Part B | 12.4
308 Part B Autonomous Ocean Vehicles, Subsystems and Control
forces against buoyancy to maintain level flight. The
concept is envisioned for circumstances where operations require short-term burst speed in very narrow
depth regimes, or when level flight is required. Level
flight capability may be required for proper operation of certain sensors like seafloor mapping sensors.
High burst speed over a limited depth extent may be
needed for certain avoidance maneuvers or for penetration in strong shallow water currents. This hybrid
propulsion system approach provides the basis for
a long range, high endurance prop-driven AUV (autonomous underwater vehicle). Design adaptations for
this role require many of the same characteristics as
depth limited roaming, with the addition of low drag
at high cruise speed to maximize prop-powered flight
performance.
12.4 Optimal Size and Shape for Horizontal Transport Efficiency
Transport efficiency varies inversely with the cost of
locomotion. It is commonly measured in transportation science by specific energy consumption or net
transport economy, which is equal to the energy consumed per horizontal distance traveled for each unit
of vehicle weight [12.7, 12]. Specific energy consumption differs from net transport economy in that the
former is based only upon the energy expended in
propulsion, whereas net transport economy includes all
forms of vehicle energy expenditure including the hotel
loads (energy consumed by control systems, communications, payload power requirements, onboard data
processing, launch and recovery systems, etc.). Similarly, the vehicle weight can be defined in various ways,
for example in terms of total dry bulk weight of the vehicle (including payload), F g D N
s gV s . In the next part
of this section, it is defined as the weight (net buoyancy force) Mg D B, due to the loaded mass M applied
to a wing (Fig. 12.1). An alternative weight normalization for specific energy consumption considers only that
of the payload/cargo, to eliminate any potential bias toward larger underwater vehicles.
12.4.1 Net Transport Economy
Analytic surveys of natural and man-made fliers provide a starting point for identification of present operating regimes of the profiling gliders and rules of scaling
to other performance regimes from those gliders. Design surveys in aeronautical design and bio-mechanics
has produced a number of useful works for this purpose [12.7, 12–15].
Analytic flight survey literature reveals that a leading order variable controlling regimes of scale is the
loaded mass. In an underwater glider, the loaded mass is
supplied by the buoyancy engine. The loaded mass (net
buoyancy/g) can be used as the normalization factor in
the net transport economy (NTE)
NTE D
P
.Bu/
D
P
.Mgu/
;
(12.5)
where P is the total time-averaged power consumed
by the flier. The net transport economy is dimensionless and smaller values indicate more efficient transport.
The dimensionless NTE is commonly compared to the
loaded mass in kilograms [12.13]. Figure 12.8 plots
NTE over 12 orders of magnitude variation in loaded
mass, covering regimes of scale from insects to jet
transports. The NTE values in Fig. 12.8 are based on
total power consumption P including both the flight
power spent overcoming drag (P e D DU) as well as
internal power consumption (P P e ), which includes
basal metabolic rates in the case of natural fliers and
all subsystem energy consumption in the case of manmade fliers (hotel loads). In [12.13] and [12.15], estimates are made of the basal metabolic rates of birds
based on the body mass, resulting in an empirical formulation of NTE
NTE D 0:898.Mg/
0:227 (Tucker Fliers) : (12.6)
References [12.12] and [12.14] also examined a limited set of data on birds and derived a similar empirical
relation
NTE D 0:914.Mg/
0:293
(Schmidt–Nielsen Fliers) :
(12.7)
These two empirical relations are indicated by the solid
sloping lines in Fig. 12.8. The most apparent scale dependent feature of these empirical relations and the
NTE data is that the energy consumed per meter traveled decreases as the loaded mass is increased – bigger
fliers (greater loaded mass) are more efficient fliers.
The profiling gliders presently operating in the range
of 100300 g of loaded mass are overlaid on Fig. 12.8
as a purple triangle. It appears that the profiling gliders are consuming relatively higher levels of energy per
horizontal distance traveled than their bird/bat counterparts operating at equivalent loaded mass. Natural fliers
are used as a standard for ultimate efficiency because
natural selection tends to eliminates all but the most
efficient mutations [12.13]. However, several factors
308 Part B Autonomous Ocean Vehicles, Subsystems and Control
forces against buoyancy to maintain level flight. The
concept is envisioned for circumstances where operations require short-term burst speed in very narrow
depth regimes, or when level flight is required. Level
flight capability may be required for proper operation of certain sensors like seafloor mapping sensors.
