Launching the AquaMAV: Bioinspired design for aerial-aquatic robotic platforms
R. Siddall and M. Kova£ Department of Aeronautics, Imperial College London, United Kingdom E-mail: [email protected], [email protected]
Abstract. Current Micro Aerial Vehicles (MAVs) are greatly limited by being able to operate in air only. Designing multimodal MAVs that can y eectively, dive into the water and retake ight would enable applications of distributed water quality monitoring, search and rescue operations and underwater exploration. While some can land on water, no technologies are available that allow them to both dive and y, due to dramatic design trade-os that have to be solved for movement in both air and water and due to the absence of high-power propulsion systems that would allow a transition from underwater to air. In nature, several animals have evolved design solutions that enable them to successfully transition between water and air, and move in both media. Examples include ying fsh, ying squid, diving birds and diving insects. In this paper, we review the biological literature on these multimodal animals and abstract their underlying design principles in the perspective of building a robotic equivalent, the Aquatic Micro Air Vehicle (AquaMAV). Building on the inspire-abstract-implement bioinspired design paradigm, we identify key adaptations from nature and designs from robotics. Based on this evaluation we propose key design principles for the design of successful aerial-aquatic robots, i.e. using a plunge diving strategy for water entry, folding wings for diving e ciency, water jet propulsion for water take-o and hydrophobic surfaces for water shedding and dry ight. Further, we demonstrate the feasibility of the water jet propulsion by building a proof of concept water jet propulsion mechanism with a mass of 2.6grams that can propel itself up to 4.8m high, corresponding to 72times its size. This propulsion mechanism can be used for AquaMAV but also for other robotic applications where high power density is of use, such as for jumping and swimming robots.
Keywords: multimodal locomotion,aquatic micro aerial vehicles, jump-gliding Aquatic Micro Air Vehicles 2
Figure 1: Artistic impression of swarm-deployed AquaMAVs mapping a toxic spill.
1. Introduction land and conserve power, or enter constrained areas such as ooded buildings. On the other side of the spectrum, Most animals use dierent forms of locomotion to move a purely aquatic robot is not able to pass over obsta- through a varied environment. This allows them to adapt cles in the water, and cannot take ight for rapid travel to nd food, escape threats or migrate, while minimis- over long distances. Having a robot that can travel in ing their energetic cost of locomotion. To do so, ani- air, move underwater and transition back to ight would mals must use the same locomotor modules to perform greatly improve search and rescue capabilities. specialised tasks that often have opposed requirements. An aerial-aquatic vehicle would also have great po- For example, an animal diving into the water to hunt tential as a sample collection or observation tool for requires a structure that is as lightweight as possible for oceanography research, a eld in which air-launched e cient ight, whilst still being structurally strong when underwater vehicles are already sought after [11, 12]. impacting the water's surface. The complexity of biolog- Research has already demonstrated the eectiveness of ical systems makes them di cult to replicate arti cially, combining aerial and aquatic robotic operation by co- but meeting the demands of two modes of locomotion ordinating separate systems as a team for the monitoring in a single robotic system has been done. Bioinspired of coral reefs [13], with an MAV using aerial video footage robots have been presented that can both y and move to guide and augment close observation by an aquatic ve- on land, either by jumping or by walking [15]. Other hicle. Murphy et al. [10] also found that use of the MAV robots take cues from amphibious locomotion in nature provided a more eective communication relay with the and can move on land and in water [68]. surface vehicle. These systems demonstrate both the feasibility and To the best of the author's knowledge, no vehicle the e cacy of multimodal robotics, and outperform currently exists with the capability to move both in the robots with only one locomotion mode. The vehicles air and beneath the water. The goal of this paper is have together covered motion in water, land and air, but therefore to oer designers a critical examination both no robot can move in both air and water, and transi- of the biological apparatus used to achieve aerial aquatic tion between the two. A robot capable of aerial-aquatic locomotion, and a sample of technology relevant to the locomotion could play an eective role in disaster relief development of an aerial-aquatic vehicle. The paper be- after oods or tsunamis, where the presence of water gins with an brief explanation of the challenges of aerial- and debris will severely limit the operation of conven- aquatic locomotion, to provide context to the remainder tional vehicles. The potential of robotic systems for use of the text. To conclude the paper, we use the inspire- in emergencies was highlighted by recent events such as abstract-implement bioinspired design paradigm [14] to the Fukushima nuclear accident [9], the response to Hur- propose a bioinspired implementation strategy for the ricane Wilma [10], and the Deepwater Horizon explosion. development of an e cient aerial-aquatic vehicle: the In these situations, an MAV can travel rapidly to a AquaMAV. To enhance discussion of these proposals, a target and return aerial footage, but would be unable to brief proof-of-concept experiment has also been included. Aquatic Micro Air Vehicles 3
Figure 2: Every stage of an aerial-aquatic mission poses dierent key challenges for an AquaMAV: (A) Dry Flight, (B) Water Entry, (C) Submerged Movement and (D) Water Exit.
