The invention relates to bodies towed behind a ship. These objects
are commonly referred to as towfish. They are notably used in the field of
variable-submersion sonars. The towfish is then provided with acoustic receive
and/or transmit antennas.
5 In order to correctly fulfill its mission, the towfish is provided with
suitable hydrodynamic characteristics. Amongst other things, the towfish is
configured to develop significant vertical forces which allow it to dive to a
desired depth despite the towing speed which has a tendency to cause it to
rise back up to the surface.
10 The simplest means for developing these vertical forces is to
increase the weight of the towfish beyond the upthrust. The weight has the
advantage of being constant regardless of the speed of the ship towing the
towfish. Another means is to arrange on the towfish a set of hydrodynamic
airfoils the lift of which is directed downward. Given the density of the water,
15 these airfoils are fairly squat and can easily be housed on a towed body. The
downwardly directed hydrodynamic lift force increases with the square of the
speed of the towfish with respect to the water. As a result, the higher the speed
the greater the extent to which the towfish remains submerged.
Towed sonars are used chiefly in the military domain. It may be that
20 the ship operating with its sonar submerged to a depth has to suddenly take
evasive action, for example if it becomes hunted by a torpedo. In that case,
given the urgency, it is not possible to bring the towed body back on board.
The evasive speed of the ship needs to be able to be withstood both by the
towed body and by the towfish towing cable; the cable experiences a
25 hydrodynamic drag that increases with the speed.
The cable needs to be rated to withstand a maximum tension
generated by the ship for a given evasive speed. Other components, such as
the towing machine and its fixings that attach it to the deck of the ship need
also to be rated to withstand this maximum tension. A high evasive speed
30 requires all the elements involved in the towing operation to be overrated. The
overrating of the cable leads to its dimensions being increased, further
increasing its hydrodynamic drag and therefore the tensile load on the cable.
2
While the ship is running evasive action, in order to limit the tension
in the cable, one maneuver is to bring as much cable as possible back on
board. In order to allow such a maneuver, the towing winch also needs to be
overrated.
5 When the towed body is submerged and it is not possible to operate
the winch there are two remaining solutions. The first is to limit the evasive
speed of the ship and the second, in extreme circumstances, is to cut the
towing line to prevent ill-timed breakage. That of course leads to the loss of
the towed body.
10 Patent application WO 2016/135326 filed in the name of the
applicant company describes a towfish equipped with an airfoil that gives it a
downwardly oriented lift allowing it to remain at depth when towed. The towfish
is equipped with a latch allowing the airfoil to be released and the lift of the
towfish reduced. Once the latch has been released, the towfish rises back up
15 to the surface and the pull of the cable decreases, allowing the ship that is
towing the towfish to increase its speed. The control of the latch is rudimentary,
giving it great simplicity and therefore good reliability. Once the latch has been
released, the towfish maintains its reduced lift. When the towfish is being towed
it is not possible to reset the latch to give the towfish its downwardly oriented
20 maximum lift. Thus, once the latch has been triggered to reduce the lift of the
towfish, the submersion of the towfish decreases so that the towfish is no
longer able to continue its mission in full at the required depth. In order to reset
the latch, it is necessary to bring the towfish back onboard the deck of the ship
to lock the airfoil in the maximum-lift position. Only then can the towfish be
25 returned to the water to resume its mission.
The invention seeks to alleviate all or some of the abovementioned
problems by proposing in-water reversible means for reducing the lift of a
towed towfish. More specifically, the invention proposes to control the lift of the
30 towfish by means of the angle of the towing cable with respect to the towfish.
To that end, the subject of the invention is a towfish intended to be
submerged and towed by a cable, the towfish comprising a structure
configured to move through the water in a horizontal main direction and at least
35 one appendage configured to generate on the towfish a downwardly directed
3
hydrodynamic lift when the towfish is moving through the water under the effect
of the towing, the appendage being orientable so as to modify its lift. According
to the invention, the towfish comprises a bracket capable of rotational
movement with respect the structure about a horizontal axis perpendicular to
5 the horizontal main direction, the cable being intended to be attached to the
bracket. The towfish comprises a non-motorized mechanism configured so
that an orientation of the appendage, allowing it to alter the lift, is dependent
on an angle formed between the bracket and the structure defined on the
basis of the horizontal main direction. A law connecting the angle to the
10 orientation of the appendage is configured so that when the value of the angle
decreases, the orientation of the appendage is increased in such a way as to
reduce the hydrodynamic lift of the towfish.
Advantageously, over a range of values for the angle the law is
unstable so that a given value for the orientation of the appendage leads to a
15 reduction in the angle.
In one preferred configuration, beyond a first given angle value, the
orientation of the appendage is fixed so as to generate what is referred to as
the operational lift of the towfish, and below the first given angle value, the
orientation of the appendage is increased as the value of the angle decreases
20 so as to reduce the lift compared with the operational lift.
Advantageously, below a second given angle value less than the
first given angle value, the orientation of the appendage is fixed so as to
generate what is referred to as an evasion lift lower than the operational lift.
