"Is it possible to power an ultralight airplane with a single-cylinder weed-whacker engine?" -- I've seen someone actually try it. The engine was still attached to the weed-whacker. The string was replaced by a propeller. The unit was strapped to the pilot's waist, projecting backwards. The aircraft was a paraglider. The pilot foot-launched off a mountain and then started the contraption in the air. Unfortunately I never heard what sort of climb rate he was able to achieve once he flew away from the ridge lift.
