The propellers
are collapsible, although in the latest craft of this class they
are semi-rigid.
The mechanical equipment is also interesting. There are two
propellers, and two motors, each nominally driving one propeller.
But should one motor break down, or motives of economy, such as
husbanding of fuel, render it advisable to run upon one engine,
then the two propellers may be driven by either of the motors.
The inventor has perfected an ingenious, simple, and highly
efficient coupling device to attain this end, but to ensure that
the propeller output is of the maximum efficiency in relation to
the engine, the pitch of the propellers may be altered and even
reversed while the engine is running. When one motor only is
being used, the pitch is lowered until the propellers revolve at
the speed which they would attain if both engines were in
operation. This adjustment of the propeller pitch to the most
economical engine revolutions is a distinctive characteristic,
and contributes to the efficiency and reliability of the Parseval
dirigible to a very pronounced degree.
Steering in the vertical plane is also carried out upon
distinctive lines.
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