Below the second given angle value, the orientation of the
25 appendage may be positive or zero.
There is defined a third angle value intermediate between the first
and second angle value, and, between the first and the third angle value a law
connecting the angle to the orientation of the appendage (27) is
advantageously configured to keep the angle at a stable value.
30 The range of values for the angle in which the law is unstable is
defined between the third and the second angle value. Over this range, the law
is advantageously configured in such a way as to orient the appendage in order
to achieve the evasion lift.
The towfish may comprise a lift-inducing airfoil forming the
35 appendage, and a stabilizing empennage configured to keep a pitch attitude
4
of the towfish substantially constant during changes to the orientation of the
appendage.
The appendage may be able to move in rotation with respect to the
structure about a second horizontal axis, the mobility of the appendage
5 allowing the lift of the towfish to be modified, and the second horizontal axis of
rotation of the appendage is positioned substantially at the instantaneous
center of rotation of the towfish when the latter pivots as a result of a change
in the orientation of the appendage.
The towfish advantageously comprises a cam and a cam follower,
10 one being secured to the appendage and the other to the bracket and the cam
follower presses against a shape on the cam as the bracket rotates about the
horizontal axis.
The towfish advantageously comprises a first latch configured to
keep the appendage in a fixed orientation when the value of the angle is
15 beyond the first given angle value.
The towfish advantageously comprises a second latch configured
to immobilize the bracket with respect to the structure when the value of the
angle formed between the bracket and the structure is below the second given
angle value.
20
The invention will be better understood and further advantages may
be apparent from reading the detailed description of one embodiment given by
way of example, which description is illustrated by the attached drawing in
which:
25 figure 1 schematically depicts a ship towing an active sonar;
figure 2 depicts a towed body of the sonar of figure 1;
figure 3 depicts the ship of figure 1 towing the towed body in two
hydrodynamic configurations of the towed body;
figures 4 and 5 depict one example of the control law controlling the
30 angle of orientation of an appendage of the towfish as a function of an angle
of a bracket of the towfish to which bracket the towing cable is attached;
figure 6 depicts a first embodiment of the control of the orientation
of the appendage as a function of the angle of the bracket;
figures 7a, 7b and 7c depict a second embodiment of the control of
35 the orientation of the appendage as a function of the bracket angle;
5
figures 8a to 8f depict various orientations of the appendage as a
function of the angle of the bracket for the second embodiment.
For the sake of clarity, throughout the various figures the same
elements will bear the same references.
5
The invention is described with respect to the towing of a sonar by
a surface vessel. It must of course be understood that the invention can be
implemented in respect of other towed elements and other towing means.
Figure 1 depicts a ship 10 towing an active sonar 11 comprising an
10 acoustic transmit antenna 12 commonly referred to as a towfish and an
acoustic receive antenna 13 commonly referred to as a streamer. The sonar
11 also comprises a cable 14 allowing the two antennas 12 and 13 to be towed.
The cable 14 also carries the signals and power supplies between the ship and
the antennas 12 and 13 of the sonar 11. It is also possible to provide two
15 distinct cables, one for towing the towfish 12 and the other for the streamer 13,
the cable towing the streamer then being attached to the towfish 12. The
invention relates more particularly to the towfish 12 and can be employed
without a streamer 13.
The antennas 12 and 13 are mechanically anchored and electrically
20 and/or optically connected to the cable 14 in a suitable manner. In the
conventional way, the receive antenna 13 is formed of a linear antenna of
tubular shape identical to those found in passive sonars, likeable to the shape
of a flute, while the transmit antenna 12 is incorporated into a bulky structure
of a shape likeable to that of a fish. The receive streamer is generally
25 positioned to the rear, at the end of the cable 14, the towfish being positioned
on the part of the cable 14 closest to the ship 10. During an underwater
acoustic mission, the antenna 12 emits sound waves into the water and the
receive antenna 13 picks up any echoes bouncing back from targets on which
the sound waves from the antenna 12 are reflected.
30 The launching and retrieval of the antennas 12 and 13 is performed
using a winch 16 positioned on a deck 17 of the ship 10. The winch comprises
a drum 18 rated to allow the winding of the cable 14 and of the receive antenna
13. The winch 16 also comprises a chassis intended to be attached to the deck
of the ship. The drum 18 is able to pivot with respect to the chassis so as to
35 allow the hauling-in of the cable. The hauling-in of the cable 14 allows the
6
towfish 12 to be hauled onboard the ship 10, for example onto a rear platform
19 provided for that purpose.
A fairlead 20 guides the cable 14 downstream of the drum 18. The
fairlead 20 constitutes the last element guiding the cable 14 before it descends
5 into the water. The cable 14 for example comprises a core made up of
electrical and/or optical conductors transmitting power and data between the
sonar equipment situated on board the ship 10 and the antennas 12 and 13.
The core of the cable 14 is generally covered with stranded metal wires
providing the cable 14 with its mechanical integrity, notably tensile strength.
