In comparing the different methods of transport for efficiency,
the resistance or thrust required is compared as a percentage of
the total weight. The result obtained is known as the
"co-efficient of tractive resistance." Experiments have shown
that as the size of the airship increases, the co-efficient of
tractive resistance decreases to a marked extent; with a
proportionate increase in horse-power it is proportionally more
economical for a 10,000,000 cubic feet capacity rigid to fly at
80 miles per hour than for a 2,000,000 cubic feet capacity to fly
at 60 miles per hour.
As the ratio "weight structure/total lift" is in airships fairly
constant, it follows that the ratio "disposable lift/total lift"
increases with the dimensions.
It is therefore obvious that increased benefits are obtained by
building airships of a larger size, and that the bigger the ship
the greater will be its efficiency, providing, of course, that it
is kept within such limits that it can be handled on the ground
and manoeuvred in the air.
The proportion of the useful lift in a large rigid, that is the
lift available for fuel, crew, passengers, and merchandise, is
well over 50 per cent when compared with the gross lift. When the
accompanying table is studied it will be seen that with airships
of large capacity the available lift will be such that
considerable weights of merchandise or passengers can be carried.
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