Page 64 - Theory and Design of Air Cushion Craft

2

Air cushion theory

2.1 Introduction

Development of air cushion theory is closely related to the development of hovercraft
themselves, particularly of flexible skirts.
Early air cushion vehicles were conceived with the object of drag reduction for a
marine craft. The concept used was air lubrication of the wet hull surface. From prac-
tical experience with high-speed stepped hull planing boats in the early part of this
century, it was realized that air lubrication did reduce the water friction; however,
it was difficult to reduce wave-making resistance. For this reason, the concept of
injecting lubricating air over the wetted surface was replaced by use of a thicker air
cushion.
In China, this theory was put into effect as a plenum chamber hovercraft with thin
'sidewalls' at the edge of the cushion, the hovercraft '33', built at the Harbin
Shipbuilding Engineering Institute (HSEI) in 1958. The air clearance under the side-
walls was very small, so it 'skated' over the water or ground and it is difficult to say
that this craft had real amphibious qualities.
Sir Christopher Cockerell invented the principle of the peripheral jet hovercraft,
whereby the air cushion was fed with air and also maintained by means of air flow
momentum change at a high-velocity peripheral air curtain. Air cushion efficiency
(depth of air cushion and thus obstacle clearance) was enhanced several times com-
pared with that of the plenum chamber hovercraft fed with air directly into the cush-
ion. Even so, prototype craft barely had amphibious ability. The Saunders Roe SR.N1
weighing 3.4 t, with an air clearance of approximately 200 mm, could only operate on
smooth surfaces such as concrete, level beach or smooth water. The initial principles
were nevertheless proved and a test bed made available for further development.
In practical terms, the amphibious capability of the SR.N1 was almost non-existent.
It required another technological leap before the concept could be used for practical
purposes. Nevertheless, the various theories to explain the peripheral jetted hovercraft
were enthusiastically followed up in the 1960s by the scientific community. These
include thin jet theory, thick jet theory, jet equilibrium theory and the various research
efforts to understand the effect of different cushion plan-forms and compartmentation.
Flexible skirts to contain and deepen the cushion were the key idea needed to
step forward. This concept began its development on hovercraft such as the SR.N1 as