Non-return fabric valves known as crab-pots are fitted
in this fabric hose on either side of their junction with the air
scoop. Two automatic air valves are fitted to the underside of
the envelope, one for each ballonet. The air pressure tends to
open the valve instead of keeping it shut and to counteract this
the spring of the valve is inside the envelope. The springs are
set to open at a pressure of 25 to 28 mm.
Two gas valves are also fitted, one on the top of the envelope,
the other at the bottom. The bottom gas valve spring is set to
open at 30 to 35 mm. pressure, the top valve is hand controlled
only.
These valves are all very similar in design. They consist of two
wooden rings, between which the envelope is gripped, and which
are secured to each other by studs and butterfly nuts. The valve
disc, or moving portion of the valve, is made of aluminium and
takes a seating on a thin india rubber ring stretched between a
metal rod bent into a circle of smaller diameter than the valve
opening and the wooden ring of the valve. When it passes over
the wooden ring it is in contact with the envelope fabric and
makes the junction gastight. The disc is held against the rubber
by a compressed spring.
The valve cords are led to the pilot's seat through eyes attached
to the envelope.
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