Tuesday, December 25, 2007
As I am wandering through these books, I am constantly thinking of how these principles apply to a trebuchet. There is a good explanation on the strength of a material and, its elasticity and its density and how that relates to its strength to weight ratio.
Mr. Gordon points out that the specific modulus of elasticity (that is its stiffness per weight) is virtually the same for all the common structural materials. (With the possible exception of carbon fiber composite). This explains why there are several choices of materials to construct a throwing arm with no clear cut winner in stiffness to weight. Whether one uses steel, aluminum, wood, fiberglass, titanium, etc... there is not a lot of differences.
I want to make one important distinction between strength and stiffness, they are not the same! When it comes to strength and weight there is quite a difference from material to material. Titanium is a clear winner of all the metals. Carbon fiber is a clear winner of all the non-metals in regards to strength to weight.
That comes to the next question. What is more important in designing a throwing arm? Is stiffness or strength more important? The obvious answer is that both are important. The throwing arm must be strong enough not to break and at the same time be stiff enough not to deflect too much on the pull down as to produce too much of a recoil upon release of the trigger.
Note: Yankee Siege has developed an annoying recoil of the arm upon release of the trigger. We have added so much weight to the counter weight that upon release, the throwing arm snaps back and creates a "hop" of the pumpkin off the ground and a subsequent oscillation of the arm that leads to a quite unpredictable release of the projectile. We are trying to time the release of the projectile to an arm that is oscillating in its own mode of frequency.
This brings home the point of the difference between strength and stiffness. Stiffness relates to elasticity. A throwing arm may be quite strong and at the same time quite flexible (elastic), leading to greater deflections when cocked. (Note: an increase in the cocking angle will cause less deflection, with no deflection if cocked to 90 degrees).
For example: two identical size throwing arms made out of different materials will exhibit different amounts of deflection even if they had the same tensile strength. Mild steel and some aluminum alloys have identical tensile strength but aluminium will deflect three times as much upon pull down (aluminium has one third the elasticity of steel).
If deflection is deemed an important criteria, then choice of an appropriate modulus of elasticity is of critical importance.
Yankee Siege is trying to figure out a way to take advantage of the elasticity of the beam and figure out the frequency of the elastic oscillation and to take advantage of the "forward snap of the arm".
Perhaps a very elastic beam would be a help not a hindrance! If we could only time it right and design an arm to oscillate at the right frequency so that the release would coincide with the forward "snap". Cellulose is very elastic-maybe Trebarbaric knows something we don't! I'm still looking for the ultimate material!
A simpler approach would be to design an arm that is very stiff (steel would be a good choice as it has the highest modulus of elasticity of all the common structural materials). A very stiff arm would deflect very little and would not be a nightmare to time the release because the deflections are insignificant (they would be very small but high frequency).
We are still in the design phase for the new throwing arm. We have not made any final decisions as to the materials or combination of materials or configuration. I have now finished re-reading Gordon's books for the third time and I am still learning something new each time.
As you can probably tell from these musings, the Yankee Siege team is somewhat obsessed with designing an energy efficient arm. I spend the majority of my waking hours thinking and reading and imagining. I keep thinking that maybe if read just one more book, one more page, one more article that the ultimate design will suddenly become obvious.
In reality, there are probably a dozen ways to construct an efficient arm. We may end up with two or three different arms that can be bolted on and see which one works best.
I think the hardest part of the whole design process is making a final decision! Committing to a final design means that you have said to yourself that this is the best design that I can come up with at this time, given my level of knowledge at this time. Perhaps I should wait a little more time, gain some more knowledge, and redesign. At what point do you say enough is enough. At what point do you pull the trigger and let gravity take over!
I'll try to keep everyone posted monthly. I enjoy hearing from you all. Yankee Siege team would like to hear your feedback. If you think we are proposing some utterly foolish idea, we would like to hear from you. I hope some of these discussions will help other teams, as well as Yankee Siege. I would especially like to hear from Pumpkin Hammer, Trebarbaric and Merlin. I think that we should keep in touch throughout the year. I call these teams, members of the "1000 Foot Club". We are the members of an exclusive club that have broken the 1000 foot mark (no easy accomplishment). I hope I haven't left anyone out. Anyone may join the club-you just have to throw more than a thousand feet in any competition. By the way, Yankee Siege fully recognizes Trebarbaric's throw of 1866 feet!
Yankee Siege hopes to start a new club in November, (the 2000 Foot Club)! Wishful thinking!!! Hope to have several new members!
