Page 84 - Theory and Design of Air Cushion Craft

Air cushion theory

where Urj is the leaking air flow speed in the horizontal direction. It is clear that the
flow rate leaking under the bow skirt can be represented by

then
a = M = 2/3B c<f>(z b ~ t { (2.32)

As a consequence, the air cushion flow for craft statically hovering on a water surface
is equal to 2/3 of that on a rigid surface, because of the action of back pressure of
leaking air.
This estimate is approximate, but realistic and is generally applied as a method of
estimation of the flow rate of an SES, because of its simplicity and the difficulty in
measuring the steady flow in an SES on a water surface. By the same logic, the flow
leaking from the stern seal can be obtained by this method; consequently, the total
flow for craft hovering statically on the water surface can be obtained.
It is useful to note that the same reduction in air leakage rate also applies to an
ACV hovering over water rather than land.

The static air cushion performance of ACVs on a water surface

The difference between the ACV and SES for static air cushion performance is that
the sidewalls provide buoyancy. The typical static hovering attitude of an amphibious
ACV can be seen in Fig. 2.19. If one neglects the reaction of the perpendicular com-
ponents of jets flowing from peripheral and stability nozzles (the value of which is
small in the case of small skirt clearance with a bag-finger type), then the cushion lift
can be written as

(2.33)
I
S c = CB C
where l c and B c are the cushion length and beam, which can be measured from the line
on the plan of the water surface, which the lower tip of skirt is projecting on.
From Fig. 2.19, it is found that the craft weight is equal to the weight of water dis-
placed from the depression; for this reason, the actual skirt clearance is equal to the

Fig. 2.19 ACV hovering static on water.