Mission Phase Design Challenges Dry Flight ( gure 2.A) Low energy ight, low weight, robustness and stability Water Entry ( gure 2.B) Accurate sensing and control (soft landing) or high impact loads (direct dive) Submerged ( gure 2.C) Deep dive for underwater exploration, low buoyancy, waterproo ng and autonomous navigation Water Exit ( gure 2.D) High power density (direct launch) or low surface drag (taxiing takeo)
2. Challenges 2.2. Water Entry
Transitioning between ying and swimming presents sev- eral key design challenges. To analyse them, it is helpful The challenges associated with entering the water depend to examine the robot's mission in the context of distinct on the strategy for transition. Plunge diving directly mission phases ( gure 2): First the robot has to perform through the surface carries over ight momentum into high e ciency ight, then must transition from air to wa- the water, and can allow depths to be reached rapidly. ter when at a target. After transition, it must spend a pe- However, in order to do so a vehicle must be as slender riod moving beneath the surface for water sampling and and streamlined as possible to minimise impact loads. video footage, before transitioning back to ight from the Diving impact loads have been found to scale approxi- water. mately with the square of impact velocity [18], and re- sult largely from the accelerated mass of water on impact. The added mass will scale with the body cross sectional diameter to the third power (an oversimpli cation, but 2.1. Dry Flight one that highlights the importance of minimising cross section during impact). This makes eective, deep diving The key challenge when in ight is the minimisation of highly impractical for xed-wing aircraft, and no animal power requirements. This is achieved by maximising attempts to dive directly through the surface without the aerodynamic e ciency of the vehicle, which becomes folding or morphing its wings. more di cult at the small scale. Micro Aerial Vehicles A body impacting at too shallow an impact angle typically operate in the Reynolds number range 104-105, will experience a large bending moment, and if the dive and at these low ight Reynolds numbers the aerody- has insu cient momentum it may rebound from the wa- namic e ciency of ight is reduced due to increased vis- ter surface rather than fully penetrating, due to the im- cous eects, and air perturbations will have a much more pact pitching moment and rapid deceleration [18, 19]. signi cant eect on the vehicle [15]. Furthermore, the de- For dives at shallow angles, the eccentricity of the im- sign constraints of ight must be satis ed simultaneously pact load also has a tendency to ip a vehicle, which is with those imposed by the need to swim. These addi- a signi cant problem for soft oatplane landings. tional constraints come from the need for dual propulsion modes, a stronger superstructure and waterproof design, Alternatively, a vehicle can perform a soft land- all of which add to the ight mass. ing, whether by gradual descent or deep stall just above Materials considerations for the wings will be essen- the surface. This approach reduces structural require- tial, not only for the minimisation of structural mass. Se- ments and may mean folding wings are not necessary, but lecting materials to add exibility to the wing structure an additional system for underwater propulsion is then also oers many enhancements for low Reynolds number needed, and the robot cannot bene t from ight momen- vehicles, from aerodynamic increases by allowing both tum when diving into water. This entry strategy will also passive and active adapting of wing morphology [16], to require more accurate control capability and sensing of structural increases by improving durability [17]. altitude, which adds mass and complexity. Aquatic Micro Air Vehicles 4
2.3. Submerged Movement Translational Review of creativity Once the vehicle is in the water, it must contend with biological systems Applications in robotics resistance from buoyancy and drag increased by several Interdisciplinary INSPIRE orders of magnitude, due to the increased density and vis- curiosity Openness for cosity of water compared to air. The bene ts of reducing unconventional buoyancy for e cient swimming must be traded against Design designs low weight for e cient ying. For non-zero buoyancy, the validation
Bioinspired A
centre of volume as well as the vehicle's hydrodynamics T Experimental System B
will determine its stability, which will be important if a N analysis
Robot Design S
optimization E
plunge dive and passive, buoyancy driven ascent is de- T
M R
E Biomimetic
sired. As the robot travels deeper, hydrostatic pressure A L
Prototyping C
P exploration loads will also become signi cant, and place harsher re- T
and fabrication M quirements on the robot's structure and the waterproof- I Modelling ing of electronic components. The limited propagation Material and simulation of electromagnetic waves in water [20] will also make re- selection Product Principle mote control of microrobots highly di cult, forcing the design def nition vehicle to instead rely on autonomous control. frameworks Figure 3: The Inspire-Abstract-Implement paradigm for 2.4. Water Exit bioinspired robot design. Figure reproduced from [14].
The key challenge here is building up speed for ight. scale with surface area. Because surface area to volume However, most MAVs in regular use are not launched un- ratio decreases with size, this may represent a signi cant der their own power, but rely on gravity or being thrown barrier to the minitaturisation of aerial-aquatic robots. in order to generate their initial ight speed. This di - Flight trajectory planning will also be important, as culty is only compounded by aquatic locomotion, where the ground eect can be exploited near the water surface hydrodynamic drag limits the speeds achievable [21], es- to reduce induced drag, which has been shown to alter pecially at the surface, where additional eects such as maximum range gliding ight paths [24]. However, water wave and spray drag can increase resistive forces by 5 waves and spray will restrict the minimum ying height times their value when fully submerged [22]. Wave drag and aect the local air ow near the surface which will is caused by the formation of a wave pattern around a create ight perturbations, so ight must be robust in body, the wavelength of which increases with speed. As order to do this. the wavelength exceeds the body's length, the stern be- gins to sit in the wave trough, and the vehicle is forced to 3. Biological Design Strategies climb its own bow wave, dramatically increasing power requirements and restrict maximum speed relative to For animals, one of the biggest evolutionary pressures body length. is the need to reduce their energetic cost of locomotion, Furthermore, the takeo power requirements will while still being capable of moving through unstructured be signi cantly increased by the added weight of water and varied terrain. A myriad of animals have evolved a [23], which must be shed as quickly as possible. Wind seamless transition between powered locomotion in water driven surfaces waves can also be a signi cant problem and air, and move through both media with the e ciency for small scale aerial-aquatic vehicles; just as the rela- required to forage and migrate eectively. Bioinspira- tive size of gust perturbations increases for micro scale tion is a particularly powerful tool in the design pro- ight, so does the relative wave height for micro scale cess, where conventional, single-mode design strategies surface swimming. Waves passing across the vehicle can become less applicable. This is especially the case for immerse lifting surfaces and cause the vehicle to crash, the design of microrobots, where energy and power den- and for a taxiing takeo, the vehicle may then have to sity is limited, and trasnport e ciency is of paramount move in the same direction as wave propagation, which importance. will require additional sensors. However, bioinspiration should not entail the blind copying of the morphology or behaviour of natural sys- 2.5. Wet Flight tems, and animals face dierent design constraints to their robotic counterparts. Evolution itself moves to- Once the vehicle has left the water with su cient veloc- wards su cient rather than optimal design, and the evo- ity, it then needs to return to e cient ight. To do so lutionary pressures faced by animals and robots are not it will be important to shed any water weight picked up always the same. For example, many of the features com- during swimming. The amount retained surface water mon in gliding vertebrates, such as a dorsoventrally at- will depend on the surface's properties, but will broadly tened body Aquatic Micro Air Vehicles 5 A. B. C.
