CL = Coefficient of Lift
Camber = Aerodynamic Shape of a Wing Section
Entomopter = (as a noun, loosely) "insect flight" vehicle
MEMS = Microelectromechanical Systems
Ornithopter = (as a noun, loosely) "bird flight" vehicle
RCM = Reciprocating Chemical Muscle
UAV = Unmanned Aerial Vehicle
µAV = Micro Air Vehicle, also MAV or MicroFlyer
I. Introduction
Efforts are underway to develop a mechanical "Entomopter" (entomo as in entomology + pteron meaning wing, or a "winged insect machine"), capable of short distance trimmed
flight and ground locomotion using a "reciprocating chemical muscle" (RCM) technique.
In addition, autonomous navigation schemes (homing) based on Georgia TechÕs
integrated-optic interferometric waveguide sensor will be developed as a means
of directing controlled flights and crawling in the futureÐ initial efforts
at autonomous guidance will concentrate only on directed crawling behavior .
Significance of the Research
Because truly "micro" air vehicles (µAV) must integrate smart materials, micro
electromechanical systems (MEMS) sensor and actuator elements, energy sources,
power converters, as well as miniaturized computing and communication circuits
to achieve intelligent navigation, self-stability, and propulsion in a palm-sized
automaton, structures will have to be multipurpose to meet the density constraints
imposed. For example, trailing antennas might double as trim stabilizers and
legs might perform not only as landing gear and ground locomotors, but as fuel
storage to increase the moment of inertia as an aid in roll stability during
flight.
Prior Art and Understanding
Currently all SBIR-funded µAV efforts as well as those proposed by other universities
and industry are geared toward fixed wing micro air vehicle applications. Nothing
in creation exhibits fixed wing flight behavior or propeller-driven thrust.
Everything that maintains sustained flight, uses flapping wings. Even though
there has been considerable analysis in the literature of mechanisms for bird
flight (Ellington1, 1984) and insect flight (e.g., Azuma2, 1992, and Brodsky3,
1994), and ornithopter-based (bird flight) machines have been demonstratedÐ
nothing at the size level of the entomopter has been tried.
Some Potential Applications
The creation of a mechanical insect capable of even simple trimmed flight will
go beyond the 'gnat robotics' of Rod Brooks at MIT (Flynn, Brooks6, 1989) which
were only able to crawl. The addition of the potential for homing behavior sets
the stage for advances in intelligent flight control and improved propulsion/endurance
which will make the entomopter a useful tool for urban military missions in
which remote eyes and ears will need to penetrate buildings and bunkers. The
same behavior could be used commercially in farming to make tiny "Terminators"
that would seek out and kill harmful insects individually by homing in on the
gases released by specific points of crop damage or by sensing the pheromones
issued by a particular species of harmful insect.
Initially however, the domain for µAVs will be as key elements of indoor missions. Major, and perhaps insurmountable obstacles confront µAVs that fall prey to the forces of the environment. Wind and rain can prevent outdoor µAV flight from taking place as the tiny air vehicle could expend its entire energy store getting nowhere in an attempt to fly at 20 mph in a 20 mph head wind. Similarly, rain will not only attenuate signals from the necessarily high frequency command links but may even push the tiny craft to the ground. Besides, assets exist for most outdoor reconnaissance missionsÐ why use a µAV? But indoors the environment is controlled, and there are no existing airborne reconnaissance craft that can negotiate hallways, crawl under doors, or navigate ventilation systems in an attempt to complete a reconnaissance mission.
II. Flapping Flight
A. Description of Flapping Flight
Four degrees of freedom in each wing are used to achieve flight in nature: flapping,
lagging, feathering, and spanning. This requires a universal joint similar the
shoulder in a human. Flapping is an angular movement about an axis in the direction
of flight. Lagging is an angular movement about a vertical axis which effectively
moves the wing forward and backward parallel to the vehicle body. Feathering
is an angular movement about an axis in the center of the wing which tilts the
wing to change its angle of attack. Spanning is an expanding and contracting
of the wingspan.
