Page 116 - Theory and Design of Air Cushion Craft

100 Steady drag forces

Fig. 3.14 Deflection of flexible skirts contacting water surface.

= (p e = PcR = R s
where D is the tension of skirt fabric per unit width (N/m), p c the cushion pressure
the atmospheric pressure and which equals zero and the water friction
(N/m"), p 0 R sf
of the skirt per unit width (N/m).
Since the Reynolds number, Re is large for the skirt, the skirt fabric can be consid-
ered as a rough surface for friction, then
* = 0.032 (k/pf-p z (3.12)
in which k is the coefficient due to equivalent roughness and q w the hydrodynamic
pressure (N/m"), = 0.5 /? w v". Thus equation (3.11) can be written as
1-cos
(3.13)
sin 0 sin 0 57.29
The values of R sf and can be defined from equations (3.12) and (3.13), i.e. the two-
1 2
dimensional equations. It is usual to assume that d = when calculating the wet sur-
1 2
face of the skirt. In fact, this creates some errors due to neglecting the definite radius
of curvature R, which is not equal to zero, but relative to the water friction and skirt
fabric tension this is a small error. In the case of calculating the friction of stern skirts,
the determination of the friction coefficient is in fact very complicated because the
immersion depth of the stern skirt of a craft running over water is so small and it also
makes a large amount of spray due to the pressure of the air cushion. Therefore, the
problem becomes the drag concerned with the dynamics of two-phase flow - thus R^ k
can be written as
>, Q,d/p c, (3.14)
where Re } is the Reynolds number for jet air, Q the flow rate coefficient, which affects
the spray, W e the Weber number = p w v] t/a { which also affects the initiation of spray,