274 H. R. Chaplin 
SIMPLE AIR CURTAIN 
The air curtain (or “annular jet” or “peripheral jet”) concepts have received by far the 
most attention and serious study. They are correspondingly the furthest advanced, in terms 
of practical understanding and engineering data. The simple air curtain is illustrated in 
Fig. 1. As in all the ground cushion concepts, the major source of lift force is the ground 
C = Perimeter 
Simplified Ideal Theorys 
Cruise 
L=ApS p V; GCV, 
Aph= pV? G (1 - sin 6) vacate Yo 
P.=V,GC(% pV? +% Ap) (1-3) VGC (% pV? +% Ap — q) 
soa IE VV 2 
pV s ps Vo. | 
s 
Fig. 1. Simple air curtain 
cushion itself, a region of positive gage pressure trapped between the vehicle and the 
ground. With the air curtain types, this ground cushion is generated and contained by a jet 
of air exhausted downward and inward from a nozzle at the periphery of the base. With the 
simple air curtain, the resultant jet reaction force is vertical, and propulsion must be pro- 
vided by a separate device (shown schematically in Fig. 1 as a propeller). 
Hovering 
The jet reaction force contributes to the lift, but this contribution is small compared to 
the cushion lift, so long as the vehicle is near the ground. The lift is approximated by the 
product of cushion pressure times base area: 
L=Aps. (1-1) 
The cushion pressure also reacts against the air curtain with a force which must bal- 
ance the momentum change within the curtain. This relationship is approximated by 
Aph = pV? G(1 - sin @) (1-2) 
