Forum: Catapult Message Board

on 06/07/2010 06:34 am, by Russell 3 stars

I finally tried out an idea I've thought about and talked about for, well years really.

Basically it's all just based on the idea that the spin direction on a treb's projectile is determined by the positions the sling cords latch onto the beam end.
With the standard fixed cord behind and released cord at the end arrangement you get backspin of course - with the spin axis horizontal and at right angles to the direction of flight.

Mounting the fixed point and the release pin the same distance apart but now side by side should do something entirely different.. at release the sling and sling cords should also be rotated 90 degrees to the usual so when the release pin lets go of the loose cord and spins up the projectile that spin axis will be pointing downrange and tilted upwards (more or less at the projectile launch angle.

I used old Son of Cheesechucker with a reduced counterweight launching a multicoloured juggling ball (to make the spin easier to see)

It seems to work, but unfortunately even the reduced spin revs defeated my attempts to photograph and video the action.
I'll revisit the experiment when I work out how to speed up the digital "shutter speed".

The main difficulty I see so far is that the projectile either has to be standing on its head in the launch trough or lying sideways normally to start and rotating 90 degs after it clears the trough... likely to cause wobbles in flight.
On top of that the sling pouch won't be holding the projectile so snugly during launch as it does in a normal shot.

Anyone feel keen enough to scale this little experiment up a notch?
.. maybe with that football..

cheers
Russell

ps attached photos show the adapter clipped on over the normal release pin - and showing that it is the right cord that releases.
The second photo shows the ball leaving the pouch, its blurred image changing colour as it spins.
Not enough detail to nail down the spin orientation, unfortunately.

on 06/07/2010 07:24 am, by Walker 4 stars

Russell,
Nice work! Good on ya, Mate.
(Please excuse the American love of Australian idioms.)

I won't be putting your theory to the test any time soon but I do support the thought behind it.
We have all seen how a twisted sling line can turn the projectile sideways in it's spin axis to target orentation and cause it to hook or slice.

But I have a question; If we lower the angle of trajectory in order to take advantage of the Magnus Effect, what if we reversed the concept and over elevated the release angle to allow the forward spinning Magnus effect pull the projectile forward.
It seems to me that using the power of the engine to get the projectile aloft and as high up as possible AND letting the weaker Magnus effect pull it forward instead of against gravity would be the more efficient usage of the concept.

Illustration: We see it in golf balls. A straight shot lifts but a slice pulls a lot more because it doesn't fight gravity.

Walker

on 06/07/2010 08:57 am, by Russell 3 stars

Hi Walker
Hey buddy, you're welcome to use the Australian idiom - we've been adopting Americanisms for decades!

Today's test was about sideways sling cord mountings and I tried to keep things simple.
OTOH Setting up a treb for forward spin should not be a big adaption - I'd say you would mount a loop where the release pin was and fit a hook below the beam where the fixed cord was previously mounted. (ie just reverse the anchor positions)

My original idea was a rotatable rig holding the fixed cord mount and the release pin (which always points parallel to the beam. Who knows, maybe you could dial up your preferred slice or hook!

One thing today's activities showed was that even very little throwing power imparts a lot of spin to the projectile.
You tend not to see it when the treb is large or throwing fast.

on 06/07/2010 12:08 pm, by Brian_L. 3 stars

> It seems to work, but unfortunately even the reduced spin revs defeated my attempts to photograph and video the action.
> I'll revisit the experiment when I work out how to speed up the digital "shutter speed".

How about just some plain static shots of the device at the arm tip and with it loaded and cocked so we can get an idea of what we're looking at in the blurriy "moving" pictures?

on 06/07/2010 08:01 pm, by Russell 3 stars

> How about just some plain static shots of the device at the arm tip and with it loaded and cocked so we can get an idea of what we're looking at in the blurriy "moving" pictures?

I took some of those. Here's a back view showing that it's just a standard sling mounted in a rotated position.

It's a very simple mod - a small piece of wood "bungied"* across the beam end and fitted with its own release pin off to one side and a hole to mount the fixed line on the other side.

okay, not an actual bungy - this is a small model treb so I used one of my daughters' ponytail "Poneo" elastics.

on 06/07/2010 04:14 pm, by Thomas 5 stars

It's been discussed, but this is the first time I know of that anyone has actually worked with it.