D. E. F.
Air Water
Figure 4: Strategies for aerial-aquatic locomotion: (A) Two stage transition: Flying Fish, (B) Hydroplaning: Ducks, (C) Jet propulsion: Flying Squid, (D) Plunge diving: Northern Gannet, (E) Morphing wings: Common Guillemot, (F) Foot propulsion: Great Diving Beetle. Images used with permission from: US NOAA (A), Cappi Thompson (B), Kouta Muramatsu (C), David Tipling (D), Lock et al. [25] (Unaired BBC footage) (E) and Warren Photgraphic (F) or webbing between limbs may be used by other animals during emergence [2]. The wing itself has pointed tips, for display or disguise, rather than any aerodynamic pur- which will reduce induced drag without increasing the pose [26]. Because of this, it is important not to simply root bending moment (which must be supported by mus- duplicate nature, but to examine and abstract key prin- cle tension) [30]. The wing membrane is supported by ciples which guide practical, optimised design. many cartiliginous n rays, with an L-shaped cross sec- In this paper, we adopt the Inspire-Abstract- tion [27]. This structure will enhance wing stiness to Implement paradigm for bioinspired robot design ( gure weight ratio, but also allows tesselation on folding. The 3) [14], and focus on the Inspire and Abstract phases of deployed wing is also given a non-smooth surface by this the process, examining and evaluating the performance structure, which may also oer aerodynamic performance of animals with a view to advancing robotics. For a de- bene ts in the low Reynolds number range [15]. tailed description of this bioinspired design strategy, in- Underwater, a typical 30cm ying sh cruises at low terested readers can refer [14]. speed (1m/s), but is capable of burst swimming at up to 10m/s when leaving the water. However, gliding speeds 3.1. Flying Fish are in the range 15-20m/s and the sh are better able to reach these high speeds in air, where drag is lessened Over 40 species of marine sh have evolved the ability [27, 31]. Additional speed is then gained through a `taxi- to perform extended ights above the water's surface. ing' phase, with wings deployed clear of the water to These animals leap from the water's surface and glide balance body weight with lift, while only the caudal n on enlarged pectoral ns ( gure 4.A), and are capable remains submerged, and can continue to propel the an- of spending over 40s airborne [27]. Beneath the water imal ( gure 4.A). This also enables prolonging of ights the oceanic ying sh (family Exocoetidae) are conven- by intermittently taxiing and `topping up' airspeed, and tional swimmers, with their pectoral ns folded against allows the sh to take more advantage of the ground ef- their body. But when deployed in ight, these ns form fect [24]. As is common with most sh, their skin secretes high performance [28] membranous wings. The wing a mucus which is viscosity-reducing, a surfactant and hy- exibility also allows the sh's body to rotate relative drophobic [32]; features which can reduce drag, ease wing to its wings, providing further stability [29], particularly unfolding [27] and help to shed water in ight. Aquatic Micro Air Vehicles 6
3.2. Flying Squid suggesting that muscle tension may not be limiting in ight. However, apsects of the planform shape could also The only fully aquatic animals displaying what can be be a structural artifact of stretching the wing membrane considered true powered ight are the ying squid (sev- to maximise wing area. eral species mainly from the family Ommastrephidae, g- ure 4.C). Flying squid have two sets of deployable wings. Their rear, main lifting surface is formed by their tenta- 3.3. Aquatic Birds cles, splayed apart to stretch a membrane, and the for- A wide variety of ying birds can also move beneath the ward wing is formed by a pair of ns, which are also used water. To takeo again, many birds will be able to lift for apping propulsion underwater at low speeds [33]. themselves from the water purely by apping, but many The forward ns of ying squid species have a higher as- heavier birds that have adapted for swimming must rst pect ratio than those of non- ying species, which is likely accelerate. To avoid wave drag penalties while doing this, a result of adapting for ight [31]. birds will lift themselves out of the water as they acceler- To power their ight, these animals use a pressurised ate, and hydroplane on the water surface through a com- water jet to provide thrust in the air [34]. This airborne bination of apping and foot propulsion [41]. This can be jet provides the sh with a nal momentum boost, al- seen in gure 4.B, in which the suppression of wave drag lowing them to perform an aerial jump-glide in a similar is made apparent by the absence of a bow wave in front of fashion to ying sh. These ights can last for 7-8 sec- the birds. However, the large amount of spray scattered onds [35] (a full power jet expulsion by a similarly sized by the birds also gives an indication of how energetic a quid takes around 200ms [36]) at peak speeds of up to process this is. Most aquatic birds execute this kind of 11m/s [37]. When swimming underwater, jet propulsion taxiing takeo, with foot propulsion supplementing their is fundamentally less e cient than locomotion through apping power. Where diving birds dier the most is the the beating of ns and tails seen in teleost sh and manner in which they move below the surface. Broadly cetaceans, due to the larger mass of water that must speaking, underwater hunting methods in birds can be be moved for propulsion. However, the thrust response divided into plunge diving directly from ight, and pur- of jet propulsion is much faster, which gives squid advan- suit diving, in which underwater locomotion is initiated tages in escape and manoeuvreability [38]. Furthermore, from a stationary position on the water surface. the jetting mechanism is uniquely applicable to aerial- aquatic missions because it can provide equal thrust in 3.4. Plunge Diving both water and air. The squid's water jet is driven by contractions of Gannets (genus Morus) have one of the more spectacu- its mantle cavity, which can undergo volume changes lar strategies for catching prey below the water surface. of 400% during a full power jet cycle. Driving the jet These birds plunge dive from the air at heights of up to with radial contractions maximises the volume of water 30m, and entry speeds of up to 24m/s [42]. To max- a squid can expel relative to its size, and reduces the in- imise their penetration depth, gannets have a long slen- ternal ow velocity and associated viscous