Nano and Micro Aerial Vehicles

Nano Aerial Vehicles and Micro Aerial Vehicles are primarily used for military applications which include Intelligence, Surveillance and Reconnaissance (ISR missions), and used for both indoor and outdoor missions, which represents a challenging environment factor to overcome. The use of MAVs and NAVs reduce human risk as it does not require an on-board pilot for the mission, they also tend to improve the assessment of danger and have visibility in the dark when equipped with infrared cameras.

Other payload equipment include numerous tactical and strategic advantages integrated with the use sensors for high risk applications to locate and analyse biological and chemical gases as well as nuclear radiation and other threats as stated by Petricca, Ohlckers and Grinde (2011).

Historically, the development of unmanned aerial vehicles is credited to advances of model airplanes throughout the 19th and 20th century; Mueller (2011) states that the 19th century brought about the development of model sized airplanes which lead to the 20th century integrating its technological advancements in radio receivers and propulsion systems to develop radio controlled airplanes. Moreover, it should be noted that propulsion systems were enriched drastically from simple rubber band propellers to liquid fuel internal combustion engines and electrical motors powered by batteries and fuel cells.

However, in order to comprehend the various types propulsion systems employed on board the Unmanned Aerial Vehicles (UAVs), certain parameters and configurations of Nano and Micro Air Vehicles must be explored along with its aerodynamic constraints. Firstly, NAVs and MAVs are subclasses of UAVs; it is critical to note that NAVs are limited to being less than or equal to 7. 5 cm in length, width or height, while maintaining a weight which is less than 10 grams as defined by Defence Advanced Research Projects Agency (DARPA) in 1997 .

Whereas, MAVs are limited to having a maximum wingspan length of 15cm while maintaining a weight which is less than 20 grams as outline by DARPA. Figure 1: Reynolds Number for Air Vehicles sourced from T. J. Mueller, “Aerodynamic measurements at low raynolds numbers for fixed wing micro-air vehicles,” Tech. Rep. , University of Notre Dame, Notre Dame, The Netherlands, 2000. With the current technology and understanding of aerodynamic factors for these miniaturized air vehicles, the configuration of the Nano and Micro Aerial Vehicles tend to determine or rather limit the range of applications they are used for, here are four main configurations; fixed wings, rotary wings, flapping wings and passive. However, the latter configuration will be excluded from this report as it does not have a propulsion system on-board to analyse, but rather uses a crude method of being dropped or hand launched. Figure 1, on the right depicts one of the major aerodynamic challenges with the use of NAVs and MAVs (Mueller 2000).

It can be seen that for a large commercial aircraft such as the 747 the Reynolds number is around 2,000,000,000 whereas the NAVs and MAVs have low Reynolds numbers; In cases where there is a low Reynolds number (typically less than 100,000), it is proven that the aerodynamic efficiency (L/D ratio) decrease at a more rapid rate. Other challenges include system integration of the airframe, communication, processor, sensors and power for operational function-ability with in the specific weight limits to keep consumption of power to a minimum.

The fixed wing configuration is designed for high speed flight where hovering and flying at low speeds are unnecessary; it is essential for the fixed wing configuration to have a thrust to weight ratio of less than 1, as it requires less power to fly since the configuration of the wings impart an additional lift component. Since the fixed wing configuration is limited to high speed flight, it is typically used for outdoor applications. It should be noted that there are two subclasses; rigid wing and flexible wing.

The flexible wing which has a ‘deformable wing [and] is expected to harvest an intrinsic benefit: a portion of the energy that would normally be lost to the wing-tip vortices and wake, downstream of the MAV, now is stored as elastic strain energy in the wing’s structure’ (Zhang et al 2008, p. 252) which contributes to a better L/D ratio at an angle of attack less than 10. Furthermore this configuration is limited to current MAVs, as existing prototypes exceed a wingspan of 15cm.

The fixed wing configuration is able to employ propulsion systems such as an electric motor, internal combustion engine or micro gas turbine engine (Ling et al 2006; Petricca et al 2011). The rotary wing configuration is designed for both high speeds and for hovering; it is also capable of vertical take-off and landing. It is electrically powered, and has high power consumption, due to the miniaturization process the efficiency of the rotor systems is decreased and it also has a low thrust to weight ratio, hence a limitation on endurance.

However, it is capable of highly manoeuvrable actions and is regarded the most controllable of the configurations. The rotary wing configuration is able to employ electric motors to turn the rotary blades. Rotary wings configuration has six subclasses configurations which basically employ a number of rotor wings at certain geometric designations, however the use of more than one rotor increases the overall weight and thus increases the power consumption (Petricca et al 2011).

Figure 2: Rigid-body representation of the parallel crank-rocker mechanism. Sourced from Ling, CS, Hyde, R, Conn, A, ;amp; Burgess, S, 2006, From natural flyers to the mechanical realization of a flapping wing micro air vehicle, (online), http://ieeexplore. ieee. org/xpls/abs_all. jsp? arnumber=4141906. The flapping wing configuration is designed to overcome challenges presented by the other two configurations regarding the low Reynolds number; it adapts techniques from nature by observing insect and bird flight characteristics.

Hence there are two sub classes; entomopters- insect like vehicles and ornithopters – bird like vehicles, both try to generate lift by flapping its wings. However, entomopters flap their wings at a greater and faster variation of angle of incidence thus creating more lift which allows them to hover as well as being able to vertically take off and land whereas ornithopters flap their wings in a more coordinated manner at smaller changes of angle of incidence, which creates a forward thrust, but leaves them incapable of hovering.

