Page 190 - Theory and Design of Air Cushion Craft

Dynamic stability, plough-in and overturning 173

0.2-
water
ground
0.1 -

5 10 0(°)

Fig. 4.39 Influence of air flow rate on static transverse stability. 1: lift for speed n — 3000 rpm; 2: n = 4500
rpm; 3: n = 5000 rpm.

4.6 Dynamic stability, plough-in and overturning of

hovercraft
hfiwiarrraft
Introduction

We will now introduce the dynamic stability of ACVs travelling over water. During the
early development of ACVs (see Chapter 1) hovercraft had very limited stability. The
concept being developed was to use air jets to allow these machines to 'fly' close to the
ground. This approach had to be revised once flexible skirts were introduced.
When operating over water, particularly at speed, the forces induced from wetting
of the lower part of the skirts could produce significant changes of trim. The early
design of skirts was such that these changes of trim did not create additional righting
moments, but instead they were reduced. Unless the hovercraft pilot took action to
maintain level trim, the craft would eventually plough-in and either come to a stop,
or overturn.
There were a number of overturning accidents in those early days. As an example
the Chinese experimental ACV type 711 overturned during a trial in May 1966. The
craft was travelling at the speed of 50 km/h and had to make a sudden turn in order
to avoid collision with a small boat. When the craft made the turn, yawing and heel-
ing to a large angle occurred, and the craft capsized.
Some British ACVs also overturned at a high speed on calm water. The process of
capsizing was as follows: bow pitching down, drifting, yawing and combined heeling,
high-speed plough-in and then overturning. Figure 4.40 shows plough-in of the
British ACV SR.N6 and Fig. 4.41 shows the plough-in that induced the capsizing of
a radio-controlled ACV model recorded by cine camera.
The sequence of plough-in events was stated in the driver instructions of SR.N5
as follows:

Contact of the flexible skirts with water surface leads to a bow pitching down
moment and bow pitch down, which will be magnified as the water resistance
increases at increased speed. The features of the plough-in phenomenon are