Saturday, November 15, 2008
Now is the time for thanks and reflection of another season gone by. Thanks that nobody got hurt, thanks to good neighbors, thanks to good sportsmanship, thanks to charity, thanks to all those good people that had a hand in putting on and participating in such an event. Thanks to all my team members who worked so hard over this past year to help set a new world record. Thanks to my wife (aka, Trebuchet Widow) who puts up with this odd hobby. Yankee Siege would not survive, if not for her.
Winter and spring are a time for reflection and a time to recharge the batteries (six weeks of public demos at the farm stand can be quite draining, mentally. One is always a little on edge, concerned that something will break or an errant pumpkin will land somewhere it shouldn't). It's nice to have some "free" time to read about physics and plan for the next season.
It never ceases to amaze me the good sportsmanship I have seen at Punkin Chunkin. Teams are so willing to help out one another, to repair a broken machine, to give suggestions as how to improve performance or to just give encouragement.
What is encouragement? Think about the word for a minute. What does it mean?
Encouragement-TO GIVE COURAGE. To give the courage to not be afraid to try. To give courage to not be afraid to fail. Failure is difficult for everyone. It's very difficult for anyone to see their machine fail, especially in front of a crowd. But fail all of us must. Failure makes success all the more sweet. We should expect failure. Failure is good, as long as we don't give up. We should be encouraged by failure. Failure is how we find out what doesn't work. Failure is a path to learning. Failure builds character and can either break you or make you dig deep into your inner being and find a strength you didn't know existed.
In talking about encouragement, I have to share with the rest of the world one of the most touching moments I have witnessed at Punkin Chunkin. Matt (I don't know his last name) the captain of Medieval Postal Service had just finished throwing with his machine (floating axle trebuchet). This is a very well designed FAT. Matt had the courage to compete this year and did exceptionally well as a first time competitor and finished in 5th place. He is a mechanical engineering student and he is obviously quite bright and has a great attitude. He is one of those people you meet and instantly you know that this "kid" is "going places". He has a very positive outlook and is very open. On the last throw, his throwing arm jumped past the stops on the slider and ended up in a heap in front of the machine. Matt handled the breakage in stride and with grace and you could see the wheels turning in his mind as to how to solve the problem in the future. Matt, I think, was a bit disappointed and perhaps needed some encouragement at that moment. Rich Foley (team Pumpkin Hammer) then did something that I will never forget. He calmly went over to Matt and congratulated him for his success and "encouraged" him, saying to him that designs always have to be changed. Flaws and weaknesses in design always show up and this was a normal process that Pumpkin Hammer and other teams have to work through. Rich acted very paternally towards Matt, giving encouragement and congratulations and reassurance. Encouragement from a Punkin Chunkin veteran must have meant a lot.
Rich Foley and the whole Pumpkin Hammer team know very well that if you keep on trying, keep on redesigning, keep on failing, keep on succeeding, that sooner or later they will the winners (or perhaps Matt will beat us all).
This is an example of great sportsmanship. It's not so important if you win or lose, it's how you play the game. Don't get me wrong, Yankee Siege, wants to win but we realize that we have been very lucky the past five years. Winning is only a temporary condition. The friendship and camaraderie will last longer and are far more important.
My hat is off to Rich Foley and the whole Pumpkin Hammer team for the leadership that they have shown. Great leaders don't only think about themselves but the whole division. The trebuchet scoreboard was a great idea (except for Friday if your in last place). I know that one day that Pumpkin Hammer will be on the top of the scoreboard. But right now they are already in first place in leadership.
THE BAR HAS BEEN RAISED!
Who would have thought a year ago that a 1589 foot throw would only get you third place? Both Pumpkin Hammer and Merlin have made great improvements in their machines this year. Both have doubled the distance of their last year throws. If this improvement continues, Yankee Siege will end up in third place or worse next year. Medieval Postal Service is the new "kid" on the block and may surprise us all.
I can't help but notice a parallel with giant pumpkin growers. It was just a few years ago that they broke 1000 lbs for the biggest pumpkin. Now 1000 lbs is only a mediocre size. The largest pumpkin is now over 1600 lbs. How things change. Records were meant to be broken. Competition focuses the mind and leads to innovative designs such as Merlin.
