Page 195 - Theory and Design of Air Cushion Craft

178 Stability

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Fig. 4.44 Experimental results of rolling angle of hovercraft models as a function of craft speed.

Bow pitching down and plough-in are common terms used by the hovercraft
designer often without clear distinction between them. The former means the craft
bow pitching down at the bow skirt but still in a condition where the skirt segments
are air lubricated and 'plough-in' means the craft is pitching bow down with so large
a negative angle as to lead the skirt undergoing significant wetting giving large local
drag forces and accompanied by the tuck-under of the bow skirt. Here we will inves-
tigate this problem in three phases, i.e. the progression, the reasons and the measures
for improvement.

The progression during plough-in
The typical progression at plough-in can be described as follows (Fig. 4.45):
• Fig. 4.45(a): An ACV normally travels with a definite positive trim angle (bow up),
and spray blown out under the fingers of bow skirt can be observed.
• Fig. 4.45(b): As the craft speeds up, the positive trim angle decreases as the thrust
of the air propulsor increases. In the case where the craft is travelling downwind
and a gust occurs, this causes the thrust to increase suddenly and lead to contact
of the bow skirt fingers with the water surface. Meanwhile the spray blown from
the delta zone between the tips of the skirt fingers can also be observed. The craft
is still running with positive stability.
• Fig. 4.45(c): Hydrodynamic resistance increases as the wetted surface of skirts
increases, thus the dynamic pressure of oncoming flow on stagnation will be so bal-
anced by the cushion pressure, as to lead to the deformation of skirt fingers and
cause the skirt to plane without spray. Eventually this can produce a suction acting
on the skirt fingers, which leads to the fingers actually touching the water surface.
The hydrodynamic drag increases further, light tuck-under occurs on the fingers,
leading to the bow cushion being pulled back and the cushion area decreased. Sub-
sequently the total force from the cushion drops while the cushion pressure rises