4 O. H. Oakley 
The two bottom illustrations of Fig. 2 have received a great deal of attention. The lit- 
erature is too extensive for complete reference here; the International Symposium at Prince- 
ton did present, however, a substantial cross section of the efforts going on in the world. 
A large percentage of the research effort has been applied to the peripheral jet type. In 
this case, air jets are located along the periphery of the craft. When starting, the jets hit 
the water, and split, part of the jet goes inward and is trapped. The pressure in the air 
cushion is thus built up. Equilibrium is reached when the pressure in the air cushion is suf- 
ficient to bend the jets so that no more air goes inward. A simple analysis indicates that 
the required lift power for a particular craft depends on the weight of the craft times the 
velocity in the jet for a constant jet momentum. If a heavy fluid, such as water, is used in- 
stead of air, constant jet momentum implies a higher mass flow and a lower jet velocity and 
thus lower required lift power. Air, for the air cushion, must be supplied separately; the 
water curtain simply provides the seal to contain the air. 
The amount of power required to supply the air cushion is a function of many parameters; 
among the most important is how high the craft flies and the peripheral length of air seal. 
Flight over water permits the use of sidewalls extending into the water (lower left in Fig. 2). 
The air loss can be greatly reduced by this device. The penalty for this is lower speed and 
loss of ability to “fly” onto a beach. The drag of the sidewalls apparently limits this type 
to about 50 knots, beyond which the peripheral jet offers a more efficient use of installed 
power. Fig. 3 shows in a highly qualitative manner the relative speed/horsepower charac- 
teristics for the four basic types. 
L SIDEWALL 
/ PLENUM 
CHAMBER 
ae 
PERIPHERAL JET 
HORSEPOWER 
RAM WING 
SPEED 
Fig. 3. Speed-power curves for the four forms of ground-effect craft 
The problems to be solved are numerous. As implied by the previous discussion, most 
of the past efforts have gone into the determination of the mechanism and parameters affect- 
ing hovering. An examination of dynamic effects associated with forward motion has just 
recently begun. 