It's interesting to see what happens after the last throw of the season. The mind starts to think, what change can I make to the machine to make it throw a greater distance? How do I keep first place? How do I stay ahead of the competition. How do I beat Yankee Siege? What will Yankee Siege do? Will they sit on their laurels? How much will modifications cost? Will my machine be able to take additional stress? Will I have to totally redesign? Can I do just a little more to keep ahead of the competition. How much more can the competition improve? How radical do I want to be? Do I need to know more about the physics of a treb? Do I need to know more about materials? Where does it end? How much time do I spend? How bad do I want it? Is it worth it? Do I try something totally off the wall? Will I have a "EUREKA" moment!
Winter is a time for learning, reflection, and most of all planning. Most people don't realize that to be one of the top three competitors requires thousands of hours of work every year. You can't expect to practice a week before the competition and win. All machines require maintenance and modifications to stay competitive! I can only image how many hours Chris Gerow has into Merlin. It's a good thing he's retired, it's a full time job.
I hear through the grapevine that Rich Foley, team Pumpkin Hammer is going to retire soon (not from Punkin Chunkin but from his real job). That will give him more time to think and play (not a good thing for Yankee Siege, 1700 feet is too close, Great Job).
The Yankee Siege team has been batting around different ideas for modification for next year. We really want to break the 2000 foot barrier. If conditions are favorable the 2000 foot barrier will probably be broken this coming year by one of the big three or somebody else- I don't want to forget about Trebarbaric with their 1866 foot shot in 2007. I wish they could bring their machine East. Image the big four all in a row! Wes Frank, Trebarbaric may be building a more portable machine that can travel to Punkin Chunkin.
Enough said! Hope everybody's winter planning goes well, stay in touch.
Remember- Gravity is the most far reaching force in the Universe and keeps us all grounded!
Steve Seigars, YS
P.S. I will be posting monthly updates on our new plans.
P.S.S. A special thanks to John Huber and Hypertension for their help in donating a hydraulic hose that was torn in practice.
P.S.S.S. A special thank you to BSA Troop 672 for pulling the trigger on our first world record breaking throw of the 2008 competition. To see BSA Troop 672 slide show of this world record throw click here Punkin Chunkin World Record Slide show via Boy Scout Troop 672 from Severn, MD.
P.S.S.S.S. Also, another special thank you must go out to team Sister Slinger and company for pulling our 1st place and final world record throw on Sunday of the 2008 competition.
Sunday, November 2, 2008
Yankee Siege results;
Friday 10/31/08 - 0 - Pumpkin Pie, pumpkin fell out of the sling.
During our free throw we tore out a hydraulic hose from hydraulic motor after the whole machine fell off our planks. We are in last place, some of the smaller trebs were bragging that they were beating Yankee Siege!
1st Place - Pumpkin Hammer - 1596.
2nd Place - Merlin - 1589.
Saturday 11/01/08 - New World Record throw - 1894 feet!
We made the decision to eliminate our new trough after another pumpkin fell out of the sling during our practice throws in the A.M. before competition. We feel that the pumpkin is contacting one edge of the trough and spinning out of the pouch!
Merlin bent their throwing arm and had to replace it with a spare arm that they brought with them.
Pumpkin Hammer added an undisclosed amount of weight to their counterweight. It looks like it helped.
2nd Place - Pumpkin Hammer - 1640
3rd Place - Merlin - 1564
Sunday 11/02/08 - New World Record throw - 1897 feet!
The 2 previous days had a slight headwind but today we had a strong tailwind in the A.M. that slightly calmed down by the time we fired in the afternoon.
2nd Place - Pumpkin Hammer (Welcome to the 1700 foot club) - 1700 feet!
3rd Place - Merlin (Highest shot of the trebuchet class) - 1260 feet!
Steve Seigars, YS
PS We promised the Trebuchet Widow (Kathy Seigars) that if we reached 2000 feet we would retire from competition!
See you next year at the World Championship Punkin Chunkin Competition.
Tuesday, October 28, 2008
Yes, it's in New York state somewhere broken down (broken alternator belt)!
Trying to find a replacement at AutoZone!!!
Do you know how dry your field is at Punkin Chunkin?
Yes, ours is very wet and they do not know if we can go onto the site tomorrow at 10:30AM when the crane arrives (First State Crane Service)!
Today, Tuesday, has been very windy, following a very substantial amount of rain on Monday night and early Tuesday AM. Hopefully the wind will help to dry up the site so we can setup on Wednesday! Pray for no rain, how often do you hear a farmer saying that!
News Alert!!! Tuesday 10/28/08 7:18PM - Our tractor trailer carrying trebuchet is now back on the road and heading for Bridgeville, DE!
