Page 135 - Theory and Design of Air Cushion Craft

Total ACV and SES drag over water 119

X9.8N

16000

14000

12000

10000

8000

6000

4000

2000
20 40 60 80 100 120 140 v(km/h)
20 40 60 80 v(kn)

Fig. 3.33 The drag and thrust curves of SR.N4.

etc.) is included in the air profile drag, because in general the air profile drag co-
efficient C a, which can be obtained either by model experimental data or by the
data from prototype craft or statistical data, implicitly includes appendage drag in
the coefficient.
• Similarly to conventional ships, model drag can be converted to drag of full scale
craft according to the Froude scaling laws (see Chapter 9).
• Taking the Chinese ACV model 7202 and 711-11A as examples, we calculate the
drag components for these craft as shown in Figs 3.31 and 3.32. The propeller
thrust in the figure was calculated according to the standard method for predicting
the air propeller performance published by the British Royal Aeronautical Society.
If K T is assumed equal to 1.23 and 1.1 for craft 7202 and 711-IIA respectively and
method 3 is used, then the calculated results agree well with the trial result.
When MARIC used method No. 1, taking K 7 as 1.65 for craft 7202 and 1.5 for
craft 711-IIA, then the calculations agreed with test results. It can be seen that
method 1 is approximate, because of the large K^ value.
• A typical resistance curve for the British SR.N4 can be seen in Fig. 3.33.

SES

There are many methods for calculating the drag components of an SES as are men-
tioned above, though one has to use these methods carefully and not mix them with