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Shaka
2010-Oct-18, 03:15 AM
I have written two Java Applets which do ballistic calculations for subsonic projectiles that are not fin-stabilized.

The first one asks for the following information and then calculates the straight range in meters, the wind deflection in meters, the impact speed in meters per second and the flight time in seconds.

Altitude above sea level in meters
Air temperature in degrees Celsius
Barometric pressure in inches of mercury
Muzzle velocity in meters per second
Launch angle in degrees
Diameter of steel ball in millimeters
Height of muzzle above ground in meters
Grade of hill in percent (+ uphill, - downhill)
Wind speed in meters per second
Wind direction in clock angles

This software is specifically for homemade mortars firing steel ball bearings. It can also be used for trebuchet-launched pumpkins by using a simple formula to determine the diameter of steel ball that is ballistically equivalent to your pumpkin.

This software assumes that drag is proportional to the square of velocity. A lot of software packages say this but, in the fine print, they add that they are going to “make some simplifying assumptions” or “drop the higher order terms.” Then they proceed to solve the much simpler case of drag being directly proportional to velocity. A word to the wise: If drag is proportional to the square of velocity, range can only be determined by an iterative method. If they are using table look-up (as I did in my pumpkin program) then they are rounding the results off in a way that is only acceptable for vegetables, not steel balls. If they have a formula in closed form – not a DO/UNTIL loop – then they are assuming that drag is proportional to velocity, which is dishonest unless they made that assumption clear.

This program uses an iterative method, so drag is indeed proportional to the square of velocity. This assumption is valid for velocities up to 240 m/s. From 240 m/s to 295 m/s, drag is proportional to velocity cubed. My program now takes account of this, which allows for higher initial velocities. It does not accurately model supersonic projectiles that decelerate through the speed of sound, 343 m/s, however, and should not be used for this purpose.

Air density is re-calculated at each step so, if the ball flies so high that the air is significantly thinner at its apex, this is taken into consideration. The coefficient of drag is also re-calculated at every step so it increases from 0.2 to 0.45 at the exact moment that the velocity falls below the critical point. Altitude, air pressure and temperature are all taken into consideration. Note that, so the isobars on weather maps are all in the same units, barometers are usually normalized to what they would read at sea level. This is also what you want because, since altitude is a separate parameter, if you reported the actual number of inches of mercury at your location, you would be over-estimating the effects of altitude.

I also have a Java Applet that is specifically for trebuchet design. It asks the user to input two of these three parameters and then finds the other one:

efficiency
angle
range

Both Java Applets can be accessed from this page (http://www.axiomaticeconomics.com/pumpkins.php), which contains a short paper on trebuchet design and explodes some common myths.

vonmazur
2010-Oct-18, 03:23 AM
Shaka: All I have is for rifles and pistols, so this is most interesting to me, I was in the Division Artillery of the First Armored Div, and even though I was an Aviator, they made me take the Artillery Officers technical course....I was amazed all all the variables, the probability charts, and all the technical info...a most rewarding course...

They did not cover Pumpkin Chucking though....

Dale

Trebuchet
2010-Oct-18, 04:45 AM
Please note that the link is to the same page as the subject of this thread (http://www.bautforum.com/showthread.php/95380-The-Ballistic-Coefficient-of-Pumpkins).

Shaka
2010-Oct-20, 11:12 PM
Shaka: All I have is for rifles and pistols, so this is most interesting to me, I was in the Division Artillery of the First Armored Div, and even though I was an Aviator, they made me take the Artillery Officers technical course....I was amazed all all the variables, the probability charts, and all the technical info...a most rewarding course...

They did not cover Pumpkin Chucking though....

Dale

No pumpkin chucking? Ha! Those Army guys just don't understand the devastating psychological effect on the enemy of getting pelted with vegetables.

slang
2010-Oct-20, 11:58 PM
Those Army guys just don't understand the devastating psychological effect on the enemy of getting pelted with vegetables.

Or cows. Or saying their mother was a hamster.

HenrikOlsen
2010-Oct-21, 06:03 AM
or the heads of their colleges.

Trebuchet
2010-Oct-21, 03:32 PM
Since this thread doesn't appear to going anywhere anyhow, I'll take the opportunity to point out the the World Championships of Punkin Chunkin take place November 5, 6, and 7 in Delaware. The Science channel is advertising a special on Nov 1, presumably of last year's event.

cjl
2010-Oct-21, 05:23 PM
I have written two Java Applets which do ballistic calculations for subsonic projectiles that are not fin-stabilized.

The first one asks for the following information and then calculates the straight range in meters, the wind deflection in meters, the impact speed in meters per second and the flight time in seconds.