Note to our Yankee Farmer Customers, Thank you for our most successful season at the farm stand! Without you we could not attend the annual Punkin Chunkin competition in Bridgeville, DE. Hopefully we will bring back a trophy to Greenfield, NH.
Monday, October 20, 2008
We added 500 pounds of counterweight on Sunday morning to bring the total counterweight up to 12,000 pounds. There was a 10 mile per hour headwind but the extra weight seemed to over power the wind. We had fairly consistent throws, with one throw at the 2100 foot mark after changing to a 24 foot sling instead of our standard 25 foot sling.
On Monday we had four schools visit our site. These were high school physics and engineering classes, as well as, some grade school students. Winds were very calm but temps were only in the high 40's. We tried a slightly longer sling (26 feet) to see how the machine would react. The throws were very good, with one shot around 1950 feet but very high.
Results of the 19 throws October 18, 19 & 20, 2008
Note: We added 500 pounds of counterweight (Sunday) for a total of 12,000 pounds.
Saturday October 18, 2008
Throw One - 1650 feet
Throw Two - 1400 feet - low trajectory
Throw Three - 1800 feet - high trajectory
Throw Four - 1400 feet
Throw Five - 1800 feet - good height
Throw Six - 1800 feet - good height
Throw Seven - 1800 feet -good height and calm wind
Sunday October 19, 2008
Throw One - 1900 feet - good height
Throw Two - 2000 feet - good height 15 mile an hour headwind
Throw Three - 1400 feet - low trajectory
Throw Four - 2100 feet - good height 10 to 15 mile an hour headwind and change to 24 foot sling
Throw Five - 1650 feet - slightly low trajectory
Throw Six - 1800 feet - good height
Throw Seven - 1700 feet - high hard left
Throw Eight - 2000 feet - slightly low
Monday October 20, 2008
Note: Changed to 26 foot sling.
Throw One - 1850 feet
Throw Two - 1850 feet - good height
Throw Three - 1950 feet - very, very high
Throw Four - 1450 feet - slight pie
PS This will be the last posting of throws before the competition. We will be practicing next weekend but we won't be posting results because we will be dismantling the machine starting on Sunday night and Monday day and that will consume all our time. The logistics of transporting a 52,000 pound machine and associated support on four separate vehicles can be mind numbing. (I have nightmares about forgetting some critical part).
I wish every team the best of luck in their practice sessions. I hope everything goes well and safe and nothing breaks. This year promises to be quite competitive, now that Pumpkin Hammer and Merlin have had a full year to tweak and tune their new machines. Both these teams have designed very sophisticated and efficient machines. Their efficiency far surpasses Yankee Siege. They are smart machines, finesse machines. Yankee Siege is just brute force. Yankee Siege is probably the least efficient machine in the whole competition (dumb machine). Any one of these three machines could win, or maybe some unknown machine will come along. Win or lose, this year will be exciting!
Steve Seigars, YS
Monday, October 13, 2008
Several thousand onlookers witnessed Yankee Siege throw throughout the weekend. Weather was sunny and in the high sixties. It was almost a perfect weekend. We had one errant throw that came dangerously close to our nearest neighbor, hitting a tree and raining down pumpkin guts. Luckily, we have great neighbors who are very understanding and we promptly repositioned the treb.
We took seven more throws on Monday with some interesting results. We wanted to see what would happen if we put more slack in the sling. We wondered if more slack would give us more consistent distances. We suspect that the elastic recoil of the throwing arm may be producing a harmonic in the sling, leading to inconsistencies in the timing of release. We tried 12 inches of slack in a 25 foot sling. There were no very short throws. All throws were 1600 feet or above. The slack seems to work (we need more testing to verify).
Results of the 23 throws October 11, 12 & 13, 2008
Note: We added 500 pounds of counterweight (Sunday) for a total of 11,500 pounds.
Note: All pumpkins are orange and used a straighter release pin.
Saturday October 11, 2008
Throw One - 1200 feet - low trajectory
Throw Two - 1300 feet - low trajectory
Throw Three - 1400 feet - low trajectory
Throw Four - 1700 feet - change to straighter pin
Throw Five - 1500 feet
Throw Six - 1700 feet
Throw Seven - 1700 feet
Throw Eight - 1650 feet
Sunday October 12, 2008
Throw One - 1800 feet - good height
Throw Two - 1850 feet
Throw Three - pumpkin fell out of sling
Throw Four - 1650 feet - hard left
Throw Five - 1450 feet - low trajectory
Throw Six - 1600 feet
Throw Seven - 2000 feet - good height
Throw Eight - 2200 feet - in the stratosphere!!!
