Centre of Pressure

Centre of Pressure

Th point on a surface exposed to pressure through which the
resultant force acts is called the centre of pressure. The centres of
pressure with which we are concerned relate to the pressure difference,
often called the gas pressure, unevenly spread over part of an
envelope separating gas from the atmosphere. Gas pressures are
small at the bottom of an envelope and reach a maximum at the top,
as illustrated in Fig. 6, and positions of the centres of pressure are
usually high.

The high centres of the total gas pressures exerted on walls which
restrain a gas-bag, as in the case of the wire bulkheads or transverse
frames of a rigid airship, lead to moments internal to the structure.
BCDE (Fig. 8) is a (full) gas-bag of an airship which is pitched at
angle a from a level keel. The longitudinal thrusts P, P' from the

* gas pressure

' are supported by bulkheads EC and DE of areas A,

A', assumed plane, B and E being lowest and C and D highest points.

The gas is assumed to be at rest, so that pressure is constant over

horizontal planes, and its pressure at B, the bottom of the bag, is

taken as equal to that of the atmosphere. Let p be the excess

pressure at height h above the level of B. Then from (3) p = pigA,

where p x is the difference in the densities of the gas and the surrounding

air.

Lower Bulkhead BC. Let 8A be the area of a narrow horizontal

strip of BC distant y from a horizontal axis in its plane through B.

Then h = y cos a, and the total thrust on BC is given by :

re re P = p dA = pig cos a y dA

JB JB = P!# cos a . AyQ