10 The cable 14 may be covered with scales configured to adapt its hydrodynamic
profile in order to limit its drag.
Figure 2 depicts the towfish 12 in profile. The towfish 12 comprises
a bracket 21 configured to allow the towfish 12 to be attached to the cable 14.
15 The bracket 21 comprises a mechanical connector, possibly removable, and,
if need be, a connector, for example an electrical or optical connector, for
passing data and/or power between the towfish 12 and the cable 14. The
towfish 12 is configured to move through the water in translation in a direction
25 represented as being horizontal in figure 2. The towfish 12 comprises a
20 structure 23 and at least one airfoil 27 allowing a downwardly directed
hydrodynamic lift P to be generated on the towfish 12 when the towfish 12 is
moving in the direction 25. The towfish 12 itself and other appendages of the
towfish 12 contribute to generating the overall hydrodynamic lift of the towfish
12. The airfoil 27 chiefly generates the overall hydrodynamic lift of the towfish
25 12. The airfoil 27 for example has a profile that is symmetrical about a direction
28. In order to generate the lift P, the direction 28 of the airfoil 27 is inclined by
an angle γ with respect to the direction 25. The angle γ is oriented negatively
in the counterclockwise direction in order to direct the lift P downward. The
towfish 12 may also comprise an empennage 29 situated toward the rear of
30 the towfish 12 according to the direction in which the towfish travels in the
direction 25. The empennage 29 ensures the stability of the movement of the
towfish 12 in the direction 25.
The bracket 21 is able to move in rotation with respect to the
structure 23 about a horizontal axis 30 perpendicular to the main direction 25.
35 The axis 30 is perpendicular to the plane of figure 2. The horizontal axis 30
7
and the direction 25 are defined with respect to the exterior shapes of the
towfish 12 orienting it in the water when it is being towed by the cable 14. The
towfish 12 for example comprises a pivot connection 31 articulating the bracket
21 and the structure 23 to one another. The pivot connection 31 allows the
5 bracket 21 to rotate with respect to the structure 23 about the horizontal axis
30. The bracket 21 for example has a sleeve through which the cable 14 is
passed. The sleeve extends along an axis 32. In the vicinity of the bracket 21,
the cable 14 also extends along the axis 32. More specifically, the cable 14
and the bracket 21 are secured to one another. The connection between the
10 cable 14 and the bracket 21 is of the fully restrained connection type also
known as a built-in connection. An angle β is defined between the axis 32 and
the main direction 25 about the horizontal axis 30. In the vicinity of the towfish
12, the cable 14 makes an angle β with the horizontal main direction 25.
The connection between the bracket 21 and the structure 23 may
15 comprise more than one degree of freedom in rotation. It may involve a pinned
ball joint with two degrees of freedom or a ball joint with three degrees of
freedom. For implementing the invention, only the angle β defined about the
horizontal axis 30 is taken into consideration.
As it moves in the direction 25 the towfish is subjected to various
20 forces aside from the lift P: its weight G and the upthrust FA, both represented
by the same vector in figure 2, its drag T and the traction C exerted by the
cable 14. In order not to overload figure 2, it is assumed that the antenna 13 is
absent. The potential presence of an antenna 13 will increase the drag P. To
a first approximation, when the towfish 12 is advancing at constant speed in
25 the direction 25, the vector sum of the various forces to which it is subjected is
zero. In order to ensure static equilibrium of the forces exerted on the towfish,
the angle β satisfies the following relationship:
β = arctan(
G−FA+P
T
) (1)
30
The towfish 12 comprises means for modifying the hydrodynamic
lift of the airfoil 27 and therefore the overall hydrodynamic lift P of the towfish
12. The lift P is considered here to be positive when directed downward. During
operational use, the towfish 12 has a lift referred to hereinafter as the
35 operational lift. This lift is provided by an angle γ of inclination of the airfoil 27
8
oriented downward. The sign of the angle γ is considered to be positive in the
counterclockwise direction. The angle γ therefore has a negative value, for
example of the order of -8°, in order to direct the lift P downward and allow the
towfish 12 to perform its mission under operational conditions. Naturally, this
5 angle value can be adapted according to the desired lift and according to the
profile of the airfoil 27.
In order to reduce the traction C that the cable 14 has to absorb in
order to tow the towfish 12, the lift P of the towfish 12 is reduced by modifying
the angle γ. Specifically, by reducing the lift P, the vector sum of the forces
10 exerted on the towfish 12 to balance the traction C is reduced, as therefore is
the modulus of the traction C.
The reduction in lift P can be brought about by means of a onepiece airfoil the inclination of which can be modified. It is also possible to attach
a mobile flap to the end of a fixed airfoil.
15 The reduction in lift may of course go so far as to cancel the lift and
even render same negative, namely directed upward and tending to cause the
towfish 12 to rise back toward the surface. However, a lift that is excessively
negative would tend to increase the traction C on the cable 14. A raising of the
towfish 12 may offer an advantage if a raised bottom appears.
