The GEN H-4 is arguably the smallest working co-axial helicopter ever created. It weighs just 155 pounds.
(GEN H-4 personal copter)
It uses four two-cylinder, air-cooled engines with 125 ccs displacement; each contributes about 10 horsepower. It uses about five gallons per hour. The rotors are about thirteen feet in diameter and rotate at roughly 800 rpm; they are made from a carbon-kevlar composite with a foam core.
This devices is obviously the copter harness from Robert Heinlein's 1954 juvenile The Star Beast:
They were half way home when a single flyer, hopping free in a copter harness, approached the little parade. The flier ignored the red warning light stabbing out from the police chief's car and slanted straight down at the huge star beast. John Thomas thought that he recognized that he recognized Betty's slapdash style...
(Read more about Heinlein's copter harness)