GEM Research in the U.S. 273 
Numerous economic and psychological reasons have been advanced to account for this 
remarkable growth of GEM activity. Certainly a major factor is the fascinating simplicity of 
the basic ground cushion concepts. The most casual technical training affords qualitative 
understanding of the basic ground cushion principles, and innumerable arrangements of 
readily available mechanical components which can produce a ground cushion come easily to 
mind. It is not really surprising that so many individuals should have felt the urge to put 
something together and putter around with it a bit. Even so, it may take something more to 
explain the scope and vigor of GEM activity —and the financial investment which it repre- 
sents — at a time when the GEM’s place in the transportation field is still highly uncertain. 
This same simplicity of the ground cushion concept occasioned many a remark of sur- 
prise, in those early days of 1957, that “someone hadn’t thought of it before.” Of course, it 
turned out that someone had, notably, Kaario of Finland, Weiland of Switzerland, Cockerell 
of England, and Frost of Canada. Even in the U.S., a rather astonishing number of GEM 
models and vehicles of various types were soon found to be already in existence in widely 
scattered back yards, basements, and garages. Nevertheless, nearly all of the many GEM 
programs now in progress in the U.S. can be traced to the NACA report of 1957 and to the 
subsequent government activity which it inspired. 
Equally as interesting as the volume of GEM work now in progress is its diversity. At 
least seven different ground cushion concepts, with almost countless combinations and 
variations, are under serious study. Sizable independent programs are being devoted to 
studies of engines, compressors, ducting problems, and structural design considerations. 
While in itself one of the most interesting aspects of the U.S. GEM picture, this diversity 
makes a comprehensive review of GEM research and development impractical. The present 
paper will be confined to a review of seven of the more significant vehicle concepts, in 
terms of simple fluid mechanics principles, with sketchy indications of the present stage of 
development and prospects of each. Sources of more detailed information will be given 
wherever possible. 
THE GROUND CUSHION CONCEPTS 
Seven ground cushion concepts have been chosen as representing, either directly or in 
combination, most of the basic ideas under active consideration in the U.S. Several of these 
concepts are at a rather primitive stage of development. In the analyses which follow, aa 
attempt is made to reduce the various concepts to their lowest common denominator, in order 
that their similarities and distinctions will be readily apparent. To this end, the analyses 
follow what is termed, throughout this paper, “simplified ideal theory.” This entails assump- 
tions of inviscid, incompressible flow, liberal application of the one-dimensional flow 
approximations, and neglect of aerodynamic pressures and forces induced by the external 
flow field. It might be argued with some justice that these analyses should be termed “over- 
simplified ideal theory.” Certainly, there is a deplorable lack of rigor, and the results are 
obviously not directly applicable to engineering problems. Nevertheless, it is believed that 
the essential features of the basic principles involved are retained in the results; and that 
the useful purposes of providing clear understanding and meaningful comparisons are served. 
In the case of the most highly developed concept, the air curtain, an engineering analysis 
(still highly simplified) will also be given, and compared to experimental results, to illus- 
trate the relation between the simplified ideal theory and physical reality. 
