Why does "beat in" = less stable.

[ElementZ]

The above is a hand waving argument. "It happens because it happens". This solution does not address why it is understability that is caused and not overstability.

You have to look at it this way, throwing a disc with drag makes it more understable in the same sense that throwing it into a headwind does.

The speed of the disc compared to how fast it's spinning are the two factors that affect it's stability.

If a disc is spinning super fast and traveling slowly, it won't flip.

On the other hand, if a disc is traveling much faster than it's spinning, it will flip (headwind).

That's how I've been thinking about it and seems to make sense. It also explains why some people can flip discs others can't, since they can throw much harder with the same amount of spin.

 

[Nick481]

In thinking about the magnus force (a.k.a. the force that makes a curveball curve), the disfigurement of a disc should lead to an increase in understability.

The force that pushes a disc sideways due to it's spin is directly proportional to the cross-sectional area of the disc. Any bends, scrapes, or dings would increase the surface area, and consequently increase the magnus force as well.

In my envisioning of the phenomenon, I see it like this: consider the far left and right edges of a spinning disc in flight. For a right handed person, the left side spins against the oncoming air, while the right side is retreating from it. The pocket of air that encounters the left side gets disrupted when the disc travels through it due to friction between the disc and air. Similarly, the pocket of air on the right side does as well, but not as severely because that side of the disc is spinning away from the oncoming air.

The air that gets disturbed on either side can only get pushed out away from the disc. Since the left half pushes air away more, the unbalance results in a net force to the right due to the conservation of momentum. If you were to increase the surface area of the disc, this net force would also increase.

I pretty much agree with this. Think about if you took a golf ball with dimples and one without dimples. The magnus effect will cause both to curve up during flight after hit with a golf club because the bottom side of the ball is rotating against the direction it is moving, causing higher pressure on the bottom of the ball and lower pressure on the top of the ball, much like how lift with an airplane wing works, only it's rotationally oriented. Of course the ball with dimples is going to have a greater magnus effect because it has "pockets" for the air to go into which causes more air to be "pushed" against the airflow from the flight of the ball.

Now translate this to a disc. Think of it as a sideways golf ball. For a rhbh thrower (the disc rotates clockwise), the left wing of the disc is going to be like the bottom of the golf ball because it is rotating against the direction the disc is moving. This is naturally going to cause a higher pressure zone, and vice versa on the right wing of the disc, it will be creating a low pressure zone, similar to the top of the golf ball. So, what this is creating is essentially higher pressure below the left side of the wing, and lower pressure below the right wing. This is what causes the right wing to dip and make a disc what we call "understable" or make it "turn". It is mostly dependent on rotational speed of the disc and how fast it is traveling too. At it's highest speed and spin (which is right after it leaves your hand) is when the greatest pressure difference is going to occur because you are combining two objects (air and disc) that are moving in different directions at a higher rate, causing more turbulence.

So, when a disc gets beat up, it is developing surface imperfections, which you can compare to the dimples of a golf ball. It is going to create more anti directional airflow on the left wing causing an even greater pressure. It does not effect the right wing as much however because that is rotating with the direction the air is passing the disc by while in flight.

I also theorize this is why discs fade as they slow and lose rotation. The left wing is progressively going down and down in combined oncoming velocity because not only is it rotating slower, but the air the disc is passing through is not "passing by" as quickly. This causes the left and right wing to even out in pressure, and eventually there is a very small if any pressure difference between the left and right wing. Combine the discs off center balance (there is some, although it is very small) with a discs natural weak gyroscopic property while rotating very slowly, and the disc turn left or fades.

I also believe this is why OAT or wobble out of the hand makes discs turn over very fast. Even if it is rotating and traveling forward at the same speed as a clean release, the left and right wing are wobbling up and down very rapidly. This enhances greatly how much air is being displaced and creates more turbulence enhancing the off balance of the two wings (ever been trough turbulence on a plane, it causes the plane to lose lift and drop a little, making your stomach "tickle" more a moment).
All of this is a theory of mine, it might not be entirely correct but I know some of it's a baseline for why discs fly like they do. It seems pretty logical to me and hopefully it's right, if not, well I'm only 15 so hopefully I can get it fixed after physics my junior year.

 

[FrigYallHos]

What I got out of all of this is do not pay attention to a discs rating at all.

