Part B | 12.1
304 Part B Autonomous Ocean Vehicles, Subsystems and Control
larger payload and cargo-carrying capacity, and emphasize point-to-point, cross-country, transport efficiency.
Various designs of these new cross-country gliders
have been built and are currently undergoing sea trials,
including two examples of flying wings, the Liberdade/XRay and Liberdade/ZRay gliders (Fig. 12.4).
The XRay and ZRay designs use a seawater-based
buoyancy engine analogous to the ballast system on
submarines, while other cross-country glider designs
use larger versions of the type of oil-based buoyancy
engine diagrammed in Fig. 12.2.
As discussed earlier, underwater gliding is a buoyancy driven form of locomotion in which the power
needed to overcome the drag (D) on the vehicle as it
moves at a speed U through water is supplied by gravity in the form of positive or negative net buoyancy
(C= B). Horizontal translation using the vertical force
of gravity is made possible by the lift (L) produced by
a wing that acts perpendicular to the trajectory of the vehicle. Inclination of the glide slope from the horizontal
gives rise to a horizontal component of lift that provides
the force of forward propulsion. In steady-state flight,
this force is balanced by the horizontal component of
the drag, which yields the relationship that the glide
slope is equal to the inverse of the lift-to-drag ratio. The
lift-to-drag ratio (L/D) of a wing is also referred to as
its finesse [12.7]. In the vertical direction, inclination
of the glide slope from the horizontal also allows the
net hydrodynamic force of lift and drag (F) to balance
the net buoyancy in steady-state flight, but implies that
a net vertical motion will result, (Fig. 12.1). This vertical motion is referred to as the sink rate (w ). During
each of the descending or ascending slopes of the sawtooth glide path, the power needed to overcome drag
(P e D DU) is equal to the rate of work by gravity acting
down (or up) (P g D Bw). Thus
P g D P e D DU D Bw :
(12.1)
Because the force triangle and the speed triangle in
Fig. 12.1 are proportional, the power expenditure per
horizontal distance traveled scales in direct proportion
to the glide slope (w =u D=L), or inversely with the
lift-to-drag ratio (L=D). Therefore, by imparting the
underwater glider with low drag and high lift properties, its energy consumed to produce locomotion in the
horizontal direction can be minimized. In other words,
almost all the energy in forward propulsion is consumed at the bottom of the dive cycle where the glider’s
displaced volume must be increased against ambient
pressure. By decreasing the glide slope, i. e., increasing L=D, the number of times this change in displaced
volume must occur is minimized for a given horizontal
distance traveled.
The buoyancy engine generates a variable displaced
volume increment, or net buoyancy volume ˙V b ; such
that the total displaced volume of the glider is V d D
V s ˙ V b , where V s represents the volume of the glider’s
rigid body. By varying the net buoyancy volume, the
buoyancy engine causes the net buoyancy force to alternate between positive and negative states, B D ˙gV b .
Typically, the underwater glider is trimmed to be approximately neutrally buoyant in seawater when V b D 0;
so that the average density of the glider approaches the
ambient seawater density, N
s ! , and the net buoyancy reduces to B D g.V d V s / D 0. In addition to the
volume of the glider’s rigid body and buoyancy engine
displacement volume, the total volume of the vehicle,
V 0 , also includes void-space water V void , that enters into
the freely flooding internal spaces. Thus, the total volume of the vehicle is represented by: V 0 D V s ˙ V b C
V void . Because the total vehicle volume is fixed, the
void water is a function of the net buoyancy volume,
V void D f .V b /, and the hull must accommodate ventilation of the void water with the outside water. Under
neutral buoyancy, the total vehicle mass becomes M 0 D
.V s ˙ V b C V void /. A variety of buoyancy engine technologies have been employed, including closed-loop
liquid-based engines [12.2–4] for which buoyancy engine capacity is in the neighborhood of 0:1% Ä V b =V 0 Ä
4%; open-loop seawater-based buoyancy engines, as
with XRay and ZRay having capacities typically ranging
from 1% Ä V b =V 0 Ä 8%; open-loop compressed gasbased systems [12.8] having 5% Ä V b =V 0 Ä 20%; as
well as the open-loop gas-based buoyancy engines that
consume gas-generating compounds, such as those used
in Concept Whisper [12.5] for which 11% Ä V b =V 0 Ä
28%. The profiling gliders use a closed-loop oil-based
buoyancy engine as diagrammed in Fig. 12.2, while the
cross-country gliders typically use open-loop seawaterbased buoyancy engines. The closed-loop liquid-based
engines have the disadvantage that the weight of the
working fluid (oil) is always onboard, limiting the net
buoyancy capacity to about half that of an open circuit
buoyancy engine that uses seawater (e.g., as in a submarine buoyancy system). Also, oils can undergo phase
changes from liquid to vapor under large decreases in
pressure, which can occur in a closed-loop system when
the oil is transferred from the external bladder to the internal reservoir to initiate ascent from deep depths. This
phase change phenomenon limits the ability to evacuate all of the oil in the external bladder. Regardless,
both close- and open-loop buoyancy engines typically
are driven by small electric pumps that produce only
low levels of intermittent self-noise, ideally suited for
passive underwater monitoring applications or those requiring stealthy behavior.
