Part B | 12
320 Part B Autonomous Ocean Vehicles, Subsystems and Control
namics, gliders with wings can fly cooperatively with
other winged vehicles and structures to further improve transport efficiency, in contrast to prop-driven
vehicles. In addition, gliders are capable of readily operating throughout their full speed regime, from top
sprint speed to creeping flight to free-drifting at neutral
buoyancy in which no propulsion energy is consumed.
Consequently, gliders offer a persistent, high-endurance
solution for many ocean sampling and surveillance missions. Buoyancy-driven gliders also are silent (acoustically and electromagnetically) throughout most of the
dive cycle, including when operating at top sprint speed.
They create low levels of self-noise for a few percent
of the total dive time due primarily to buoyancy engine operation. However, gliders are generally slower
than prop-driven AUVs, typically operating in the speed
regime below 3 kts. (Much of a glider’s on-station persistence is attributable to these low operating speeds.)
In addition, a glider must change depth in order to
move forward and so is incapable of level flight. On the
other hand, these depth changes allow vertical profiles
of ocean properties to be collected and for energy to be
extracted from the temperature gradients in the ocean.
In addition to being slower and an inability to conduct level flight, gliders are less maneuverable and
more balance sensitive than prop-driven AUVs. However, possibly the greatest disadvantage is the additional
vehicle interior volume consumed by, and the additional
complexity of, a buoyancy engine compared to a propdriven system.
The calculus of gliding has been explored for ways
of increasing the horizontal point-to-point transport efficiency and speed of underwater gliders. An analytic
solution for the minimum specific energy consumption
(maximum L=D) indicates that increasing the net buoyancy volume of the vehicle V b increases the speed as
p
V b while specific energy consumption E e declines as
E e V
=4
b
, where 1=5 Ä Ä 1=2. Since buoyancy engine capacity is some fraction n b of vehicle volume, V 0 ,
this finding immediately argues for larger gliders for
improved horizontal transport efficiency. The challenge
therefore reduces to finding the most efficient geometries for large volumes. Of all the geometric properties
of the glider, the wetted surface to wing area ratio,
N A D A t =A 0 , has the strongest influence on specific energy consumption, with E e decreasing as E e N
1=2
A
with decreasing N A . The flying wing glider offers a 9fold reduction in the wetted surface to wing area ratio
over existing profiling gliders, which offsets the 3-fold
(at most) increase in profile drag resulting from the
thick wing section of a flying wing. The other benefit derived from concentrating the vehicle volume in
the wing is that a large wing area reduces the magnitudes of lift coefficient C L required to support a given
V b , thereby reducing the associated induced drag, the
largest component of drag at maximum L=D. However,
increasing wing area indefinitely becomes mutually exclusive with high aspect ratio, N R D S
2
=A 0 , where S is
the wingspan. Specific energy consumption was shown
to decline with decreases in the ratio of induced drag
factors at a rate given by E e C
=4
L =N
.1=2=4/
R
.
Other factors that favor flying wing geometries for
many classes of underwater gliders are higher Reynolds
number on the wing section due to larger wing chord,
leading to higher maximum lift-to-drag ratios and
avoidance of rapid L=D degradation occurring in the
10
4 Reynolds number regime, and increased structural depth of the center section allowing increases in
wingspan (and aspect ratio) with fewer weight penalties
compared to winged bodies of revolution [12.31–33].
Analytic and numerical comparisons, supported by atsea results, for large gliders (V 0 1000 L) of comparable net buoyancy demonstrate that a flying wing glider
is about 43% more efficient in horizontal transport,
but that a winged body of revolution glider has about
a 30% advantage in maximum cross-country speed.
All gliders designed for long-duration, long-distance
flights certainly would benefit significantly from using
a buoyancy engine that can harvest energy from the
temperature gradients in the ocean.
References
12.1
S. A. Jenkins, D. E. Humphreys, J. Sherman, J.
Osse, C. Jones, N. Leonard, J. Graver, R. Bachmayer:
Underwater Glider System Study, Scripps Institution of Oceanography, Tech. Rep. 57 (University of
California, San Diego, La Jolla 2003) online available at http://repositories.cdlib.org/sio/techreport/
53/
12.2
J. Sherman, R.E. Davis, W.B. Owens, J. Valdes: The
autonomous underwater glider spray, IEEE J. Ocean.
Eng. 26(4), 437–446 (2001)
12.3
C.C. Eriksen, T.J. Osse, R.D. Light, T. Wen,
T.W. Lehman, P.L. Sabin, J.W. Ballard, A.M. Chiodi:
Seaglider: A long-range autonomous underwater
vehicle for oceanographic research, IEEE J. Ocean.
