ABOUT THE AUTHOR
5.1 INTRODUCTION
For centuries, humans have been inspired by the flight of birds, bats, and insects. Early attempts at building aircraft by replicating the shape of
bird and bat wings, without understanding the underlying principles of aerodynamics that govern flight, resulted in failure. It was only systematic observations of bird flight and morphology, in conjunction with experiments on models, that led to successful glider designs by pioneers such as Otto Lilienthal, finally cul-minating in the powered flight by the Wright brothers.
The aircraft that we see around us today have little in common with biological flyers except for the general shape and basic aerodynamic prin ciples. There are no aircraft in production today that incorporate flapping wings. There are num erous aircraft that bear little resemblance to the planform of birds in flight. Modern aircraft have far exceeded biological flyers in many aspects of performance, such as maximum speed and payload. However, they still cannot compete with biological flyers in other aspects such as maneuverability, gust recovery, and autonomy.
In general, modern aircraft development has seen an increase in maximum gross weight, payload fraction, and maximum speed of the aircraft. However, over the past decade, there has been considerable interest in miniaturizing C H A P T E R
aircraft to create a class of extremely small, remotely piloted vehicles with a gross weight on the order of tens of grams and a dimension on the order of tens of centimeters. These are collectively referred to as micro-aerial vehicles, or micro air vehicles (MAVs).
The concept of using miniaturized, remotely piloted aerial vehicles for covert surveillance is not new. In the 1970s, the Central Intelligence Agency (CIA) developed an insect-sized, mechanical dragonfly to carry a miniature listening device [1].
The flapping wings of the dragonfly were actuated by a miniature engine powered by a liquid propellant. This MAV was designed to be steered using a laser beam. Due to difficulties in controlling the dragonfly, the project was not pursued beyond the fabrication of a flying prototype, which is now on display in the CIA museum.
In the early 1990s, a research project at Los Ala-mos National Laboratory theoretically investi-gated the feasibility of microrobots fabricated using microlithographic techniques for military uses [2] such as intelligence gathering and sen -sing or disruption of a variety of environmental stimuli (electrical, mechanical, and chemical). The small size of these systems would have made them difficult to detect, and the intent was to increase the probability of mission success by deploying a large number of microrobots. Mass production, similar to the process used to fabri-cate integrated circuits, was expected to keep the costs of each individual robot low. During the course of this research, the conceptual design of a rotary-wing vehicle was explored using the smallest commercially available electromagnetic motors (∼1.5 g in mass), with different rotor blades, thin-film batteries, and miniaturized video cameras, acoustic sensors, and communications chips. Four different flying vehicle configurations were also investigated: fixed-wing, rotary-wing, microairship, and a passive, autorotative device based on a maple seed. The conceptual fixed-wing vehicle had a total mass of 4 g, with 1 g of sensors and a cruising speed of 900 cm/s. The conceptual rotary-wing vehicle had a counter-rotating rotor
configuration with a sensor mass of 1 g, a cruising speed of 200 cm/s, and an endurance of 5 min.
The conceptual microairship had a total mass of 1.8 g, featured an almost transparent film enve-lope filled with hydrogen, and had a cruising speed of 200 cm/s. The conceptual autorotating device had a total mass of 0.3 g, with a wing area of 1.5 cm × 5 cm. These were designed to remain aloft for the maximum time possible after being deployed by a stealthy mother vehicle.
In late 1992, the RAND Corporation con-ducted a workshop for the Advanced Research Projects Agency (ARPA) on “Future Technology-Driven Revolutions in Military Operations” [3].
The objective of this workshop was to identify breakthrough technologies that could revolu-tionize future military operations. The identified applications included a “fly on the wall,” or a miniature fly-sized vehicle carrying sensors, navigation, processing, and communication capabilities. The vehicle design featured the ability to move around by flying, crawling, or hopping. Another application involved the addition of a “stinger” on the vehicle that was intended to disable enemy systems. For concep-tual design, a vehicle mass on the order of 1 g, with a size on the order of 1 cm, was selected.
The power required for hovering and for for-ward flight was estimated, using momentum theory, to be ∼30 mW/g and ∼45 mW/g. In comparison, the hovering power requirement of large insects ranges from ∼9 mW/g to 19 mW/g, and for hummingbirds, ∼19 mW/g to 26 mW/g.
Based on using a 530 J thin-film lithium polymer battery, this was calculated to yield an estimated hover time of 4.9 h and a flight time of 3.3 h, covering 80 km.
