Page 71 - Theory and Design of Air Cushion Craft

Practical formulae for predicting air cushion performance 55

5
5
45
3
~ A)) ]° = (2°' r//z s)[l + (hj[2t cos 9 + 2.75 ((//„ - h s)/t sin #)~°' ]}] 05
N[pJ(p c
45 a5
X {h s/[2t cos 6 + 2.75 ((h b - /z s)/t sin 0)~°- ]} (2.16)
where m is the flow rate per unit air curtain length and N the lift power per unit air
curtain area.
While A. A. West's theory could be applied to real craft with a bag and finger skirt,
the disadvantages of this approach would be as follows:
• The assumption made by West that the bag and finger type skirt may be simplified
as a simple single wall skirt and the air curtain jetted from a nozzle stuck to the
skirt finger does not agree with practice.
• The theory does not consider the effect of viscosity of air as a real fluid. Real flow
conditions can be illustrated as in Fig. 2.5. Thanks to the viscosity of flow a lot of
air will be ingested from the atmosphere into the air curtain to form a combined
flow, namely the curtain jet flow ra c which will separate into two curtains, one to the
atmosphere and another into the cushion.
• The theory does not consider the flow energy losses from nozzles in the bag. Clearly
this is not reasonable for bag and finger skirts, though it is acceptable for open loop
designs.
• It is not reasonable to assume zero energy loss between the flow streamlines AA'
and BB', i.e. losses in a two-dimensional jet in a real fluid as against an infinitely
thin jet.

2.3 Practical formulae for predicting air cushion
performance

The various theories described above to predict the static air cushion performance of
craft over ground, have disadvantages as follows :
• The theory based on a thin nozzle correlated with experimental results in the early
research stage of hovercraft has precision at large hover heights (clear air gap), but
is not realistic for small hovering heights, as is the case of craft with bag and finger
skirts. This is because the air curtain jetted from under segments or fingers will be
distorted by the proximity of the ground and the complex geometry of the finger
itself.

Fig. 2.5 Actual air streamlines including air viscosity.