losses (relative der body, and wings that can be swept fully backward to a piston driven expulsion, for example). During the to minimise their cross section when entering the water jetting phase, the water pressure within the squid man- ( gure 4.D) . They also have subcutaneous air sac struc- tle reaches around 25kPa in a typical squid with a 21cm tures, pressurised through connection to the lungs. These mantle length [36]. The squid's cavity muscle cannot structures streamline their bodies, cushion impact and provide su cient power over its full contraction range, provide some means of buoyancy control [43]. Record- and so additional elastic energy is stored in collagen - ings from data loggers attached to the backs of diving bres as the cavity is expanded and lled with water [39]. gannets record no measurable impact deceleration at a The out ow nozzle of the jet is also muscular, and the sampling rate of 32Hz, as argued by Ropert-Coudert et squid can adapt its geometry during jetting to optimise al. [44]. propulsive e ciency. This also allows it to vector its jet Interestingly, a recent computational simulation of thrust to steer. gannet water impact [18] has predicted high impact de- As muscular hydrostats, the wing muscle tension celerations (up to 23g for a 24m/s dive) with force peaks required during ying may limit the duration of squid lasting on the order of 0.1s, which should be measurable ights [37], and the animals are observed to actively by the loggers in [44]. The simulation treats the gan- brake and fold their wings to return to the water [40], net as rigid, and the disparity could be accounted for to diving in a streamlined, nose forward con guration. The some extent by the gannet's air structures and feathers. squid's main wing is formed by soft tentacles, which could The feathers of the gannet are also both air lled and hy- allow the squid to adapt its planform shape. It is sug- drophobic, so will entrain and release an air layer around gested that the squid has 'chosen' a near elliptical plan- the bird's body during a dive, likely to an extent that was form [31], which would minimise induced drag, but shape not captured in [18]. This has been suggested as a source adjustments could reduce the wing bending moment [30], of drag reduction through air lubrication in penguins [45], Aquatic Micro Air Vehicles 7 but it should also be noted that [18] predicts a minimal loons dive with folded wings, propelled by their feet. Cor- contribution to impact force from skin friction. Another morants (Phalacrocoracidae) are some of the most capa- consideration is the fact that the gannet will be exceed- ble foot propelled divers, and are widely studied. Adap- ing the surface cavitation speed on impact [46], though tations in these birds' bone and muscle structure make well below the speed to form a natural supercavity. their speci c buoyancy around half that of other ying While capable of some swimming, the birds gen- seabirds [53], and cormorants are able to achieve neutral erally achieve depth purely through the momentum of buoyancy by swimming to depths greater than 50m, at their dive [47]. When they have reached their desired which point the air in their feathers and lungs is suf- depth, they redeploy their wings as brakes, before allow- ciently compressed by water pressure [54]. At depths ing buoyancy to propel them back to the surface. While where they are still positively buoyant, cormorants use this strategy of minimal swimming limits the birds' un- an intermittent `burst and coast' swim pattern to coun- derwater range, it is noteworthy that compared to other teract upthrust e ciently while swimming [55]. They diving animals the gannet has a much higher foraging do not dive from ight, but will execute a half dive by success rate in terms of food energy per unit time, some- leaping in an arc from the water surface to achieve some thing necessitated by the energy required to remain air- initial momentum. borne whilst seeking a target. These birds are more ef- Buoyancy control is important to diving birds be- cient yers than most aquatic birds, with large, high cause they are obliged to carry with them a certain vol- aspect ratio wings for e cient loitering whilst seeking ume of air when diving. This is for both insulation a dive target. The downside of such large wings comes and respiration, as well as the need for dry plumage when attempting to leave the water, where the apping when returning to ight. As mentioned previously, in stroke length is constrained. Gannets on the surface of peguins (Spheniscidae) the trapped air compressed be- rough seas often must wait for the water to calm before neath feathers can escape during ascent from a dive, they are able to takeo [48]. To compensate for their which may reduce drag by as much as 70% through the limited apping power at the surface, the birds takeo creation of a lubricating air lm [45]. This allows the into the wind whilst holding their wings aloft to gener- birds to leap onto high vertical ice shelves from the wa- ate initial lift, and use dynamic soaring ying techniques, ter. However, the presence of feathers is overall likely to gaining altitude using air currents de ected by surface constitute a drag penalty in steady submerged swimming waves. [56] for thermal insulation, and the ability to y e - Plunge diving behaviour is not limited to gannets ciently [57]. While most pronounced in cormorants, pur- and boobies (Sulidae), but is also exhibited to a less suit diving ying birds have all evolved less waterproof striking extent in smany other bird families, such as plumage than their non-aquatic counterparts to some de- king shers (Alcidinae) [47], pelicans (Pelicanidae) [49] gree, and many of these birds must dry their wings on and some shearwaters (Procellariidae) [50]. These birds land before attempting long ights [58]. The air retain- do not plunge as deeply, and the shearwaters rely more ing features of feathers are a result of their hydropho- heavily on propulsion from their wings to forage [50]. bic microstructure, which is also of great importance for ight from water, where takeo power requirements in 3.5. Pursuit Diving birds can be increased by around 30% with a 10% mass increase from retained water [23]. Auks (Alcidae), particularly guillemots (genus Uria) are almost equally at home moving in air or in water. These 3.6. Diving Insects birds swim by apping in both media, while adapting their wing morphology dynamically to their environment. Many insects are able to swim beneath the water, and Flight favours large wing areas, while underwater ap- some have also retained their wings and can still y in ping is better achieved with smaller, shorter wings [51] air. Beneath the water, diving beetles (Dytiscidae) use and to maximise their e ciency guillemots will morph their legs to kick, with air trapped beneath their wings their wings to a highly swept, lower aspect ratio con gu- for breathing ( gure 4.F)[59]. Without the soft, com- ration when swimming ( gure 4.E) [52]. A consequence pressible air retaining structures of birds, the trapped of adapting smaller wings for apping underwater is an air forces them to constantly swim downward to counter increased wing loading. A Common Guillemot (Uria buoyancy. Across the several thousand known species aalge) weighs 0.9kg and has a wing loading of around of diving beetle, a broad range of morphologies exist. 