Page 114 - Theory and Design of Air Cushion Craft

98 Steady drag forces

Wtany'
(a) SL-Sea Level

Fig. 3.13 Cushion wave-making drag and drag due to the difference of air momentum leaking from bow and
stern of cushion.

differential air momentum leaking from the bow/stern skirts R a is equal to the trim
drag minus the wave-making drag.
• When i//' < 0, i.e. the craft operates with bow down, then
R a.,= -
which means R a becomes a thrust and makes a net thrust of Wtan y/' .

The composition of skirt drag can be written as
=
^sk ^sf + ^sp + ^ss + ^si
where R sk is the total skirt drag (N), R sf the skirt water friction (N), R sp the skirt
pressure drag (N), 7? ss the skirt spray drag (N) and R si the skirt inertia drag (N).
is the wave-making drag induced by the high frequency vibration of the skirt.
R s[
For this reason, this drag is relative to the mass of the skirt, therefore this drag is also
called inertia drag.
R sp is the result of the cushion pressure and internal loads on the deformed part of
the skirt above the water level locally balancing the water pressure acting on the part