Page 70 - Theory and Design of Air Cushion Craft

54 Air cushion theory

I ___^LJ.__^r-r:

Fig. 2.4 Hypothesis for air jet streamlines based on A. A.West's single wall theory.

skirt, the lower the pressure and nearer to atmospheric pressure. However, globally it
is reasonable to assume that the static pressure mentioned above is constant, namely
cushion pressure p c and then the above formula may be written as
fj (cos 9 + M e/Mj) = (p c - p Q)h s (2.13)

Based on the Mayer velocity distribution and boundary layer thickness for a two-
dimensional jet with enclosing wall and turbulent flow, the ratio of flow momentum
at section e to that at section j has been derived by A. A. West as follows:
0 45
0 45
M e/Mj = 2.75 (lit)' ' = 2.75 ((/j b - h s)/(t sin 6>))~ - (2.14)
where /z s is the air clearance of the nozzle.
Upon the substitution of equations (2.11) and (2.14) in equation (2.13), we have

45
A - A) 1 + (hj[2t cos 0 + 2.75 ((h b - h s)/(t sin 0))~°' ]}
Thus, the flow rate per unit air curtain length can be written as

or
m 1
(2.15)
,0.5
[p*(p c - A))] h s[ cos 0 + 2.75 ((h b - /z s)/t sin 0)"
The lift power per unit air curtain area can be written as