Page 140 - Theory and Design of Air Cushion Craft

Skirt/terrain interaction drag 123

where Q is the lift air flow (m ), /_, the peripheral length of the skirts (m), h the skirt
clearance, including the equivalent clearance regarding the air leakage from the delta
area of fingers, </> the air flow discharge coefficient and/? c the cushion pressure (N/m~).
Different terrain conditions can radically change the effective discharge coefficient,
(see Table 3.5). Grass or rock have the greatest effect. It is inappropriate therefore to
characterize the air gap by h alone, since rough terrain and stiff grasses or reeds will
reduce the skirt clearance significantly at the same air flow.
Fowler [36] defined h { K as the gap height instead of using h alone (i.e. h f K = h),
where K is referred directly to the terrain condition. This gap height for various craft
is shown in Table 3.4. Then it can be seen that a high gap height K is normal for a
h {
high-speed ACV and low h f K for hover platforms.
Test results demonstrating the relation between skirt/terrain interaction drag and
K as well as the terrain conditions are shown in Fig. 3.36 [36]. It is clear that the
h f
skirt/terrain interaction drag is very strongly sensitive to lift air flow.
Skirt/terrain interaction drag will increase at a higher rate as the skirt air gap is
reduced below a critical value. For this reason, an optimum skirt air gap has to be
selected as shown in Table 3.5 [37], recommended by Fowler.
Figure 3.37 shows the relation between the skirt/terrain interaction drag and craft
speed. Figure 3.38 shows the drag for craft running on an ice surface in relation to the
Froude number. These test results are provided for reference.

Table 3.4 Gap height h ( K of various ACV
Item Craft Type h f K
1 SR.N5 ACV 0.08
2 SR.N6 ACV 0.07
3 SR.N4 ACV 0.084
4 SR.N4 Mk2 ACV 0.073
5 Voyageur ACV 0.08
6 Viking ACV 0.068
7 LACV-30 ACV 0.062
8 ACT 100 ACP 0.019
9 Sea Pearl ACP 0.018
10 Yukon Princess ACP 0.012
11 Hex-55 ACP 0.018
12 Hex- IB ACP 0.015

Table 3.5 The suggested gap height K for various ACV terrain conditions [36]
h {
Ground terrain h { K K Drag coefficient %

Smooth concrete, slow speed 0.0035 1.0 2
Firm snow 0.0055 1.5 2.5
Short grass 0.02 6 2
Moderate grass 0.02 6 2
Long reedy grass (1st pass) 0.022 6 40
Long reedy grass (10th pass) 0.022 6 5
Crushed rock 0.02 6 15-30
Mudflats 0.016 5 2-5
Concrete, high speed 0.013 4 2