GEM Research in the U.S. 281 
staggering amount. It is obviously necessary to compromise in favor of a much, much 
thinner nozzle. In both of the cases cited, simplified ideal theory gives the power as pro- 
portional to the quantity 
1 A 
Vipt-ae 25 
h 
The compromise to thinner nozzles thus destroys much of the theoretical advantage. (For 
example, if G/h = 1/200 instead of 1/2, the power required is increased by a factor of five, 
leaving the water curtain with roughly a 6:1 theoretical advantage over the air curtain com- 
pared to the original 29:1 advantage.) Furthermore, experiments show that a thin water 
curtain provides an imperfect seal, so that air must be pumped into the cushion at a quite 
substantial rate to maintain the cushion pressure. This makes further serious inroads into 
the theoretical advantage. Just how much advantage is left, if any, is not quite clear at 
present. Certainly, the water curtain concept should not be discounted without careful 
study, and a vigorous study program is, in fact, underway. 
More detailed information on the water curtain concept is found in Ref. 8. 
PLENUM 
The plenum is by far the simplest of the ground cushion concepts, both in principle and 
in physical embodiment. The plenum concept is represented schematically in Fig. 5. The 
vehicle has a recessed base. Air is simply pumped into the recess and allowed to leak out 
along the ground. 
Hover 
Neglecting internal velocities in comparison to the full-expansion velocity, V_, gives 
Lift EL = ApS (5-1) 
Cushion pressure Ap = + p v2 (5-2) 
Cushion power cE = VD he Ap (5-3) 
Ao Ss 
Fj f merit MzVWaas 5-4) 
igure of meri 2 2D, hC ( 
where D, is the discharge coefficient of the leakage flow, D, AC being the flow area at full 
expansion. Using a typical value of 0.61 for D, gives 
S 
M=0.58 Fe: (5-5) 
