure distribution around the object also imparts a torque, or moment, on the object. If a flying airfoil is not controlled in some way it will tumble as it moves through the air.

If we consider an airfoil at angle of attack, we can (theoretically) determine the pressure variation around the airfoil, and calculate the aerodynamic force and the center of pressure. But if we change the angle of attack, the pressure distribution changes and therefore the aerodynamic force and the location of the center of pressure and the moment all change. So determining the aerodynamic behavior of an airfoil is very complicated if we use the center of pressure to analyze the forces. We can compute the moment about any point on the airfoil if we know the pressure distribution. The aerodynamic force will be the same, but the value of the moment depends on the point where that force is applied. It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack. Engineers call the location where the aerodynamic moment remains constant the aerodynamic center (ac) of the airfoil. Using the aerodynamic center as the location where the aerodynamic force is applied eliminates the problem of the movement of the center of pressure with angle of attack in aerodynamic analysis.

For symmetric airfoils, the aerodynamic moment about the ac is zero for all angles of attack. With camber, the moment is non-zero and constant for thin airfoils. For a positive cambered airfoil, the moment is negative and results in a counter-clockwise rotation of the airfoil. With camber, an angle of attack can be determined for which the airfoil produces no lift, but the moment is still present.

 

[armiller]

These threads always enter that twilight zone where the discussion is either above my head or totally outside the realm of relevance. I'm inclined to believe it's the latter, but I'm not above admitting that my understanding of fluid mechanics and solid body dynamics isn't all that good. Good for DGCR? Maybe. But I'll leave it at maybe.

That's correct that lift is only pointing up when the disc is flying flat. It is at all times as you mentioned pointing perpendicular to the direction of flight. The question becomes what is the lift on a vertical disc? It can be pointing in one of two direction, either to the right, assuming the disc is generating positive lift, or to the left, assuming the disc is generating negative lift. More than likely there is very little or no lift vector, since most vertical throws are probably released at near the zero lift (and minimum drag) angle of attack.

I don't understand what makes you say this. You might even be right in SOME cases, but I think it's far more likely that lift is pointing toward the "top" of the disc. At some point, if the disc's angular momentum changes direction (i.e. banks/rolls/rotates), doesn't that mean that there's some force acting through the disc but not through the center of mass? If that's not lift or drag, what else could it be?

Even if they were generating lift one way or the other, the trajectory will quickly change to the minimum drag angle of attack, since the gravity vector is perpendicular to the lift vector for a vertical disc.

I don't understand what the gravity vector has to do with this. I do think gravity is very important in the flight path of tomahawks and thumbers, but I think it's because of how much the y-direction velocity changes (more than with standard backhand or forehand throws). This, in turn, causes the angle of attack and lift/drag forces to change dramatically from beginning to end of an overhand throw.

In an earlier post I mentioned a very simple thumber example, thrown vertically. It was just for illustration, as I don't think any decent overhander would ever throw like this. Dang, I just wish I had the time and equipment to study this stuff, cuz I really do think these things are interesting. What I'd really love to see (at least while reading this forum) is a video showing a tomahawk or thumber with lift, drag, velocity, and angular momentum vectors at all points during flight from release to landing. One can always dream...

I'm leaving the rest of the quote below, for completeness, but I think the premise is wrong. Also, the more I read it, the more confused I get. Earlier in your post you had mentioned that aerodynamic center doesn't relate to disc golf. That sounds possible to me. But I think what you're talking about below is the center of pressure, right? And the lift/drag forces that act through that center of pressure should be the ONLY forces causing a torque on the disc, right? If I'm wrong, I'd love to be corrected. Am I missing something?

At this point the question becomes, why does the disc pan out if there is no lift pointing in either direction? In spite of the fact that there is no lift, there is still a torque being generated. A cambered wing generates a pitch down torque when at the zero lift angle of attack (as well as at a negative lift angle of attack). You can think of it this way: There are a whole lot of various pressures pushing in different directions on different parts of the disc. If you sum all these forces in the lift direction vector, they add up to zero. However, they are still generating a torque on the disc, because there's more up pushing pressure on the back of the disc and down pushing pressure on the front. The nose down torque is going to cause the disc to precess in the "turn over" direction so to speak. So a thumber will pan right and a tomahawk pan left. As soon as it starts to tilt over, the gravity vector is no longer going to be perfectly perpendicular to the lift vector. At this point the disc is going to start generating negative lift (since it's upside down), continuing to generate a pitch down torque, and continue to precess in the "turn over" direction.

 

[DG_player]

These threads always enter that twilight zone where the discussion is either above my head or totally outside the realm of relevance. I'm inclined to believe it's the latter, but I'm not above admitting that my understanding of fluid mechanics and solid body dynamics isn't all that good. Good for DGCR? Maybe. But I'll leave it at maybe.