Not all flying animals implement all of these motions. Unlike birds, most insects do not use the spanning technique. Insects with low wing beat frequencies (17-25 Hz) generally have very restricted lagging capabilities (Brodsky3, 1994). Insects such as alderflies and mayflies, have fixed stroke planes with respect to their bodies, and the only way these insects can alter the stroke plan with respect to gravity is to change their body angle (Brodsky3, 1994). Thus, flapping flight is possible with only two degrees of freedom: flapping and feathering.
Using only these two degrees of freedom, there are 3 important variables with respect to wing kinematics: wing beat frequency, wing beat amplitude, and wing feathering as a function of wing position. When coordinated, these motions can provide lift not only on the down stroke, but also on the up stroke. The ability to generate lift on both strokes results from a change in the angle of attack of the wing whose tip which inscribes an ellipse when considered relative to a body-referenced point. The ability to generate lift on both the up- and down-stroke leads to the potential for hovering flight in entomopters and ornithopters.
Wing beat amplitudes vary in nature from approximately 25¡ to 175¡. In general,
as wing beat frequency increases, wing beat amplitude decreases. The feathering
of the wing as a function of wing position is crucial to the flight dynamics.
A schematic representation of this motion is shown in the adjacent figure. This
figure (Figure 1) is a schematic representation of combined lift and thrust
generation for a flapping wing in horizontal flight (Ward-Smith4,1984). Each
line represents the wing section at some arbitrary position across the span
of the wing (Ward-Smith4,1984). Generally insects with a constant, vertical
stroke plane must use a large angle of attack on the descending part of their
wing trajectory.
Other techniques such as optimizing wing shape, using elastic wing deformation, and employing the Weis-Fogh clapping mechanism (Lighthill5,1975) can be used to enhance the wing kinematics, and thus produce more efficient flapping flight.
B. Facts of Life for Tiny Flapping Wing µAVs
Large flying creatures use their wings to overcome gravity through lift. Tiny
flyers instead use the drag of their wings by vigorously "paddling" to stay
afloat in the air mass. This is because the Reynolds number (inertial force
of body Ö viscous force of air) for large flyers is relatively high (stork approx.
= 4 E5), while it is very low for tiny flyers (fruit fly approx. = 200). Therefore,
µAVs using wing beating for propulsion should be expect to beat continuously
to maintain flight, as soaring will not be possible except for the lightest
of wing loadings (around 25 N/m2 for butterflies).
Larger, faster flying creatures can save energy through various power-saving flight regimes such as gliding, flying in formation, or flying in ground effect. Other than some gliding butterflies, all small flying creatures expend energy continuously to stay aloft in still air. While, the aerodynamic advantages of formation flying will probably be negligible for µAV operations, flight in ground effect may prove beneficial. The design of an entomopter µAV should therefore assume continuous flapping as the norm.
C. Benefits of Flapping Flight in a µAV
Although admittedly more complex than a fixed wing design, there are many reasons
to explore the possibilities of flapping wing flight. Throughout creation, all
terrestrial life that is capable of initiating lift-generating flight, does
so through the flapping of wings. Nowhere do we observe fixed wings, propellers,
or jets for sustained flight in a living creature, nor can we find evidence
of such flight mechanisms in the fossil record. Thus, it is a proven technology,
with no indication of an evolutionary advantage, or even a hint at attempts
by extinct creatures to fly by alternate means. Perhaps the only thing that
can be said for wing flapping as a universal means of sustained biological flight
propulsion is that scale may be a factor in the advantage of wing beating over
fixed wing alternatives. The largest flying creatures today are the Andean Condors,
but size may not be the key as much as weight.
The heaviest flying creature in existence also happens to be the Andean condor, averaging 9 - 11 kilograms (20 - 25 lbs). Another very heavy and powerful bird is the Berkut eagle which has an average weight of between 6.8 and 9 kg. The killing powers of the Berkut are out of proportion to its size. In the Asian equivalent to falconry, Berkuts are normally flown at wolves, deer, and other large prey. It would therefore be to the hunterÕs advantage to have as large a Berkut as possible. Efforts to breed larger Berkuts have been an obsession since the time of Genghis Khan to realize a larger bird of prey. In over 700 years, no Berkut has been bred beyond about 11 kg gross weight. All birds of greater weight are incapable of flight.