High burst speed over a limited depth extent may be
needed for certain avoidance maneuvers or for penetration in strong shallow water currents. This hybrid
propulsion system approach provides the basis for
a long range, high endurance prop-driven AUV (autonomous underwater vehicle). Design adaptations for
this role require many of the same characteristics as
depth limited roaming, with the addition of low drag
at high cruise speed to maximize prop-powered flight
performance.
12.4 Optimal Size and Shape for Horizontal Transport Efficiency
Transport efficiency varies inversely with the cost of
locomotion. It is commonly measured in transportation science by specific energy consumption or net
transport economy, which is equal to the energy consumed per horizontal distance traveled for each unit
of vehicle weight [12.7, 12]. Specific energy consumption differs from net transport economy in that the
former is based only upon the energy expended in
propulsion, whereas net transport economy includes all
forms of vehicle energy expenditure including the hotel
loads (energy consumed by control systems, communications, payload power requirements, onboard data
processing, launch and recovery systems, etc.). Similarly, the vehicle weight can be defined in various ways,
for example in terms of total dry bulk weight of the vehicle (including payload), F g D N
s gV s . In the next part
of this section, it is defined as the weight (net buoyancy force) Mg D B, due to the loaded mass M applied
to a wing (Fig. 12.1). An alternative weight normalization for specific energy consumption considers only that
of the payload/cargo, to eliminate any potential bias toward larger underwater vehicles.
12.4.1 Net Transport Economy
Analytic surveys of natural and man-made fliers provide a starting point for identification of present operating regimes of the profiling gliders and rules of scaling
to other performance regimes from those gliders. Design surveys in aeronautical design and bio-mechanics
has produced a number of useful works for this purpose [12.7, 12–15].
Analytic flight survey literature reveals that a leading order variable controlling regimes of scale is the
loaded mass. In an underwater glider, the loaded mass is
supplied by the buoyancy engine. The loaded mass (net
buoyancy/g) can be used as the normalization factor in
the net transport economy (NTE)
NTE D
P
.Bu/
D
P
.Mgu/
;
(12.5)
where P is the total time-averaged power consumed
by the flier. The net transport economy is dimensionless and smaller values indicate more efficient transport.
The dimensionless NTE is commonly compared to the
loaded mass in kilograms [12.13]. Figure 12.8 plots
NTE over 12 orders of magnitude variation in loaded
mass, covering regimes of scale from insects to jet
transports. The NTE values in Fig. 12.8 are based on
total power consumption P including both the flight
power spent overcoming drag (P e D DU) as well as
internal power consumption (P P e ), which includes
basal metabolic rates in the case of natural fliers and
all subsystem energy consumption in the case of manmade fliers (hotel loads). In [12.13] and [12.15], estimates are made of the basal metabolic rates of birds
based on the body mass, resulting in an empirical formulation of NTE
NTE D 0:898.Mg/
0:227 (Tucker Fliers) : (12.6)
References [12.12] and [12.14] also examined a limited set of data on birds and derived a similar empirical
relation
NTE D 0:914.Mg/
0:293
(Schmidt–Nielsen Fliers) :
(12.7)
These two empirical relations are indicated by the solid
sloping lines in Fig. 12.8. The most apparent scale dependent feature of these empirical relations and the
NTE data is that the energy consumed per meter traveled decreases as the loaded mass is increased – bigger
fliers (greater loaded mass) are more efficient fliers.
The profiling gliders presently operating in the range
of 100300 g of loaded mass are overlaid on Fig. 12.8
as a purple triangle. It appears that the profiling gliders are consuming relatively higher levels of energy per
horizontal distance traveled than their bird/bat counterparts operating at equivalent loaded mass. Natural fliers
are used as a standard for ultimate efficiency because
natural selection tends to eliminates all but the most
efficient mutations [12.13]. However, several factors