304 Part B Autonomous Ocean Vehicles, Subsystems and Control
larger payload and cargo-carrying capacity, and emphasize point-to-point, cross-country, transport efficiency.
Various designs of these new cross-country gliders
have been built and are currently undergoing sea trials,
including two examples of flying wings, the Liberdade/XRay and Liberdade/ZRay gliders (Fig. 12.4).
The XRay and ZRay designs use a seawater-based
buoyancy engine analogous to the ballast system on
submarines, while other cross-country glider designs
use larger versions of the type of oil-based buoyancy
engine diagrammed in Fig. 12.2.
As discussed earlier, underwater gliding is a buoyancy driven form of locomotion in which the power
needed to overcome the drag (D) on the vehicle as it
moves at a speed U through water is supplied by gravity in the form of positive or negative net buoyancy
(C= B). Horizontal translation using the vertical force
of gravity is made possible by the lift (L) produced by
a wing that acts perpendicular to the trajectory of the vehicle. Inclination of the glide slope from the horizontal
gives rise to a horizontal component of lift that provides
the force of forward propulsion. In steady-state flight,
this force is balanced by the horizontal component of
the drag, which yields the relationship that the glide
slope is equal to the inverse of the lift-to-drag ratio. The
lift-to-drag ratio (L/D) of a wing is also referred to as
its finesse [12.7]. In the vertical direction, inclination
of the glide slope from the horizontal also allows the
net hydrodynamic force of lift and drag (F) to balance
the net buoyancy in steady-state flight, but implies that
a net vertical motion will result, (Fig. 12.1). This vertical motion is referred to as the sink rate (w ). During
each of the descending or ascending slopes of the sawtooth glide path, the power needed to overcome drag
(P e D DU) is equal to the rate of work by gravity acting
down (or up) (P g D Bw). Thus
P g D P e D DU D Bw :
(12.1)
Because the force triangle and the speed triangle in
Fig. 12.1 are proportional, the power expenditure per
horizontal distance traveled scales in direct proportion
to the glide slope (w =u D=L), or inversely with the
lift-to-drag ratio (L=D). Therefore, by imparting the
underwater glider with low drag and high lift properties, its energy consumed to produce locomotion in the
horizontal direction can be minimized. In other words,
almost all the energy in forward propulsion is consumed at the bottom of the dive cycle where the glider’s
displaced volume must be increased against ambient
pressure. By decreasing the glide slope, i. e., increasing L=D, the number of times this change in displaced
volume must occur is minimized for a given horizontal
distance traveled.
The buoyancy engine generates a variable displaced
volume increment, or net buoyancy volume ˙V b ; such
that the total displaced volume of the glider is V d D
V s ˙ V b , where V s represents the volume of the glider’s
rigid body. By varying the net buoyancy volume, the
buoyancy engine causes the net buoyancy force to alternate between positive and negative states, B D ˙gV b .
Typically, the underwater glider is trimmed to be approximately neutrally buoyant in seawater when V b D 0;
so that the average density of the glider approaches the
ambient seawater density, N
s ! , and the net buoyancy reduces to B D g.V d V s / D 0. In addition to the
volume of the glider’s rigid body and buoyancy engine
displacement volume, the total volume of the vehicle,
V 0 , also includes void-space water V void , that enters into
the freely flooding internal spaces. Thus, the total volume of the vehicle is represented by: V 0 D V s ˙ V b C
V void . Because the total vehicle volume is fixed, the
void water is a function of the net buoyancy volume,
V void D f .V b /, and the hull must accommodate ventilation of the void water with the outside water. Under
neutral buoyancy, the total vehicle mass becomes M 0 D
.V s ˙ V b C V void /. A variety of buoyancy engine technologies have been employed, including closed-loop
liquid-based engines [12.2–4] for which buoyancy engine capacity is in the neighborhood of 0:1% Ä V b =V 0 Ä
4%; open-loop seawater-based buoyancy engines, as
with XRay and ZRay having capacities typically ranging
from 1% Ä V b =V 0 Ä 8%; open-loop compressed gasbased systems [12.8] having 5% Ä V b =V 0 Ä 20%; as
well as the open-loop gas-based buoyancy engines that
consume gas-generating compounds, such as those used
in Concept Whisper [12.5] for which 11% Ä V b =V 0 Ä
28%. The profiling gliders use a closed-loop oil-based
buoyancy engine as diagrammed in Fig. 12.2, while the
cross-country gliders typically use open-loop seawaterbased buoyancy engines. The closed-loop liquid-based
engines have the disadvantage that the weight of the
working fluid (oil) is always onboard, limiting the net
buoyancy capacity to about half that of an open circuit
buoyancy engine that uses seawater (e.g., as in a submarine buoyancy system). Also, oils can undergo phase
changes from liquid to vapor under large decreases in
pressure, which can occur in a closed-loop system when
the oil is transferred from the external bladder to the internal reservoir to initiate ascent from deep depths. This
phase change phenomenon limits the ability to evacuate all of the oil in the external bladder. Regardless,
both close- and open-loop buoyancy engines typically
are driven by small electric pumps that produce only
low levels of intermittent self-noise, ideally suited for
passive underwater monitoring applications or those requiring stealthy behavior.