Then the motor of the big toy was warmed up. Flimsy blades, that a moment ago had hung limp, now stiffened with the whirl of rotor force to a firmness that would withstand a hundred horse power. Centrifugal force did it! When the whir of its blades gathered power, the thing rose--not with a glide, or a slanting run to take the air, not at all--it rose straight up.