Thursday, November 07, 2013

Picking the Perfect Punkin for Chunkin

Credit: Punkin Chunkin Website punkinchunkin.com

Last weekend, thousands watched as competitors from around the country competed for the 28th annual World Championship Punkin Chunkin trophy. Trebuchets, resembling something out of a Lord of the Rings film, flung their pumpkins while air cannons, rivaling some cranes in size, fired the helpless gourds toward their inevitable, squashy doom.

American Chunker Inc. launched this year’s farthest-flying pumpkin at a distance of 4,694.68 feet, about 585 feet shy of a mile. The team sent their winning pumpkin flying on the first day of the event when conditions were more favorable for chunkin than the final two days of the competition. The trick to winning Punkin Chunkin relies more on just the machine.

The machine controls the pumpkin’s launching speed and angle and therefore is a major determinant of how far the pumpkin travels. However, there are other components, which affect distance, that chunking competitors cannot control. Wind velocity is a major one. Although they cannot influence the weather, punkin chunkers can attempt to minimize the effect from air and wind by launching the most aerodynamic pumpkin possible. But what would that look like? It depends.

“If air cannons were able to rifle the barrel, the best pumpkin shape would be like a bullet,” said Eli Lansey a Research staff member at Riverside Research and physics consultant for the Punkin Chunkin competitor, Team Chucky, who use a catapult-style launcher. “For other machines like trebuchets and catapults, you want a small, dense, spherical-shaped pumpkin with tiny divots.”

Divots, like those in a golf ball, amplify an important force for flying pumpkins, and other objects, called the Magnus effect. The Magnus effect is a force that changes the original flight path of an object moving through a fluid, but only if that object is rapidly spinning. For example, whenever your golf ball curves to the left or right instead of going straight, blame the Magnus effect - as long as the wind has nothing to do with it.

Another way in which the Magnus effect changes flight path is to generate lift. Lift elevates the spinning pumpkin, golf ball, what have you, so that it can travel farther than if the object were not spinning. Moreover, the faster the object spins upon launch, the more lift it generates. Therefore, chunkin competitors try to put as much spin on their pumpkin as possible, and divots can enhance that lift.

“There’s a balance to be struck,” Lansey said. “If the pumpkin is too bumpy you add drag, but the right combination of spinning and texture will give you the best shot.”

Below are two plots. The first compares how the Magnus effect influences distance traveled for a perfectly spherical pumpkin. The second shows how divots play a role.


Credit: Eli Lansey, Research staff member at Riverside Research and physics consultant for Team Chucky


Scenario 1: The dotted line in the above plot is of a perfectly spherical pumpkin in a vacuum launched at 20.7 degrees with a speed of 224 mph (similar values to what a trebuchet machine might use).

Scenario 2: The red line uses the same launching parameters but with a certain amount of drag acting against the pumpkin. This more realistic scenario is akin to if the pumpkin was traveling through air. Drag force reduces distance traveled by about 1000 feet, or 55 percent.

Scenario 3: The blue line depicts the projection of the same pumpkin under the same drag force but with an initial spinning rate of 41 revolutions per second. Compared to Scenario 2, the pumpkin travels roughly 150 feet farther or about 10 percent more.



Credit: Eli Lansey, Research staff member at Riverside Research and physics consultant for Team Chucky


The red and blue lines in the second plot compare the distance a perfectly-spherical pumpkin travels in the presence of drag while spinning at 41 revolutions per second with a dimpled, sphere-like pumpkin under the same conditions. The dimples add an extra 275 feet to the total distance.

Therefore, a spinning, dimpled pumpkin can travel about 25 percent farther than a smoother, non-spinning pumpkin could under the same conditions.

Of course, Lansey’s estimations do not include other factors air currents, wind velocity and the fact that pumpkins are not perfect spheres. However, they present a good idea of what spinning, dimply pumpkins are capable of.

To compete for the National Punkin Chunkin trophy, teams must use a pumpkin that weighs between eight and ten pounds. Since drag is proportional to cross-sectional area and momentum is proportional to mass, the ultimate aerodynamic pumpkin for a catapult-style machine should be the smallest, heaviest, most sphere-like pumpkin with a speckle of divots, that upon launch is spinning as fast as physically possible without flying apart.

For the physics of Punkin Chunkin machines, there is a nice piece by Dot Physics blogger Rhett Allain on Wired.










1 comment:

  1. What an interesting sport. I'm glad people are coming together to have a good time nonetheless

    ReplyDelete