Page 98 - Theory and Design of Air Cushion Craft

82 Air cushion theory

z = z m sin co t
= sin (cot + s)
Ap c Ap cm
= Ap cm sin cot cos e + zf/? cm cos cot sin £ (2.52)

and
_ a/7 c a/7

sin cot + -T • z m co cos cot (2.53)
dp c

then
a/? zJ/7 cm sing

cose
dp_ = Ap cm

The stability and damping coefficient can then be obtained using equations (2.50),
(2.51) and (2.54) from measurements taken on the rig using known values.

Comparison of calculations with test results

1. A comparison of calculations with test results is shown in Figs 2.28 and 2.29 and
it is found that the calculated values are in good agreement. The latter are higher
than the former, because of the air leakage from the connecting parts of the flexi-
ble skirts (the results are uncorrected). The flow in test is consequently larger than
the actual value, and the calculation values are smaller than the test results.
2. From equation (2.47), it is found that because dQ/dp c < 0 and both Q 0 > 0 and/? c
> 0, this means that the steeper the fan characteristic curve (H rQ), the smaller
dQ/dHj and the larger the damping coefficient.
3. From the formulae, one can see that the heave stability is proportional to flow rate
and inversely proportional to skirt clearance; and that the relation between the
heave stability and the characteristic curve of fan/air ducting is similar to the rela-
tion between the damping coefficient and the characteristic of fan/air ducting,
namely that mentioned in (2) above.