Monday October 13, 2008
Note: Put one foot of slack in the sling.
Throw One - 1650 feet - slightly low trajectory
Throw Two - 1850 feet - good height
Throw Three - 2100 feet - great height, long flight time
Throw Four - 1900 feet - slightly lower
Throw Five - 1700 feet
Throw Six - 1600 feet - tried underweight Lumina pumpkin
Throw Seven - 1850 feet - good height
PS Next weekend (weather permitting) we will try varying sling length and adding more weight to the counterweight.
Steve Seigars, YS
Sunday, October 5, 2008
We tried out some new items this weekend. First, we tried out a new 10 inch steel trough. We fabbed it by cutting a 10 inch diameter steel pipe into equal halves with a metal-cutting "skill saw". This saw looks and cuts exactly like a wood cutting saw. It leaves a very smooth cut with very little cleanup.
Next we tried to cure our problem of the pumpkin falling out of the pouch. Apparently, what is happening, is that the throwing arm is elastically recoiling when the trigger is released. This is followed by a rebound in the opposite direction, causing a momentary slacking of the sling and a subsequent rolling of the pumpkin out of the sling. We have solved the problem by putting about six inches of slack into the sling and allowing the recoil of the throwing arm to occur before the pumpkin moves down the trough. (Our high speed camera shows the sling actually becoming slack as the throwing arm rebounds).
We threw fourteen pumpkins this weekend with the following results:
Saturday 10/4/08 10,300 pounds counterweight.
Throw One - 1550 feet - slightly low trajectory
Throw Two - 1400 feet - low trajectory
Throw Three - rolled out of sling - didn't put any slack in the sling
Throw Four - 1700 feet - broke 3/16 inch cable stay
Throw Five - 1600 feet
Throw Six - 1500 feet
Throw Seven - 1800 feet - good height
Throw Eight - 1700 feet
Sunday 10/5/08 Added 700 pounds of rail road track to counterweight. Counterweight now 11,000 pounds.
Throw One - 1800 feet - good height, broke cable, bend aluminum extension.
Throw Two - 1900 feet - changed to carbon fiber extension and 1/4 inch cable.
Throw Three - 2000 + feet - good height
Throw Four - 1700 feet - slightly low
Throw Five - 1200 feet - pied into two pieces - cable stay strut slightly bent.
Throw Six - 1750 feet - good height.
Next week we will try adding more weight and trying different pin angles and sling length.
PS All throws were done with orange pumpkins no Lumina pumpkins.
Steve Seigars YS
Monday, September 29, 2008
Saturday was totally rained out by Hurricane Kyle. We tried to salvage part of the weekend (Sunday), and managed to get off six throws between rain showers. This was absolutely, positively the worst weekend ever for Yankee Siege! We couldn't do anything right. I hate to even post the results, but I have promised to post all results (good or bad) after each weekend of practice.
Of the six throws, four actually fell out of the sling. One went 1200 feet (a line drive), and one mediocre throw went 1650 feet.
We have been practicing with our eight foot aluminum cable-stayed extension and our new steel based arm with 10,300 pounds of counterweight. The new steel arm appears to flex a little more than our old throwing arm. Upon release of the trigger, the throwing arm springs back suddenly, creating a "hop" of the pumpkin and subsequent rolling out of the side of the pouch. (Yankee Siege has never used a trough). We hope that by using a trough we can cure the problem. Next weekend we will be practicing with a trough and our cable-stayed carbon fiber extension. Hopefully no more rainy days!
Steve Seigars, YS
Saturday, September 20, 2008
Yankee Siege's new steel throwing arm has been completed and tested. Many thanks to American Steel Erectors, and to Dave Webb the welder. It weighs in at about 700 lbs., a savings of over 400 lbs. over the old throwing arm. This throwing arm will require much less torque to rotate (lower moment of inertia). American Builder (Comcast) came up to the site September 8th to film the unveiling of the new throwing arm. Things fit perfectly, with the throwing arm neatly nestling into the trigger with no interference. They (American Builder) will be following us to Punkin Chunkin to see how the new carbon fiber extension performs.