Eng. 26(4), 424–436 (2001)
12.4
D.C. Webb, P.J. Simonetti, C.P. Jones: SLOCUM: An
underwater glider propelled by environmental energy, IEEE J. Ocean. Eng. 26(4), 447–452 (2001)
12.5
R. H. Oversmith, R. E. Leadon: Concept Whisper: A buoyancy-propelled, multiple cycle under-
320 Part B Autonomous Ocean Vehicles, Subsystems and Control
namics, gliders with wings can fly cooperatively with
other winged vehicles and structures to further improve transport efficiency, in contrast to prop-driven
vehicles. In addition, gliders are capable of readily operating throughout their full speed regime, from top
sprint speed to creeping flight to free-drifting at neutral
buoyancy in which no propulsion energy is consumed.
Consequently, gliders offer a persistent, high-endurance
solution for many ocean sampling and surveillance missions. Buoyancy-driven gliders also are silent (acoustically and electromagnetically) throughout most of the
dive cycle, including when operating at top sprint speed.
They create low levels of self-noise for a few percent
of the total dive time due primarily to buoyancy engine operation. However, gliders are generally slower
than prop-driven AUVs, typically operating in the speed
regime below 3 kts. (Much of a glider’s on-station persistence is attributable to these low operating speeds.)
In addition, a glider must change depth in order to
move forward and so is incapable of level flight. On the
other hand, these depth changes allow vertical profiles
of ocean properties to be collected and for energy to be
extracted from the temperature gradients in the ocean.
In addition to being slower and an inability to conduct level flight, gliders are less maneuverable and
more balance sensitive than prop-driven AUVs. However, possibly the greatest disadvantage is the additional
vehicle interior volume consumed by, and the additional
complexity of, a buoyancy engine compared to a propdriven system.
The calculus of gliding has been explored for ways
of increasing the horizontal point-to-point transport efficiency and speed of underwater gliders. An analytic
solution for the minimum specific energy consumption
(maximum L=D) indicates that increasing the net buoyancy volume of the vehicle V b increases the speed as
p
V b while specific energy consumption E e declines as
E e V
=4
b
, where 1=5 Ä Ä 1=2. Since buoyancy engine capacity is some fraction n b of vehicle volume, V 0 ,
this finding immediately argues for larger gliders for
improved horizontal transport efficiency. The challenge
therefore reduces to finding the most efficient geometries for large volumes. Of all the geometric properties
of the glider, the wetted surface to wing area ratio,
N A D A t =A 0 , has the strongest influence on specific energy consumption, with E e decreasing as E e N
1=2
A
with decreasing N A . The flying wing glider offers a 9fold reduction in the wetted surface to wing area ratio
over existing profiling gliders, which offsets the 3-fold
(at most) increase in profile drag resulting from the
thick wing section of a flying wing. The other benefit derived from concentrating the vehicle volume in
the wing is that a large wing area reduces the magnitudes of lift coefficient C L required to support a given
V b , thereby reducing the associated induced drag, the
largest component of drag at maximum L=D. However,
increasing wing area indefinitely becomes mutually exclusive with high aspect ratio, N R D S
2
=A 0 , where S is
the wingspan. Specific energy consumption was shown
to decline with decreases in the ratio of induced drag
factors at a rate given by E e C
=4
L =N
.1=2=4/
R
.
Other factors that favor flying wing geometries for
many classes of underwater gliders are higher Reynolds
number on the wing section due to larger wing chord,
leading to higher maximum lift-to-drag ratios and
avoidance of rapid L=D degradation occurring in the
10
4 Reynolds number regime, and increased structural depth of the center section allowing increases in
wingspan (and aspect ratio) with fewer weight penalties
compared to winged bodies of revolution [12.31–33].
Analytic and numerical comparisons, supported by atsea results, for large gliders (V 0 1000 L) of comparable net buoyancy demonstrate that a flying wing glider
is about 43% more efficient in horizontal transport,
but that a winged body of revolution glider has about
a 30% advantage in maximum cross-country speed.
All gliders designed for long-duration, long-distance
flights certainly would benefit significantly from using
a buoyancy engine that can harvest energy from the
temperature gradients in the ocean.
References
12.1
S. A. Jenkins, D. E. Humphreys, J. Sherman, J.
Osse, C. Jones, N. Leonard, J. Graver, R. Bachmayer:
Underwater Glider System Study, Scripps Institution of Oceanography, Tech. Rep. 57 (University of
California, San Diego, La Jolla 2003) online available at http://repositories.cdlib.org/sio/techreport/
53/
12.2
J. Sherman, R.E. Davis, W.B. Owens, J. Valdes: The
autonomous underwater glider spray, IEEE J. Ocean.
Eng. 26(4), 437–446 (2001)
12.3
C.C. Eriksen, T.J. Osse, R.D. Light, T. Wen,
T.W. Lehman, P.L. Sabin, J.W. Ballard, A.M. Chiodi:
Seaglider: A long-range autonomous underwater
vehicle for oceanographic research, IEEE J. Ocean.
Eng. 26(4), 424–436 (2001)
12.4
D.C. Webb, P.J. Simonetti, C.P. Jones: SLOCUM: An
underwater glider propelled by environmental energy, IEEE J. Ocean. Eng. 26(4), 447–452 (2001)
12.5
R. H. Oversmith, R. E. Leadon: Concept Whisper: A buoyancy-propelled, multiple cycle under-