Trebuchet (Wendy Banner)

Author

Wendy Banner : Paul Revere Middle School Physical Science and Robotics.

NGSS Engineering Standards

ETS1A: Defining and Delimiting the Engineering Problem

ETS1B: Developing Possible Solutions

ETS1C: Optimizing the Design Solution

Delimiting Constraints on Base Construction:

• Tools : Our miter boxes can cut only 90, 45 degree angles. Designs with other angles will not be approved.

• Materials : Students must design using structural support principals to compensate for the very weak 1/4" balsa wood. No other materials may be used on the base (frame.)

• Cost : Students must design their base with only 5 sticks of balsa wood; 5x91cm = 455 linear cm.

• Dimensions : The Basla wood base has a maximum height of 25 cm and width of 38 cm.

Constraints on Home-Built Elements:

• No LEGO, K'NEX or other ready made "purpose-built" parts, (i.e. toy car wheels.)

• Each of the 2 major elements;1) the throwing arm with counterweight and 2) the sling with it's chute, are built by an individual member to the two-person team. They share responsibility for coordinating their design efforts.

Materials needed for the base (framework).

• Balsa wood: 36" x1/4" x 1/4"; 5 sticks per student/group

• Graph paper print outs 8.5 by 11 ; 2 sheets per student/group.

• Elmer's wood glue or white glue, and craft picks.

• wax paper 11" x 17" long sheet per student/group.

• 8-10 Miter boxes (with saws) 5.99 $ each at IKEA curtain rod department.

• Fresh cranberries or other low mass projectile, and safety goggles.

Procedure

• Preparation

  • Students watch Medieval Siege -Secrets of Lost Empires by NOVA, about the efforts of engineers and woodworkers to recreate the Warwolf trebuchet. Discuss the engineering process and the components of the weapon. Identify variables.

  • Webquest on the trebuchet.

  • Students search trebuchet designs and images to brainstorm and rough design sketches. Get one of them approved.

  • Students produce a scaled drawing on 8x10 sheet of graph paper to ensure that angle are workable, (isosceles right triangles) and that their design can be built with 455 linear cm of balsa.

  • Students redraw 1:1 scale blueprints of base side piece on a 11x17" sheet of 1cm graph paper, (two 8x11 sheets of photocopied graph paper are glued together).

  • To differentiate, (for less measuring and converting) you can use 1/4" graph paper.

• Base (framework) construction

  • Constraints;

    • Maximum size: 25cm tall, 38cm long.

    • Maximum materials: 5 pieces of balsa wood; 5x91cm= 455 cm.

    • Glue only, no nails, pins, string, rubber bands, other - to hold it together.

    • Angles limited to 45and 90 degrees, (due to miter box set angles).

  • Methods

    • Use MESA balsawood bridge building techniques; create strength with triangular structures. The 2 sides are built as single layers, then erected with cross-support.Glue is best when allowed to dry up a bit for tackiness before using. Apply glue a craft pick.

• Trebuchet Completion

  • Students design and build the following elements at home;

    • Throwing arm with axle.

    • Counterweight (fixed or swinging)

    • Sling, (pouch and straps)

    • Wheels , if any

  • Students bring their individually made components to school to to combine with there partner's and the already constructed base. Testing and iterative optimization begin with this assembly.

• Competition

  • Distance: Which group can capture the most gravitational potential energy and convert it into projectile motion? The farthest flying cranberry wins.

  • Targeting: Students with adjustable elements on their weapons may compete for aiming at specific distances.

  • Melee; The class is divided in 2 teams, behind barriers(shop tables in our case) for showball-fight style combat. Goggles required.

Questions

(with answers)

• Explain the advantage(s) of a swinging counterweight. Are there any disadvantages or tradeoffs?

  • The swinging counterweight will stabilize lateral motion by changing it's angle to the throwing arm - hence dropping the mass in a straighter line down. By doing so it also less force. A fixed counterweight starts higher and ultimately drops farther, gathering more velocity with acceleration. The Swinging mass can also be a loose cannon by knocking into the framework of the base.

• Besides transportation purposes, why do some trebuchet bases have wheels? Which trebuchet are less likely to have them?

  • The wheels are usually seen on Trebuchet with fixed counterweights. They can dampen the lateral force and help keep the base-frame upright. This is similar to base isolation of earthquake construction. Most often the trebuchet with swinging counterweights do not have wheels.

• What are the advantages of a taller trebuchet? (specifically higher throwing arm axle) Are there any disadvantages or tradeoffs?

  • Overall the higher the throwing arm axle, the longer the throwing arm specifically effort arm radius)can be and the farther the counterweight can accelerate. Both of these will increase projectile velocity. Unfortunately, the greater the distance between mass-in-motion and the center of gravity, the greater the instability. The longer throwing arm will add a great deal of mass to strain the axle and overall structure.

Photos

Early 8"x5" rough draft and scale drawings on 1/5" graph paper, for the purpose of working out the design and learning to work within the constraints imposed by our miter boxes. All cuts must be either 90 or 45 degrees. Therefor all triangles must be isosceles right triangles. Other angles simply cannot be cut.