Page 184 - Theory and Design of Air Cushion Craft

Calculation of ACV transverse stability 167

Fig. 4.32 Coordinate system of craft for stability.

to satisfy the flow rate continuity equation. Thus the equations of weight and
moments can be expressed in matrix form as follows:

W
(4.20)
IM J
where [A n] is the parametric matrix for cushion geometry of the craft and can be
written as

I ^cr
(4.21)
Y AY j
l
1 * pi ^cr pr
where W is the craft weight, M the heeling moment, M = M c + M g, M c the righting
moment due to the cushion pressure at the heeling angle of 0, M % the heeling moment
due to the height of CG at the heeling angle of 9, i.e. the moment of W about the
intersection point of line GZ with the line linking to the lower tip of skirts, A d the area
of left cushion, suffix 1 denotes left cushion, A cr the area of right cushion, suffix r
denotes right cushion, F pl the vertical distance between the CG and centre of left
cushion area and F pr the vertical distance between the CG and centre of right
cushion area.
The total pressure-flow rate characteristic of the fan and duct can also be expressed
by

CQ (4.22)
where P t is the bag pressure, Q the flow rate of the fan and A, B, C the parameters for
fan characteristics.
Expression (4.22) defines the relation between bag pressure and flow rate. As a
matter of fact, we have to put the total pressure of fan H } into the foregoing equation
instead of bag pressure P t. Here we neglect the pressure of the air duct from the out-
let of the fan to skirt bag. The bag pressure will be different in the case of different
duct configurations. One has to consider this matter in practical calculations accord-
ing to the specific geometry of the craft cushion air distribution system.