20 For a given angle γ, the lift P and the drag T are functions of the
speed V of the towfish 12 in the direction 25. This speed is, to a first
approximation, equal to the speed of the ship 10 when the towfish 12 is in
equilibrium. The more the speed P increases, the greater the lift P and drag T.
In order to maintain equilibrium between the forces exerted on the towfish 12,
25 the algebraic value of the traction C increases and the angle β increases also.
For a given angle γ, there is a function connecting the angle β and the algebraic
value of the traction T. In practice, the angle β varies according to the lift P and
drag T. The dimensions of the towfish 12 are defined in such a way that the
angle β varies as a function of the speed V. In the case illustrated, when the
30 speed V increases, the algebraic value of the drag T increases more than the
algebraic value of the lift P. Thus, when the speed V increases, the angle β
decreases. Alternatively, it would be possible to define a towfish such that
when the speed V increases, the algebraic value of the drag T increases less
than the algebraic value of the lift P.
9
The invention takes advantage of this relationship to control the
orientation γ of the airfoil 27 as a function of the angle β. By reducing the angle
γ and thereby the lift P of the airfoil 27, in order to keep the towfish 12 in
equilibrium, the modulus of the traction C is reduced.
5 In other words, according to the invention, the orientation of the
airfoil 27, defined by the angle γ, is a function of an angle formed between the
bracket 21 and the structure 23 about the horizontal axis 30. For the sake of
simplification, this angle will be considered to be the angle β between the cable
14 in the vicinity of the towfish 12 and the main direction 25. In practice, the
10 angle between the bracket 21 and the structure 23 which is the angle used to
modify the orientation of the airfoil 27 may be offset by a fixed value according
to the configuration of the means of attachment of the cable 14 to the bracket
21. To ensure correct operation of the invention, a variation in the angle β leads
to a variation in the orientation of the airfoil 27.
15
Figure 3 depicts two configurations in which the ship 10 is towing
the towfish 12. The first configuration is the operational configuration and the
towfish 12 is identified as 12-1. In the operational configuration, the airfoil 27
provides a downwardly-directed lift. The vector sum of the forces due to the
20 weight G, to the upthrust FA, to the drag T and to the lift P is identified 35-1.
This vector sum is opposing the traction C of the cable 14 on the towfish 12.
At the ship 10, the resultant 35-1 leads to a traction force 36-1 on the cable 14.
The second configuration is referred to as the evasion configuration, and the
towfish is identified 12-2. In the evasion configuration, controlled by a variation
25 in the angle β, the lift of the airfoil 27 has been reduced. The vector sum of the
forces due to the weight G, to the upthrust FA, and to the drag is identified 35-2.
It is considered that, in the evasion configuration, the lift P is zero. At the ship
10, the resultant 35-2 leads to a traction force 36-2 on the cable 14.
The modulus of the traction force 36-2 is less than the modulus of
30 the traction force 36-1. If the entire towing setup (winch, cable and towfish) is
rated to operate with a force 36-1 at a given speed for the ship 10, the act of
reducing the lift of the towfish 12 allows the given speed to be increased until
a force 36-2 is reached of which the modulus is equal to the modulus of the
force 36-1 at the lower speed.
10
Figure 2 depicts just a single airfoil 27 on one side of the towfish 12.
It is conventional practice for the towfish 12 to comprise two airfoils 27 each
positioned symmetrically with respect to the direction 25. In that case, the
means for modifying the orientation γ apply advantageously to the two airfoils
5 27 in a coordinated manner. More generally, the towfish 12 may comprise
more than two airfoils all generating a downwardly directed lift. The invention
is already beneficial in reducing the lift on one of the airfoils. Conversely, the
towfish 12 could comprise just a single airfoil, for example positioned on the
nose of the towfish 12. The invention then consists in reducing the lift of this
10 single airfoil.
In figure 2, the airfoil 27 is able to move in rotation with respect to
the structure 23 about a horizontal axis 33 parallel to the axis 30 of rotation of
the bracket 21. The empennage 29 of the towfish 12 is in this instance fixed
with respect to the structure 23. This configuration allows the overall pitch
15 attitude of the towfish 12 to be maintained when the angle γ is modified. The
directions of the weight G and of the upthrust FA are unchanged with respect
to the structure 23. The towfish then finds itself in the best conditions for
withstanding an increase in its speed. Alternatively, it is possible to keep one
airfoil fixed with respect to the structure 23 and to act on the orientation of the
20 empennage in order to modify the lift of the airfoil. This mobile-empennage
configuration limits the forces that need to be exerted on the mobile
appendage, in this instance the empennage, in order to reduce the lift of the
towfish 12.
The appendices intended to vary the pitch attitude of an object
25 traveling through a fluid, through air or through water, are usually articulated
about an axis situated in the vicinity of their leading edge, in order to ensure
their stability. To implement the invention, it is advantageous to position the
axis 33 of rotation of the airfoil 27 with respect to the structure 23 substantially
at the instantaneous center of rotation of the towfish 12 as it pivots as a result
30 of a modification to the orientation γ of the airfoil 27. This arrangement limits
the torque that has to be applied in order to cause the airfoil 27 to turn.
