Page 112 - Theory and Design of Air Cushion Craft

96 Steady drag forces

3.4 Aerodynamic profile drag

The aerodynamic profile drag can be written as

- c v nfi\
^
R a -
A C — • 9 a V p.OJ
where 7? a is the aerodynamic profile drag (N), C a the coefficient for the aerodynamic
2
profile drag and S a the frontal projecting area of the hull above the water (m ). C a is
highly sensitive to the aerodynamic profile of the craft's hull, inclusive of the inflated
skirt. Its value is generally obtained from wind tunnel tests for detailed design. Table
3.1 gives a selection of C a data for various models and craft based on wind tunnel
data. It can be seen that C a for an ACV (particularly for passenger craft) is in general
small. C a can be estimated based on the geometry of a new craft, using data such as
presented by Hoerner [23] and comparison with known data such as that in Table 3.1.
ACV aerodynamic drag is generally a significant proportion of total drag because
of high design craft speed and the low water drag. For example, the aerodynamic drag
of ACV 711-A built in China is 30% of the total drag of this craft at a speed at 85
km/h. Therefore it is very important to design an ACV craft superstructure with care.
In general, the C a can be taken as 0.4-0.6 for an ACV, with extreme values of 0.3
for fine lines and 0.75 for poor lines. For example, C a for US Voyageur and JEFF(B)
is 0.75 due to their open well deck for accommodating tanks or large loads. C a is
typically 0.5-0.7 for SES. In the final analysis, it is best to use test results from a wind
tunnel if possible, to determine this fraction of the total drag force accurately.

Table 3.1 The aerodynamic profile drag coefficient Ca for various craft (models)
Item Craft name Craft type C, Source of data
1 SR.N2 ACV 0.25 ADAO 22583
2 SR.N4 ACV 0.30 ADAO 22583
3 SR.N5 ACV 0.38 ADAO 22583
4 SKMR.l ACV 0.398 AIAA 73-318
5 SK-5 ACV 0.28 AIAA 73-318
6 JEFF(B) ACV 0.495 AIAA 73-3 18
7 Voyageur ACV 0.75 AIAA 73-3 18
8 N500 ACV 0.30 AIAA 73-3 18
9 SES-100B SES 0.32 ADAO 22583
10 Model 71 9 SES 0.63 Marie Report

3.5 Aerodynamic momentum drag

Pressurized air has to be blown into the air cushion to replace air leakage out from
the cushion under the skirt or seals in order to maintain the ACV/SES travelling on
cushion. Thus, this mass of pressurized air contained in the cushion will be acceler-
ated to the speed of the craft. The drag due to the momentum change of this air mass
is called the aerodynamic momentum drag and can be calculated as