170N/m2. This can compared to 55N/m2 for a plunge These dierences can be related to the relative coverage diving Brown Booby (Sula leucogaster) of the same mass, of their habitat by water, and that water's ow speed. or an expected 100N/m2 for a 0.9kg ying sh. The high Fast swimming beetles exhibit slender, more streamlined wing loading impedes the birds' manoeuverability and body shapes, and exoskeletons with fewer protuberances, takeo performance, forcing them to ap at much higher to reduce turbulence [60]. rates and y at high speeds. Dragon ies (Anisoptera), arguably one of the natu- Other birds such as cormorants, grebes, ducks and ral world's most capable yers, lay their eggs underwater Aquatic Micro Air Vehicles 8
1.5 3 2.1kg 10 1.0kg 1.8kg 3.3kg A. 1.4 B. 0.9kg ) 36kg 6 0.6kg 1.3 2.5kg 0.5kg 1.0kg (x10 0.2 60g 240g 300g 1 10 1.1 57g 1.0 Flying Re Flying 0.9 Cost of Transport of Cost 4g 10 -1 0.8 10 -6 10 -2 10 5 0.2 10 20 30 Body Mass Swimming Re (x10 5)
Foot Propelled Birds Wing Propelled Birds Flying Squid Swimming Flying Running Plunge Diving Birds Flying Fish Figure 5: (A) The metabolic costs of dierent locomotion strategies (reproduced from [51]). (B) Average airborne and submerged Reynolds numbers for aerial-aquatic animals, with body mass indicated. and will dive directly through the surface, but adults observation. are unable to swim and instead rely on crawling along Flight favors low density, while swimming favours submerged material [61]. While underwater, breathing neutral buoyancy, and pursuit diving birds lose ying air trapped by their wing microstructure allows them ability as their size increases [64]. The largest auk and to remain submerged for hours at a time, but to leave cormorant (Pinguinus impennis and Phalacrocorax per- the water, they struggle to free their wings from surface spicillatus) were ightless and near ightless respectively. tension and must spend a period drying before further Within a given locomotion strategy, most aquatic ani- ight. Dragon y nymphs are fully aquatic, and larger mals show little variation in size and speed ( gure 5.B). larvae (approximately 50mm in length) are able to swim However, squid range in length from 20mm to well over using jet propulsion similar to that of cephalopods. By 10m, and ight has been documented in squid ranging doing so, the animals can produce pulsed water jet thrust from 60mm to over 1m [40], a variation in size far sur- at around 2Hz, with up to 150mN amplitude. However, passing that of any other aerial-aquatic animal. This the jetting mechanism is tied to the larval form's under- suggests a scalability that could make taking inspiration water breathing apparatus, and is abandoned once the from ying squid highly applicable to robot design. dragon y metamorphoses into its adult stage [62]. It has also been suggested that ight in squid and Most insects leave the water by simply climbing out sh oers energetic advantages for migration over aquatic and drying themselves, but several very small species of locomotion alone [34]. This is due to the reduced drag in jumping insect have also adapted to leap from the wa- air, and is against the overall trend in gure 5.A. While ter's surface. Pigmy mole crickets (around 7mg mass) are the subject of contention, this highlights the fact that able to leap to around 60 times their own height from the size is not the only consideration for transport e ciency, water surface [63], but such small insects are reliant on and careful selection of locomotion strategy will also be surface tension to support themselves on the water while key to AquaMAV design. static, and do not have any swimming ability. 4. Existing Aerial-Aquatic Robots 3.7. Sizing and Energetics While no truly aquatic ying vehicle exists, studies have In nature, aerial and aquatic animals show marked dif- recently been conducted on the implementation of plunge ferences in metabolic costs ( gure 5.A). Transport costs diving capability into a ying vehicle [18, 65, 66]. Liang in water are reduced because self-weight does not need et al [66] have developed a testing platform able to fold to be counteracted during motion due to the presence its wings before diving into the water. However, this de- of buoyancy [51]. Figure 5.A also highlights the impor- vice is currently an experimental platform being used to tance of size as a design consideration, and as size is evaluate the requirements for a plunge diving vehicle and reduced increased eects from viscosity, surface tension does not yet have ight capability. and natural perturbantions act to reduce locomotion ef- Flapping propulsion has been demonstrated in many ciency. However, many of the envisaged applications robots, and has many advantages, both in air [67] and in for an aerial-aquatic vehicle (section 1) require a vehi- water [68]. Lock et al. [25] used experimental data to de- cle that can be rapidly transported to an area of inter- velop a model for the eect of wing sizing and morphing est for deployment, and this will be facillitated by small on the power requirements in air and water of a guille- size and low weight. Smaller robots can also function- mot inspired apping wing robot. This enabled them to ally locomote in smaller spaces, which will allow closer propose dierent Aquatic Micro Air Vehicles 9 A. B.
A.
C. D. E.
