294 H. R. Chaplin 
Similar demonstrations have been or could be made, from data available in the refer- 
ences, that the hovering performance (figure of merit) is also, experimentally, almost 
directly proportional to the size/height ratio in the cases of all the air curtain types, the 
water curtain type, and the plenum type. The hovering performance is less significant, in 
terms of vehicle usefulness; but this lends additional support to the essential conclusions 
of the simplified ideal theory. 
RESUME 
The simplified ideal analyses furnish valuable insight into the fundamental ideas of the 
air cushion concepts. They furnish, further, the invaluable information that all of the air 
cushion concepts have in common the property of rapidly improving performance with increas- 
ing size/height ratio. Unfortunately, they leave unresolved —as does also experience to 
date —the essential question: Which concept is best for which application? 
The following remarks are in large part merely the author’s opinions: 
Air Curtain — Most thoroughly studied, and correspondingly best understood of the air 
cushion concepts; most logical choice for early applications. 
Plenum — Simplest and cheapest to build but ranks low in performance; very poor pros- 
pects for favorable high-speed performance unless combined with another concept, such as 
ram wing. 
Ram Wing — Interesting only in combination with some other concept to provide low- 
speed flight capability; requires much additional research to evaluate practical problems and 
merits; appears to have great potential, if stability and control problems prove tractable. 
Diffuser-Recirculation System — Intriguing idea, but unlikely to prove practical. 
Water Curtain — Falls far short of theoretical expectations, but may eventually prove 
advantageous for moderate-speed over-water applications; not amphibious 
Skegs — (Previously considered in combination with air curtain, but equally suitable for 
combination with any of the other concepts.) Likely to prove advantageous for moderate- 
speed over-water applications; not amphibious. 
THE GEM’S PLACE IN TRANSPORTATION 
In every one of the ground cushion concepts considered, elementary analysis indicates 
a direct dependence of the performance on the size/height ratio. Except for the air cushion 
types, the available experimental evidence is rather limited; but every bit of data which is 
available tends to confirm this direct dependence. This alone is enough to identify the 
natural form and habitat of ground effect machines: very large vehicles operating very close 
to the earth’s surface. Since land areas are, in general, topographically unsuitable for the 
operation of large, high-speed vehicles at low heights, this means ocean-going GEM’s. A 
more specific idea of the sizes and heights involved is afforded by Fig. 11. For a GEM 
with LV, /P = 0.9 S/AC (appropriate to a highly developed air curtain vehicle) the equivalent 
lift/drag ratio is plotted in comparison with existing aircraft, and the size is given for each 
of several operating heights. The argument implied in Fig. 11 is, of course, somewhat 
superficial. There is the possibility that one of the other ground cushion concepts will 