So wing beating may be a successful solution to animal flight in the present atmosphere of Earth for animals under 11 kg in total gross weight. Therefore, one might expect that moving down in scale to mimic lighter fliers such as insects could become a function of factors other than subtle wing aerodynamics over complex alterable wing surfaces like those of birds. Instead, simpler wings will work if sufficient power can be delivered to the wing economically. As will be shown below, various factors contribute to this economy such as wing chord, beat frequency, and flight speed, but the predominant influencing factor is still gross weight.
The size constraints placed on this design, as well as future plans of achieving even smaller air vehicles, also provides motivation for a flapping mechanism. The smaller the vehicle the less reasonable a fixed wing solution because fixed wing vehicles rely strictly on lift generated by airflow from the vehicle moving through the air to support the weight of the vehicle. This lift is directly proportional to wing area and velocity of air flow over the wing. Thus, the smaller the vehicle, the less lift it can supply. Presently, most designs counter this effect by increasing the velocity of the vehicle. This increase in velocity is unacceptable in situations such as indoor missions where a µAV makes the most sense.
A flapping wing design can rely on lift generated by airflow created by both vehicle speed and wing flapping to support the weight of the vehicle. Therefore, if the scale is reduced the frequency of the beating can be increased without affecting the minimum velocity of the vehicle. This design is inherently forgiving to scale changes. Conceivably, the size of an air vehicle could be reduced to a size on the order of millimeters, as observed in nature.
Another advantage to wing flapping, relates to the minimum speed of the vehicle and ability to perform short takeoffs and landings. Provided with enough power, a vehicle with flapping wings could actually takeoff and land vertically. As described above, the fixed wing solution is very limited at slow speeds, thus requiring significant distances to increase its speed before reaching flight speeds.
D. Necessary Power
The power necessary to achieve flapping flight can be calculated by using formulas
derived by Azuma2, 1992. This power is mainly a function of the following variables:
vehicle mass, flapping frequency, forward speed, wing chord, wing span, and
wing beat amplitude. Example calculations for a vehicle weighing 50g and having
an ideal 100% efficient RCM are given in the
Appendix. Based on this analysis, just over a watt of power would be necessary
to propel such an entomopter. For comparison, several plots of the necessary
power versus forward velocity and mass are provided in Figure 2. Note that doubling
the mass of the entomopter results in almost eight times the required power.
III. Reciprocating Chemical Muscle
A. Description
The Reciprocating Chemical Muscle is a mechanism that takes advantage of the
superior energy density of chemical reactions as opposed to that of electrical
energy storage which is the approach currently being taken by most other µAV
researchers. For example, the energy potential in one drop of gasoline is enormous
compared to that which can be stored in a battery of the same volume and weight.
The RCM is a regenerative device that converts chemical energy into motion through a direct noncombustive chemical reaction. Hence, the concept of a "muscle" as opposed to an engine. There is no combustion taking place nor is there an ignition system required. The RCM is not only capable of producing autonomic wing flapping as well as small amounts of electricity for control of MEMS devices and the "nervous system" of the entomopter, but it creates enough gas to energize circulation-controlled airfoils. This means that simple autonomic (involuntary, uncontrolled) wing flapping of constant frequency and equal amplitude can result in directional control of the entomopter by varying the coefficient of lift (CL) on each of the wings, thereby inducing a roll moment about the body of the entomopter while in flight.
B. Benefits of the Reciprocating Chemical
Muscle
The implementation of a Reciprocating Chemical Muscle is motivated chiefly by
the basic necessity for very high rate of energy release from compact energy
sources. The goal of creating a µAV with a wingspan of less than 15 cm places
a great constraints on the size of the muscle mechanism. The use of a solid
propellant seems unfavorable in this application since the entire solid charge
must then be contained at all times within the systemÕs combustion chamber.