 

[BIGHAFNER]

^True, well, I honestly think, I don't know, that it's the same reason that if you throw a disc harder than intended it is more understable. You are throwing it faster so the velocity of the air passing it is greater. Force is Mass * Velocity so if the same mass of air is passing the disc at a faster speed, the force of the air on the disc is greater. This force being drag. The same thing happens with the disc getting beat in, the drag is increased so there is more force acting upon the disc, thus causing it to be more understable than it's non beat in counterparts of the same mold. Or more understable than it was before it was beat in.

This also is why tailwinds cause a disc to be more overstable. The velocity of the air is moving in the same direction of the disc, thus it's value is lower, or negative even, causing the force acting upon the disc to be lower than it would be if it were thrown with no wind. So there is less drag in a tailwind, which causes discs to be more overstable than the discs thrown in no wind.

So pretty much, every disc has it's flight ratings based on the disc being brand new, thrown with the appropriate power, in no wind. Thus, the ratings are based on the disc receiving a certain amount of drag, when that amount is increased, by being thrown too hard, thrown into a headwind, or the disc being beat in, it acts more understable. When the amount of drag is decreased, by not throwing it to the power required or by throwing into a tailwind, the disc acts more overstable.

P.S. I didn't feel like typing that earlier.

 

[Nick481]

Your right too, you kinda touched up on where i failed to elaborate much, that throwing a harder throw makes it spin and travel faster making a greater pressure difference than a less powerful throw, obviously causing a disc to turn more if thrown harder.

 

[DavidSauls]

This thread is way over my head.

I do wish someone would do an experiment---buy a matched set of DX discs. Keep one pristine. Scuff up one with steel wool, screwdrivers, whatever, but don't hit anything with it. On another, bend down the "nose" a bit but don't scuff it up at all.

Then throw all 3 and see which is most understable.

I'm curious but....not enough to try this myself. Any volunteers?

 

[jenb]

This article is helpful in understanding the effect of dimples on a golf ball: http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml

One thing to note is that the same structure on an aerodynamic wing does not have the same net effect by reducing pressure drag because the skin friction drag is much more prominent. However, the rough surface on top of the flight plate, especially near the rim, may act as a vortex generator to increase lift.

Meanwhile, the description above of the right and left edges of the disc with respect to the wind passing to the sides of the disc is good, but I think it is a little off. Slower moving air has more pressure than faster moving air, but I like to the think of the two side undersurfaces of the wings of the disc, rather than the disc edges. The one traveling towards the oncoming airflow (i.e. to the left on a clockwise spinning disc) will have skin friction drag that opposes and slows the oncoming airflow. The other wing will have the opposite effect. So the pressure under the wing on the left is greater, and the disc flips at sufficiently high speeds. It may be that dings and scrapes in the plastic under the wing could increase the skin friction drag and enhance this effect.

 

[mashnut]

This thread is way over my head.

I do wish someone would do an experiment---buy a matched set of DX discs. Keep one pristine. Scuff up one with steel wool, screwdrivers, whatever, but don't hit anything with it. On another, bend down the "nose" a bit but don't scuff it up at all.

Then throw all 3 and see which is most understable.

I'm curious but....not enough to try this myself. Any volunteers?

FWIW, I had a friend try to artificially break in a dx disc just by scuffing and roughing the edge without impacts, it made very little difference in the flight.

 

[ManU]

this thread is half not helpful and half correct

IF drag = understability then someone answer me why the big azz blunt nosed and beaded gator (i.e. a disc with huge drag) is so UNDER stable

it's not

the nicks and dings on a disc themselves do NOT create the understability

what happens when you hit a solid object like a tree and get a nick or a ding?
the nose of the disc gets bent down

the more the nose of the disc gets bent down then the lower the PLH effectively

discs with lower PLH are less stable

here is an article on tuning

http://aerobie.com/Products/Details/EpicTuningArticle.htm

yes it deals with the epic but the theory holds across discs

 

[tripp]

this thread is half not helpful and half correct

IF drag = understability then someone answer me why the big azz blunt nosed and beaded gator (i.e. a disc with huge drag) is so UNDER stable

it's not

the nicks and dings on a disc themselves do NOT create the understability

what happens when you hit a solid object like a tree and get a nick or a ding?
the nose of the disc gets bent down

the more the nose of the disc gets bent down then the lower the PLH effectively

discs with lower PLH are less stable

here is an article on tuning

http://aerobie.com/Products/Details/EpicTuningArticle.htm

yes it deals with the epic but the theory holds across discs

Does the nose always get bent down? Doesn't it some of the time get bent up?

