The aeroplane is thus particularly weak as to stability, launching,
and descending: but it is economical in power because it uses the
air to hold itself up. The dirigible balloon is lacking in power and
speed, but can ascend and descend safely, even if only by wasteful
methods; and it can carry heavy weights, which are impossible with the
structurally fragile aeroplane. The helicopter is wasteful in power,
but is stable and sure in ascending and descending, providing only that
the motor power does not fail.
Why, then, not combine the types? An aeroplane-dirigible would
be open to only one objection: on the ground of stability. The
dirigible-helicopter would have as its only disadvantage a certain
wastefulness of power, while the aeroplane-helicopter would seem to
have no drawback whatever.
All three combinations have been, or are being, tried. An Italian
engineer officer has designed a balloon-aeroplane. The balloon is
greatly flattened, or lens-shaped, and floats on its side, presenting
its edge to the horizon--if inclination be disregarded. With some
inclination, the machine acts like an aeroplane and is partially
self-sustaining at any reasonable velocity.
The use of a vertically-acting screw on a dirigible combines the
features of that type and the helicopter. This arrangement has also
been the subject of design (as in Captain Miller's flexible balloon)
if not of construction. The combination of helicopter and aeroplane
seems especially promising: the vertical propellers being employed for
starting and descending, as an emergency safety feature and perhaps for
aid in stabilizing. The fact that composite types of flying machine
have been suggested is perhaps, however, an indication that the
ultimate type has not yet been established.