Figure 6: Aerial-aquatic robotics. Aquatic unmanned aerial vehicles: (A) The University of Michigan's `Flying Fish'and (B) WarriorAero's `Gull'. (C) TACMAV underwater MAV launcher. (D) 'Sea Robin' submarine launched UAV launch sequence. (E) Dropsonde air launched atmospheric reconnaissance device.Images used with permission from: University of Michigan (A), Warrior AeroMarine (B), ARA Force Protection (C), US NCAR (E) optimised geometries depending on the relative amount use a conventional wing layout, execute soft landings and of time the robot would spend in air and water during a are incapable of fully submersing themselves in the wa- mission, but this work was not developed into a practical ter. They are also well out of the micro scale, all having robot. wingspans of around 2m. To takeo, these vehicles rely Robotic apping ight in air currently relies on very on highly buoyant pontoons to keep their propellers po- light, low inertia wings with high apping speeds [69], sitioned clear of the water surface, require large landing and those wings and their associated actuators would be and takeo areas and are not able to move through di - unsuited to use underwater. Furthermore, the loss of cult terrain, such as ooded buildings or areas of oating stroke range when oating on the surface would make it debris. highly di cult to take o without pontoons, the use of The Gull UAV ( gure 6.A) does have the ability to which would then hugely inhibit submerged movement. fold its wings, though this is used for manoeuvering and We therefore suggest that hybrid apping propulsion is storage rather than diving. It also is able to exploit sur- not an immediately feasible design strategy for an Aqua- face waves (0.3m, or 15% of its size) at takeo, using MAV, principally because of the di culty in making such them as ramps. However, this UAV is large in size, and a system transition between locomotion modes. How- those same waves would be a far bigger impediment to ever, future advances in technology may well make this a micro scale vehicle attempting to perform a taxiing more feasible. takeo or a shallow descent. Many robotic aerial vehicles exist which use the wa- While none of these robots are able to operate be- ter's surface as an extended runway. Three examples neath the surface, systems do exist for deployment of un- of UAV seaplanes are the Oregon Ironworks `SeaScout', derwater vehicles from the air. Such systems are reliant Warrior Aeromarine's `Gull' and the University of Michi- on single use, disposable equipment such as parachutes, gan's `Flying Fish' ( gure 6.A/B). The former two ve- airbags and discardable wings ( gure 6.C). However, the hicles are remotely operated, but are able to land and existence of a demand for air launched underwater vehi- takeo autonomously, while the Flying Fish is fully au- cles [11] is important nonetheless, and a realised Aqua- tonomous and is designed to operate as a mobile surveil- MAV would have inherrent capability for air launched lance buoy [70]. All of these aircraft are propeller driven, aquatic response. Aquatic Micro Air Vehicles 10 Hydrophobic mucus helps shed water. Intermittent taxiing to increase speed Flying time limited by muscle tension and oxygen, ight is ended deliberately by air braking and diving.[40] Water repellent feathers shed moisture. Foraging upwind of nest so tailwind aids return ight, when food and water add to weight. Drying period on land [76]. Foraging upwind of nest for e cient return ight. Drying period on land [23]. Foraging upwind of nest for e cient return ight. Drying period on land. Not Possible Sensing of weather conditions to determine appropriate timing for ight Currently a non- ying test platform Taxiing acceleration with only propulsive n submerged [27]. High speed leap followed by water jet thrust to accelerate [37]. Hydroplaning taxiing by apping and foot propulsion. Hydroplaning taxiing by apping and foot propulsion. Hydroplaning taxiing by apping and foot propulsion. Flight from standstill on land. Discardable torpedo and deployable wings Take-o into the wind. Taxi by hydroplaning on pontoons Slender hull, use of surface waves as ramps, pontoons for stability Oscillating tail n propulsion, swim bladder allows changes in buoyancy.[51] Water jet propulsion [72] or apping wings/ ns [33]. Intermittent locomotion used to conserve energy [36]. Little swimming [47], depth gained by dive momentum, buoyancy used for ascent [44]. Foot propulsion with wings folded. Depth compresses air to reduce buoyancy [54]. Intermittent kicking/gliding for e ciency [75]. Flapping propulsion, with wings morphed shorter. [25] Depth compresses air to reduce buoyancy. Kicking foot propulsion with wings folded. Air trapped beneathfor wings breathing. Not Possible Not Possible Not Possible Passive plunge dive Not yet possible , air ◦ Table 1: Key design principles from nature and robotics Wings lie at against the body [27]. Fore wings lie at against the body. Hind wings streamlined backward [37]. Wings swept to 90 sacs cushion impact [43]. Flight into wind for low groundspeed and fast dive response [73]. Slowed descent and soft, oating landing. Short dive-initiating leap from the surface. Slowed descent and soft, oating landing. Dive begins from standstill. Dive begins from standstill. Wings fold beneath forewings/shell [77]. Landing is nal; craft not designed to relaunch Slow, autonomous descent. Landing on twin oats. Slow, autonomous descent. Landing on twin oats. Plunge diving, impact reduced by variable wing sweep Dry FlightFlight in ground eect todrag reduce [24]. Thin exiblefor wings stall performance and stability [28, 71]. Water jet propulsion provides thrust [34]. Elliptical wingreduce to Water wing Entry tension [31]. High aspect ratio wings for e cient loitering.[73] Feathered wings adapt to wind perturbations[71]. Submerged MovementRelatively high weight compensated for by large wings.[74] Feathered wings. Small wings Water adapted Exit for swimming, [51] compensated for by fast apping and short ights [73]. Feathered wings. Insect ight, by Wet rapid Flight apping of hind wings. Dry FlightFuel cell powered propeller thrust Electric propeller with solar cells. Conventional wing and tail layout Conventional seaplane layout, combustion engine, propeller Water Entrythrust. Currently a non- ying test platform Submerged Movement Water Exit Wet Flight ) e) e) ) ) ) Exocoetida Ommastrephida Sulidae Phalacrocoracidae Alcidae Dytiscidae Animal Flying Fish ( Flying Squid ( Gannet (plunge diving) ( Cormorant (foot propelled) ( Guillemot (wing propelled) ( Diving Beetle ( Robot US NRL 'Sea Robin' U Virg. `Flying Fish'[70] WarriorAero `Gull' Beihang U `Bionic Gannetl' Aquatic Micro Air Vehicles 11 A. B.
C.
Figure 7: Bioinspired principles for AquaMAV: Plunge diving by gannets (A), Jet propelled take o by squid (B), Plunge diving AquaMAV concept sketch. Images used with permission from: Alexander Safonov (A) and Bob and Deb Hulse (B).