 

[joe_kool315]

I'm a ball golfer so I can relate to the references there. What some failed to mention is what "causes" a ball that's round to fade slice balloon or nose dive. Maybe that will help the OP understand what's going on with the disc its self somewhat if he hasn't already. Rotational spin, in a golf balls case, is the culprit. Someone hit on the correct terminologies and scientific names earlier but in plain speak it's the spin. if you hit a golf ball off-center and cause it to spin, say left to right, that causes the ball to travel right to left (draw). This is the same reason a disc fades off to the left when thrown RHBH when it slows down and to the right RHFH. It's the same disc ... only the direction of rotation changed.

Now as a disc gets used and "beat in" its air flow properties change as well as the physical properties of the disc. As already stated, PHL is a HUGE determining factor in overall stability of a disc. Two discs of the same mold BRAND NEW of the shelf will fly differently if the PLH doesn't line up. a lower PLH on one will cause that disc to fly under stable compared to the disc with a higher PLH.

Lots of good information above ... some that just doesn't hit the mark though.

 

[joe_kool315]

Does the nose always get bent down? Doesn't it some of the time get bent up?

"Usually" when the discs hit they'll get blunted in and/or down. You'd have to throw at a pretty awkward angle to get it to bend up.

I have had some hyzer spiked discs that impacted a sidewalk that bent up a bit but I was able to tweak it around to get it back in line.

 

[BIGHAFNER]

With a disc like the gator, the weight distribution and rim shape is such that it is overstable. It was made that way, expecting the drag it receives. I'm not saying that PLH and the nose getting bent down doesn't contribute to understability, I'm just saying that it could be a combination of both.

 

[bikinjack]

At least 99% of a disc being beat in and becoming understable comes from the combination of the lowering of the PLH (as ManU explained) and the flashing (and bead if the mold is beaded) getting worn down and/or off.

 

[mjoyce]

Drag doesn't make a disc understable. Drag makes it overstable. When you lower the PLH you create more lift, lift makes a disc understable. Because as someone stated earlier the pressure differece between the left side and right side imparted by the spin of the disc. A disc is a wing, if you give the disc more lift it's going to want to turn to the right more(RHBH). You're in the ball park thinking about lift and drag but beating a disc in doesn't give it any more drag. And drag isn't making a disc understable. A bead on the bottom of a disc adds drag which makes a disc more overstable. (roc, wasp ,wizard, bb avaiar) Or if the nose is made more blunt (increasing drag banshee, firebird, XXX) The disc is more overstable.

You're confusing lift and drag, The turn (understability) would be caused by creating greater lift. When you lower the PLH the wing pulls more air under the disc increasing the lift. (For reference I've had two rivers that from day 1 were completly different discs because the PLH of one was much lower than the other. The one with the lower PLH(170 Opto River) is significantly more understable than the higher PLH (160 GL River) Conventional L64 wisdom states that heavier + Opto means more stable. But if the PLH is lower it's going to be more understable than an identical disc with a higher PLH.

 

[atvan]

To contribute more physics to the dings argument , dings and scratches reduce friction. Read on, but be warned, nerd material ahead.

There are two types of flow- laminar and turbulent. They are what they sound like. Laminar flow is when all the particles travel in a straight line parallel to each other, and slight distubances are neutralized because the particles form a sort of track for each other to put them back in line. Because the particles are all moving together, this type of flow creates the least friction against a surface, because the particles are not bouncing around. Turbulent flow is the opposite. If too great a disturbance is introduced into laminar flow, the particles start bouncing off of each other and a chain reaction begins, quickly causing the flow to become chaotic and turbulent. It is much easier to make this transition than to transition back to laminar flow. Because turbulent flow is bouncing all over the place, it pushes harder against a surface, creating more drag.

At this point, you may be wondering why the dings reduce friction, if they create turbulence, which increases drag. The answer is that it is more complicated than just laminar and turbulent flow. When a fluid passes over a surface, there is something known as a boundary layer. This is the layer of (in our case) air that is caught up in the friction of the disc, and not quite moving with the surrounding air. At the very surface of the disc, there is a layer of air that moves with the disc. As you progress outwards, the difference between the speed of the disc and the air increases, until the point at which the air is no longer moving with the disc at all. This is where the boundary layer ends.

