276 H. R. Chaplin 
and the propulsion system must expend energy at the rate 7, = Dam Vo to overcome this drag 
(Fig. 1, Eqs. (1-6), (1-7). Second, while the cushion power P. is still the product of air 
volume flow rate times compressor pressure rise, the required pressure rise is now reduced 
by the amount q, of the free-stream dynamic pressure recovered by the inlet (Hq. (1-8), 
Fig. 1). 
The cruise performance is expressed by the dimensionless “equivalent lift/drag ratio” 
LV /P (Both the range of the GEM and its direct operating cost per ton-mile are directly 
proportional to the equivalent lift/drag ratio.). Combining Eqs. (1-7) and (1-8) with (1-1) and 
(1-2) and solving again for optimum G/h (= 1/2 (1 + U2)) and 6 ( = —90°) gives 
= 
0 S U 
— = 2 nt (1-10) 
es si hC Ne amiy2 
where V is the dimensionless velocity parameter K)/ /L/ ps). 
Since optimum nozzle width ratio G/h decreases with increasing forward velocity, Eq. 
(1-10) represents an envelope curve for possible designs (or possible settings of an 
adjustable-nozzle design). 
Equation (1-10) describes a curve which rises asymptotically to the value 2 S/hC as 
the forward velocity increases without limit. Consideration of the parasite drag will, of 
course, cause the equivalent lift/drag ratio to fall below this simplified ideal solution and 
to peak out at some value of V) corresponding to the “optimum cruise speed.” This, and 
other practical considerations, will be discussed further when a simplified engineering 
analysis of the air curtain vehicle is presented in a later section of this paper. It may be 
said here that actual vehicles of the simple air curtain type will probably be limited to 
equivalent lift/drag ratios of about 0.7 S/AC, at optimum cruise speeds corresponding 
roughly to V = 1.0. 
More detailed information on the simple air curtain is found in Refs. 2 to 5. 
AIR CURTAIN WITH SKEGS 
If the GEM is to operate exclusively over water, and at moderate speeds, a substantial 
power saving is effected by the use of side plates, or skegs, which extend into the water as 
sketched in Fig. 2. The equations are exactly the same as for the simple air curtain except 
that the air curtain is furnished to only the portion 2b of the total perimeter 2b + 2/. Both 
the figure of merit and the equivalent lift/drag ratio are hence improved in proportion to the 
factor 1 + 1/b (Fig. 2, Eqs. (2-1), (2-2)). 
In practice, of course, the drag on the submerged portion of the skeg becomes very sig- 
nificant at high speeds. The exact breakeven point between the simple air curtain and the 
air curtain with skegs depends on the roughness of the water surface (which determines the 
minimum skeg submersion necessary to effect a seal). It is generally felt, however, that the 
application of submerged skegs is confined to speeds below 50 knots. 
A compromise between the simple air curtain and the air curtain with skegs is also 
being studied wherein side plates extend below the base part way to the water, with air 
curtains issuing from nozzles at the lower extremities of the side plates. The total curtain 