Altitude above sea level in meters
Air temperature in degrees Celsius
Barometric pressure in inches of mercury
Muzzle velocity in meters per second
Launch angle in degrees
Diameter of steel ball in millimeters
Height of muzzle above ground in meters
Grade of hill in percent (+ uphill, - downhill)
Wind speed in meters per second
Wind direction in clock angles

This software is specifically for homemade mortars firing steel ball bearings. It can also be used for trebuchet-launched pumpkins by using a simple formula to determine the diameter of steel ball that is ballistically equivalent to your pumpkin.

This software assumes that drag is proportional to the square of velocity. A lot of software packages say this but, in the fine print, they add that they are going to “make some simplifying assumptions” or “drop the higher order terms.” Then they proceed to solve the much simpler case of drag being directly proportional to velocity. A word to the wise: If drag is proportional to the square of velocity, range can only be determined by an iterative method. If they are using table look-up (as I did in my pumpkin program) then they are rounding the results off in a way that is only acceptable for vegetables, not steel balls. If they have a formula in closed form – not a DO/UNTIL loop – then they are assuming that drag is proportional to velocity, which is dishonest unless they made that assumption clear.

This program uses an iterative method, so drag is indeed proportional to the square of velocity. This assumption is valid for velocities up to 240 m/s. From 240 m/s to 295 m/s, drag is proportional to velocity cubed. My program now takes account of this, which allows for higher initial velocities. It does not accurately model supersonic projectiles that decelerate through the speed of sound, 343 m/s, however, and should not be used for this purpose.

Air density is re-calculated at each step so, if the ball flies so high that the air is significantly thinner at its apex, this is taken into consideration. The coefficient of drag is also re-calculated at every step so it increases from 0.2 to 0.45 at the exact moment that the velocity falls below the critical point. Altitude, air pressure and temperature are all taken into consideration. Note that, so the isobars on weather maps are all in the same units, barometers are usually normalized to what they would read at sea level. This is also what you want because, since altitude is a separate parameter, if you reported the actual number of inches of mercury at your location, you would be over-estimating the effects of altitude.

I also have a Java Applet that is specifically for trebuchet design. It asks the user to input two of these three parameters and then finds the other one:

efficiency
angle
range

Both Java Applets can be accessed from this page (http://www.axiomaticeconomics.com/pumpkins.php), which contains a short paper on trebuchet design and explodes some common myths.

Do I really need to post my code demonstrating that you are wrong about the "common myths"? I thought we'd already been over this...

Shaka
2010-Oct-22, 02:58 AM
Do I really need to post my code demonstrating that you are wrong about the "common myths"? I thought we'd already been over this...

I remember you conceding this point:


They drop below 240m/s in a fraction of a second? From 295 m/s? Let's say a fraction is ~1/4. In that case, the deceleration is 220 m/s^2, which I find somewhat hard to believe.


A 25 mm ball fired at 295 m/s will decelerate to 240 m/s in 0.6887 seconds. Since the total flight time is 22 seconds, the first 0.6887 seconds have relatively little effect on the range - it was about ten meters off (out of 1300 m) before I updated the software.

This is assuming 15 degrees Celsius, 29.92 inches of murcury and a sea level firing range.


After doing some calculations, that doesn't actually appear to be that far off. If anything, it's a bit low, though it does depend strongly on reynolds number.

I don't remember you successfully defending either of the common myths I explode.

Is this the myth you want to defend?


You need a lower angle to get the most distance. That 45° nonsense only works in a vacuum because the horizontal component must remain constant (no drag) for it to work out that way. Go talk to any — let me repeat that last word... ANY — college freshman-level engineering physics professor. I dare you.

C. B. Daish (1972, p. 51) writes:


For a cricket ball thrown at 30 meters per second in still air, the maximum range is achieved if it is projected at an angle of 42˝° with the horizontal, but there is only a very slight variation, amounting to 30 cm or so, between the lengths of the throws over the range of angles between 40° and 45°. This variation is of no practical consequence at all.

The professional version (http://www.sniperflashcards.com/mortars.php) of my software confirms Daish’s result. The optimal angle is 43°, but no angle between 40.8° and 45.1° is more than 0.5 m short of that.


Or is this the myth you want to defend?


Go to any - once again, ANY - hurling competition and watch the pumpkins spin like crazy out of the pouch. Listen to the whistle of the bowling balls, and tell me that thing ain't spinning. I dare you.

According to Daish (1972, p. 62), a golf ball requires as much as 7800 rpm to overcome gravity.


The spins imparted to golf balls by the use of lofted clubs are surprisingly high… With a 5-iron, it is about 100 revolutions per second [6000 rpm], rising to 130 [7800 rpm] with a 7-iron.

