288 H. R. Chaplin 
G 
P. 1 (1 - sin AM ett 1 
L3/2//' 6S int 
4 
; 3/2 Cr % 
(1 ee v7 - nq? (1 — v4 (3-8) 
' S : G 
2 (1 =*sin sane (1 -- sin 2 lee. cos? B 
D 
. Vv (8-9) 
a ae - sin @) = Ce. 
2 
(1 - sin @) 74 
aS 
(Ss 
i} 
Ss 
+ 
tan “L 21 
an 
sg ae ie ac Sate (1 = 2 = (8-10) 
(ths "Sai G3) = e 
Ac A 
LV P_.P 
Svan peed SePeele (8-11) 
P L3/2// 0S 
These last four equations form a system from which realistic estimates of air curtain per- 
formance (whether simple, integrated, or semi-integrated air curtain) can be made — if esti- 
mates of the parameters 74, Cy 9, and Cp¢ can be obtained. The problem of providing a 
reliable basis for estimating these three parameters occupies much of the systematic GEM 
research now underway in the U.S. (The internal-, duct-, and propulsive-efficiency problems 
are, of course, age-old problems, not peculiar to GEM’s, and highly developed techniques for 
dealing with them are available in the literature.) Some progress has been reported in the 
references. A good deal of additional progress can be expected in the immediate future. 
It will suit our present purposes to side-step the question of how these estimates can 
be made, and to use, for our discussion, a source of data for which the necessary param- 
eters have been evaluated to some reasonable degree of accuracy, experimentally. Such a 
source is Ref. 5, which presents data from wind-tunnel tests of the David Taylor Model 
Basin’s Gem Model 448. This model represents the nearest approach to a realistic, 
practical GEM configuration for which laboratory-controlled performance test data are 
presently available. 
Photographs of the model are presented in Fig. 8. The sketch in Fig. 3, used to illus- 
trate the integrated air curtain concept, is also representative of the geometric arrangement 
