Page 80 - Theory and Design of Air Cushion Craft

64 Air cushion theory

2.0
Exponential theory MARIC
A. A. West Plenum chamber theory
with flow coefficient of 0.53

0.5 1.0 1.5 2.0 hit

Fig. 2.13 Comparison of air flow coefficient between various theories: exponential theory; A. A. West theory;
MARIC theory; plenum chamber theory with air flow rate coefficient of 0.53.

follows the angle of the wall of the finger, consequently contracting the cushion
outflow jet so as to reduce the cushion flow rate.
From the figure the greatest difference in p t is found between the test results and
the results calculated by the exponential formula and the West formula in the case
of large hit values. However, all of them will be closer in the case of smaller hit. The
test result for the cushion lift coefficient (Fig. 2.14) is higher than that calculated
from the exponential and A. A. West's theories, because the experiments accurately
simulate the losses through the feed holes of bag-finger skirts and the weak effects
of the jet enclosing the walls.
6. The effect of the skirt material on the static air cushion performance. Reference 13
listed eight test samples with different materials which are shown in Table 2.5. The
test results showed the effect of specific weight of skirt material on the static shape
of skirts.
It is found that skirt material characteristics do not greatly affect the static air
cushion performance in the case of any given configuration and hovering height of
the skirts. The effect of skirt material characteristics on the hovering height can
therefore normally be neglected and the hovering height can be measured directly
during tests. The effect of specific weight of skirts on the geometric shape will be
described in Chapter 7. From such tests it is also found that the effect of elasticity
of skirt material on the shape of the skirt is weak.

Table 2.5 Experimental samples of skirt for various skirt materials [14]

Model Scale Width of air duct/ Equivalent nozzle Skirt material Test
ratio A width of bag width/0.095 equipment
1 1 0.625 1 95 14 Rubber core Large rig
2 1 1 1 95 14 Rubber core Large rig
3 1/2 1 1 6408 Rubber core Large rig
4 1/2 1 1/2 6408 Rubber core Large rig
5 1/2 1 1/2 1533 Rubber single Large rig
surface coating
6 1/2 1 1 1533 Rubber single Large rig
surface coating
7 1/8.9 1 1/8.9 Plastic diaphragm Small rig
8 1/8.9 1 1/8.9 95 14 Rubber core Small rig