284 H. R. Chaplin 
P = DranVo (6-3) 
“. 
LVy_\., See 
p -1AGe —- - (6-4) 
This elementary solution gives equilibrium flight at only one speed, corresponding to 
Vo 
= —————.- / 2. = 
i YL7 (eS) 2 (6-5) 
In practice, one of the other ground cushion concepts must be combined with the ram wing to 
maintain the ground cushion during acceleration to and deceleration from this equilibrium 
speed. For equilibrium at speeds higher than given by (6-5), the height 4 must increase 
until the cushion pressure falls to something less than the full ram pressure. Even at V = 2, 
the lift contributed by suction pressure induced on the upper surface of the wing (neglected 
by the simplified ideal theory) is significant, but it does not dominate the problem. At 
higher speeds this suction-pressure lift does begin to predominate, and the simplified ideal 
theory must be abandoned in favor of approaches along the line of conventional wing theory. 
It is interesting to note, from Eq. (6-4), that the ram wing, like the conventional aircraft 
wing, gives much better performance with high aspect ratio, b/I. 
There has been practically no development effort devoted to the ram wing in the U.S. 
However, several U.S. groups have recently begun study programs, and a rapid expansion of 
activity in this area is not unlikely. 
Kaario’s discussion of the ram wing concept is found in Ref. 11. 
DIFFUSER-RECIRCULATION SYSTEM 
From an academic point of view, the diffuser-recirculation system is perhaps the most 
interesting of all the ground cushion concepts. It is the only one which can, in principle, 
sustain a vehicle at a finite height above the ground without dissipating any power. 
The diffuser-recirculation system is represented schematically in Fig. 7. A recirculat- 
ing flow, rather like a standing ring vortex, is maintained within and under the vehicle. At 
the periphery of the base, the flow passes through a nozzle G, and is then exposed to atmos- 
pheric pressure until it passes through the gap & between the underside of the base and the 
rim. The static pressure at the gap A is thus essentially atmospheric. The geometry of the 
base and internal passages is so arranged that the highest velocity (and thus the lowest 
pressure) of the flow occurs at the periphery, where the flow is exposed to atmospheric 
pressure. The average static pressure under the vehicle is therefore higher than atmospheric, 
giving a net lift force on the vehicle. Under the assumption of inviscid flow, this is a 
closed system with no energy dissipation. Simplified ideal theory thus gives 
M = 1 Lyk .. 
oor i apie taal Slows 