Figures 4 and 5 depict one example of a control law controlling the
angle γ of orientation of the airfoil 27 as a function of the angle β of the bracket
21 with respect to the structure 23. In these two figures, the angle β is
35 represented on the abscissa axis and the angle γ on the ordinate axis. For this
11
law, figure 4 represents the change in angle γ as the angle β decreases and
figure 5 as the angle β increases. The law is depicted on an array of curves
each representing, at a given speed V, the angle β that the bracket 21 naturally
adopts as a function of the angle γ of orientation of the airfoil 27 in the absence
5 of a control law. In other words, for a chosen speed V and a chosen angle γ,
the various forces exerted on the towfish 12 balance one another for a given
bracket angle β. These curves and the associated numerical values are given
only by way of example and are dependent on the geometry and weight of the
towfish 12. In figures 4 and 5, the array of curves is represented in steps of 1
10 knot between 13 knots and 20 knots. It will be recalled that a knot is equivalent
to approximately 0.514 m/s. Knots are used here rather than the si units for
speed because knots are more commonly used in the maritime domain.
The law is defined in such a way that, between the two extreme
speeds of 13 and 20 knots, there is a transition between the operational lift
15 value and the evasion lift value.
For example, for a speed of below 16 knots, the airfoil 27 is oriented
downward in order to obtain operational lift. In figures 4 and 5, the angle γ is
set at a first value, for example a negative value of -10.5°, in order to obtain
the operational lift. For a speed lower than 16 knots, the bracket angle is
20 greater than 52° and the angle γ remains constant at the value of -10.5°. The
law is defined in such a way as to reduce the angle γ when the bracket angle
drops below 52°. The threshold at which the reduction in the lift of the airfoil 27
is triggered is set at 52°. This threshold bears the reference 41 in figure 4. In
other words, because there is a relationship between the bracket angle β and
25 the speed V, the angle γ is reduced when the speed becomes higher than
16 knots.
A second threshold 42 for the angle β is also defined, and below
this threshold the angle γ of orientation of the airfoil 27 is fixed at a second
value higher than the value defined when the bracket angle β is above the first
30 threshold 41. The second angle value γ may remain negative or positive or
zero. In the example illustrated, the second value is 0°. A positive value may
offer the benefit of generating an upward lift and of partially compensating for
the weight G. In the example depicted, at the threshold 42, the value of the
angle β is 19°. At this value, the lift is reduced compared with the operational
35 lift. This lift may be referred to as the evasion lift.
12
Between the two thresholds 41 and 42, the control law controlling
the angle γ may adopt various forms. It is for example possible to define stable
and unstable parts of the control law. A part of the law is said to be stable for
a given value of β when the derivative of the function γ = f(β) is lower than the
5 derivative of the speed curve in the β-γ frame of reference. Conversely, a part
of the law is said to be unstable, for a given value β, when the derivative of the
function γ = f(β) is greater than the derivative of the speed curve in the β-γ
frame of reference. In a stable part of the law, when the towfish 12 passes
through a given speed V, there is the same value for the angles β and γ both
10 when the speed is increasing and when the speed is decreasing. By contrast,
in an unstable part of the control law, when the towfish is accelerating and
reaches a given speed, the angle γ of orientation of the airfoil 27 is such that
the angle β of the bracket 21 has a tendency to reduce automatically, at the
same time leading to an increase in the angle γ until a stable part of the control
15 law is reached.
In the example depicted, in figure 4, when the angle β decreases,
namely when the speed of the towfish 12 increases above 16 knots, the control
law is stable up to a bracket angle of 40° corresponding to a speed of 19 knots.
This stable part is identified 43 in figure 4. Beyond the bracket angle β of 40°,
20 the control law enters an unstable part 44. In other words, if, at 19 knots, the
towfish 12 tends to accelerate, the angle γ of orientation of the airfoil 27 tends
to increase together with a reduction in the angle β of the bracket 21 until the
threshold 42 is reached, keeping the 19-knot speed substantially constant,
give or take its acceleration. In the example illustrated using figure 4, the
25 threshold 42 corresponds to a bracket angle β of 19° and to an angle of
orientation γ of the airfoil 27 of 0°. The threshold between the stable and
unstable parts is identified as 45 in figure 4. The unstable part of the curve
allows the lift of the airfoil 27 and the bracket angle β to be reduced rapidly.
The modulus of the traction C also decreases rapidly, thereby allowing the ship
30 10 to accelerate further for better evasion.
