Summary

Make it climb

https://youtu.be/3sg1ElKSOP0

Greg Gzumwalt (gzumwalt) brings 3D printed parts to life. He demonstrates that in more than 100 designs on Thingiverse. Inspired by all this motion, this ”thing” is a modest contribution. It’s a version of the power source of that very old (first seen in 1903) children’s toy, the Lehmann climbing monkey. The engine is constructed in such a way that it can be inserted in toys. A bear, a helicopter or whatever. In this thing we construct an Angry Bird, or something that looks like one.
The two wheels of the engine snap-lock together . Make sure the holes for fixing the strings are at the outside. Shift them into the housing and push the square shaft from one side of the housing to the other side. However, before you do that, cut a small piece from both ends of the shaft to remove print imperfections and remove sharp edges with sandpaper. Move the end caps with “wing” over the square shaft ends. They should go into the openings of the housing since they act as bearings. As you can see, I am not a great wing designer (although I am an aerospace engineer). So, replace the wings with your own design.
The string that comes from the larger diameter goes through the bottom hole, while the string wound on the smaller diameter goes through the top hole.
Wind the string once around the small wheel and leave it long. Make as many windings as possible on the larger diameter and you only need some remaining length to fasten it to the pull ring. Both strings should leave the wheels at the same side when they go to the escape holes. The exact length of the strings is determined by experiment. Just pull the lower string and change the number of windings on the small diameter until you are happy with it.
Although supports are not really necessary for the engine housing, you might want to have some support for the shaft bearing to make it look nicer. But since it is at the inside of the housing, it is not really important. The wheel combination should rotate without friction. The Angry Bird is formed by two simple spheres. By printing the x- and y-axis at 100% but the z-axis at 200%, it fits nicely around the engine. This part is printed twice, with supports. Eyes and beak are improvised, I am sure you can do better.
More climbing "Things":
http://www.thingiverse.com/thing:1918477
http://www.thingiverse.com/thing:1930503
http://www.thingiverse.com/thing:1943901