220 G. J. Wennagel 
of the gas turbine power plant. Air flow utilized from each port is approximately 0.20 lb/ 
sec. Natural paths, for air flow from the atmosphere, are also provided. The blunt base of 
the tail strut extends through the water surface. Similarly, air can flow from the atmosphere 
to the sides of the afterbody on the forward two struts. At high speed, therefore, only the 
parabolic forebodies of the forward struts are wetted. The cavities along the afterbody 
sides can interconnect with cavities on the upper surface of the forward cruise foil elements. 
A natural air path to the cavity regions of the forward cruise foil elements is also provided 
along the blunt trailing edges of the diagonal and cruise foils. These blunt bases are con- 
nected to each other at the intersection of the two foil elements. In addition, the cavity 
regions of the cruise foils, at high speed, open directly to the atmosphere since these ele- 
ments are surface piercing and this forms their primary source of air under cruise conditions. 
A further possibility for air supply to these regions is through vortices from the foil inboard 
tips which can extend aft and up to the water surface. 
Air from the atmosphere flows to cavity regions with velocity and associated pressure 
drop. Resultant cavity pressures are thus intermediate between vapor and atmospheric 
pressures. 
A goal of testing at Grumman Aircraft and Dynamic Developments, Inc. is to develop 
hydrofoil systems which can make a smooth and stable load transition from subcavitating 
conditions at low speeds to cavity flow conditions at high speeds. Each type of flow is 
thereby utilized over the speed range where it best affords maximum lift-to-drag ratio. Both 
surface-piercing and fully submerged foil systems are envisioned which can accomplish this 
smooth flow transition, and it is expected that several foil arrangements will eventually be 
tested on the XCH-6. - 
PERFORMANCE 
Curves of estimated vehicle drag and thrust available are plotted vs velocity in Fig. 8. 
These are typical curves which pertain to current vehicle operation at a gross weight of 
2550 pounds. In practice, both thrust available and drag curves can be altered. 
THRUST AVAILABLE AT CONSTANT 
PROPELLER TORQUE OF 2,100 IN. LB. 
800 / 
600 
DRAG AND 4np 
THRUST, LB. 
si SMOOTH WATER DRAG AT 
GROSS WEIGHT OF 2,550 LB. 
0 10 20 30 40 50 60 70 
FORWARD VELOCITY, KNOTS 
Fig. 8. Estimated smooth water drag and thrust available 