I don't understand what makes you say this. You might even be right in SOME cases, but I think it's far more likely that lift is pointing toward the "top" of the disc. At some point, if the disc's angular momentum changes direction (i.e. banks/rolls/rotates), doesn't that mean that there's some force acting through the disc but not through the center of mass? If that's not lift or drag, what else could it be?

I may have been unclear, so I'll go into more detail. Let's draw some vector axes on the disc: X is the vector pointing through the back of the disc to the front. Y is the vector pointing from left to right across the disc, and Z is the vector pointing straight up from the center of the disc and straight down from the bottom. Lift by definition always points through the Z vector, and drag always points through the X vector. In actuality there are a bunch of forces in a lot of different directions who's net value is pointing in some direction between X and Z. The X portion is called drag, and the Z portion called lift. The point where are all of these forces summed up produce a zero moment (or zero torque) on the disc is the center of pressure. You are correct that this point is very rarely the center of mass. Just because lift points up in the Z direction, does not mean that it's pushing up from the center of the disc.

The lift is always pointing in the Z axis, even if the disc is tilted. This is essentially why a hyzer goes left. Gravity is pointing down, and lift is pointing both up and to the left. If a disc is at a 45 degree angle the lift vector is pointing at a 45 degree angle to the ground. If a disc is vertical the lift vector is pointing parallel to the ground.

I don't understand what the gravity vector has to do with this. I do think gravity is very important in the flight path of tomahawks and thumbers, but I think it's because of how much the y-direction velocity changes (more than with standard backhand or forehand throws). This, in turn, causes the angle of attack and lift/drag forces to change dramatically from beginning to end of an overhand throw.

Gravity is always important because it's a significant force on any throw. The big reason why it's important is that it's counteracting lift.

Let's take 3 throws for example, one by Simon Lizotte, one by me, and one by my mom. All these throws are going to be released at a shallow angle of attack, let's just call it 5 degrees.

Simon throws fast so his disc at 5 degrees is generating lift > gravity.

I throw pretty slow so my disc at 5 degrees is generating lift = gravity.

My mom has a noodle arm so her disc at 5 degrees is generating lift < gravity.

What's going to happen to these throws. Simon's throw is going to have a net upward acceleration (because lift is greater than gravity), so his disc maybe after a few seconds will reach an angle of attack of 1 or 2 degrees where lift = gravity.

My throw is going to continue on it's trajectory shortly after release because lift = gravity.

My mom's throw is going to gain downward velocity (because gravity > lift), such that her disc will be at a higher angle of attack shortly after release.

Eventually they will all reach a point where the lift force is nearly equal to the force of gravity. For Simon his disc will be at a low angle of attack and ascending, for me, it'll be flying at a higher angle of attack neither ascending or descending, and my mom's will be falling out of the sky at a steep angle of attack.

Now tilt these all of these discs to vertical so that the gravity vector is perpendicular to the lift vector. Now gravity is no longer pulling down against lift, the only force acting on the disc along the Z vector is lift. Eventually all these throws trajectories will change until the angle of attack reaches the zero lift point, because there is no gravity fighting it.

In an earlier post I mentioned a very simple thumber example, thrown vertically. It was just for illustration, as I don't think any decent overhander would ever throw like this. Dang, I just wish I had the time and equipment to study this stuff, cuz I really do think these things are interesting. What I'd really love to see (at least while reading this forum) is a video showing a tomahawk or thumber with lift, drag, velocity, and angular momentum vectors at all points during flight from release to landing. One can always dream...

I'm leaving the rest of the quote below, for completeness, but I think the premise is wrong. Also, the more I read it, the more confused I get. Earlier in your post you had mentioned that aerodynamic center doesn't relate to disc golf. That sounds possible to me. But I think what you're talking about below is the center of pressure, right? And the lift/drag forces that act through that center of pressure should be the ONLY forces causing a torque on the disc, right? If I'm wrong, I'd love to be corrected. Am I missing something?

You are correct, I'm talking about the center of pressure.

I'll explain the aerodynamic center as well so you can understand why it's not really important in disc golf:

As you increase the angle of attack of a wing the lift coefficient increases, so basically a wing flying really fast at a low angle of attack is generating the same amount of lift as a wing flying slow at a very high angle of attack. For a cambered wing, at a low angle of attack the center of pressure is towards the rear of the wing. As the angle of attack increases it moves forward toward a point about half way between the center and the front of the wing.

Let's now imagine our wing is a wrench and this point half way between the center and the front is a nut. If we push up slightly on the back of the wrench, we generate a certain amount of torque. We have a long lever, and a little bit of force. If we move close to the nut and push really hard, we end up with same torque, because we have a much shorter lever, but are compensating with increased force. This is essentially what is happening at the aerodynamic center, force increases as we move center of pressure closer, but the distance decreases so you end up with the same torque.

