Forum: Catapult Message Board

Well, Victor the Terrible, you spent a fair amount of verbiage "immortalizing" the BAT video. I have to challenge your conclusion, though.

You state:

"It is possible that people are not analyzing their own videos. Or, possibly, the fact that their projectiles are released well over their heads gives them the illusion that their projectile’s altitude is due to a steep angle rather than to initial height. But, whatever the cause of the problem, designers need to put more work into tuning their trebs to launch at 45° before committing their inventions to YouTube."

The introductory paragraph to the video reads:

"An experiment in improving trebuchet performance by increasing the dropping distance of the counterweight. This is a rough prototype made from bits and pieces of wood that was on hand and is in a state of development.. A number of issues still need to be dealt with, but it works pretty well."


The stuff actually on ballistic properties of pumpkins is OK, if pretty basic. However, the suggestion that a pumpkin be filled with water for added density would be against the rules as set down by WCPC. Most other events follow the WCPC rules.

"8. Pumpkins are not to be altered in any way, excluding P.C.A. marker paints. All pumpkins must be in their natural state.
Pumpkins fired from machine during competition will be measured from the front of or the farthest point of machine. Your pit #
may be written on your pumpkin to help in the identification at the landing zone."

Grozny,

Please stop.
You know not whereof you speak and it's embarrassing.

Walker

flying pumpkins are weard things, i'll give some examples:

a pumpkin fired started to turn sideway's, ending up making an 90° turn ito the wind...

a pumkin fired allmost horizontal started to climb and then dropped like a wounded bird...

the turning sideway's happened often to us, some went left, some went right, with same wind conditions, it all depends on projectile shape, center of gravity and the ammount of spin the sling gave...

there is no way to predict the way it will behave after release, that's the reason why i like hurling a lot, you never know what's gonne happen

If a Base Ball, thrown at 80 MPH can perform a drop or lift or curve by reason of imparted spin (and the position of the stitches),
Then is it so hard to accept that a pumpkin may also have sufficent spin imparted to it at 120+ MPH whith which to cause positive lift?

UFO photo attached.

Really, Groz', I take it you were trying to disfuze the tension rather than trying to be a jerk with the UFO comment.

If you want UFOs, please visit my website: Evil Genius Labs
Your computer will be scanned and if no government spyware is found, we will open upon your hello.
However, if your computer is found to contain any form of government spyware, you will be ignored.


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Edit: add on 25SEP09.

Grozny asked: "It is possible that people are not analyzing their own videos (?)."

Yes, it is possible. That is why a number of us have taken to analyzing each other's works. My video library holds 40 test videos from JohnN and another 37 from Thomas. And others, too.
I edit and reproduce these into slo-mo, super slo-mo, super-duper schlow-mo, etc.
I consider that a fair question.

Chuck's last throw at WCPC climbed for about 2-3 seconds after launch, then set a new record. Pumpkins are imperfect and behave weirdly.
Tom

Seems like it's Bash Grozny Time. Why? Well, we have someone from Academia telling people who have real-world knowledge of a subject "how it is".

The article in question is basically fluff. Other than data taken from one pumpkin, statistically an invalid sampling, and the previously discussed video insertion, not particularly germane, it's unattributed cribbed data (unless Grozny did the wind tunnel tests himself, in which case I apologize).

Missing data include what variety of pumpkin was tested, of the many available. Some are more suitable for hurling than others, a fact well known to hurlers, apparently not to others.

Among the missing was any evidence that the ballistics of spinning objects was looked into. There is a large amount of research on that subject, much of it by the manufacturers of golf balls. Spin is an important factor in the flight of many objects. "Bend it like Beckham", hooking or slicing a golf ball, a popup in baseball.

Proceeding from that, the effect of the shape of various pumpkins needed to be tested. Does a ribbed one fly better than a smooth one? Dimpled golf balls do better than smooth ones, as is well known. Some pumpkins are flatter than others. Will a flat one thrown edge-on do as well as a round one? With a round pumpkin, is the orientation of the stem important?

When the paper sticks to its subject it's correct, if overly-limited in its treatment of that subject. Unfortunately it strays into other areas and reaches conclusions that are questionable.

Thanks to you guy's highly accurate and unimpeachable eyewitness accounts, I have able to add the following paragraphs to my paper:

I see no evidence that the opening of a nylon pouch imparts spin to pumpkins. Balls are given spin when they roll; for example, a baseball rolls off the pitcher’s fingertips or a golf ball rolls up the angled face of a club. A golf ball is violently struck by a steeply angled clubface made of unyielding steel and with very aggressive grooves cut into it to gain traction and assure that the ball rolls rather than slides up the face. It accelerates from a standstill up to speeds over 60 m/s in less than 10 ms, which is a very different thing than a pumpkin gently nestled in a smooth nylon pouch and taking about 50 to 60 times longer to accelerate up to speed, at which point the pouch just opens, without any twisting motions. Anyway, pumpkins lack the raised stitches or dimples that cause baseballs or golf balls to curve.