Pitched baseballs rotate at up to 1600 rpm (Daish, 1972, p. 58) and do not have positive lift. A “rising fastball” just falls slower than we expect to see things fall and thus it creates the illusion of actually rising.

Since the throwing arm of a trebuchet rotates at no more than seventy rpm, it is indeed hard for this author to accept that it may impart on a pumpkin sufficient spin to cause positive lift.

vonmazur
2010-Oct-22, 03:12 AM
Put fins on the pumpkins, with a slight angle, like some rockets....then we'll see some difference! :)

Dale

cjl
2010-Oct-22, 06:20 AM
I remember you conceding this point:







I don't remember you successfully defending either of the common myths I explode.

Is this the myth you want to defend?



C. B. Daish (1972, p. 51) writes:



The professional version (http://www.sniperflashcards.com/mortars.php) of my software confirms Daish’s result. The optimal angle is 43°, but no angle between 40.8° and 45.1° is more than 0.5 m short of that.


Or is this the myth you want to defend?



According to Daish (1972, p. 62), a golf ball requires as much as 7800 rpm to overcome gravity.



Pitched baseballs rotate at up to 1600 rpm (Daish, 1972, p. 58) and do not have positive lift. A “rising fastball” just falls slower than we expect to see things fall and thus it creates the illusion of actually rising.

Since the throwing arm of a trebuchet rotates at no more than seventy rpm, it is indeed hard for this author to accept that it may impart on a pumpkin sufficient spin to cause positive lift.

I did concede that one point, for the simple reason that I was wrong. On the other hand, the main part of my point was that the optimum angle significantly deviates from 45 degrees for a pumpkin thrown to >1000 feet, which I believe I quite successfully defended (and my code, which accounts for quite a few factors, shows this fairly well).

Oh, and spin is significant. Pumpkins don't spin because of the rotation of the throwing arm, they spin because they roll out of the pouch. Also, you're setting up an impressive strawman with your claims about positive lift - an object doesn't need lift exceeding weight to have a significant deviation from a liftless trajectory.

(Of course, it also depends on what you consider to be significant - depending on input parameters, my code shows pumpkin throws to have ideal angles of anywhere from 35 to 42 degrees, which I consider a significant deviation from 45)

Shaka
2010-Oct-29, 11:14 PM
spin is significant. Pumpkins don't spin because of the rotation of the throwing arm, they spin because they roll out of the pouch.

The soft nylon pouch goes flaccid when the string is released - completely unlike a baseball pitcher's hand, which is not flaccid at all. In fact, the 1600 rpm achieved by major league pitchers is accomplished with a tremendous snap of the wrist that ordinary men's hands are not strong enough for.

And a "rising fastball" is only an inch or two above where it would have been without any spin - enough to confuse the batter, but not enough to affect the distance of the throw if the catcher were not there and you marked the spot in the dirt where it landed.

It is absurd to think that this flaccid pouch is putting enough spin on the pumpkin to have a measurable effect on its range.


my code shows pumpkin throws to have ideal angles of anywhere from 35 to 42 degrees, which I consider a significant deviation from 45)

35 degrees is about right for a .30-06. Unless you're firing pumpkins at rifle velocities, I would have to say that your code is wrong.

For pumpkins launched at 54 m/s, which represents quite a powerful trebuchet, the optimal angle is 43°, but no angle between 40.8° and 45.1° is more than 0.5 m short of that. More typical trebs that fit in the bed of a pickup truck achieve velocities closer to the 30 m/s cricket ball that Daish investigated.


For a cricket ball thrown at 30 meters per second in still air, the maximum range is achieved if it is projected at an angle of 42˝° with the horizontal, but there is only a very slight variation, amounting to 30 cm or so, between the lengths of the throws over the range of angles between 40° and 45°. This variation is of no practical consequence at all.

Shaka
2010-Oct-29, 11:20 PM
Put fins on the pumpkins, with a slight angle, like some rockets....then we'll see some difference! :)

Dale

Sorry, but my software does not work for fin-stabilized projectiles.


Spheres are just pushed sideways by the wind while projectiles with fins may turn their nose into the wind if the fins are too big. This makes windage adjustment vastly more complicated than it is for spheres. Also, such projectiles typically go high enough that the wind is stronger at their apex. There are software packages available to the designers of model rockets that may be helpful in finding the smallest fins needed to stabilize a projectile in still air. One can then use the software on this page (http://www.sniperflashcards.com/mortars.php) for launching mortars short distances in mild winds.