In the late 1990s, the Defense Advanced Research Projects Agency (DARPA) released a solicitation for MAVs that would have a dimen-sion no larger than 15 cm, a mass of about 100 g (with a payload of 20 g), and a mission endur-ance of around 1 h.
These vehicles were intended to be man-portable robots that could fly to a target and
5.1 INTRODUCTION 109
relay video and audio information back to the operator. In this way, the MAVs would enhance the situational awareness of the soldier. Other possible civilian applications of MAVs included sensing of biological/chemical agents in an accident zone without risk to the human operator, fire rescue, traffic monitoring, mobile communications links, and civil structure inspection. Responses to this solicitation included several fixed-wing MAVs such as the MicroStar by Lockheed Martin and the Black Widow by Aerovironment. Rotary-wing MAVs included the LuMAV by Lutronix Inc. In general, it was observed that the fixed-wing MAVs outperformed the rotary-wing MAVs in terms of cruise speed, range, and endurance. However, the major advantage of the rotary-wing MAVs is their hover and low speed capability, which is very useful for surveillance indoors or in confined areas. The key design requirements for a MAV as described by this solicitation are listed in Table 5.1. Note that these specifications are very stringent; over the years, the term MAV in published literature has been used to refer to unmanned aerial vehicles with a range of dimensions, from palm-sized to meter-sized.
Recently, DARPA released specifications for the nano air vehicle (NAV) program [4]. The goal of this program was to develop a vehicle even smaller than the MAV specifications. The gross mass of the NAV was specified as 10 g, with a payload of 2 g. Configurations that were selected for Phase 1 of this program were a coaxial heli-copter, a flapping-wing vehicle inspired by a hummingbird, a flapping-wing vehicle inspired
by a cicada, and a single-bladed rotary-wing vehicle (typically called a monocopter) inspired by a maple seed. The size, mass, and performance requirements of the NAV were intended to push the limits of aerodynamics, propulsion systems, and electronics. The vehicle based on the hum-mingbird was selected for further development in Phase 2 of the program. The final prototype was capable of stable, controllable flight indoors as well as outdoors, with an onboard camera and a fuselage fairing that made it look like a real hummingbird. The prototype met all of the original specifications except for the gross mass.
However, all the components were commercially available and it is expected that developing components specifically optimized for this application will enable a significant reduction in gross mass.
The Air Force Research Laboratory has also released a future-vision plan that describes a fully autonomous robotic bird by the year 2015 and a fully autonomous robotic insect by 2030. Several research groups are currently investigating a variety of issues related to such vehicles, specifi-cally focusing on flapping-wing aerodynamics, wing aeroelasticity, gust response, stability, and control as well as autonomous flight.
Bioinspiration and biomimetics form a com-mon theme of many of these micro and nano air vehicles (referred to as microflyers), for two key reasons. The first reason is the belief that a microflyer performing a surveillance mission can remain undetected by looking like a real bird or insect and literally hiding in plain sight.
The second reason is that by virtue of their size, microflyers fall in a size regime that is naturally populated by large insects and small birds. It is believed that by copying several of the charac-teristics of these natural fliers, man- made micro-flyers can improve several aspects of their performance such as flight endurance, maneu-verability, and gust tolerance. However, it is important to caution against blindly copying biological systems without properly under-standing their function. It is quite tempting to TABLE 5.1 Key MAV design requirements.
Maximum dimension <15.24 cm
Take-off mass <100 g
Range Up to 10 km
Endurance (loiter time) 60 min
Payload mass 20 g
Maximum flight speed 15 m/s
conclude that if a certain feature exists on a bird wing, and the bird flies well, then that feature is essential for flight. An example of this reasoning is to conclude that feathers on birds, by virtue of their beneficial aerodynamic properties, must have evolved to enable flight. However, it is now a widely accepted fact that birds evolved from theropod dinosaurs, and feathers evolved for several reasons before the ancestors of birds could fly. Some of these reasons include thermal insulation, water repellancy, and coloration to attract a mate. Numerous fossils have confirmed that feathers existed in nonavian dinosaurs.
These early feathers reflect the stages of feather development predicted by theoretical reasoning based on evolutionary developmental biology [5]. It has been stated that “proposing that feath-ers evolved for flight now appears to be like hypothesizing that fingers evolved to play the piano” [6].
This chapter describes recent developments in the area of manmade microflyers, along with fundamental limits to their performance.
Because the focus is on biomimicry, scaled-down versions of conventional aircraft, such as fixed-wing micro air vehicles and micro-heli-copters, are not discussed.