Since the whole resistance, in either type of flying machine, is
approximately proportional to the square of the velocity; and since
horse-power (work) is the product of resistance and velocity, the
horse-power of an air craft of any sort varies about as the cube of the
speed. To increase present speeds of dirigible balloons from thirty to
sixty miles per hour would then mean eight times as much horse-power,
eight times as much motor weight, eight times as rapid a rate of fuel
consumption, and (since the speed has been doubled) four times as rapid
a consumption of fuel in proportion to the distance traveled. Either
the radius of action must be decreased, or the weight of fuel carried
must be greatly increased, if higher velocities are to be attained.
Present (independent) aeroplane speeds are usually about fifty miles
per hour, and there is not the necessity for a great increase which
exists with the lighter-than-air machines. We have already succeeded in
carrying and propelling fifty pounds of total load or fifteen pounds
of passenger load per horse-power of motor, with aeroplanes; the ratio
of net load to horse-power in the dirigible is considerably lower; but
the question of weight in relation to power is of relatively smaller
importance in the latter machine, where support is afforded by the gas
and not by the engine.