Introduction: Crossbow Powered Glider
First Prize in the
Launch It! Contest
The current schoolproject where I am working on is a human powered glider. That means that all the energy that the plane uses to fly comes from rubberbands, airpressure, a flyweel,... In a short version: NO ELECTRONICS. My idea was using a crossbow as a launchmechanism. I wanted to be original, so i made a custom crossbow that was only designed to fire my glider. For the glider itself, I started with an aluminum strip and built a body with wings around it.
Step 1: Crossbow: Make a Rough Design
The first step, the basic shape, determines the looks and the functioning of the crossbow. I did the drawing directly on a wooden (ayous) beam that had a width of 3.5 cm and a height of 10 cm. In fact, you are free to choose the form of the body of the crossbow, but you have to mind that the body needs a barrel with a flight groove, a place for the trigger, a stronger part to attach the bow, a groove where the string runs and a stock. You can see the design that i used in the picture.
Step 2: Mill the Grooves
First of al you have to do the milling, because al the edges are still straight in this fase. Start with the groove for the trigger. The cutter has to have a diameter of 1 cm. The groove has to be as deep as possible and has a length of 10 cm. The folowing step is making the flight groove on top of the bow. You can do this with a milling machine or with a tablesaw. I did it with a table saw. Make shure that the groove goes from the front to the groove for the trigger and that it lays just in the middle of the beam. I made a groove with a dept of 3.5 cm and a width of 0.7 cm. The last groove you should make is the groove for the string. The center of the groove lays about 2 cm under the upperside of the beam. The groove should go trough the beam with a width of 43 cm and a height of 1 cm.
Step 3: Cut Out the Basic Shape
Use a jigsaw to cut out the body you designed in the beginning. Make also a smal cut out after the string groove. That cut out is going to be your string lock.
Step 4: The Trigger
For the trigger you need a plastic cutting board. Before you start to cut the trigger you want to be shure that your trigger actualy dous it's job. Therefore you take a piece of cardboard and draw the trigger on it. Use a form that looks like the one in the picture. The task of the trigger is pushing the string out of the cut out so make shure that the shape and the positioning is right. Also search the right place for the axle that goes trough the trigger. When you found the right place for the axle, drill a hole trough the cardbord and troough the body of the crosbow. Now use the cardbord form to draw the shape of the trigger on the plastic and cut it out with a jigsaw. Dril the hole trough the trigger and secure the trigger in the trigger groove with an axle. You can put stoppers before and after the trigger as you can see in the picture but they are not realy necessary.
Step 5: Bow Attachmentpoint
Drill a hole with diameter 1cm in the center of the front end of the body. Then insert a metal plug with M8 thread inside it.
Step 6: The Bow
Start by drilling a hole with a diameter of 0.8cm in the middle of a 75 cm plastic bar. Then attach at both ends of the bar a strip of wood with a thicknes of 2mm a width of 3 cm and a length of 25 cm. Make at both ends cut outs to attach the string. Put a bolt with a washer trough the hole in the plastic bar. The last step is attaching fiberglas rods, I used 3 glasfiber rods from bikeflags. You can easily attach the rods with duct tape.
Step 7: The String
Put a nylon string trough an aluminum tube and make a loop at both ends of the string.
Step 8: Put the Bow Together
Insert the bolt from the bow inside the provided hole in the body. Then atach one loop to one end of the bow, pull the string trough the string groove and attach the other loop to the other end of the bow. You can load the bow by pulling the tube backwards and then locking it in the cut out. Fire the bow with the trigger.
Step 9: The Glider
For the glider you can use all kinds of materials, you should opt for lightweight stuf like balsa, aluminum, polystyrene, etc. I started from a strip of aluminum. The strip is necessary to launch the glider from the crossbow, this strip glides trough the flight groove. I gave the strip a round and smooth shape at the bottom and cut holes at the back end to reduce weight.
Step 10: L-profiles
The folowing step is attaching L-profiles to the upper side of the aluminum body. I attached them with rivets. You should attach profiles from the front to the back of the strip. The best is to attach the rivets mostly in the front to keep the center of gravity at the front.
Step 11: The Wings
To make the wings I used polystyrene. To cut smooth wings, you need a hot wire cutter and two identical molds. First you should place the molds at each side of the piece of polystyrene. Than you go with a long hot wire allong the shape of the two molds. Keep the wire perpendicular with the two molds and try to cut in one smooth curve. Don't worry if it doesn't work the first time, i tried it at least 5 times! When you eventualy have the wings, reinforce and connect them with eachother with the help of a sheet of balsawood (2mm) that you aply under the polystyrene whings. I used hot glue to put it all together. Then sand the whing with fine sandpaper until you become the right shape.
Step 12: Attach the Wings
Attach the wings with hot glue 7cm before the center of the aluminum strip. (in fact you can choose where you want to place the wings but make shure that left and right are in balance)
Step 13: Counterbalance Platform
Attach a wooden platform in the front that acts as a counter balance and where you can ad more weight later on to get the center of gravity in the right place. I attached the platform with 2 smal screws trough the aluminum L-profiles
Step 14: The Tale
Sand a sheet of balsawood as shown in the pictures. Attach it to the back of the plane and make shure that it's perfectly centered.
Step 15: Make a Body
Cut out the desired shape with the hot whire and cut out a space for the wings. Then sand everything down until it has the shape you want. Now reinforce the body with scewers. The last step is is covering the body with shrink film. That last step is not that easy without experience but the principle is: put the foil on the body and heat it. You can do that with the wings as well, but that's your own choice. Now attach the body to the rest of the plane.
Step 16: Balance the Plane
You want the center of gravity at 1/3 of the wing (see picture) , to realise that you can ad counterweight on the platform in the front. My counter weight was a piece of lead and an aluminum nose.
Step 17: THE END
Enjoy your plane!
Excuse me for the moderate English..
Glider IO1: http://youtu.be/vQBSe7ZxZnU
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