Page 88 - Theory and Design of Air Cushion Craft

72 Air cushion theory

higher pressure bag is improved by the arrangement of a larger number of small feed-
ing holes. This design improves the strength of skirt bags by reducing stress concen-
trations and thus the tendency to tear after fatigue due to operation.
The air cushion characteristics of such skirts are closer to those represented by
plenum chamber theory. Moreover, the take-off performance and obstacle clearance
ability is improved, therefore the flow for the take-off to the planing condition over
water is not such an important parameter as concerned designers in the early stage of
ACV/SES development. For this reason, rather than spend time on deriving the math-
ematical expressions for predicting the static air cushion performance, we take the
flow rate coefficient Q as the factor to represent the static air cushion performance of
craft. The relation for Q can be written as
(2pM (2-34)
In general, we take the values of Q to be [15] :

Q = 0.015 - 0.050 for ACV
Q = 0.005 - 0.010 for SES
The required value of Q is related to the following performance factors:
1 . craft drag at full or cruising speed on calm water;
2. take-off ability;
3. seaworthiness;
4. longitudinal/transverse stability of craft;
5. resistance to plough-in, etc.
Acceptable craft performance can normally be obtained if the cushion air system is
designed with Q in the range above. The quoted range is rather large when designing
a large SES or ACV and so it is normally best to start with the lower value (suitable
for calm water operation, medium-speed craft) and then assess the additional flow
required for items 2 to 5. These factors will be discussed further in following chapters.
As an alternative, particularly for amphibious ACVs, one often takes the skirt clear-
ance of the craft hovering on a rigid surface as the factor to characterize its hovering
ability and so to design the lift system. This is a common approach of designers
because it is easy to measure the skirt clearance of an ACV both in model and full-
scale craft. Although it is not accurate for the reasons outlined in the discussion of the
various air jet theories above, it is easier to compare with other craft (or models).
Typically, for smaller amphibious craft the following relation is used:
Q = V cD chL(rn/s)
where
= (2/? c//? a), the cushion air escape velocity (m/s),
V c v
2
3
= 1.2257 kg/m /9.8062 = 0.12499 (kg m/s )
p. A
3
= (0.07656 Ib/ft /32.17 = 0.00238 slug/ft 3 in imperial units)
D c = nozzle discharge coefficient (2.3.4 item 5), D c = 0.53 for 45° segment, L =
peripheral length of cushion at the ground line (m) and h = effective gap height, typ-
ically 0.125 X segment width, or if it may be assumed that segment width is approxi-
mately h c/2.5 then h = 0.05 h c. Thus