A laminar boundary layer is thinner (and thus lighter in weight and more aerodynamic) than a turbulent boundary layer. However, there is a phenomenon called boundary layer seperation that occurs on bodies with very low Reynold's Numbers (read: less than a few feet from leading to trailing edge depending on the shape) In which this slowed down air actually seperates from the body, creating a tremendous amount of drag. Because of the chaotic movement of the particles, a turbulent boundary layer is much less likely to seperate than a laminar one, greatly out-weighing the additional drag is causes.


And now to how this effects stability.

As we know there are two components that reflect the stability of a disc, turn (the aerodynamic component) and fade (the gyroscopic component). dings affect both of these. As well as what i stated above, any ding, especially in the rim (where most of them happen), reduces the gyroscopic effect to some degree becuase of the removal of some of the spinning weight that causes it.

However, turn works differently. Turn is caused because of a lift imbalance stemming from the spin of a disc. For a RHBH row, the disc is spinning clockwise. This means that, relative to the air it is moving through, the left side of the disc is moving faster than the right. Because faster movement (usually) results in more lift the left side of the rises faster (or falls slower) than the right side, causing the lift vector to contain an upward component and a rightward component- this rightward component is the turn.

So why don't discs just always turn and burn? Because of the gyroscopic effect! Because I have never taken a physics class with any of this type of stuff (surprising huh?) I admit that I don't understand this part, other than that more spinning weight, especially farther out (like a lever) increases the force of the effect.


And I probably missed a few things, so be sure to ask about any holes in my description.
Edit: forgetting why the first part applies is kind of a biggie.
Basically, the decreased drag from the dings increases the amount of time in which turn overpowers fade, so it has time to turn farther before it slows down.

And credit for all this fluid dynamics knowledge goes to a book called Model Aircraft Aerodynamics, essentially discussing the aerodynamics of RC planes. The sizes are similar, meaning that many things apply to both, though the spin thing is exclusive to the discs.

/nerd rant

Also, I agree that PLH probably is probably the major factor, but there is at least some truth to the dings theory.


Edit: Didn't realze how long it was. Scary, I know, but if you want, you can skip over all the stuff in the middle.

 

[The dude on the bike]

It's the wing getting bent down. I did a fun demo of this with a pro D Venom, with much exaggeration.

 

[captain jack]

Well, everybody so far is clueless. The question is why does a beat in disc become less stable.
You PLH people are the most annoying, although its true PLH can initially determine stablilty, beating in a disc does not necessarily mean the PLH comes down.
The nick and surface camp makes sense, but it still doesnt explain why a disc thats repeatedly taco'd into a tree, but has no extreme surface damage, can still fly less stable than a new version.

The only real answer is, and I dont know why 3P hasnt piped up yet , repeated impacts change the properties of TPE's at a molecular level.
If we had any physics smarties that participated here, they could tell the rest of us exactly whats happening here, on a molecular level, to cause the changes.

My guess is the TPE's become less dense, due to more air between molecules as it gets crashed, thereby becoming less gyroscopic.

Okay, thats my BSOMA theory, you guys can tear it down.

 

[The dude on the bike]

I can show you on a piece of dx plastic, or you can try it yourself. All old school players know how to fix a taco.

 

[mashnut]

Well, everybody so far is clueless. The question is why does a beat in disc become less stable.
You PLH people are the most annoying, although its true PLH can initially determine stablilty, beating in a disc does not necessarily mean the PLH comes down.
The nick and surface camp makes sense, but it still doesnt explain why a disc thats repeatedly taco'd into a tree, but has no extreme surface damage, can still fly less stable than a new version.

The only real answer is, and I dont know why 3P hasnt piped up yet , repeated impacts change the properties of TPE's at a molecular level.
If we had any physics smarties that participated here, they could tell the rest of us exactly whats happening here, on a molecular level, to cause the changes.

My guess is the TPE's become less dense, due to more air between molecules as it gets crashed, thereby becoming less gyroscopic.

Okay, thats my BSOMA theory, you guys can tear it down.

Out of curiosity, have you ever tried bending a disc then throwing it? Bending the nose down (effectively lowering the plh) makes discs fly more understable, bending it up makes them fly more overstable. Every time I've ever hit a tree hard enough to see a bend in the disc, it's bent the nose down in the area of the impact. Put those two together and I'm a whole lot more convinced of that as a cause than your theory.

If the material of the disc was becoming less dense, either the disc is getting bigger with impacts, or it's losing material and getting lighter, you'd need a pretty significant change to actually affect the disc flight. I've never seen either of those actually happen from normal wear and tear, have you?