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

Yankee Siege Update October 20, 2009

Yankee Siege is back in operation! The throwing arm axle bearings have been replaced with new and beefier bearings. We were able to get new bearings from in Nashua, NH. The new bearings were not the same dimensions as the old bearings and required some reworking of the supports and the throwing arm. Thanks to team members and ace mechanics Chuck Willard and Ken Holland, they made short work of the job and were able to get Yankee Siege fixed without missing a single weekend of demos at the farmstand. Also, thanks to my wife Kathleen for not murdering me after picking up the bearings and finding out the cost.

The four new pillow-block bearings are performing flawlessly and are holding the axle firmly in position with no slipping or axial movement of the towers. We now have six massive bearings supporting the 4" diameter solid steel axle!

Monday, October 5, 2009

Yankee Siege Destroys Main Axle Bearings - October 5, 2009

Just finished a weekend of throwing. Saturday was raining and no throws. Sunday was a beautiful fall day in the 60's and perfectly calm. Several shots exceeded the 2000 foot range, but some shots released very late and were very low. We don't know what exactly is causing the late release (we did throw some rather light pumpkins in the 8 pound range and that may account for the late release but even some of the ten pound pumpkins would release somewhat late).

Today (Monday) we entertained a high school physics class. This was a small class with very attentive students. It's nice to see a well behaved class with students that appear to be interested in physics and math. I'm afraid I may have talked too long. I have a tendency to be long winded and perhaps too technical when it comes to trebuchets. I get very excited. Sometimes it's hard to talk to a class that has just been introduced to projectile motion and not be too technical. I could talk for days. They had to shut me up and get on with the demo or we would have run out of time.

Bearing Failure

Just before the class, Chuck and I went to inspect the main axle bearings for the throwing arm. We have been noticing that the bearings on one side had appeared to be loosening and causing one of the towers to move inward, causing a binding of the throwing arm against the frame. Upon closer inspection we found that two of the four bearings had cracked housings. These bearings are forty year old Dodge pillow block bearings obtained from a junk yard. We suspect that the new throwing arm is the culprit. The new throwing arm is so light and has such a low moment of inertia, that the counterweight literally "falls" for 12 feet and slams to a near halt at the end of its travel. This action of the counterweight causes a significant increase in the reaction force on the axle leading to failure of the bearing. (We don't know exactly when this cracking occurred, it may have started last year and went unnoticed until this year with a new throwing arm). There is about a 3/16 th's inch wide crack on the lower part of the bearing housing.

Now we have to figure out how to solve the stress on the bearings. We have decided to put six bearings on the axle (we now have 4 bearings). We will also move the two inner bearings on each side as close to the throwing arm as possible (the inner bearings take most of the load).

This is just another example of unanticipated stresses with a change in the configuration of the machine. A lighter throwing arm can lead to vast increases in the reaction force on the axle. Yankee Siege has a four inch solid steel axle through the throwing arm and turned down to 3 and 7/16 th's inches in the bearings

We have ordered four new bearings that will be here Thursday. Changing the bearings will not be an easy task. It will require removing the new throwing arm, removing the counterweight and stub throwing arm, beating off the old bearings and then reassembling. Hopefully we will be ready for Saturday's demos. Actually, the hardest part of the whole project is telling my wife how much I just spent on the new bearings! Wish me luck!

Steve Seigars, YS

Yankee Siege Update - September 27, 2009

This spring and summer have been a very busy time for Yankee Siege. I haven't had the time to update our progress. We have been visited by four TV shows. Time Warp (Discovery Channel), Lock and Load (History Channel), All Jacked Up (Country Music TV), and The Science Channel following teams to Punkin Chunkin (more about the shows later).

This year's Punkin Chunkin will almost surely see the two thousand foot barrier broken by a trebuchet. Yankee Siege's goal is to break the 2500 foot barrier! We have fabbed a brand new throwing arm and unveiled it for the first time in front of the cameras for Discovery Channel. The arm is very light and fast! We have reduce the weight of the arm to a mere 600 pounds, down from our previous arm of 800 pounds, down from the original arm of 1200 pounds. Not only have we reduce the weight but we have moved some of the mass of the throwing arm considerably closer to the axle, with the effect of significantly lowering its moment of inertia (resistance to a change in rotational velocity). We are still running 12,000 pounds in the counterweight (94,000 foot-pounds of torque, max). We have only done two throws at the time of this posting. I don't know if the arm can take the stress. If the arm holds up, we could potentially break 2500 feet this year. The cable stayed throwing arm has remained straight as an arrow with these first two throws.

We are unable to accurately measure the distance of the throw. At the 1700 to 1800 foot range the pumpkin disappears into some pine trees on the far side of a ten acre wetland. We can only estimate how far into the pines the pumpkin has flown. The first two throws were very far. They were also very low. Almost a line drive. They left the machine at tremendous speed. We will have to adjust the pin angle or sling length to get a higher path. The second throw appeared to rise about halfway through its flight and just kept climbing and didn't reach a peak till somewhere around the 1500 mark.

The extremely low trajectory would seem to indicate that the throwing arm is rotating significantly faster (higher angular velocity) than our previous throwing arm (the throwing arm is able to "get ahead" of the sling causing the sling to release a little later (similar to the effect of adding more counterweight).

We will be doing public demos at the farm stand for the next five weekends. These demos serve as our practice sessions for Punkin Chunkin.

P.S. Lumina pumpkins are very hard to find in New England, we had a very wet and cold June and July. We will have to practice with orange pumpkins. Does anyone have a souce for Lumina's? If you have a source please e-mail us, Thanks.

P.S.S. A special note to Merlin and Pumpkin Hammer, Good Luck in your practice sessions, stay safe and hope nothing breaks. See you in November.

Steve Seigars, YS