Software for fin-stabilized projectiles that considers their tendency to turn into high winds and considers long-range projectiles that go high enough to encounter different wind conditions than are present near the ground is expensive and its purchase will attract the attention of law enforcement.

Shaka
2010-Nov-06, 12:40 AM
spin is significant...
35 to 42 degrees

Will you concede, or are we going to have to do this again next October?

Trebuchet
2010-Nov-06, 05:15 AM
Will you build yourself a trebuchet and actually get a little practical experience?

cjl
2010-Nov-07, 12:09 AM
Will you concede, or are we going to have to do this again next October?

Since you really seem unwilling to admit that trebuchets cause projectiles to spin when launching them, I don't see much of a point.

(Oh, and I know my code is right for a number of reasons - one of which is that it does agree with yours for a spin-free projectile)

Trebuchet
2010-Nov-07, 05:32 PM
Perhaps I'll try taking a small treb to the park and see if I can catch the spin on video. Probably a softball with black/white markings to make it visible. I expect it'll be a while though.

While I agree they spin, I'm not convinced that it helps performance. Most times the pumpkin tends to be wider than it is tall and start out on the trough sitting on it's base. That means that instead of spinning, they tumble. The last time I hurled full size pumpkins I was trying to get tall ones so they could start on their sides, but then they tend to roll sitting on the ramp.

Pouches, by the way, are all over the place in terms of materials. One team at the NW events this year had an animal hide, complete with fur on the outside. Mine was duct tape!

Shaka
2010-Nov-07, 11:45 PM
Perhaps I'll try taking a small treb to the park and see if I can catch the spin on video. Probably a softball with black/white markings to make it visible.

The whole point of Bad Astronomy is debunking myths, like one about the moon landing being faked. If you and cjl are going to claim that pumpkins spin, then you MUST provide video evidence. It flies in the face of everything that Bad Astronomy stands for if you just SAY that they spin.

I cannot do this for you. You're the one claiming spin, so YOU prove it. If I showed you a video of a trebuchet-launched pumpkin that is not spinning, you would just say that there was a flaw in my design that caused it to not achieve the terrific spins that master designers like yourself are seeing. ;)

From my pumpkin page (http://www.axiomaticeconomics.com/pumpkins.php):


http://i596.photobucket.com/albums/tt42/Grozny/golfball.jpg

Daish (1972, p. 62) writes:


The photograph shows that, during this interval, the ball was rotated through an angle of about 180°, corresponding to a rate of spin of about 150 revolutions per second [9000 rpm].

If a car’s engine ran this fast, it would throw a rod. And Ron Toms would have us believe that “a pumpkin fired at a depressed angle (below the horizon) will climb to a 15° ascent and continue for about 2,000 feet?” Get real. A pumpkin is a vegetable. If it were possible to spin pumpkins this fast – which it is not – one would create instantaneous pumpkin pie.

9000 rpm is enough to achieve positive lift. I'd like to see you videotape a pumpkin spinning at even a tenth that. If you paint one hemisphere of a softball orange (not black - clay pigeons are orange for visibility), then it will alternate white and orange every thirtieth of a second if it is spinning at 900 rpm.

Video cameras operate at 30 frames per second so, if you stand directly behind your trebuchet, every frame should show a different color. You might want to sit in your car and videotape it through the windshield. If your sling breaks, you don't want to wind up wearing a softball for a hat.

Trebuchet
2010-Nov-08, 01:30 AM
I've SEEN them spin -- or at least tumble. NOT at 9000 or 900 RPM, probably more like 50 or 100 RPM. I'll have to see if I can find a video. Perhaps I'll try the really small treb in the backyard with a tennis ball tomorrow. If it stops raining. I've no idea how to post a video here, of course!

I actually agree that there is unlikely to be any performance advantage from the spin. And I see no relevance to your golf ball photo. If anything, the ball acquiring spin from hitting the ground is rather like what happens when the treb projectile leaves the pouch.

HenrikOlsen
2010-Nov-08, 02:47 AM
http://vimeo.com/6807356 here's one, the spin't quite easy to see on the first slo-mo.

Trebuchet
2010-Nov-08, 03:34 AM
Thanks, Henrik. the slo-mo shows the tumbling pretty well. And also illustrates how difficult it is to actually catch the pumpkin in flight!

hurler
2010-Nov-08, 12:19 PM
Let's not forget that the big trebuchets can easily hurl a 10 lb. pumpkin 2000 feet. They do indeed come out of the pouch in a tumble, but they pretty quickly stabilize into a non-tumbling spin orientation. Here's a video for a "spin-stabilized" orientation -
http://www.youtube.com/watch?v=6sMIO15gCQI
The bag of flour got nicked on launch, and the trail of escaping flour shows it all.