To achieve propulsion for ornithopters flapping wing configuration an electric motor with a crank rocker mechanism is employed as can be seen in figure 3, when the electric motor with the crank rocker mechanism is in motion (complete revolution); the flexible wing creates a flapping motion in the left and right direction. Whereas to achieve propulsion for entomopters, insect wing muscle movements must be mimicked and this can be done with the use of linear actuators or with electroactive polymers (EAP) which delivers high efficiency and high energy density (Ling et al 2006; Petricca et al 2011).

Figure 3: Coreless Motor sourced from CITIZEN MICRO CO. , LTD. , 2009, http://www. citizen-micro. com/tec/corelessmotor. html. Electric motors exhibit desirable qualities such as reliability, high efficiencies as well as easy to control characteristics; specifically coreless motors which also contribute a reduction in weight and even higher efficiencies due to the absence of the iron core compared to that of the DC iron core motor. Figure 3 depicts the structure of a coreless motor; it can be seen that the magnet is placed inside the coil, and no iron material is used.

The depicted coil in figure two is very light hence is limited to a small inertia which permits rapid accelerations and decelerations. However the drawback of employing a coreless motor (without the iron core) is that; it is susceptible to overheating since the use of iron core also functions to dissipate the heat. Electric motors are used for all three configuration; propeller driven electric motors, rotor driven electric motors and flapping wing using crank rocker mechanism with electric motors (Citizen Micro Co. ,Ltd. , 2009).

Figure 3: Components of MTE Sourced from ONERA, the French Aerospace Lab http://www. onera. fr/defa-en/thermal-mems-micro-machines/decawatt. php Miniaturized Internal Combustion Engine has higher energy densities for flight, which extends the range of missions. Since it has a noise factor to consider, it is in adequate for stealth and tactical NAV missions. Miniaturized Internal Combustion Engines have two subclasses; an intermittent combustion cycle which uses pistons and a continuous combustion cycle which uses turbine blades, i. . Micro Turbine Engine. These types of engines can only be employed on the fixed wing configuration on MAVs as it is designed for high speed flight and long ranges. Figure 4 depicts a single stage micro gas turbine engine, it can be seen that it’s very similar to that of larger turbine engines in both component-wise and function-wise, however micro turbine engines follow principles of micro combustion and have higher power to weight ratio than that of other ICE.

A noticeable drawback of this system is that it tends to lose efficiency at lower power levels (ONERA – The French Aerospace Lab 2011; Petricca et al 2011). Reciprocating Chemical Muscle (RCM) is primarily a technique used for MAV entomopters in mimicking insect like wing beating using a mechanism that transforms chemical energy into motion without combustion and it can also power electricity to the control systems on-board at small amounts. It improves lift on the wings as well as pitch and roll. It has greater energy densities obtained from the chemical reactions.

Similarly Electroactive Polymers (EAP) can be used as artificial muscles to replicate the insect wing beat muscles, however this technology uses an electric field to stimulate and change the size and shape of the polymer. Although these fields are relatively fresh, further advancements would likely optimize the current entomopter (Deyle 2009; Michelson et al 2011). Hybrid propulsion systems may include the use of electric motors and Internal Combustion Engine motors, and although it is possible for UAVs to integrate both, it is not applicable to NAVs as size and weight will be compromised.

It should be noted that during the testing phase cooper-harper rating is used to evaluate and determine control characteristics of MAVs and NAVs, which makes it easier to identify whether that specific MAV or NAV is capable of performing a specific application (Petricca et al 2011). It is evident that the propulsion system employed and the wing configuration play a significant role in determining and limiting the scope of applications that Micro and Nano Aerial Vehicles can perform.

In this report, it can be seen that propulsion systems with the use of electric motors are currently the most desirable as it can be employed in all three configurations, however the electric motors for the rotary wing configuration stands out the most, as it is highly reliable, has high efficiencies and high manoeuvrability and control while being able to VTOL and hover, furthermore the six different sub configurations also adds to the wider range of applications.

Although its major drawback is its lack of endurance, we will see a change in this aspect in the near future; as energy storage improves with development of Ni-Cd Batteries and fuel cells. By overcoming this hurdle, the electric motor could possibly be the most desirable. However it is also important to note that the propulsion system in endomopters will be competing to be the most desirable (perhaps much later in the future) as RCM and EAP technologies are advanced and are integrated to outperform the electric motor.

When this done, (Davis 2007; Deyle 2009) flapping wing endomopters will be the most desirable because it will be capable of doing everything the rotary wing configuration can do (high manoeuvrability and control, VTOL, hover and etc. ) but will also be much smaller in size and thus more appealing to ISR missons. As for micro turbine engines, it can be said that there will always be a demand for it, as it is the best propulsion system specifically designed for fixed wing long range flights at high speeds, development in this field will continue as both weight and power consumption are decreased.

The engineering field of UAVs continues to grow, in the last two centuries technological leaps have been made from no propulsion systems to rubber band propulsion systems to finally ICE and electrical motors which makes up majority of the systems implemented for propulsion; the 21st century has just begun, and it is fascinating to see how new technologies are given birth to in order to overcome various aerodynamic hurdle i. e. RCM and EAP.