Monday, September 29, 2008

Rainy Day in New Hampshire September 29, 2008

Our second weekend of public trebuchet demos was a washout, in more ways than one!

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

Late September 2008

It's been a very busy summer at the farm stand. We grow about 9 acres of pumpkins and that has consumed a good portion of my time and as a result I have neglected the Yankee Siege web site. P.S. I have had no luck in growing Lumina pumpkins. I hope all your practice sessions are going well!

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