Page 78 - Theory and Design of Air Cushion Craft

62 Air cushion theory

vorticity, even though the range of vortices is rather large. Simultaneously, the test
cushion pressure distributes uniformly.
• According to analysis of the streamline charts, it is found that the location of the
feed holes in the bag will affect the jet flow enclosing length and consequently the
air cushion performance.
may
For this reason, an optimum hole location xlB and relative area of hole S/B b
be found. The best location is generally close to the segment upper attachment,
though some experimentation is necessary for optimization, once the overall skirt
section has been designed. This is always a worthwhile exercise.
• Owing to the enclosing effect of the jet flow assisting sealing of the air cushion, this
is equivalent to a reduced flow coefficient during leakage of air from a small hole
to the atmosphere, consequently the approach will be to concentrate on defining
the relative flow coefficient m and cushion pressure coefficient/^ at different ground
clearances (air gap under skirt tip).

Experimental results and analysis

The loss of air (leakage) between the skirt and box wall has to be considered, which
leads to the equivalent air clearance of the skirt h, and can be written as

h = h 0 + h'
where h Q is the actual air clearance of the skirt, namely, the vertical distance between
the lower edge of the skirt finger and the ground, and h' the equivalent air clearance
of skirt considering the air leakage loss between the skirt and box wall as well as the
gap between the fingers. The air leakage therefore has to be corrected in the case of
fitting a regression line for cushion flow derived from the test results. The following
observations were made at MARIC:
1. It was found that the test results are concentrated, having a small standard devia-
tion, which was also verified by test results gathered for another series which was
carried out one year later. The test data were considered stable and the selected
parameters to be reasonable.
2. For a conventional bag and finger type skirt, with parameters such as H {/H 2 = 1;
x/B = 0.512; S/B b = 0.0289; the cushion pressure coefficient^, cushion flow rate
coefficient ra, bag and cushion pressure ratio p t and cushion lift power coefficient N
can be regressed to the following expressions (see Figs 2.12, 1.14 and 1.26):
4 3
PC = PJ4j = 0.602 (/z/0~° ° (2.22)
A = P c/Pt = 0.638 (h/ty OA9 (2.23)
while

h/t<\.05 m = m/(p ap cf 5 = 0.625 (hit] -0.415 (2.24a)
h/t>l.Q5 m = m/(p ap c) 05 = 0.848 - 0.147 (hit) (2.24b)
hit = 1.05 ra = m^pzPc) 05 = average of (equations 2.24a and b)

These expressions are suitable for hit between 0.4 and 2.5; alternatively, the follow-
ing may be used: