Page 147 - Theory and Design of Air Cushion Craft

130 Steady drag forces

3.15 Effect of various factors on drag

The themes to be discussed here are the problems related to the drag of craft running
over calm water. The performance on rough sea will be discussed in Chapter 8. Since
the hovercraft, especially ACVs, travels close to the water surface at high speed, the
drag will increase dramatically as soon as the skirts come into contact with the water
surface occasionally making the drag unstable in magnitude. The effect of various fac-
tors on the craft performance are as follows.

The effect of position of LCG

The effect of LCG on craft drag, mainly skirt drag, is deterministic. A slight change
of LCG will lead directly to varying craft drag due to its effect on bow and stern skirt
friction, especially in the case of poorly designed bow/stern skirts.
With respect to SES, especially SES with thin sidewalls, the change of LCG will
also lead to a draft change at bow/stern seals, increasing seal drag, in the case of no
transverse cushion compartmentation on an SES. Figure 3.44 shows the effect of
LCG on drag of an SES based on model testing data. It can be seen that when the
centre of gravity of the model is moved just 3% of L c, a drag increase of about 70%
may result. Figure 3.45 [30] shows the relation between lift-drag ratio and trim angle
of the US SES-100B. It is clear that the deviation of trim angle from an optimum of
about 2° leads to an increase in drag about twice at a speed of 40 knots.
Figure 3.46 shows the influence of LCG on the speed of SES model 717C during
trials. For this reason, it is necessary to determine the LCG carefully. Based on

1.5

oo
cK 1.0
X

0.5
4
v(m/s)

Fig. 3.44 Influence of longitudinal centre of gravity on total drag of SES model. 1: P cll c = 15.4,
2: P cll c= 17.3,3:^/4= 19.4.