Steve Seigars, YS
Thursday, November 22, 2007
Now is the time for reflection and to decide to what direction we want Yankee Siege to go. Yankee Siege has always been known as the brute force machine. We have enjoyed throwing 200 to 300 pound pumpkins as well as 10 pound pumpkins. We have now come to a cross road. We have come to the realization that a single machine can not be built that can efficiently throw a large as well as a small projectile. The massive throwing arm needed to throw a heavy projectile severely increases the moment of inertia, which requires a massive counterweight to accelerate the arm (we have a 14,000 pound counterweight). Our throwing arm weights 2600 pounds. Most of the potential energy of the counterweight is used up in accelerating the arm. A minuscule amount of energy actually gets into the pumpkin. (A paltry 5.9 percent of the potential energy of the counterweight). A machine that is only 5.9 % is a very poorly designed machine! We are, by far, the most energy inefficient machine in the whole competition. By contrast, King Arthur, is 57 percent efficient. King Arthur is a very well designed machine from an energy efficiency viewpoint.
Early in the spring of 2007, we cut the end of the throwing arm off about 3 feet from the axle and mounted a companion flange in anticipation of building a new lighter throwing arm. We now have the option of bolting on a newly designed arm or if that arm does not work out to default back to the original arm. This, bolt on modular design, would give us total freedom to test new arms and see what works. If the new arms failed, we could always go back to the old "tried and trued" arm. Our basic machine (winch, frame, axle, counterweight, etc...) have proven themselves over the years reliable and quite robust, so the decision was made to keep the basic machine "as is" and modify the throwing arm to be lighter. The new throwing arm will be specifically designed to throw 10 pound pumpkins and therefore giving up our ability to throw 300 pound pumpkins. To remain competitive, we are forced to make major changes in the throwing arm. The throwing arm is the "energy hog". It is robbing us of energy to accelerate the pumpkin.
The team has been racking our collective brains trying to come up with the "ultimate throwing arm".
What would it be made of?
What would be its shape?
What would it weigh?
Would it be able to be repaired in the field?
Would it be able to be modified?
Would it break?
What would it cost?
Would we be able to build it?
All these are questions to be answered and hopefully solved. I'm still looking for the massless arm! (I'm told that somewhere, perhaps in Area 51, or on another planet there have been sighted massless throwing arms designed by an advanced civilization)! Or maybe we could locate one in a junkyard, we all know you can find anything in a junkyard. We'll keep on looking!
We all know there is no material or combination of materials that will satisfy all design considerations. Light and strong are two incompatible parameters.
By contrast, Yankee Siege is so inefficient(5.9%), that a small increase in efficiency will bring large increases in distance thrown. If we could just get Yankee Siege to be 10% efficient we would be throwing 2881 feet(14000 lbs. dropping 12 feet and throwing a 10 lb. projectile).
Anyone who has been working with trebs for a long time knows that designing a treb to throw long distances is no easy task and is full of potential pitfalls. The best thought out plans can sometimes fail by overlooking the smallest detail. The weakest link will always show up sooner or later. There are often unexpected events happening at such high speeds that can't be seen by the naked eye. It's hard enough to design a structure that is static. It becomes exponentially harder to design something from a dynamic viewpoint.
We all learn from failure (we learn more from failure than success). When something breaks, we know for sure it wasn't strong enough. When something doesn't break, we don't have a clue as to how much we have "over-engineered". After all we trying to push our machines up to ,but not beyond the breaking point, just light enough for maximum acceleration, but not so light as to break. So break a few arms and find the limit!
We'll keep you posted on our progress.
Steve Seigars, Yankee Siege
Tuesday, November 6, 2007
The convoy all packed up and departing area 64 for the long drive back to New Hampshire.
Friday, November 2, 2007
First two shots fell out of the pouch. And the controversial 1st successful practice shot was thought by the forward observers who didn't see the pumkin but heard it land at 2k plus! We switched to a more encompassing pouch and and tried some 1/8th wire cable and had moderate shots at the 15 to 1600 range. The last shot of the day was very high (highest ever from YS). Our good friends Pumkin Hammer and The Magic of Merlin arrived. Pumkin Hammer got off two shots and had one slip off the sling. Merlin arrived late and didn't get off a shot before dark.
Weather was warm and very little wind! Field is beginning to dry out although our two left wheels are slowly sinking into the soil. Bridgeville had several days of rain prior to our arrival. We will shore up the left side of the treb and wheels early Friday a.m.
Friday November 2, 2007 (Day one of competition)!
Very windy day, hurricane Noel off the coast. Strong north west winds!
Yankee Siege was the first of the trebs to throw! We decided to use a lower tragectory because of the strong head wind.
Good shot hooked strongly to the left at 1658 feet. Happy with the shot considering the windy conditions. Pumpin Hammer was next at 700 plus and Merlin 800 plus. Artemis threw 4th.
Costa Mantis has been filming about 20 teams for the past year and has hired a high speed camera crew that will give slow motion view of the machines at 1000 frames p/second. Can't wait to see the slow motion video of what's going on and can not be seen by the human eye of shot one!
A special thank you to Pumkin Hammer for their trebuchet running scoreboard. They have great team spirit and always think of the trebuchet division as a whole. I think the scoreboard is a great idea and it helps clear up some of the confusion during the pumkin chunkin competition. Also, thank Pumkin Hammer for always making reservations for Saturday night dinner at Big Fish.
Wishing everybody luck for day two!