What Shaka doesn't understand is pouch dynamics. The pumpkin is rolling in the pouch during the entire launch. The friction between the pouch and the pumpkin makes it roll as the pouch swings around the tip of the arm. How much roll there is depends on the attachment points of the sling lines. The wider apart they are, the more spin you'll get. The spin is imparted long before the pumpkin ever leaves the pouch. I don't know how significant the spin is to the range, but (to use golf terminology), I've seen pumpkins "slice" and "hook" considerably on windless days and consecutive throws.

Speed matters. Baseball pitchers can throw curve balls, sinkers and risers, but they cannot hurl a baseball with the same spin and sheer velocity that a trebuchet can hurl a pumpkin. When was the last time a human being threw a ball 2000 feet with his bare hands?

Let's debunk the myths, yes, but always remember - in theory, there is no difference between theory and practice. In practice, however, there is. When theory does not agree with observable phenomenon, it's time for a new theory. Unless, of course, you're just a shady software salesman trying to push an inferior product. In which case, it's our job to point this out. Why does Shaka only promote his pumpkin trebuchet software in October?

Trebuchet
2010-Nov-08, 10:05 PM
Ok, I took the little treb out in the backyard, blackened one hemisphere of a tennis ball, and tossed a few. Very low power and short distance so I could see. The shot definitely spins. I'm satisfied. Anyone who isn't, I suggest you go build your own.

I did try to take video. To my utter lack of surprise, I failed to get anything useful. I did get one with the ball falling through the scene, but it's backlit against the sky and you can't see the markings. The Mk1 eyeball is still superior to my level of technology.

The interesting thing about the spin is that it's backspin. I think that would actually make the shot drop, if the velocity and spin were fast enough.

Shaka
2010-Nov-09, 01:39 AM
http://www.youtube.com/watch?v=6sMIO15gCQI
The bag of flour got nicked on launch, and the trail of escaping flour shows it all.


I count seven complete rotations in a three-second flight. That is 140 rpm.

140 rpm is insignificant compared to the 9000 rpm of a golf ball coming off a 7-iron. It is less than 9% of the spin achieved by a baseball pitcher.


Pitched baseballs rotate at up to 1600 rpm (Daish, 1972, p. 58) and do not have positive lift. A “rising fastball” just falls slower than we expect to see things fall and thus it creates the illusion of actually rising. Since misjudging the fall by even a few centimeters is the difference between a strike and a base hit, the effect of spin is important in baseball. But, if the pitcher were asked to throw for distance, this little bit of spin would be negligible and he would use the same 45° angle that a slingshot of the type used for fielding practice uses.

You need to post a video that is not just slow motion but frame-by-frame in 1/30th of a second increments. If you have painted one hemisphere a different color, then a spin of 900 rpm (about half what a baseball pitcher can do) will result in every frame showing a different hemisphere.

You're not even close.

cjl
2010-Nov-09, 02:15 AM
Ok, I took the little treb out in the backyard, blackened one hemisphere of a tennis ball, and tossed a few. Very low power and short distance so I could see. The shot definitely spins. I'm satisfied. Anyone who isn't, I suggest you go build your own.

I did try to take video. To my utter lack of surprise, I failed to get anything useful. I did get one with the ball falling through the scene, but it's backlit against the sky and you can't see the markings. The Mk1 eyeball is still superior to my level of technology.

The interesting thing about the spin is that it's backspin. I think that would actually make the shot drop, if the velocity and spin were fast enough.

By backspin, do you mean that the top of the ball is receding from the direction of travel, and the bottom is advancing? If so, that makes the shot rise, not drop.

Trebuchet
2010-Nov-09, 03:32 AM
Yes, top is rotating back from the direction of travel. I was thinking that reduces relative velocity on top and increases it on the bottom, resulting in reduced pressure on the bottom. Is that incorrect? Good thing I didn't work in the Aero group before I retired!

ETA: Never mind, I just looked up Maguns effect (http://en.wikipedia.org/wiki/Magnus_effect). Obviously works differently from what I thought.

HenrikOlsen
2010-Nov-09, 03:13 PM
ETA: Never mind, I just looked up Maguns effect (http://en.wikipedia.org/wiki/Magnus_effect). Obviously works differently from what I thought.
Just to get random Googling to find this page, it's the Magnus effect:)

Anyway: Shaka, will you admit that pumpkins thrown by a trebuchet does spin?

I know it's a bummer to realize that you have to account for an extra factor in a program you though was complete and the easiest response is to go somewhere dark with your fingers in your ears and cry "it isn't significant, it isn't significant", but that's not really an honest reaction, it it?

