I printed it just as it is using the spiral vase settings I posted on the Thing details page and then I made a hole using the soldering iron. So i didn't measure the exact center when I made the hole and the diameter was somewhere between 1.5 and 2 millimeters. the paper clip wire was 1mm thick. After installing the propeller, I placed a drop of sewing machine oil on the nose hole just to make the movement smoother. I guess the model could be improved. I could make a hole into the solid and then mach the number of the solid layers with the depth of the nose hole and then have the spiral vase section from that height up. But when I designed it I never thought of efficiency. I only wanted to have a proof of concept and something that can get your kids excited. I noticed that because the hole is not long enough and of proper size, the propeller spins a little bit noisy and wobbly. I know this means energy loss and that I could make it spin for a longer period of time by redesigning the nose, but like I said, it was enough for the purpose it was created for.
I am very excited by the fact that you are going to attach an engine to it. Tell us if it flies and post pictures :D