Note: A video that demonstrates how to build an indoor catapult launch-glider is located at the end of this article.
This particular indoor catapult-launch glider was originally designed for use in the 2013 Science Olympiad challenge, but it makes a great glider for anyone.
View the 2014 Science Olympiad page for the Elastic Launched Glider here.
To construct an indoor catapult-launch glider, you will need the following:
- Ballpoint pen
- Balsa wood*
- Clay for nose ballast
- Indoor catapult-launch glider plan
- Lip balm cap for dimensioning the nose
- Sanding sticks
- Sharp knife
- Steel ruler in metric
- Very fine marker
*Refer to the plan for balsa wood sizes; 1/16 ” thick for wings, 1/32” thick for tail surfaces, and the fuselage uses hard 1/8” x 1/4 ” x 300mm long. Additional pieces of 1/8” and 1/32” thick wood will be to create the “launch tooth”.
Need materials? Look for a local model hobby shop in your area or visit our preffered online supplier, AC Supply.
- Referring to the plan, use the wing design that states “Indoor/Outdoor Wing”, “Indoor Stabilizer”, “Fin” and “Fuselage” to cut the parts for assembly. Start by marking outlines with a very fine point marker and straightedge. Be sure to include the lines named “Dihedral Joint”, as they are critical junctures for new dihedral wing-tips.
Note: No glider will fly without a layout that specifically includes dihedral or angles in the wing that allow the wingtip to be higher than the middle or centerline of the wing.
- Below are the various balsa sheet blanks that have been marked and are ready for cutting. The fuselage stick is cut to length as well. A very light slice should be made along the dihedral joint line. DO NOT slice through the wing along this line! The line will be there after the airfoil has been sanded so that we can continue to finish the dihedral joint.
These shots below show additional parts marked and ready to be cut to shape and sizes.
- Carefully cut out each part along the outlines with a steel straight edge ruler and sharp knife (X-Acto #11). DO NOT cut along the centerline or along the dihedral joint line.
- Once the wing outline is cut, it is necessary to use sanding tools to finish its aerodynamic shape. The wing needs to have an airfoil or cambered surface over the top of the wing to enhance lift. This means the leading edge of the wing is rounded and is the starting edge of the airfoil.
This picture on the right shows the sanding stick being used to shape the trailing edge of the wing. Note the taper! Sanding against the grain removes the material with adding undo friction, which can induce a warped edge (very un-aerodynamic!). Be patient while sanding the airfoil shape as we want to have a consistent airfoil the full length of the wing.
This picture shows the end view of a wing with an airfoil. The leading edge is on the left, while the trailing edge is below.
- The wing now has a consistent airfoil shaped by sanding. The next critical operation is to add dihedral to the wing. The simplest overall is to use “tip dihedral”, where the wing tips end up higher than the centerline of the wing. The dihedral is necessary as it allows the glider to self-correct in the “roll axis”. The tools needed at this point are a ballpoint pen and a straight edge.
- Using the straightedge, create a groove about 1/32” deep along the dihedral joint line. DO NOT gouge completely through the wood. It is best to take three passes instead of trying to accomplish the task in one pass. Do this operation for both tips.
- While pushing down onto the main section of the wing, lift the wing tip until a crack is heard. This means that the tip can now be positioned in the preferred dihedral angle at about 30 degrees. Do the same for the opposite wing tip.
When both tips have an equal amount of dihedral angle, they should appear much like the picture below.
- The dihedral joints may be permanently bonded using glue. The best choice for glue is a medium viscosity. Another option is to use cyanoacrylate (super glue). If cyanoacrylate is used, it should be used very sparingly to minimize weight gain. When you gain more experience, using classic acetate (model airplane cement, such as Ambroid) will save even more weight.
- Next, we tackle the layout of the fuselage. Mark the various locations, such as where the launch tooth is positioned, the wing’s leading edge and the location of the stabilizer.
- We can then proceed with the launch tooth. Cut the tooth as shown here and glue to the bottom of the nose. Note that the grain is perpendicular to the grain on the fuselage. Since this launch tooth takes so much loaded energy, it is paramount to reinforce the joint, so laminate a piece of 1/32” squares on both sides.
- Note how these 1/32” thick “caps” cover both the fuselage, as well as the launch tooth. Glue these on both sides securely!
Refer to the new rules that show how the cap of a tube lip balm is used as a gauge. The complete nose (fuselage front) assembly must be blunt and cannot not fit inside the cap.
- Using the plan for reference, mark the nose and tooth so that it can grip the rubber loop for launching.
After cutting and sanding, the nose should appear similar to that on the plan.
- The Wing can be positioned onto the top of the fuselage. Once it is located, lift it off and apply a thin layer of glue on top of the fuselage. Carefully lower the wing onto the fuselage using the wing’s centerline as the reference for centering on the fuselage. Press together and count slowly to 60. The wing should now remain in place but let it cure 24 hours before attempting any launches.
- Much like the cutting the wing outline, the two tail surfaces may be cut to size and readied for sanding. It is important to lose mass but still maintain strength in these surfaces. Patiently sanding and then polishing with a 600 grit Emory paper or stick is an absolute.
Too much mass at the opposite ends of fuselage makes flight trimming difficult, impact breakage common and reduces potential flight times.
As a reference, the stabilizer shown on the right is half of its original thickness, making it very translucent. It still is strong enough to function and, most importantly, has a trailing edge that will be able to be bent to adjust flight attitudes.
A special feature that will be added is called “stab tilt”. When a glider has its stabilizer set at a different angle than the wing, the glider will have a natural turning orbit while in descent. This is important as the glider must fly within the confines of a big bow without hitting the walls. This picture shows sanding a slight angle on the rear of the fuselage, where the stabilizer will be mounted.
From this aft looking forward view, we see how the stabilizer has a tilt with the port tip higher than the starboard side. Since the stabilizer tends to fly parallel with the floor, it will induce the rest of the glider to turn. In the configuration shown, this glider will turn left.
- Be sure to allow for a small section of the fuselage beyond the railing edge of the stabilizer. This will be used as a finger grip during launch.
- It is paramount that the glider balances at a point halfway between the leading and trailing edges of the wing. This will be the center of gravity (CG). To assure this balance, the nose will need some non-metal ballast. Modeling clay is an excellent form of ballast and can be located in the “pocket” behind the launch tooth against the fuselage. The photo below shows the glider in balance and ready for the process of test flying.
This is a dowel stick and rubber loop (1/16” cross section rubber) launcher attached to the launch tooth.
This is the typical configuration when launching an indoor catapult-launch glider.
Rick Crosslin (AMA member and science teacher) and Tom Sanders (AMA member and Science Olympiad expert) demonstrate how to build an indoor catapult-launch glider.
To view a video of how to fly and trim an indoor catapult-launch glider, click here!