Trebuchet
2010-Nov-09, 06:04 PM
Just to get random Googling to find this page, it's the Magnus effect:)

Anyway: Shaka, will you admit that pumpkins thrown by a trebuchet does spin?

I know it's a bummer to realize that you have to account for an extra factor in a program you though was complete and the easiest response is to go somewhere dark with your fingers in your ears and cry "it isn't significant, it isn't significant", but that's not really an honest reaction, it it?

Dang tpos. Er, typoos. typops. You know what I mean.

Shaka
2010-Nov-09, 11:23 PM
I know it's a bummer to realize that you have to account for an extra factor in a program you though was complete and the easiest response is to go somewhere dark with your fingers in your ears and cry "it isn't significant, it isn't significant", but that's not really an honest reaction, it it?

I'll calculate the effect of a 140 rpm spin just as soon as I finish calculating the effect of sunspots. And have we even considered the gravitational pull of Neptune yet?

HenrikOlsen
2010-Nov-09, 11:47 PM
Cries of "it isn't significant, it isn't significant" it is then. OK.:)

Trebuchet
2010-Nov-10, 03:50 AM
I'll calculate the effect of a 140 rpm spin just as soon as I finish calculating the effect of sunspots. And have we even considered the gravitational pull of Neptune yet?

My very low powered test had the ball spinning at least 5 revs/sec or 300 RPM. Of course it's also very small, so the launch still happened quickly.

Tog
2010-Nov-10, 07:49 AM
Couldn't the rate of spin be calculated by knowing the circumference and speed of the "ball" at the point of release?

I mean, a 100 cm circumference ball would have to roll five times to get out of 0.5 meters of sling, right?
If that ball was moving at 30 m/s, then it should pass out of the sling in about 1/(0.5/30)th of a second. If I did that right: 1/60 seconds.
Since it spins five times in that span of time, that means each spin takes 1/300th of a second, which would be 18,000 rpm.

Hmm. Would it work with accurate numbers? I know it would slide a little and that would take some of the rotation off of it, but the speed of rotation should still be something that could be calculated this way isn't it?

What did I do wrong?

cjl
2010-Nov-10, 08:24 AM
In theory, yes, but it could be very difficult knowing the degree of slip, as well as the actual relative velocity at release. I think just filming it would be a better way, especially if anyone has a high speed (ish) camera available.

hurler
2010-Nov-10, 03:20 PM
Shaka ... Victor,

Don't be ridiculous. That bag of flour only sailed about 120 feet. It doesn't compare to a pumpkin being hurled 2000 feet. The energies, velocities and scales are not in the same ball park. The bag of flour video was just to prove a point - that spin happens. It wasn't a definitive example.

I've read your web site, the one that has my name so prominently displayed. Your attempts at ridicule are pathetic, and you are clearly a charlatan. You blame the observed hook and slice behavior as "loose screws" and shoddy construction. How can you claim to be such an expert on machines you've never seen? And how much time did it take to dig up that ridiculous reference from a 1972 textbook about 45 degree launch angles, just to make me look wrong? It only makes you look like a fool when everyone who knows anything about ballistics knows the truth.

Look, you're clearly a bright and productive guy. Please, put your ego and arrogance aside and you'll probably be able to do something worthwhile and good for the world, and maybe even earn a little well deserved recognition in the process. We've got some very real and serious problems to solve. I'm working on one right now that you'd probably enjoy - trying to solve an economic problem of making solar power cost effective in rural environments - and it's a really big project too. I've read your work, and I'm looking for someone with your talents and insights, but I'd never hire someone who only wants to prove to himself how smart he is, even when he's clearly wrong about something. No matter how truly smart that guy really is.

Clean up your act and you could easily earn the respect you're looking for. Or stay the course, and suffer the ridicule of the very people you're trying to impress. I won't be back to this thread. If you want to contact me, you know how to find me.

-- Ron Toms

Swift
2010-Nov-10, 04:15 PM
hurler,

I strongly advice you to read the Rules for Posting to this board (http://www.bautforum.com/showthread.php/32864-Rules-For-Posting-To-This-Board), particularly Rule 2, on politeness. I don't know what your beef with Shaka is (I don't care), but we do not debate people on this board. If you want to discuss balistics or software or pumpkin hurling, that's fine, but don't debate ego and arrogance.

Further posts like that one and you will be infracted, suspended, or banned.

Shaka
2010-Nov-10, 11:50 PM
My very low powered test had the ball spinning at least 5 revs/sec or 300 RPM. Of course it's also very small, so the launch still happened quickly.

You would need at least 3000 rpm for me to consider the Magnus effect significant.