This requires that the combustion chamber be large enough to hold the amount
of charge needed and still strong enough to resist the pressures exerted on
its inner surfaces, resulting in a very massive device. In contrast, a liquid
propellant could be metered into a reaction chamber that would need to be only
as large so as to provide the desired reaction rate. The liquid not immediately
being used in the reaction could be held in a lightweight storage vessel. In
addition to weight and size reduction, this paradigm also allows the power output
to be regulated by simply controlling the flow rate of the liquid propellant
into the reaction chamber.
Particular benefits of the RCM are that it:
C. Reciprocating Chemical Muscle Testbed
A nonflying testbed was constructed by the authors to demonstrate the RCM principal
on a larger scale. No attempt was made to create a device light enough to flyÉ
the goal was merely to show that a single degree-of-freedom autonomic wing flapping
could be produced from the RCM.
The adjacent figure
(Figure 3) shows the entomopter testbed using the RCM concept. Composite spars
with simple aluminum wings were included not to generate lift, but to emphasize
the reciprocating motion produced by the RCM.
Empirical analyses performed with the entomopter testbed showed the RCM to be responsive and powerful. Wing beat frequencies of 10 Hz were demonstrated as was the ability to generate thermoelectric power. At the time of this writing, experiments to modify the coefficient of lift (CL) on each of the wings with the RCM gas by-product had not be conducted, however experiments have led to an estimation of the potential power available to drive the wing beating in an entomopter.
D. Available Power
The fuel used in the entomopter testbed was not the most energetic fuel type
available, nor was the fuel aspiration method at all optimized, so the fuel
flow rate calculations subsequently derived are conservative. Based on an average
fuel flow rate of 0.5 ml/s at ambient temperatures, we calculate maximum power
to be the rate of RCM reaction multiplied by the work associated with the expanding
gas produced in the RCM.
The rate of reaction is thus determined to be 1.888 E-2 mol/s
The work performed by an ideal RCM (neglecting friction) is determined from the formula W = PDV where an ideal gas is assumed, and P is a constant atmospheric pressure of 1.013 E5 Pa. From this and the calculated changed in volume of gas produced within the RCM, we determine that the work performed is 6.825 E3 J/mol. This is equivalent to an available power of 128.8 Watts.
If 8.16 W is required for flight of a 100g device (see Figure 2, above), then 8.16W Ö 6.825 E3 J/mol = 1.196 E-3 mol/s which is the flow rate of fuel required were an ideal RCM scaled down to actuate a 100g entomopter. In turn, this translates into about a 32 second flight of a 100g vehicle using 1 ml of fuel.
As a demonstration of how important weight reduction is, were the same calculations applied to an entomopter of half the weight (i.e., 50g), then the expected flight duration would be just over three minutes for a milliliter of fuel. Of course one must realize that these calculations are based on several ideal assumptions and might not fare quite as well when reduced to practice. It should be noted however, that the power available to various flying insects (which weigh much less than 50g) falls in a range from about 1 E-3 to 13 W/kg. Compare this to the specific available power of a reciprocating engine (1,500 to 1,800) or a gas turbine engine (3,700 to 7,400) (Azuma2, Table 4.1-2, pg. 92).
IV. Conclusions
Indoor missions are where µAVs show the greatest promise. The lack of severe wind and precipitation make µAV flight easier, however the confined spaces found indoors will necessitate not only the ability to fly slowly at times, but also the need for a multimode vehicle which can crawl or roll as well as fly.
Slow flight indoors where a multimode µAV has little room to takeoff and land, is facilitated by a flapping wing propulsion paradigm. Presently fossil and chemical fuels offer a greater energy density than stored electrical energy for driving a wing-flapping engine.
A reciprocating chemical muscle has been conceived which shows promise in entomopter applications. Weight is the major factor in preventing low power entomopter flight. The RCM concept is scalable to take advantage of MEMS components wherein an entomopter of 50g or less could be constructed.
Initial experiments with an RCM testbed have been promising. Continued research efforts will be focused on the development of a flying entomopter capable of trimmed flight "across the room," followed by homing experiments using a trinary steering actuator and the RCM for prime propulsion. Longer term goals involve the use of RCM exhaust gas to increase the coefficient of lift CL of the individual wings by circulation control techniques for roll stability and in-flight navigation.