In other words, in the stable part of the control law, in the β-γ frame
of reference, the gradient of the curve defining the control law is shallower than
the gradient of each of the curves of the bundle defining the equilibrium of the
towfish 12 as a function of the angles β and γ. More specifically, the curve
35 defining the control law intercepts a number of curves of the bundle and, at
13
each intersection, the gradient of the curve defining the control law is shallower
than the curve of the bundle. By contrast, in the unstable part of the control
law, the gradient of the curve defining the control law is steeper than the
intercepted curve of the bundle. When applied to the example of figure 4, when
5 the speed increases and reaches 19 knots, the towfish 12 automatically rises
back up until the point of equilibrium defined by the threshold 42 is reached. In
practice, the towfish 12 can take several minutes to pass through the unstable
part and cross from the point 45 to the point 42 as a result of its inertia.
If the ship 10 continues to accelerate beyond 19 knots, the angle β
10 of the bracket 21 will continue to decrease, but the angle γ of orientation of the
airfoil 27 remains fixed at a value of 0°. The ship can exceed 20 knots with a
reduced traction force on the cable 14 compared with the traction force that
would be generated with an operational lift obtained with an angle γ of
orientation of the airfoil 27 of -10.5°.
15 When the speed of the towfish 12 is decreasing, reference is made
to figure 5. The control law γ = f(β) is, in the example considered, the same for
acceleration and for deceleration. It is also possible to define different control
laws for acceleration and deceleration, depending on the desired effects.
As long as the speed of the towfish 12 is above 16 knots, the angle
20 γ of orientation of the airfoil 27 remains at 0°. At 16 knots, equilibrium of the
towfish 12 is obtained for β = 25° and γ = 0°. This point of equilibrium is
identified 47 in figure 5. When, at 16 knots, the towfish 12 begins to decelerate,
the angle γ of orientation of the airfoil 27 tends to decrease while at the same
time the angle β of the bracket 21 tends to increase until the threshold 41 is
25 reached, with the speed of 16 knots remaining substantially constant, give or
takes its deceleration. When the speed of the towfish 12 is decreasing, the
control law between the two angles γ of orientation of the airfoil 27 from 0° to
-10.5° is completely unstable. Specifically, in the transition from the point 47 to
the point 41, the curve defining the control law intercepts the curves of constant
30 velocity higher than 16 knots of the bundle. Because the speed of the towfish
12 is lower than the curves of the bundle through which curves the control law
curve passes, the towfish 12 automatically tends toward the point 41 where
β = 52° and γ = -10.5°.
The unstable part of the control law provides hysteresis between
35 acceleration and deceleration. More specifically, during acceleration, the
14
speed of 19 knots needs to be achieved in order to enter the unstable part and
reach the threshold 42. By contrast, in deceleration, as long as the speed
remains above 16 knots, the orientation γ of the airfoil 27 remains at its value
of 0°.
5 In the example depicted, the control law comprises a stable part and
an unstable part. It is possible to define the entirety of the law in a stable
domain. There is then no hysteresis. Conversely, it is possible to define the
control law so that it is completely unstable between the two thresholds 41 and
47, and this makes it possible to reduce the traction on the cable more quickly.
10 A control law defined as being completely unstable has a tendency to increase
the hysteresis. Too much hysteresis would require the ship 10 to slow too
much in order to recover operational lift.
It is preferable to avoid defining a control law that follows one of the
curves of the bundle, namely between the stable and unstable domains. This
15 is because if it did, and the ship 10 maintained that speed, there would be a
risk of the airfoil 27 oscillating between its two threshold orientations.
Figure 6 describes a first embodiment of the control of the
orientation of the airfoil 27 as a function of the angle β. The towfish 12
20 comprises an angle sensor 50 for determining the angular position of the
bracket 21 with respect to the structure 23. The sensor 50 is, for example, a
resolver positioned in the pivot connection 31. Any other type of angle sensor
can be used. The towfish 12 also comprises a computer 51, a memory 52 and
an actuator 53 for modifying the orientation of the airfoil 27 about its axis 33.
25 The actuator 53 may be an electric stepping motor orienting the airfoil 27. It is
possible to use a microcontroller to perform the functions of the computer 51
and of the memory 52. The computer 51 is configured to control the actuator
53 as a function of the angle β determined by the sensor 50. The memory 52
contains the control law controlling the actuator 53 as a function of the angle β
30 and a program for implementing the control law.
The chief benefit of the first embodiment lies in its operational
flexibility. Specifically, it is possible to modify the control law easily by replacing
the contents of the memory 52. By contrast, the presence of electron
components may impair the reliability of this embodiment.
35
15
Figures 7a, 7b and 7c describe a second embodiment of the control
of the orientation of the airfoil 27 as a function of the angle β. The second
embodiment is fully mechanical without motorized drive. It is only the
hydrodynamic forces (drag and lift), the force of gravity, the traction on the
5 cable and the upthrust applied to the towfish 12 that allow the orientation γ of
the airfoil 27 to be modified as a function of the angle β. The towfish 12
comprises a cam and a cam follower. In the example depicted, the cam is
secured to the airfoil 27 and the cam follower is secured to the bracket 21.