We test fired the new throwing arm with an 8 foot aluminum extension and an empty counterweight box (8000 lbs.). We also tested it with the 9 foot carbon fiber extension. Things went well with 8000 lbs. We added 1350 lbs. to give us 9350 lbs of counterweight. The furthest throws were in the 1700 foot range.
We did note a difference in motion of the machine. The usual fore/aft movement of the machine was severely restricted. We have added crushed stone under the wheels to help with the footing of the machine (on wet days the wheels sink into the mud). The shifting of the crushed stone appear to inhibit the fore/aft movement of the machine with a consequence of a significant reduction of distance. We have now added sand to the stone to try to stabilize it from shifting. This appears to have worked.
On September 14th we increased the counterweight to 11,000 lbs. This is when weaknesses in the design started to show up! The design of the throwing arm was made on one assumption. We (or more correctly I, I like to spread the blame when something goes wrong) assumed that the maximum peak G forces on the pumpkin would not exceed 40 G's. That would give a force of 400 lbs with a 10 lb pumpkin. Four hundred pounds at the end of the cantilever would translate into a bending moment of 400 lbs. times the distance from the end. This is an easy calculation to make and the design and area moment of inertia of the extension was based on that assumption. We all know what can happen when we assume! What I thought to be a very conservative assumption turned out to be very wrong. We have severely underestimated the forces. The new throwing arm is producing G forces in excess of 60 G's. This is good news and bad news. The good news is that the new steel base is accelerating very quickly with less counterweight. The bad news is that the carbon fiber and aluminum extensions can not take the load. The carbon fiber has broken and the aluminum extension has bent!
At the time of this posting a new beefier carbon fiber extension has been fabbed on the assumption of 80 G's of force. NIM-COR company of Nashua, NH has fabbed the newest carbon fiber extension. It is a high modulus (22 million pounds per square inch fiber) with a tensile strength of 140,000 p.s.i. with a 2.75 inch outside diameter and a 2.25 inch inside diameter. It should be able to withstand 1645 lbs. at the tip. The engineer at NIM-COR was nice enough to give us a tour the carbon fiber manufacturing plant. It was fascinating to see the carbon fiber filament winding machine wrap the carbon fiber around a rotating mandrel. A computer controls the R.P.M. of the mandrel and the axial movement of the carbon fiber carriage. The fibers are first feed through an epoxy bath and then onto the mandrel. Excess epoxy drips off, leaving a high fiber to resin ratio. Next, the mandrel is hoisted into the oven to cure the epoxy.
We are trying to get a better idea of the forces we are generating at the tip of the throwing arm. We have invested in a high speed camera, 300 frames per second. If we can measure the deflection of the carbon fiber extension we can easily calculate the forces since we already know the length, area moment of inertia and the modulus of elasticity of the carbon fiber.
Deflection of a cantilever
D=lbs x (L)3
__3 x I x E
LBS=Pounds of Force
I=Area moment of inertia
E=Modulus of Elasticity
We have started our public demos on Saturday September 20. We had to revert back to our old steel throwing arm while the new stronger extensions are being finished.
I will keep you posted (I promise) weekly. We are keeping a "little black book" of all our throws. We will be posting distances (approx.) after each weekend of throws. We will also be informing everyone of our failures and successes. All of us are facing similar problems with strength of materials versus minimizing the amount of mass to rotate. Tweaking a machine to maximize the throw and still not break a component is still the name of the game. If we have any successes or insights we will try to pass them on to those who may be interested. We are not afraid of failure. We expect to fail a lot of the time before we finally get something that works. This is the first major renovation of Yankee Siege from its original design. We hope to increase our efficiency from a paltry 5.9 percent to something approaching 8 percent.
We are trying to approach our design from a more scientific/engineering angle. That means getting some hard numbers (no assuming) on the forces involved. So if you want to hear about failures this is the web site to read. Hopefully we will have a few successes. Time is running out. Only a few weeks to Punkin Chunkin. It always seems to come down to the last couple of weeks when everything will hopefully come together.
I hope everyone has great and safe practice sessions. This year promises to be very competitive with Pumpkin Hammer, Merlin, Trebarbaric and any unknowns. Pray that nothing breaks!
Steve Seigars, YS
Saturday, May 10, 2008
Yankee Siege is now for hire. We are available for rent for a day or we can travel to your site.
We have recently been contacted by Medieval Fantasies Company to shoot a commercial for a company that sells electronic monitors for power companies.
