Page 66 - Theory and Design of Air Cushion Craft

50 Air cushion theory

2.2 Early air cushion theory developments

Although the theories mentioned below may seem to be out of date, it is useful to
study them in order to understand the air cushion rationale. While peripheral air jets
are no longer used in practice, the basic understanding developed through these
theories is still equally valid for modern ACVs.

Theory of thin peripheral jet air cushion hovering on a rigid
surface
This theory was used on early ACVs with rigid jet nozzles over ground for determin-
ing the air cushion performance. It assumes that:
• The nozzles are infinitely thin, therefore the air flow is jetted uniformly perpendic-
ular to the centre line of the jet.
• The air flow jetted from nozzles is non-viscous and incompressible.
• The air flow jetted from nozzles will not combine with media around the air jet
(induced flows are not treated).
• The cushion is supported on a rigid surface.
The transverse section of such an ACV is shown in Fig. 2.1. According to the theory
for flow momentum,
=
hp c P aV]t (1 + cos 9} = p aVjXh (2.1 )
2 2 4
where p c is the cushion pressure (N/m ), p a the air density (Ns /m ), t the width of noz-
zle (m), h the air clearance (m), V- } the mean velocity of jets (m/s), 0 the angle
between the centre-line of the nozzle and the craft baseline (°) and
x = (1 + cos9)t/h

Then the total pressure of the jet can be expressed as

where P t is the total pressure of the jet at the nozzle (N/m") and/the coefficient
for relative air clearance, as shown in Table 2.1.
If we neglect the problems regarding three-dimensional flow and flow from stabil-
ity trunks (internal skirts to divide the air cushion), then the air flow rate from the jet
nozzles of the craft could be written as
(2.3)

Table 2.1 Coefficient/relative to hit

hit f
1 0.75
2 0.65
3 0.54
>4 0.50