Bullet blimp.

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Artist's conception :

Practical result.

https://www.youtube.com/watch?v=_BBxV9VUKoA

[synchro1]

Artist's conception :

Practical result.

https://www.youtube.com/watch?v=_BBxV9VUKoA

[synchro1]

Close up of the fan :

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Here's a sketch for a twin rubber band model that mounts both counter wound bands in the middle : Positioning the cylinder band grommet aft of the the collar bearing by braces gives the rubber band a secure place to anchor and spin.

The tube through which the rubber band passes mounts to the inside rim of the frame tube at the rear. The inside of the collar bearing mounts on the tube.

It would be fun to 3D print this twin rubber band model.

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Attaching a cycloidal ring wing around the blade ends of the rear propeller would quadruple the lift of the the coaxial propellers!