Also, the fact that your treb is low power is not important. What determines spin is how fast the treb operates, which is more a function of the ratio between the length of the long arm and the short arm than the sheer size of the treb.

I have repeatedly heard people cite the 2000-foot range of a large treb as though it would have more spin than a small one hurling tennis balls a hundred feet. But if the dimensions are proportionate, the spin should be the same - the big treb is just hurling heavier projectiles. If they go farther, it is because heavier objects have higher ballistic coefficients. In fact, it is easier (and safer) to get small trebs to operate fast, so it is not surprising that your low-power treb is spinning the ball up to 300 rpm while the large flour-bag-hurling treb is only at 140 rpm.


Couldn't the rate of spin be calculated by knowing the circumference and speed of the "ball" at the point of release?

You are making it too complicated. I believe (and can probably write a formal proof) that the spin of the projectile is less than or equal to the rotation of the pouch around the axle at the moment of release. Angular momentum is conserved, so how could the projectile have more angular momentum after release than it does before release?

I think I can write a simple formula for maximum spin as a function of velocity and the length of the throwing arm and sling. Actual spin would be somewhat less because the sling may not be stretched out straight from the throwing arm at the moment of release, but it will be close. It is a lot easier to measure velocity (either directly with radar or indirectly by estimating the ballistic coefficient and then using my software (http://www.axiomaticeconomics.com/pumpkins.php) to determine what velocity would have resulted in the measured range) than it is to measure spin.


Pitched baseballs rotate at up to 1600 rpm (Daish, 1972, p. 58) and do not have positive lift. A “rising fastball” just falls slower than we expect to see things fall and thus it creates the illusion of actually rising. Since misjudging the fall by even a few centimeters is the difference between a strike and a base hit, the effect of spin is important in baseball. But, if the pitcher were asked to throw for distance, this little bit of spin would be negligible and he would use the same 45° angle that a slingshot of the type used for fielding practice uses.

Since the throwing arm of a trebuchet rotates at no more than seventy rpm, it is indeed hard for this author to accept that it may impart on a pumpkin sufficient spin to cause positive lift.

In this quotation I - ahem - made a mistake. At the moment of release, the pouch is moving about twice as fast as the tip of the throwing arm. So I should have concluded with this:


Since the pouch of a trebuchet rotates at no more than 140 rpm around the axle, it is indeed hard for this author to accept that it may impart on a pumpkin sufficient spin to cause positive lift.

140 rpm is the measured spin of the treb videotaped with the bag of flour. This is probably typical of most trebs with a 3.5-to-1 ratio (or so) of their long arm to their short arm. Trebs specifically designed to hurl light objects far may get more spin. But such high-speed trebs are prone to breakage (As Siano said, it is not a good idea to try to hurl a pea across the channel) and the big trebs are known more for hurling heavy objects like flaming pianos than for hurling small objects far.

But, even if you tripled the 140 rpm, it doesn't matter. As I said above, you would need at least 3000 rpm to convince me that the Magnus effect is significant. And, unless you are building your device out of titanium and using depleted uranium for your counterweight, I don't think it is possible to get a treb to operate that fast without grenading.

Shaka
2010-Nov-21, 10:57 PM
Do I really need to post my code demonstrating that you are wrong about the "common myths"? I thought we'd already been over this...



I don't remember you successfully defending either of the common myths I explode.

Is this the myth you want to defend?


You need a lower angle to get the most distance. That 45° nonsense only works in a vacuum because the horizontal component must remain constant (no drag) for it to work out that way. Go talk to any — let me repeat that last word... ANY — college freshman-level engineering physics professor. I dare you.

C. B. Daish (1972, p. 51) writes:


For a cricket ball thrown at 30 meters per second in still air, the maximum range is achieved if it is projected at an angle of 42˝° with the horizontal, but there is only a very slight variation, amounting to 30 cm or so, between the lengths of the throws over the range of angles between 40° and 45°. This variation is of no practical consequence at all.

The professional version (http://www.sniperflashcards.com/mortars.php) of my software confirms Daish’s result. The optimal angle is 43°, but no angle between 40.8° and 45.1° is more than 0.5 m short of that.


Or is this the myth you want to defend?


Go to any - once again, ANY - hurling competition and watch the pumpkins spin like crazy out of the pouch. Listen to the whistle of the bowling balls, and tell me that thing ain't spinning. I dare you.

According to Daish (1972, p. 62), a golf ball requires as much as 7800 rpm to overcome gravity.


The spins imparted to golf balls by the use of lofted clubs are surprisingly high… With a 5-iron, it is about 100 revolutions per second [6000 rpm], rising to 130 [7800 rpm] with a 7-iron.