Alternatively, it is also possible to provide a cam secured to the bracket 21 and
10 a cam follower secured to the airfoil 27. The cam follower presses against a
shape on the cam as the bracket 21 rotates about the horizontal axis 30. The
shape of the cam defines the control law γ = f(β).
The cam and the cam follower are advantageously positioned inside
the structure 23 so that this mechanism does not alter the hydrodynamic
15 shapes of the towfish 12. Figure 7a is a perspective depiction of the bracket
21, two airfoils 27 together with the cam and cam follower. For ease of
understanding, the structure 23 is not depicted. Figure 7b is a perspective
depiction of the cam secured to the airfoils 27 and figure 7c depicts the cam
follower secured to the bracket 21.
20 In the example depicted, the cam is formed by two symmetrical slots
60 and the cam follower is formed by two pins 61, each one guided in one of
the slots 60. It was seen earlier that the axis 33 of rotation of the airfoil 27 with
respect to the structure 23 may be positioned so that it is substantially secant
with a vertical axis 34 bearing the lift force generated by the airfoil 27. That
25 makes it possible to limit the torque needed to turn the airfoil. This torque may
even be near-zero, negative or positive. In this configuration, it is beneficial to
make provision for the cam follower to press on the cam in two directions. The
slots 60 allow for this two-directional pressure. In other words, in each of the
slots, the pin 61 in question can press against one of the lateral faces of the
30 corresponding slot 60.
Other forms of cam and cam follower are of course possible within
the context of the invention.
Figures 8a to 8f depict various orientations γ of the airfoil 27 as a
35 function of the angle β of the bracket 21 for the second embodiment. The
16
description of these figures is given for an angle β that is increasing from
figure 8a toward figure 8f. Conversely, when the angle β decreases, the
relative configuration of the bracket 21 and of the airfoil 27 pass from figure 8f
toward figure 8a.
5 In figure 8a, the bracket angle β is equal to 90° and the orientation
γ of the airfoil 27 is -10.5°. Such a configuration may notably be obtained in
the event of a zero speed of the ship 10. In figure 8b, the bracket angle β is
equal to 73° and the orientation γ of the airfoil 27 is also -10.5°. In these two
configurations, the bracket angle β is above the threshold 41 and the
10 orientation γ of the airfoil 27 remains constant so as to generate the operational
lift.
As long as the bracket angle β remains above the threshold 41, the
cam profile, which is to say the shape of the slots 60 against which the two
pins 61 press, is an arc of a circle of radius R1 centered on the axis 30 of
15 rotation of the bracket 21. It is possible to secure this part of the control law by
adding a latch 65 formed of two mechanical components 66 and 67, one of
them, 66, attached to the bracket 21 and the other, 67, attached to the airfoil
27. In practice, when the towfish 12 is being launched or recovered by the ship
10, the bracket angle β is generally equal or close to 90° because the towfish
20 12 is suspended out of the water by the cable 14. During these maneuvers,
there is a risk that the airfoil 27 may be knocked. The latch 65 allows these
various knocks to be absorbed in order to prevent stressing the slots 60 and
the pins 61.
The latch 65 is visible in an inset of figure 8b, in which inset the two
25 mechanical components 66 and 67 are depicted in section. The component 66
comprises a cylindrical portion 68 centered on the axis 30 and extending in an
angular sector α1. The component 67 comprises a cylindrical portion 69
centered on the axis 30 and extending in an angular section α2. The two
cylindrical portions 68 and 69 have the same radius, give or take a functional
30 clearance, and press against one another as long as the angle β remains
above the threshold 41. In the rotation of the bracket 21, when the angular
sector α1 is overlapping the angular sector α2, the latch 65 prevents any
rotation of the airfoil 27 and the angle γ remains set at a value of -10.5°. When
the angular sector α1 is no longer overlapping the angular sector α2, the latch
35 65 is released. When the two angular sectors α1 and α2 overlap, the cam profile
17
is an arc of a circle of radius R1. It is even possible to disperse partially with a
cam. For example, for β = 90°, the pins 61 are no longer pressing against the
corresponding slots 60. The slots 60 take over from the latch 65 before the
angular sectors α1 and α2 have stopped overlapping. The shapes employed
5 for the latch 65 may also be employed to set the value of the angle γ to the
value of 0° when the angle β is below the value of the threshold 42.
In figure 8c, the latch 65 is released and the bracket angle β
reaches the threshold 41 at a value of 52°. The orientation γ of the airfoil 27 is
still -10.5°. In figure 8d, the bracket angle β reaches the threshold 45 at a value
10 of 40°. The orientation γ of the airfoil 27 is -6°. In figure 8e, the bracket angle
β is 25°. This angle β corresponds to the point of equilibrium 47 depicted in
figure 5. The orientation γ of the airfoil 27 is 0° here. In figure 8f, the bracket
angle β reaches the threshold 42 and has a value of 19° without modifying the
orientation γ of the airfoil 27 at 0°. Beyond the threshold 42, the orientation γ
15 of the airfoil 27 remains constant at 0°. A second latch may immobilize the
bracket 21 with respect to the structure 23 when the angle β reaches a
minimum value, 15° in the example depicted. The latch may be produced using
the two mechanical components 66 and 67 each comprising an end stop, 71
and 72 respectively, bearing against one another when the angle β reaches
20 the desired value. The two end stops 71 and 72 of the latch are visible in
figures 7b and 7c. Like with the latch 65, the shapes of the end stops 71 and
72 may be used to immobilize the bracket 21 with respect to the structure 23
when the angle β reaches a maximum value, for example 90° as depicted in
figure 8a.
