Depressors are winged bodies used in water towing applications to produce a downward "lift" force to overcome the effect of tow cable drag and thus achieve the required depth of operation of the vehicle without using a body of excessive weight The general mode of operation is sketched in Figure 1

A common problem with depressors is that they tend to roll owing to slightly unequal lift forces on the two wings The equilibrium roll angle is determined by the opposing rolling moment exerted about the tow point by the body weight As illustrated in Figure 2, the vehicle will roll until the hydrodynamic rolling moment is balanced by the weight moment As a result of the roll angle the depressor force exerts a sideways component which causes the tow to be displaced laterally, known as "kiting" or "tow-off", sometimes the roll angle reaches 45" or more, with consequent large offsets of the towed body

Figure 3 shows that the novel system proposed uses a slender delta wing curved laterally about a longitudinal axis on which the tow point is situated, such that all normal force components pass through this axis and are unable to exert any significant rolling moment The gravity force acting on the depressor maintains it vertically below the tow-point Gravity is thus able to tell the depressor which way is "down" without being overcome by hydrodynamic rolling moments The curved wing bps form the fins, giving directional stability.

A feature of the fluid dynamics of these curved slender delta wings is the behaviour of the leading edge vortices, which lift away from the wing as it bends up to form the fins and give non-linear variation of the sideforce and yawing moment with sideslip angle, particularly at high incidence, as shown by wind tunnel test results(Figures is available in full paper)

NOTATION

Notation is shown in Table I(available in full paper)

TRIM AND STABILITY CONSIDERATIONS
Trim

For pitch trim the nett moment of all forces (including gravity) and moments acting about the tow point must be zero For a plate wing (either flat or curved) at modest incidene, the aero/hydrodynamic resultant force due to the combination of the pressure loads normal to the surfaces and the skin friction forces tangential to the surface becomes sensibly a force normal to the centerline. The body axial force due to skin friction is negligble in comparison to the pressure loads normal to the surface This is illustrated in Figure 5.

Stability in pitch

For stability in pitch it is necessary that if the depressor pitches slightly nose down due to a disturbance, the resulting change in moments exerted about the tow point must be in the nose-up sense.

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