The shoot was in early April. Weather was cold and rainy the first day with lots of mud. The following day was bright and sunny but windy. Crushed gravel and stone had to be hauled in to control the mud. We threw 44 pound monitors which smashed into the field about 500 feet down range. We also crushed monitors with our ten thousand pound steel mace ball dropped about 20 feet high from a crane. And, did I mention, we also hit our crane (twice) with a 35 pound rock which spit out backwards from the treb and another 35 pound rock that went straight up and came down directly on the roof of the 1950 NorthWest model six crane, almost creating a new skylight! Oh well, I guess we need more practice in throwing objects other than pumpkins.
We ended up, removing 5000 pounds from the counter weight, and finally had the machine tuned to throw 40 pound objects. The second day went very well, with approximately 20 shots, with no mishaps, and very predictable throws.
The good thing about the shoot, is that it forced us to set up the machine early in the season, instead of waiting until September. The only thing bad about setting up early is the ice and mud. Ice had to be melted off the steel with a blow torch to get parts to mate. We can usually assemble the machine in four hours with a crew of six. It took us three times as long because we had to fight the weather.
We also had trouble getting the old NorthWest crane going (it hadn't been started in over a year). Something internal had froze (ice or rust) and the fuel pump wasn't working. Thanks to Chuck and Ken (two master mechanics) they finally were able to fire up the 1950 six cylinder Murphy diesel engine and get the job done.
The Museum of Science will be visiting our test site on June 30. The museum has a traveling group of teachers that visits sites of interest. They are planing a treb exhibit at the museum. They would like to learn more about trebs and to see a life-sized treb in action. I guess I will have to "bone up" on my physics!
Finally technical drawings of the new steel/carbon fiber throwing arm are completed. Nim-Cor of Nashua has fabbed the nine foot carbon fiber extension. End fittings (aluminum) will have to be fabbed and glued on with epoxy.
Copies of the final drawings of the new arm will be posted next month. These are fairly crude drawings (I'm not an engineer). I hope everyone can decipher them. I will scan them in so that everyone may see exactly what Yankee Siege is doing. And yes, these are the real drawings! We don't believe in playing poker. Yankee Siege believes in total transparency. We believe in sharing knowledge, information and insights. And yes, it's o.k. to criticize the design.
Next month I will be posting a more technical paper on the design process and the rationale for the new throwing arm.
P.S. Hope to see all the Pumkin Chunkin teams in Leesburg, Virginia, on June 7 for the grand premiere of Costa Mantis', "Flying Pumpkins the Movie".
Steve Seigars, YS
Saturday, April 12, 2008
I have spent the last two months with two treb simulators (WinTreb and A-Treb). These simulators are a great aide in developing a good understanding of the "split second" motion of these machines. The ability to slow down time is invaluable to see what is actually occurring. What one thinks ones "sees" in real time, is often quite different from reality.
These simulators appear to be quite accurate. We have inputted the Yankee Siege parameters into these simulators and they have predicted very close to our actual distances thrown (within + or - 2%). As everyone knows, these machines must obey the laws of physics. (Although sometimes they appear to defy these laws).
Variations in the distances from throw to throw appear to be related to the time of release. (Excluding, of course, wind). There is a very small window of time in which to release for maximum distance. Plus or minus one tenth of a second can make a huge difference. (Small machines are even more critical - in a small machine everything happens quicker, making timing more difficult).
I've had to use these two simulators to explore as many ways as I can, to redesign Yankee Siege. One of the limitations of A-Treb is that it does not allow very high counterweight to missile ratios. Yankee Siege is now running a ratio of 1400 to one, (14,000 pound counterweight to 10 pound pumpkin). WinTreb will allow higher counterweight to missile ratios.
I do like A-Treb's graphs of forces on the machine. One very interesting note is that Yankee Siege has a reaction force on the axle of 100,000 pounds! (The force on the axle experienced when the counterweight reaches bottom).
A-Treb's graphs of acceleration and velocity of the throwing arm, really help one understand what happens as the counterweight "bottoms out" at the lower part of it's travel, causing the throwing arm to greatly accelerate. It also helps one visualize a transfer of energy from counterweight to throwing arm to sling. Each part of the machine will momentarily "stall" and as a consequence of the principle of conservation of energy, that energy present in the one moving part will have to be transmitted to another part of the machine (conservation of energy says that throughout the cycle the total energy of the machine will remain constant). Energy is conserved - you can't get rid of it! If one part of the machine "stalls" (no kinetic energy), then another part of the machine must possess the rest of the energy (either in kinetic or potential energy). Yankee Siege, for example, has 247,000 joules of energy stored in its counterweight. (14,000 pounds raised 12 feet). Throughout the cycle, that entire 247,000 joules must be accounted for. A-Treb has some nice graphs that show the relative partitioning of energy between the counterweight throwing arm and missile throughout the cycle.