Maybe you could get some of those golf cross balls (external link)

that actually resemble footballs.

Thomas

on 06/07/2010 07:38 pm, by Walker 4 stars

Golf-Cross?
Holy Macaroni, I love it.

Thomas, you are a genius.
(Just not Evil enough yet.)

Walker

on 06/07/2010 10:42 pm, by Thomas 5 stars

> Thomas, you are a genius.
> (Just not Evil enough yet.)
>
> Walker

"Good is better than evil because it's nicer."

Mammy Yokum (Li'l Abner's mother), ca. 1950

on 06/08/2010 08:23 pm, by RIPCORD 3 stars

Most interesting. Tip of the coif to you, Russell.

To understand the potentials of forward spin, it helps to understand what is actually taking place, then how forward spin would change that.

IMHO (combined with not having thought this out for long): Back spin works in our favor due to the turbulence at the rear of the Payload being pulled under, while slightly extending the smooth air flow over the top. This produces the pressure differential between top and bottom, with the lower pressure at the top due to velocity. Mind you, the static air pressure difference between top and bottom has no appreciable effect, being such a minor difference.

With forward spin, one would expect the turbulent zone to move upward, at the rear, and other zones to move in a relative fashion. However, does this avail us anything? As Russell suggests, it would seem that there would be a low pressure zone now leading the Payload, which in turn should 'pull' the Payload along. Makes sense, if that was all there was to it.

If the low pressure zone moves forward and down enough to be of use in pulling the Payload forward, what is happening elsewhere? Does the turbulence zone move towards the top and, if so, do we get a relatively low-pressure zone at the bottom that could pull the Payload downward?
What of the on-rushing air stream? Will it raise the pressure at the front beyond the ability of the spin to reduce it?

Wow...one can go happily nuts thinking of possible "ifs".

I think what will happen is that, yes, there will be a forward pull to the Payload, but at the same time there will be so much loss of vertical lift, which will have to be compensated by a higher angle of trajectory, that there will be an over all loss of range.

Of course, all of that will depend on all the flight variable involved and, as we've seen between various sizes of machine, this can produce a considerable difference!

on 06/09/2010 02:27 am, by Melvang 3 stars

> Most interesting. Tip of the coif to you, Russell.
>
> To understand the potentials of forward spin, it helps to understand what is actually taking place, then how forward spin would change that.
>
> IMHO (combined with not having thought this out for long): Back spin works in our favor due to the turbulence at the rear of the Payload being pulled under, while slightly extending the smooth air flow over the top. This produces the pressure differential between top and bottom, with the lower pressure at the top due to velocity. Mind you, the static air pressure difference between top and bottom has no appreciable effect, being such a minor difference.
>
> With forward spin, one would expect the turbulent zone to move upward, at the rear, and other zones to move in a relative fashion. However, does this avail us anything? As Russell suggests, it would seem that there would be a low pressure zone now leading the Payload, which in turn should 'pull' the Payload along. Makes sense, if that was all there was to it.
>
> If the low pressure zone moves forward and down enough to be of use in pulling the Payload forward, what is happening elsewhere? Does the turbulence zone move towards the top and, if so, do we get a relatively low-pressure zone at the bottom that could pull the Payload downward?
> What of the on-rushing air stream? Will it raise the pressure at the front beyond the ability of the spin to reduce it?
>
> Wow...one can go happily nuts thinking of possible "ifs".
>
> I think what will happen is that, yes, there will be a forward pull to the Payload, but at the same time there will be so much loss of vertical lift, which will have to be compensated by a higher angle of trajectory, that there will be an over all loss of range.
>
> Of course, all of that will depend on all the flight variable involved and, as we've seen between various sizes of machine, this can produce a considerable difference!

Fair warning guys i am not an expert on aerodynamics or fluid dynamics (air in some aspects is considered a fluid) but from what i remember my dad saying about spin on the projectile from his golfing experience and my experience with golf on video games. The back spin will produce a longer loft with a lot shorter run on the ground. The forward spin (more commonly called top spin in the golfing world) will result in a lower flatter trajectory however is affected by the wind a lot less so wind hurling into a head wind or a strong side wind there might very well be a benefit to top spin. However with any tail wind or little to no wind the back spin should result in the most distance as we measure to the first impact. But this is just my 2 cents and thinking "out loud".