25
18
CLAIMS
1. A towfish intended to be submerged and towed by a cable (14),
the towfish (12) comprising a structure (23) configured to move through the
water in a horizontal main direction (25) and at least one appendage (27)
configured to generate on the towfish (12) a downwardly directed
5 hydrodynamic lift (P) when the towfish (12) is moving through the water under
the effect of the towing, the appendage (27) being orientable so as to modify
its lift, characterized in that the towfish (12) comprises a bracket (21) capable
of rotational movement with respect the structure (23) about a horizontal axis
(30) perpendicular to the horizontal main direction (25), the cable (14) being
10 intended to be attached to the bracket (21), in that it comprises a nonmotorized mechanism configured so that an orientation (γ) of the appendage
(27), allowing it to alter the lift (P), is dependent on an angle (β) formed
between the bracket (21) and the structure (23) defined on the basis of the
horizontal main direction (25), and in that a law connecting the angle (β) to the
15 orientation (γ) of the appendage (27) is configured so that when the value of
the angle (β) decreases, the orientation (γ) of the appendage (27) is increased
in such a way as to reduce the hydrodynamic lift (P) of the towfish (12).
2. The towfish as claimed in claim 1, characterized in that over a
20 range of values for the angle (β), the law is unstable so that a given value for
the orientation (γ) of the appendage (27) leads to a reduction in the angle (β).
3. The towfish as claimed in one of the preceding claims,
characterized in that beyond a first given angle value (β = 52°), the orientation
25 of the appendage (27) is fixed (γ = -10.5°) so as to generate what is referred
to as the operational lift of the towfish (12), and in that below the first given
angular value (β = 52°), the orientation (γ) of the appendage (27) is increased
as the value of the angle (β) decreases so as to reduce the lift compared with
the operational lift.
30
4. The towfish as claimed in claim 3, characterized in that below a
second given angular value (β = 19°) less than the first given angular value,
the orientation of the appendage (27) is fixed (γ = 0°) so as to generate what
is referred to as an evasion lift lower than the operational lift.
19
5. The towfish as claimed in claim 4, characterized in that below the
second given angular value (β = 19°), the orientation of the appendage (27) is
positive or zero.
5
6. The towfish as claimed in one of claims 4 and 5, characterized in
that there is defined a third angle value (β = 40°) intermediate between the first
(β = 52°) and second (β = 19°) angle value, and in that, between the first and
the third angle value, a law connecting the angle (β) to the orientation (γ) of
10 the appendage (27) is configured to keep the angle (β) at a stable value.
7. The towfish as claimed in claims 2 and 6, characterized in that
the range of values for the angle (β) in which the law is unstable is defined
between the third (β = 40°) and the second (β = 19°) angle value, and in that
15 the law is configured in such a way as to orient the appendage (27) in order to
achieve the evasion lift.
8. The towfish as claimed in one of the preceding claims,
characterized in that it comprises a lift-inducing airfoil (27) forming the
20 appendage, and a stabilizing empennage (29) configured to keep a pitch
attitude of the towfish (12) substantially constant during changes to the
orientation (γ) of the appendage (27).
9. The towfish as claimed in one of the preceding claims,
25 characterized in that the appendage (27) is able to move in rotation with
respect to the structure (23) about a second horizontal axis (33), the mobility
of the appendage (27) allowing the lift (P) of the towfish (12) to be modified,
and in that the second horizontal axis (33) of rotation of the appendage (27) is
positioned substantially at the instantaneous center of rotation of the towfish
30 (12) when the latter pivots as a result of a change in the orientation (γ) of the
appendage (27).
10. The towfish as claimed in one of the preceding claims,
characterized in that it comprises a cam (60) and a cam follower (61), one
35 being secured to the appendage (27) and the other to the bracket (21), and in
20
that the cam follower (61) presses against a shape on the cam (60) as the
bracket (21) rotates about the horizontal axis (30).
11. The towfish as claimed in claim 9 as claim dependent on
5 claim 3, characterized in that it comprises a first latch (65) configured to keep
the appendage (27) in a fixed orientation (γ = -10.5°) when the value of the
angle is beyond the first given angle value (β = 52°).
12. The towfish as claimed in one of claims 10 and 11 as claims
10 dependent on claim 4, characterized in that it comprises a second latch (71,
72) configured to immobilize the bracket (21) with respect to the structure (23)
when the value of the angle (β) formed between the bracket (21) and the
structure (23) is below the second given angle value (β = 19°).