In addition, angular momentum is conserved. As the sling and projectile swing around (due to centripetal acceleration) and finally overcomes the throwing arm, there is a dramatic increase in the moment of inertia of the beam/projectile combination. The projectile, as a consequence of its change in radius relative to the axle, "steals" some of the angular momentum and energy from the throwing arm, causing a slowing of the angular velocity of the throwing arm with a dramatic increase in the tangential velocity of the projectile. (Analogous to the skater who slows her spin by extending her arms).
As they say, you don't have know how a car works to drive it. But, if you want to know how to design a better car then you had better know every detail of how it functions. Yankee Siege is a "dumb design". We win only by brute force. It's time to re-design in a "smart way". We have to find a new way to more efficiently transmit a greater portion of the 247,000 joules of energy available to the projectile.
The single biggest "consumer" of energy is the throwing arm. Our throwing arm is a tapered steel built up I-beam weighing 2600 pounds! It takes a lot of energy to rotate such a large mass. Yankee Siege was designed to throw larger (300 pound) projectiles. Yankee Siege is very inefficient at throwing small (10 pound) projectiles. If we can design a less massive throwing arm, that won't break, we could throw significantly farther. Simulations with WinTreb show that with significant reduction of the throwing arm mass we could be throwing over "4000 feet"! Only one problem, we only have 2500 feet at our test site! 4000 feet seems almost too good to be true. Could the simulator be wrong? Where would we practice? Could a pumpkin take the g-force? The goal of the trebuchet division should be to beat the air cannons. Could such a machine be built? Should I wake up and smell the coffee?
Perhaps we should be a little bit more conservative and not go for the "ultimate machine" and aim for a more realistic 2500 foot machine. This would not require re-designing the whole machine but perhaps re-designing only a portion of the throwing arm. We have cut our existing long end of the throwing arm off about three feet from the axle and will be bolting on a new long end.
The new long end of the arm will be fabbed from ASTM A-572 steel, 3/16 inch thick plate, starting with a cross section of 12 inches by 12 inches and tapering to 3 inches by 3 inches over 23 feet. This will give a total weight of approximately 500 pounds. (Compared to over one thousand pounds for the old arm). 500 pounds is a huge savings in weight!
A note about A-572, grade 50, steel: this is a nobidium/vanadium low alloy structural steel with a minimum yield strength of 50,000 psi. Compare this to A-36 (mild steel) with a minimum yield strength of 36,000 psi. It (A-572) is easily cut and welded with no reduction in strength. (This is not a heat treated steel and will not degrade in strength in the heat affected zone of the weld).
The last nine feet of the tip of the arm will be a carbon fiber cantilever (clad with Kevlar for impact resistance). The carbon fiber tip be a "bolt on item", with the option of going to an aluminum 9 foot extension if the carbon fiber should fail. The carbon fiber will weigh less than 10 pounds! (A savings of over 35 pounds over the old extension).
We have decided on this "hybrid" arm for several reasons.
First, a total carbon fiber arm would be prohibitively expensive and perhaps not as forgiving as steel with regards to impact resistance. (When carbon fiber fails, it fails catastrophically without warning).
Second, steel, although heavy, is very stiff and easily fabricated and repaired in the field. An A-572 tapered steel arm, would make a relatively light strong arm. I have worked the numbers for a cabled stayed steel arm and came out with very little weight savings over a cantilever, with a lot more complexity. We have decided to keep it simple, with a pure cantilever. (I like the "looks" better, also).
Third, the part of the arm closest to the tip is the most important area of the arm to keep light. (Moment of inertia increases with the radius squared). It takes a lot of energy to rotate a mass that is a great distant from the axle. It is absolutely critical that the last nine feet of the throw arm be extremely light and stiff. What better way to solve this problem than to use a material that has the highest strength to weight ratio of all common structural materials (carbon fiber). We have decided to put the most effort where it counts the most.
Fourth, we have decided not to go with wood because of maintenance problems and reliability.