Melvang

on 06/09/2010 07:40 am, by Walker 4 stars

> Melvang wrote:
> Fair warning guys i am not an expert on aerodynamics or fluid dynamics (air in some aspects is considered a fluid) but from what i remember my dad saying about spin on the projectile from his golfing experience and my experience with golf on video games. The back spin will produce a longer loft with a lot shorter run on the ground. The forward spin (more commonly called top spin in the golfing world) will result in a lower flatter trajectory however is affected by the wind a lot less so wind hurling into a head wind or a strong side wind there might very well be a benefit to top spin. However with any tail wind or little to no wind the back spin should result in the most distance as we measure to the first impact. But this is just my 2 cents and thinking "out loud".
>
> Melvang

No worry, mate. I don't think any of us are Fluid Dynamics experts. WE just try to reason things out together.

For instance: "Top Spin" is what a golfer calls a topped shot, that is a swing that connects with the ball too high.
"Top Spin" makes it sound intensional where as @#$%^&*! is the correct nomenclature. Shanks a lot.
From my experiance, golfers will always provide bad advice to other golfers. ("Sure, son, You can reach the green from here with that 5 iron.") (You want to play real close to that tree ~THUNK~ Yeah, that tree.) Maybe your Dad is different.

Additionally, Forward rotation will not produce a flatter shot, it's just that in golf, the ball is driven at a low attitude. It actually dives quickly as it's velocity falls below air viscosity factors, it surely is not flat.
In Hurling, you would have to give your shot a lot more attitude; higher angle of ballistic trajectory to compensate for the downward curl of forward projectile rotation.

I want to know more about those spheroidical balls.

Walker

on 06/09/2010 11:35 am, by madmattd 3 stars

Trying to remember my Fluids Dynamics here. Basically, spin of any sort results in what is known as the Magnus Effect. The rougher the surface, the more turbulent the boundary layer flow around the object, increasing the effect. When given sufficient backspin, the Magnus effect can actually overcome gravity and increase the flight angle of a projectile (Chucky II in 2007 anyone?--the "3k" shot that wasn't...). When the initial trajectory is at a low enough angle, this increased flight angle can result in more distance than if fired at a more typical trajectory (ie. 40 degrees) with less backspin.

Now, reverse this. Yes, forward spin will "pull" the projectile forward, but more importantly, will pull it downward. Magus doesn't affect the horizontal component of the trajectory as much as the vertical as I recall, and this is certainly born out with classical examples and already mentioned cases.

Magnus is what allows the "4-seam fastball" in baseball to maintain a pretty straight trajectory. The vertical lift due to the backspin allows the ball to fight gravity sufficiently to stay on a straight path. Until, of course, so guy with a bat ruins your day. Imparting sidespin is what results in a "slider", that is a pitch that curves to one side or the other. Again, Magnus at work.

Matt

on 06/09/2010 11:35 am, by Thomas 5 stars

> Fair warning guys i am not an expert on aerodynamics or fluid dynamics (air in some aspects is considered a fluid) but from what i remember my dad saying about spin on the projectile from his golfing experience and my experience with golf on video games. The back spin will produce a longer loft with a lot shorter run on the ground. The forward spin (more commonly called top spin in the golfing world) will result in a lower flatter trajectory however is affected by the wind a lot less so wind hurling into a head wind or a strong side wind there might very well be a benefit to top spin. However with any tail wind or little to no wind the back spin should result in the most distance as we measure to the first impact. But this is just my 2 cents and thinking "out loud".
>
> Melvang

Not to teke anything away fron Melvan's (and Walker's) comments, but driving a golf ball and hurling can be contrasted in one significant way: in golf it's more important to gain distance overall, on the ground as well as in the air. The guys that teach long driving distance emphasize flatter trajectories and long rolls, so golf's lessons aren't directly applicable to hurling. Maybe we're lucky we don't have to deal with the other factors.

on 06/10/2010 07:16 am, by Russell 3 stars

A while ago Nasa put up a page on the aerodynamics of rotating bodies.
You can find it here:
for cylinders (external link)

and here
adapted for spheres (external link)

The animated calculators are nice too.

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Another spin orientation - axis "downrange"

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