The building of flying models has become of commercial importance.
It is not difficult to attain a high ratio of surface to weight,
but it is almost impossible to get motor power in the small units
necessary without exceeding the permissible limit of motor weight. No
gasoline engine or electric motor can be made sufficiently light for
a toy model. Clockwork springs, if especially designed, may give the
necessary power for short flights, but no better form of power is known
just now than the twisted rubber band. For the small boy, a biplane
with sails about eighteen inches by four feet, eighteen inches apart,
anchored under his shoulders by six-foot cords while he rides his
bicycle, will give no small amount of experience in balancing and will
support enough of a load to make the experiment interesting.