This is a slingshot for small planes, like the great planes from robotfriend 
https://www.printables.com/de/model/456057-f23-ultralight-glider-print-in-place
https://www.printables.com/de/model/462466-uss-enterprise-1701-f-print-in-place-glider/files

My grandchildren had difficulties to fly these planes without a slingshot. Because I couldn't find a usable model I designed this one.

The special thing about it, the tension rubber goes parallel over the end of the stick. This measure reduces the risk, 
1. that the airplane touches the slingshot during the launch and 
2. if you take care that the rubber is not twisted, the airplane will not rotate during the launch.

I printed the part with 0.2mm, 4 perimeters (3 are also ok), 15% infill, bottom and top each with 5 layers, PLA, PETG