GEM Research in the U.S. 275 
where the jet discharge angle 0 is taken, by convention, to be negative for the inward- 
inclined jet sketched in Fig. 1. 
The power required is the product of jet volume flow rate times compressor pressure 
rise, approximated by 
P_ = VGC(>pV? + Ap) (1-3) 
where the compressor pressure rise equals the jet total pressure and is the sum of mean 
dynamic pressure (approximated by (1/2) p V2) and mean static pressure (approximated by 
(1/2)A p). 
Hovering performance is expressed by the dimensionless figure of merit, 
Mie A= 1-4 
Dip ee S (1-4) 
which provides a direct indication of the important lift/power ratio L/P, and a direct com- 
parison with the ideal shrouded propeller or helicopter (with fixed figures of merit, outside 
ground effect, of 1.0 and \/1/2, respectively). Combining Eqs. (1-1) through (1-4), gives 
nal - sin 0) S 
Hil cin 7) or cgiaabheubl lias ankG 
1 pe 1 - sin @) 
which has a maximum value (when G/h = 1/2, 6 = —90°) of 
Ss 
Atty es (1-5) 
A more thorough analysis (see Ref. 3, for example) gives slightly different values for 
optimum nozzle width ratio G/h and jet angle 0, but almost exactly the same result for 
optimum figure of merit M,,. Practical design limitations, internal losses, etc., will limit 
actual vehicles to about M = 0.6 S/hC. 
The ratio S/AC is called the size/height ratio. The initiate to the GEM field will find 
that a few minutes devoted to firmly fixing the size/height ratio and its geometric meaning 
in his mind will be well spent. This ratio is of predominant importance in virtually all con- 
siderations of all types of ground effect machines. The size/height ratio may be thought of 
as an area ratio, between the “cushion area” S of the vehicle base and the “curtain area” hC 
of an imaginary peripheral screen sealing in the ground cushion. Or, it may be thought of as 
a length ratio. Noting that, for a circular plan form, the quantity S/C is equivalent to one- 
fourth the diameter, S/hC = D, /4h, where D, is the “equivalent diameter” of the plan form. 
Cruise 
When the simple air curtain vehicle moves horizontally in forward flight, two modifica- 
tions to the hovering equations occur. First, a “ram” drag D,gm equal to the air mass flow 
rate through the peripheral nozzle times the forward velocity, is experienced by the vehicle; 