Several military programmes have attempted to cre- 5.1. Jet Propulsion ate manned submersible aircraft [78, 79], but to the au- thors' knowledge these endeavours have not been com- The most challenging part of an aerial-aquatic mission is pleted. Now, the recent shift towards unmanned tech- to re-initiate ight after diving. A vehicle that has been nology has opened up opportunity for MAV designers. designed for eectively plunging would still need to be The US Navy Research lab has recently demonstrated minimally buoyant, in order to maximise its dive depth. the succesful launch of a UAV from a submarine mis- This would then make a taxiing takeo near impossible sile tube ( gure 6.D). The vehicle (called `Sea Robin' due to the wave drag acting on a vehicle with high dis- UAS) uses a discardable tube which brings the vehicle to placement. the surface and provides launching power. At a smaller Flying sh and birds are able to leap directly from scale, the ARA Inc. `TACMAV' can also be deployed the water, and avoid taxiing takeo complications How- from below the surface, by divers using an airbag and ever, Flying sh must rst reach submerged speeds of compressed gas to launch an MAV ( gure 6.E). Both of around 20 body lengths per second. This performance these vehicles feature deployable wings, but neither have represents an order of magnitude increase over what has any true aquatic capability and are solely aerial vehicles been achieved up to this point by biomimetic underwa- after takeo. ter robots [80], and the power availability of small scale mechanical actuation systems cannot yet match that of bird and sh muscle, without prohibitive sacri ces in fre- quency response or stroke rate. This will make it hugely 5. Bioinspired Design Principles di cult to takeo from the surface of the water by ap- ping, or biomimetic swimming. However, water jet propulsion has a very rapid We have summarized the advantages and limitations of thrust response that is di cult, if not impossible to several animals and robots with aerial-aquatic capabili- achieve using propellers, apping wings or beating tails, ties in table 1. While each animal has a distinct set of and as such is ideal for an impulsive takeo. Compared locomotion modes, several recurring principles can still to apping and beating, a full power squid escape jet is be identi ed. For example, all animals employ a form of also mechanically simple. Importantly, it can continue to hydrophobicity to shed water or retain air, most feature produce thrust when clear of the water surface and its as- some means of buoyancy control and all fold or modify sociated high drag, unlike n based water surface jumps, their wing structure when moving in the water. which rely on reaction forces from the surrounding uid. Here we select from our review several bioinspired The high hydrodynamic drag in water makes it un- design principles, and propose a that a vehicle capable economical to launch directly through the surface, and of a jet propelled takeo, utilising folding wings and able it is better to accelerate when out of the water. How- to plunge dive is the best route for realising AquaMAV. ever, leaping directly out of the water is far more robust, Aquatic Micro Air Vehicles 12 because it does not rely on a clear expanse of water to brane is reminiscent of recent MAV designs such as [85] accelerate in. In this sense, the ying squid's jet propul- and [86]. With regards to deployability, the wings are at- sion strategy achieves the best of both worlds, by being tractive because they are actuated entirely at their root able to produce thrust continuously while transitioning and do not contain any internal musculature, unlike the between media. This allows an airborne transition stage wings of birds and bats. This is also the case with insects without the need for the surface taxiing phase required such as dragon ies and butter ies, considered to be some by birds and sh, and when on water, a jet propelled of the highest performing ying animals. method of launch is also robust to environmental condi- Flying sh and squid both use dual pairs of wings, tions, requiring far less takeo area than taxiing takeos. which makes their ight very stable [28], and four-winged One mission an AquaMAV would be uniquely able ying sh have also been observed to glide for greater to perform is the collection and rapid return of a water distances than their two-winged counterparts [29]. Simi- sample from a hazardous area. Because a sample return larly, insects with gliding ability also tend to adopt a bi- mission only requires one takeo from water, the authors plane con guration, and biplanes have also been shown suggest that the energy required to create the jet thrust to have advantages over monoplanes in induced drag and could come from a `single-shot' system using compressed minimum speed for MAVs. The reduction in cruise ve- air. The propellant water mass can then be collected locity and resistance to stall would be very advantageous in situ after diving into the water, to keep ight mass for a momentum-limited takeo from water and we pro- down. While the energy density of such a system would pose implementing the same layout into an AquaMAV. not match that of a combustible rocket, compressed gas In ight, if the forward wings could be folded indepen- is far less hazardous to its environment. Solid rocket fu- dently of the rearward, this mechanism would also be els are highly controlled by regulations, are di cult to su cient to initiate a dive. miniaturise [81] and many situations (an oil rig accident, for example) would preclude the use of re by exploring 5.3. Plunge Diving robots. Among aquatic birds, plung diving birds show the least detriment to their ying ability as a result of adapting 5.2. Wing Folding for the water, and their landing strategy is robust, sim- To the best of the author's knowledge, no truly xed- ple and does not require additional propulsion or neutral wing animals exist, and wing folding is a feature of most buoyancy to penetrate beneath the water's surface. Es- ying creatures, but it becomes more important when pecially with regards to mechanical design feasibility, the locomotion in water is required. Folding wings reduce gannet's plunge dive represents a robust and practical the vehicle's pro le underwater, which can greatly re- strategy for entry, just as the squid's propulsion system duce drag. Folded wings also do not have to support as is an eective means of takeo. large a hydrodynamic load, which reduces structural re- The requirements of accurate control and a large quirements, and hence mass. Of the discussed animals, area during taxiing takeo also exist for a soft water land- only the guillemot does not fully collapse its wings when ing. However, plunge diving in the manner of the gannet in the water, and these birds must still morph their wings avoids the complications of soft landings. A plunging ve- into a much lower aspect ratio con guration in order to hicle would need minimal landing area, and would need swim. It is reasonable to then suggest that for an aerial only a GPS waypoint to execute a landing in an area of vehicle to submerge, some degree of wing folding will be known bathymetry. Dive velocity can be controlled by obligatory. varying initial dive height, and will allow accomodation In robotics, rigid winged underwater gliders do ex- for shallow water. The maximal dive depth will be lim- ist, but these craft are too heavy, with wings too small ited by the structural loads at impact, but the favourable for aerial ight without very high energy density propul- scaling of strength as size is decreased means that MAV sion. Many existing aerial vehicles do employ some kind size range is better suited to this type of locomotion. of morphing wing [82, 83], and others feature deploy- In order to design for eective plunging, folding able wings [3, 84]. However, wing deployment systems wings will again be essential, and the gannet's strategy are often merely for storage, without the ability to re- of sweeping wings fully backward at entry is a mechani- fold, and most morphing concepts only vary the wing cally simple way to implement this. Such a wing folding geometry slightly. Implementing high performance wing system should ideally be designed such that folding the folding into an AquaMAV will require improvements in wings shifts the centre of buoyancy aft of the centre of mechanical design over existing systems. mass. This would that the dive would be stable under Flying sh wings are a mechanically attractive in- buoyancy forces, but at the termination of the dive, un- spiration source for deployable wing design. Their wing folding the wings would move the centre of buoyancy structure, with rigid battens supporting a exible mem- forward and orient the vehicle nose up for launch. Aquatic Micro Air Vehicles 13
A. B. C. F (t)+T(t) Delrin cap int
Schrader Water Level valve core θ pgas (t) Polyethylene body 45° patm
m(t)g u (t) y jet
Delrin nozzle cap x
Nitrile rubber diaphragm
D. Trajectory E. Thrust 5 4.5 2.5 4 3.5 2 3
1.5 2.5 Y (m) Thrust (N) Thrust 2 1 1.5 1 0.5 From Land From Land (Theory) 0.5 From Water 0 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 2 4 6 8 10 12 14 16 18 20 X (m) Time (ms) Figure 8: Miniature water jet experiment: Jet Schematic (A); Launch setup, with water level during aquatic launch indicated by blue line (B); Theory nomenclature (C); Tracked launch trajectories (D); Simulated thrust (E).
