Page 139 - Theory and Design of Air Cushion Craft

122 Steady drag forces

is a high percentage of the total drag. The total overland drag of ACV can be written
as follows:

^gacv = ^a + ^m + ^sp + ^si + ^sk (3.50)
where R gacv is the total overland drag of ACV (N), R a the aerodynamic profile drag
(N), R m the aerodynamic momentum drag (N), R sp the spray (debris) momentum drag
(N), R si the slope drag (N) and R sk the skirt/terrain interaction drag (N).
R a, R m can be calculated by the methods outlined above. R sp can usually be
neglected due to the craft's low speed. The slope drag can be calculated according to
the geography of the terrain. The skirt/terrain interaction drag is very strongly sensi-
tive to lift air flow and is a function of craft speed and terrain condition. It is difficult
to determine analytically and is usually determined from experimental data.
The overland drag curve of an ACV can be divided in three modes controlled by
cushion flow rate as shown in Fig. 3.35:

1. Mode A, ACV profiles the terrain perfectly (i.e. a clear air gap between ACV and
terrain);
2. Mode B, ACV experiences strong skirt/terrain interaction effects;
3. Mode C, ACV operates in 'ski' mode.

In mode A, at high flow rates, drag is relatively low. Normally in this flow region there
is an air gap under most of the skirt periphery. In mode B, segment tips drag on the
surface, but the delta regions between skirt tips still exist. In mode C, segment tips are
pressed against the surface and the air flow acts more as a lubricant.
Figure 3.35 shows that the skirt/terrain interaction drag is closely related to skirt tip
air gap. According to Chapter 2, the lift air flow Q can be written as
0.5
l
Q = ih<j> [2p c/pJ (3.51)

Drag

Fig. 3.35 Three operation modes of an ACV over ground terrain.