There is a group of guys at the Catmess who seem to think that the phrase “Magnus effect” is a sort of magical incantation that, when spoken, will dispel any doubts about the most outlandish braggadocio. These guys are probably amateur golfers who learned this big and very scientific-sounding word from their golf pro. Observing the rather unpredictable trajectories of their own golf balls at the driving range, they concluded that ballistics has a sort of anything-goes quality that is ideally suited to internet bragging.

For example, one fellow writes, “I have seen a pumpkin fired at a depressed angle (below the horizon) which climbed to a 15° ascent and continued for about 2,000 feet.” When asked for an explanation of this gravity-defying feat, he replies, “Is it so hard to accept that a pumpkin may have sufficient spin imparted to it at 120+ mph with which to cause positive lift?”

Obviously, there is not much that I can do to help this gentleman. But, for those of you launching pumpkins at angles between zero and ninety degrees and with sling pouches that somehow manage to open without spinning the poor pumpkin up to hundreds of rpm, my software will help you create a ballistics table for your trebuchet.

Also, while I hesitate to impugn the craftsmanship that goes into these homemade devices, I must raise the possibility that they wobble. Perhaps sometimes their frames flex to the left and sometimes to the right. The resulting trajectories, being reminiscent of the hook or slice of a golf ball, conjured up talk of the Magnus effect, rather than a tightening of wood screws, which might have actually helped.

A wobbly treb is probably the explanation for this guy’s observations:

"Turning sideways happened often to us; some went left, some went right, with same wind conditions. It all depends on projectile shape, center of gravity and the amount of spin the sling gives. There is no way to predict the way it will behave after release. That's the reason why I like hurling a lot. You never know what is gonna happen."

Frankly, I do not think I would want to stand too close to this guy’s treb during a pumpkin launch. You never know what is gonna happen!

You are more than welcome to come to Delaware and meet some of the fine people that have no idea what they are talking about. Of course it would be easier to sit behind a computer and criticize those that have actually constructed something besides a "paper" and some sarcastic comments.

Groz',
Tact isn't your forte, is it.

If you have some credentials, please share.
It may make it easier to accept some guy who apparently walks in off the street and starts spouting off about how we are all wrong even though his head is so far up his inseam that his eyes have turned brown.

My creds? None you would recognize until one hit you.

Your "Paper"?
I found that the selected quotations of my fellows here tended to elevate the quality of the work.
I am quoted accurately if not attributed. (Nor do I consent to being quoted outside of this forum.)

This “gentleman” (hack, hack) did not ask for your misguided help. Furthermore I find it insulting to be labeled as such.
Fact is, Groz’, I am not a gentleman at all. I’m one of those guys who volunteered to fight so that your right to express yourself would be protected. That being said, I really hope you will find another forum in which to express yourself.

Walker

How about a 5th year Mechanical Engineer who has some Fluid Dymanics under his belt? The Magnus Effect is REAL, I have personally observed its effects with trebuchets, caused it to occur even more by adjusting the fixed position of the sling, and can even give you some rough idea of its effects on a pumpkin (way too long to go into right now, and it's late). And the 45 degree trajectory, as ANY Physics Teacher who knows what they are talking about will tell you, applies only in a vaccuum. In fact, most state that when they introduce the "optimum" trajectory as 45 degrees, mine sure did. Again, it has to do with some Fluid Mechanics/Dynamics. Please, I do not want to sound like I am attacking you, but I cannot stand people standing behind a desk/computer telling people who have real-world experience what is real and what isn't. Theory is great, but you need practical hands-on experience to back it up and to make yourself credible. I know far too many people from my time in college that do not understand that the real world does not work according to their equations.

Matt

When theory and practice colide, which is the first to go?
Groz' does not understand Zen Trebuchet.
Therefore all he has is what he remembers from his classes.

GROZNY

Walker

This got posted last week on another board I frequent. Here's a link.

Conclusions are generally similar to here. You can probably guess which member I am over there...

That one guy basically went line by line disproving his paper. Remind me never to submit anything to rocket scientists! Ouch!

Punkins do spin.

http://www.youtube.com/watch?v=Blftpqd4Gd4

Craig

I was hesitant to see ugly slime may have returned to this thread but I only found my hurling buds, nobody named after a Chechen city that the Russans used like a cheap ~um~ (family safe site?) ~er~ Ah!:
like a cheap plate of HUNGARIAN GOULASH!

Maybe he was abducted by alien pumpkins.
Maybe someone would think I had anything to do with his sudden disappearance. Who? Me?
I don't make goulash, I do Chili. (Non-Texas style.)

Footballs don't spin, either.