GEM Research in the U.S. 283 
S U 
—s— = 1.16 —~—: ———_. (5-10) 
m 1+50? 
Compared with the corresponding air curtain results, the plenum concept falls consid- 
erably short, both as to figure of merit and (especially) equivalent lift/drag ratio. Never- 
theless, it has its supporters among GEM investigators, because of its extreme simplicity. 
Of the GEM concepts, the plenum is the most compatible with cheap, lightweight, rugged 
construction. 
More detailed information on the plenum concept is found in Refs. 9 and 10. 
RAM WING 
The ram wing is believed to be the oldest of the ground cushion concepts, having been 
introduced in Finland by Kaario as early as 1935. The ram wing concept is represented 
schematically in Fig. 6. The vehicle takes the form of a box with the bottom and front side 
removed. The basic ram wing has no hovering capability, but if it moves forward very close 
to the ground (so that velocities within the ground cushion are negligible), the ram pressure 
pV7 /2 builds up beneath the base, giving a lift force 
L= 5 pves. (6-1) 
Simplified Ideal Theory: 
Cruise 
= %pV2 1b 
= pV, (21AD,) Vo 
Fig. 6. Ram wing 
Only the air which leaks out at the tips contributes to the ram drag, since the air leaked out 
the rear recovers it original rearward momentum. Assuming the discharge coefficient D, to 
be 0.61, 
Dram = PVo (21hD,) VY (6-2) 
