Page 144 - Theory and Design of Air Cushion Craft

Problems concerning ACV/SES take-off 127

• the added wave-making drag due to seal/skirt at secondary hump speed and the
flexibility of skirts to yield to waves without scooping;
• the ability of the craft to keep straight course stability and good transverse stabil-
ity during take-off through hump speed.

It is not difficult to improve the ability to accelerate through hump speed if the fac-
tors mentioned above are taken into account. According to research experience at
MARIC, we give some examples to illustrate these factors for the reader's reference.
1. The ACV model 711, the first Chinese amphibious test hovercraft, was found to
suffer difficulties on passing through hump speed in 1965. The craft, weighing 4 t,
was powered by propulsion engines rated 191 kW and obtained a thrust of 5000 N
during take-off. This meant that the thrust/lift ratio of the craft was about 1/8. It
was difficult to get the craft to take off, owing to large water-scooping drag of the
peripheral jetted nozzle and shorter extended flexible nozzle, especially at the stern
position. After a time, MARIC used the controlling valve of the air duct to adjust
the running attitude of the craft in order to decrease the water contact drag of the
skirt and the craft successfully passed though the hump speed.
2. After a time, craft model 711 had been chosen to mount a flexible skirt. The take-
off ability of the craft was improved significantly due to the enlarged air cushion
area, which reduced the cushion pressure and cushion pressure-length ratio, and
also the flexible skirts' ability to yield to the wave hump. The wave-making drag at
secondary hump speed was reduced by the same modifications.
3. The modified craft model 711 with flexible skirts was occasionally found to suffer
difficulties in passing though the hump speed. The flexible jetted bag stern skirt
with relatively larger area forward (Fig. 3.40) induced large skirt drag during take-
off because the stern skirt took a form allowing scooping. It was observed that
sometimes the original craft could still struggle to cross over the hump speed after
a long running time. A breakthough occurred (literally!) after the diaphragms of
the jetted skirt were accidentally broken and the stern skirt had changed from A to
B as shown in Fig. 3.40.
4. The probability of successful take-off for the first Chinese experimental SES, the
711 in 1967, was about 60-70%, and it could be improved by retracting the stern
seal during the course of passing though the hump speed (Fig. 3.41) and
reached 100% take-off probability. This was a successful method for the following
reasons:
(a) Drag due to water scooping was reduced by decreasing the water scooping area
of the stern seal as the stern seal is raised.
(b) Angle of attack of the stern seal was reduced with consequential reduction in drag.
(c) The running attitude was changed to a trimmed condition with bow up; as a
result the bow seal drag was reduced and the course stability was also enhanced.

It was noted that similar methods have been developed in other countries. For
instance, there was a retractable stern seal mounted on the Soviet passenger craft
Gorkovchanin and similar equipment was also mounted on the US test craft XR-3
to improve the dynamic stability during take-off, as shown in Fig. 3.42. Figure 3.42
shows that hump drag can be reduced considerably by retracting the stern seal.
5. When the Jin-Sah river passenger SES with the balanced rigid seal performed