Fifth, aluminum: Yankee Siege is seriously considering going to an aluminum arm. Aluminum has a more favorable strength to weight ratio. A 23 foot extension to Yankee Siege's base arm could be fabbed out of 1/4 inch 7075-T6 aluminum plate with a yield strength of 75,000 psi! The main drawback of aluminum is that it can't be welded without a significant reduction in strength (40 percent reduction). Fabrication would require bolted connections (austenitic stainless steel bolts). Now I finally understand why airplane wings are riveted, not welded! The heat of welding destroys the tempering of the aluminum. There would be significant reduction in the weight of the 23 foot arm. (From 1000 pounds to 330 pounds, a 670 pound savings)! Aluminum is a material to seriously consider.
Nim-Cor Company in Nashua, NH. Phone 1(888)464-6267 will be fabbing the 9 foot carbon fiber extension. They have a carbon fiber automated filament winding mandrel system with the epoxy cured in an autoclave under pressure. Nice people to talk to, very helpful. They can only fab a uniform (prismatic) beam (no hand lay-up or tapering). They suggested wrapping the arm in Kevlar to give the arm some impact resistance. (An errant steel slip ring on the end of the sling could destroy the arm).
We will start gathering materials and start fabbing as soon as good weather comes.
Last but not least, perhaps the hardest part of the whole project is telling your wife how expensive the new carbon fiber extension will be!
I would appreciate your feedback! If you think this line of reasoning to totally foolish I would like to know. Do you have any suggestions? I would like to have an open discuss. We are all facing similar problems with strength and weight.
The Yankee Siege team is very excited over the new arm. Will it work? Failure would be o.k. Just not in competition! Looking forward to your feedback.
P.S. Next posting mid-May
Steve Seigars, YS
Sunday, January 20, 2008
I have spent the past two months trying to design a new throwing arm for Yankee Siege. The team has narrowed down choices of shapes and materials to three or four different materials/configurations. (More about this later).
Before going ahead with a new lighter throwing arm, I first of all, had to determine if there would be any benefit to a lighter throwing arm. How far is Yankee Siege away from the "speed limit of a trebuchet"? I call this the SPLAT law. What is the SPLAT law? There is an upper limit of speed that no trebuchet can exceed (catapults do not have this limit!). This limit, is the speed due to the acceleration of gravity. The center of gravity of the counter weight can NEVER fall faster than the speed due to the acceleration of gravity (32 feet per second per second). This is the speed of a free falling body. The counter weight will, of course, always fall a little bit slower than a free falling body because it has to overcome the inertia of the throwing arm.
IF Yankee Siege has added so much weight to the counter weight that the counter weight speed is very close the speed of a free falling body, then making a lighter arm will only have a very slight marginal effect on the throwing arm angular velocity (speed of rotation) and therefore, will not throw significantly further. I am still searching for that mass less throwing arm which would offer no resistance to rotation and would allow the counter weight to free fall and therefore attain the maximum angular velocity.
The other major factor that limits the angular velocity (speed of rotation of the throwing arm) is the length of the short end of the throwing arm. The greater the length of the short end (counter weight end) of the throwing arm, the slower the maximum limit of the angular velocity (rotation) of the throwing arm. (It takes longer for the counter weight to drop because it has a longer distant to fall). Another way to look at this is if you increase the length of the short end to infinity, then the counter weight would have an infinite height and would never complete its fall and therefore the throwing arm would rotate infinitely slow. This is the main reason why large scale trebs have relatively slow moving (rotating) throwing arms. Their angular velocity is limited by the SPLAT law and the SPLAT law is reached at a lower angular velocity. Conversely, a small scale treb with a very small, short end of the arm, can rotate very quickly because the counter weight doesn't have very far to drop and can complete the cycle very quickly.
Yankee Siege has a 12 foot drop of the counter weight. If we are close to the ultimate speed limit, the counter weight should take .86 seconds to fall. (The time it takes a free falling body to fall 12 feet). Yankee Siege takes a little over 1 second to fall 12 feet (timed with a stop watch off of Pumpkin Hammer's website showing Yankee Siege firing). Thank you Pumpkin Hammer-by the way you have a great website.
The other consideration is the fact that the counter weight does not drop straight down. And we would expect it take a little longer to drop if it doesn't go straight down but takes another (non-vertical) path. We may be even closer to the limit than we originally thought.
The conclusion from this discussion is that Yankee Siege is approaching the speed limit and that making a lighter arm will help but not dramatically increase the angular speed of the arm because we are so close to free fall.
More on the new throwing arm details next month. Hope to hear you feed back!
Steve Seigars, YS