6. Proof of concept experiment air-water interface to the nozzle exit ( gure 8.C). Total pressure along this streamline is equal to the instanta- We have proprosed that water jet propulsion would be neous gas pressure, multiplied by an e ciency factor, the best way of launching an AquaMAV without relying η, that represents the losses due to viscosity as a pres- on hazardous combustible material. To do validate this sure drop. The gas expansion takes place over a very proprosal, a simple water jet device has been fabricated. short time period, meaning that the expansion process The jet is a miniaturisation of the ubiquitous 'water can be approximated as adiabatic [87] and the instan- rocket', adapted so that it can be actuated electronically. taneous gas pressure can be given by the isentropic gas The device contains air and water free to mix, and can be relation (equation 1). pressurised through a small SchraderTMvalve core. The γ (1) pressure can be released by bursting a rubber diaphragm pgas = p0 (V0/V ) using a hot wire ( gure 8.A). The plastic jet is not in- ˙ s V 2η(pgas − patm) (2) tended to immediately form part of an AquaMAV, but ujet = = 2 Ao ρ (1 − (A /A ) ) simply to serve as a basis for discussion (the device has w o i immediately obvious shortcomings, such as the inability η(pgas − patm) (3) to launch at shallow angles, due to the lack of separation T = 2Aexit 2 1 − (Ao/Ai) between water and air). Z I = T + Fint dt (4) 6.1. Theoretical Basis Where p0 and V0 are the initial gas pressure and The incompressibility of water means that the water exit volume, V is the gas volume, Ai and Ao are the nozzle velocity can be calculated using mass continuity and inlet and exit areas and ρw is the density of water ( g- Bernoulli's equation along a streamline running from the ure 8.C). The variation of thrust with time can then be Aquatic Micro Air Vehicles 14 obtained by numerical integration, which allows launch 5). trajectories to be computed. An additional force term, is the force experienced by the vessel due to the Vtot Fint(t) Z p V (β − βγ ) internal uid mass' acceleration during jetting. E = p dV = 0 0 (5) gas 1 − γ The trajectory is modelled by treating the jet a par- βVtot ticle with a drag coe cient of 0.79 (as measured by Ben- This gives a scalling of the form . For net [88] for a similarly sized cylinder). If friction from E ∝ pV a thin walled cylindrical pressure vessel with a given the lubricated launch ramp is ignored, the discharge co- safety factor, the vessel mass will scale in the same way. e cient is the only unknown in the model, and can be This means that the energy density ( ) of a com- estimated by tting the modelled and tracked trajecto- E/m pressed gas water jet is purely a function of the material's ries. This gives a value of 0.89. strength to weight ratio. The polyethylene used for the Integrating the propulsive force with respect to time abve jet has a speci c ultimate strength of 24 kN.m/kg, ( gure 8.E) gives the total impulse (equation 4). The op- so the use of a high performance material such as carbon timum air-waiter volume fraction can then be found by bre (2300 kN.m/kg) could potentially increase a wa- simulation. A fraction of air of 36% was found to β = ter rocket's maximum perfomance a hundredfold. This give the maximum speci c impulse ( ), and this I/mtotal also means that the jet's energy density is almost en- volume fraction was then used during tests. tirely independent of both the vessel size and internal gas pressure, a scalability which will be a great asset to 6.2. Experiment AquaMAV design. While increased pressure will not increase energy The jet has an empty mass of 2.55 grams (4.0 grams full), density, it will increase the maximum release rate of the an internal volume of 2.6ml, and is 66mm long. The de- energy stored. This is a key bene t of water jet propul- vice can be pressured to 9bar, and is the capable of a sion; while compressed gas lags behind modern batteries self propelled jump in air to a vertical height of 4.8m, in terms of energy density (0.17kJ/kg for the plastic jet equivalent to 72 times its own size. versus 84 kJ/kg for a typical 3.5g lithium polymer bat- To test jumping out of water, the jet was launched tery), the power that can be developed is much greater. at a 45 degree angle ( gure 8.B) and lmed at 120 fps to This is critical if a short, impulsive takeo is to be ex- track trajectories. Video tracking range was limited by ecuted. The unoptimised plastic jet shown here has a camera eld of view, so horizontal jump ranges were mea- power density of around 8kW/kg, which already greatly sured manually. When launched in air, the jet is able to exceeds that of conventional electromechanical systems reach a height of 2.37m, travelling a horizontal distance (around 0.5kW/kg for a typical 10g coreless dc motor and of 7.4 (40 times its size). The jet was then immersed in 0.4kW/kg for the aforementioned 3.5g lipo battery). It is a tray of water, such that the tip was just beneath the estimated by Gao et al. [21] that an at-scale biomimetic water surface ( gure 8.D). When launched from the wa- ying sh robot would need to generate 4kW/kg from its ter, additional drag reduced the jump range by 32% to swimming actuators in order to achieve the same launch 5.1m, (1.39m high). speeds as a real ying sh. This means jet propulsion is well within the feasibility range for such a system, even if Table 2: Miniature water jet performance the decreased propulsive e ciency of jet propulsion rel- ative to teleost swimming is considered. Mass (full) 4.0 g Mass (empty) 2.55 g Peak thrust 4.5N 7. Conclusion Stored energy 0.67 J We have demonstrated the e cacy of our proposed take- Energy density 169 J/kg o strategy with a proof of concept jet which can launch Thrust duration 20ms itself from both ground and through the air-water bound- Average power 34.1 W ary. This device is unoptimised, but can be easily mod- Power density 8526 W/kg elled, and an optimised jet scaled up to an appropriate Speci c impulse 9.5 s size would be su cient to launch an MAV. The signi - Vertical launch 4.8m cant challegne is then to produce a folding wing structure height capable of sustaining plunge dive loads, without com- primising the aerodynamics of the vehicle in free ight. 6.3. 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