Saturday, October 24, 2009

Yankee Siege Tune-Up October 24, 2009

We are making a concentrated effort to tune and tweak Yankee Siege with the new throwing arm. We have decided to leave the counterweight alone with a fixed amount of weight (12,000 pounds).

The pumpkin will also remain a fixed weight of ten pounds.

That leaves us with only two parameters to adjust (sling length and pin angle). We will try to tune the machine to time the release to coincide with the maximum velocity of the machine (not necessarily to the ideal 45 degree angle). As we all know, distance is proportional to the velocity squared. So it is much more important to time your machine to the maximum velocity and not be obsessed with the ideal 45 degree angle. Most machines will not have a coincident 45 degree release angle and maximum velocity.

The key, in my opinion, is to vary the pin angle and sling length in combination to get the maximum velocity and 45 degree angle to coincide. This "magic combination" may not be possible with every machine. Different geometries and masses in a traditional treb versus a FAT or a whipper or Merlin's ski jump will dictate when the maximum velocities are reached in the cycle.

It must be remembered that given two equal release velocities, any angle of release between 30 degrees to 60 degrees will give 85 percent of the ideal 45 degree maximum distance. You are better off tuning to maximum velocity. Small increases in release velocity will yield exponential increases in the distance thrown. Distance will increase by the velocity squared. A ten percent increase in velocity will yield a 21 percent increase in distance thrown.

Yankee Siege has setup sling lengths of 24 ' 6 " through 30 ' in 6 " increments. We also have 4 pins at various angles. We will only throw 10 pound pumpkins. With 4 pins and 12 sling lengths that makes 48 possible combinations. Do we have enough time to tune-in the machine? That will require 48 ten pound round pumpkins, no wind, good weather etc! Can we shorten the process? If we shorten the process are we going to miss just the right combination? (Note: most simulators will show 2 peaks of near maximum velocity with a steep valley in between. There are probably many more combinations that will probably work. There are an infinite number of combinations given small enough increments of pin angle and sling length).

We may run into some interference problems with the frame when we use a 30 ' sling. (Yankee Siege is an underslung trebuchet, withdrawing the projectile from under the frame). The sling and/or the pumpkin will come dangerously close to the frame. This could spell disaster if the sling should "fetch-up" on the frame, possible breaking the pumpkin or breaking the arm. (Note: we will be bringing 2 throwing arms to the competition, just in case). This is a basic design flaw of Yankee Siege.

Timing has become a more precise issue with a new throwing arm. It is faster than the previous throwing arms, making correct timing much more critical. A tenth of a second too "early" or too "late" will lead to significant decreases in distances thrown. We may have to accept a little greater percentage variation from throw to throw.

A Note on Sling Length

We want the longest sling length possible and still have time for the sling "to come around" and release at the proper time. A long sling will further multiply the speed of the accelerating counterweight by increasing the effective radius of the long end of the throwing arm. A longer sling will "steal" more of the angular momentum from the throwing arm, causing the throwing arm to slow and transfer more energy from the counterweight to the throwing arm to the projectile. A projectile that is allowed to move away from the axle can significantly increase the moment of inertia of the throwing arm/projectile combination. This is analogous to a spinning skater throwing out her arms and slowing her rate of spin. In a very efficient machine, the throwing arm will almost come to a "halt" at the time of release of the projectile.

Yankee Siege hopes that by increasing the sling length in six inch increments, we will find a length that will accelerate at just the right angular velocity to be close to a 45 degree angle when the counterweight "stalls" as it becomes collinear with the throwing arm.

Two Thousand Foot Holy Grail!!!

Can 2000 ' be broken for a trebuchet? Can 2500 ' be broken? Will the weather cooperate? Could all 3 of the "big three trebs" (Yankee Siege, Pumpkin Hammer & Merlin) break 2000 '? A throw of 2000 ' may only garner 3rd place! Will an unknown machine come and beat us all? That's the excitement of Punkin Chunkin. To get a good throw everything "has to go right"! To have a bad throw only one thing has to go wrong! How does one "make it happen"? How does a team make everything go right? How does a team prevent a mistake or breakage of the machine? What part does luck play? Do you make your own luck through practice, vigilance, hard work and experience?

We only have a little bit more than a week left. Yankee Siege sincerely wishes all the other trebs in the division the best of luck in the 24th Annual Punkin Chunkin competition. Hope the weather cooperates and the 2000 foot mark is blown away!

Steve Seigars, YS

P.S. A Note of Finding Good Pumpkins to Throw!

Today was a rainy Saturday. We spent the whole day looking for just the right pumpkin. Luminas are very scarce this year. Trying to find just the right weight and shaped pumpkin that is free of rot or bruising is not an easy task. Luminas are almost impossible to find. There are, however, three other choices:

1. The Cuban pumpkin called the La Estrella. This is a smooth, spherical, dense pumpkin. It is sometimes available in specialty produce stores catering to Cuban, Portuguese or Brazilian groups.

2. The White Buttercup. This is similar in size, shape and density to the La Estrella. This is not the same thing as a butter cup squash.

3. The Traditional Orange Pumpkin (many different varieties). Although not as dense as the first two choices, it may be the only one available.

We will be looking for Buttercup and La Estrella at the Chelsea Wholesale Produce Market Terminal in Boston.

P.S.S. Yankee Siege will be bringing different weight pumpkins for trading with other teams. Yankee Siege needs ten pounders, we will trade some of our lower weight pumpkins for ten pounders.

See the Yankee Siege in action, click the links below!

Yankee Siege YouTube video 1
Yankee Siege YouTube video 2
Yankee Siege YouTube video 3
Yankee Siege YouTube video 4

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