GEM Research in the U.S. 279 
slender plan form, with pointed bow and stern, so that the entire peripheral nozzle lies 
nearly parallel to the direction of flight, then the rearward velocity component produced by 
tangential deflection is, at all points along the nozzle exit, nearly equal to j sin 8B. The 
thrust component of jet reaction is hence mass flow times Jj sin 8, and equals the ram drag 
(mass flow times Vo) when 
W=V,sin8. (31) 
The base pressure and jet momentum relationship is modified from the simple air curtain 
case (Eq. (1-2)) as follows: The effective nozzle area per unit nozzle length is reduced in 
proportion to cos f, and only the fraction cos B of the jet momentum enters into reaction 
against the ground cushion (since the tangential component of jet momentum is unchanged 
as the jet curves outward). The equivalent relationship for the integrated air curtain is 
therefore 
Mph=- 0 ee G (1 - sin @) cos? B. (3-3) 
The cushion power expression is the same for the simple air curtain (Eq. (1-8)) except for 
the modified effective nozzle area: 
P.=V,GC cos 8 (5p V,2 ++ -Ap - ay) | (3-4) 
Combining, and solving for optimum nozzle width ratio G/h (= 1/2) and jet angle 6( = -90°) 
gives for the equivalent lift/drag ratio 
LV. 
) S 
ry SES olay Ge) 
opt 
Comparison of this equation with Eq. (1-10) for the simple air curtain shows that the 
results are equivalent at very low speeds, but the integrated air curtain becomes vastly 
superior at high speeds. Close examination of the integrated air curtain equations (Fig. 3) 
will reveal that the power required, compressor pressure rise, internal air mass-flow rate, 
and optimum nozzle width are all independent of speed. In practice, all of these advantages 
are somewhat diluted (but not negated) by the effects of parasite drag and internal losses. 
The best design practice might be to provide sufficient tangential jet deflection to counter- 
act the ram drag, plus a separate propulsion system to counteract the parasite drag. This 
and other practical considerations are discussed further in the simplified engineering analy- 
sis of air cushion performance presented in a later section of this paper. It may be said 
here that actual air cushion vehicles will probably be limited to equivalent lift/drag ratios 
of the order of 0.9 S/AC, at optimum cruise speeds corresponding roughly to V = 1.0. 
More detailed information on the integrated air curtain is found in Refs. 4 and 5. 
WATER CURTAIN 
In principle, a ground cushion can be contained by a peripheral jet of water in just the 
same manner as by a jet of air. This concept is represented schematically in Fig. 4. Air is 
pumped to the base of the vehicle until the ground cushion is established, at which point 
equilibrium is reached between the change of momentum within the water curtain and the 