Pitched baseballs rotate at up to 1600 rpm (Daish, 1972, p. 58) and do not have positive lift. A “rising fastball” just falls slower than we expect to see things fall and thus it creates the illusion of actually rising.

Since the throwing arm of a trebuchet rotates at no more than seventy rpm, it is indeed hard for this author to accept that it may impart on a pumpkin sufficient spin to cause positive lift.

cjl-

I wait for you to concede.

cjl
2010-Nov-22, 05:44 AM
You'll be waiting for a while then.

(Note that the reason pumpkins and similarly round projectiles spin coming out of a trebuchet is because they are rolling in the pouch, not because of the angular velocity of the pouch itself)

Swift
2010-Nov-22, 02:22 PM
cjl-

I wait for you to concede.
And I wait for you to read the rules of this board. Lose the attitude Shaka, or you will be infracted. And don't bring your fights from somewhere else on the Internet to here.

Shaka
2010-Nov-22, 03:28 PM
And I wait for you to read the rules of this board. Lose the attitude Shaka, or you will be infracted. And don't bring your fights from somewhere else on the Internet to here.

You mean this?


Unfortunately a lot of misinformation is spread about astronomy. Sometimes this information is just plain silly, but many times it makes just enough sense that people believe it. I feel obliged to right these wrongs when I can. The Bad Astronomy web pages are devoted to airing out myths and misconceptions in astronomy and related topics.

For instance, did you hear the whopper about pumpkins spinning at thousands of rpm inside closed pouches?

Like Phil Plait, I feel obliged to right these wrongs when I can:




http://www.youtube.com/watch?v=6sMIO15gCQI
The bag of flour got nicked on launch, and the trail of escaping flour shows it all.

I count seven complete rotations in a three-second flight. That is 140 rpm.

140 rpm is insignificant compared to the 9000 rpm of a golf ball coming off a 7-iron. It is less than 9% of the spin achieved by a baseball pitcher.

Swift
2010-Nov-22, 03:51 PM
Shaka has been suspended for arguing moderation in thread.

Trebuchet
2010-Nov-23, 12:29 AM
Perhaps in time for Shaka's return, The Discovery Channel will be showing this year's World Championships of Punkin' Chunkin (WCPC) Thursday night -- Thanksgiving here in the USA. The trebuchet division was the most competitive it's been in years since the monster Yankee Siege wasn't there. We probably won't be able to see pumpkin rotation, unfortunately.

I'd be interested in seeing where anyone said pumpkins spin thousands of RPM inside closed pouches, when Shaka returns. I don't recall that. I do recall Shaka saying, in the previous thread, that they don't spin at all.

And in Shaka's defense, I'll say once again that while pumpkins do spin when hurled, I doubt they get much if any benefit from the Magnus effect unless in the case of extremely overpowered machines. I do wish Shaka would build himself a treb!

slang
2010-Nov-23, 01:04 AM
Have you trebuchet guys ever considered inviting the Time Warp (http://en.wikipedia.org/wiki/Time_Warp_%28TV_series%29) people (or those of similar shows) to come film some awesome slo-mo footage at one of the festivals?

Trebuchet
2010-Nov-23, 04:01 AM
That's a pretty good idea. I've no idea if they're still making the show or how to contact them but since the Mythbusters are hosting the Discovery Channel show I wouldn't be terribly surprised to see some slo-mo. (Just occurred to me: How come slow motion and high speed video are the same thing?)

Here (http://www.youtube.com/watch?v=P2KLigdMjLU) is a link to some slo-mo from the recent Burlington event. I swear you can almost see the pumpkins spinning.

Trebuchet
2010-Nov-25, 06:34 AM
Perhaps in time for Shaka's return, The Discovery Channel will be showing this year's World Championships of Punkin' Chunkin (WCPC) Thursday night -- Thanksgiving here in the USA. The trebuchet division was the most competitive it's been in years since the monster Yankee Siege wasn't there. We probably won't be able to see pumpkin rotation, unfortunately.

Correction: Science Channel, not Discovery. "The Road to Punkin Chunkin" is on right now. They didn't get around to advertising that; I almost missed it. I wouldn't be surprised if they re-run it tomorrow before the main show.

publiusr
2010-Dec-21, 11:10 PM
Some interesting developments
http://www.google.com/patents/about?id=FX7YAAAAEBAJ&dq=7,823,510
http://up-ship.com/blog/?p=8090
A smaller railgun
http://www.bing.com/videos/watch/video/railgun-firing-by-powerlabs-compilation-video-with-several-shots/8ac097c36d554b5c92af8ac097c36d554b5c92af-286351884602?q=sci+fi+science+rail+guns&FORM=VIRE3