Now this is useful if you're designing an aircraft to simplify this one torque that is affecting it, since the wing is fastened to a larger craft. You can essentially measure the torque at the aerodynamic center and not worry about the changing lift coefficient. With a flying disc, the disc is the aircraft, so it's pretty much irrelevant. The only torque we are worried about is the torque around the center of the mass, since this is what will cause precession, not the torque as calculated at some point towards the front of the disc.

The torque around the center of mass is what causes the disc to precess. A nose down torque will cause the disc to turn over, and a nose up torque will cause it to fade. You can measure all of the forces around the disc and find a point where there is zero torque, this is what is called the center of pressure. You can visualize it as a line through the disc pointing in the direction of the net lift and drag force. If it is in front of the center of mass, it will cause the disc to nose up, and if it's behind it will cause it to nose down.

Now if you put the disc at an angle where lift equals zero, and all you have is drag, what is the net torque around the center of mass? As sidewinder pointed out for a symmetric disc, it would be zero. This is because the aerodynamic pressures pushing the disc down are exactly equal and opposite the pressures pushing up, because the surfaces are identical. For a cambered wing this isn't the case. The top and bottom are totally different, so while the total force pushing up is equal to the total force pushing down, they are not pushing in the same places. For a cambered wing this results in a nose down torque around the center of mass. So a cambered wing at the zero lift angle of attack is always going to have a nosedown torque. Hence a thumber pans right and a tomahawk pans left.

 

[armiller]

I may have been unclear, so I'll go into more detail. Let's draw some vector axes on the disc: X is the vector pointing through the back of the disc to the front. Y is the vector pointing from left to right across the disc, and Z is the vector pointing straight up from the center of the disc and straight down from the bottom. Lift by definition always points through the Z vector, and drag always points through the X vector. In actuality there are a bunch of forces in a lot of different directions who's net value is pointing in some direction between X and Z. The X portion is called drag, and the Z portion called lift. The point where are all of these forces summed up produce a zero moment (or zero torque) on the disc is the center of pressure. You are correct that this point is very rarely the center of mass. Just because lift points up in the Z direction, does not mean that it's pushing up from the center of the disc.

The lift is always pointing in the Z axis, even if the disc is tilted. This is essentially why a hyzer goes left. Gravity is pointing down, and lift is pointing both up and to the left. If a disc is at a 45 degree angle the lift vector is pointing at a 45 degree angle to the ground. If a disc is vertical the lift vector is pointing parallel to the ground.

I agree with this for the most part, though the hyzer thing is also partly explained by conservation of angular momentum (I'm guessing even more than lift). You left out angle of attack, which is important. Drag is a vector operating opposite to velocity, and lift is a vector operating perpendicular to velocity. I think we agree that drag and lift operate through that "center of pressure" point.

Gravity is always important because it's a significant force on any throw.

Agree

... The big reason why it's important is that it's counteracting lift.

...

Not in overhands... Again, I think mainly we agree, especially in regular "cambered-side down" throws.

I'm sorry. I'm guessing you have way more experience with aerodynamics than me. Just nit-picking on small details. Sometimes in the past this has exposed my own ignorance...

 

[DG_player]

I agree with this for the most part, though the hyzer thing is also partly explained by conservation of angular momentum (I'm guessing even more than lift). You left out angle of attack, which is important. Drag is a vector operating opposite to velocity, and lift is a vector operating perpendicular to velocity. I think we agree that drag and lift operate through that "center of pressure" point.

I assume with your reference to angular momentum and AoA, you are referring to the disc rolling/banking to the left or right. In that case I agree with you, that is the cause of the disc rolling or banking. What I was referring to was the disc actually flying to the left or right after it turns over or fades (as opposed to continuing to fly straight forward). This is a result of the lift vector pointing in that direction.

Not in overhands... Again, I think mainly we agree, especially in regular "cambered-side down" throws.

I agree in overhands. I was trying to show how gravity affects lift and AoA, so it would be clear as to what happens in the absence of gravity or the gravity vector being perpendicular to lift (thumbers or tomahawks). If lift isn't fighting gravity, it's always going to push the disc to the minimum drag AoA, which is also the zero lift AoA.

 

[ILUVSMGS18]

Should OH throws "flip" and not stay vertical the entire flight? My 'hawks stay vertical while my thumbers flip. The 'hawks usually end up about 75' further.

 

[DG_player]

Should OH throws "flip" and not stay vertical the entire flight? My 'hawks stay vertical while my thumbers flip. The 'hawks usually end up about 75' further.

You're probably not getting the disc vertical enough on release. If you watch a really good overhand player, their throw looks like a baseball pitcher.