Insect flight

Insects are the only group of invertebrates to have evolved powered flight. Over the past several million years, flying insects have evolved some remarkable flight characteristics and abilities, superior in many ways to anything created by mankind. Even our understanding of the aerodynamics of flexible, flapping wings and how insects fly is imperfect. The most obvious application of this research is the engineering of low Reynolds number, extremely small micro air vehicles.

Evolution and adaptation
Some time in the Carboniferous Period, some 350 million years ago when there were only two major land masses, insects began flying. How and why insect wings developed is, however, not well understood. Two main theories on the origins of insect flight are that wings developed from paranotal lobes, extensions of the thoracic terga; or that they are modifications of movable abdominal gills as found on aquatic naiads of mayflies.

The earliest flyers were similar to dragonflies with two sets of wings, direct flight muscles, and the inability to fold their wings over their abdomen. Most insects today, which evolved from those first flyers, have simplified down to either one pair of wings or two pairs functioning as a single pair, and using a system of indirect flight muscles. Natural selection has played an enormous role in refining the wings, control and sensory systems, and anything else that affects aerodynamics or kinematics. One noteworthy trait is wing twist. Most insect wings are twisted, like helicopter blades, with a higher angle of attack at the base. The twist is generally between 10 and 20 degrees. In addition to this twist, the wing surfaces are not necessarily flat, or featureless; most larger insects have the wing membrane distorted and angled between the veins in such a way that the cross-section of the wing approximates an airfoil. Thus, the wing's basic shape is already capable of generating a small amount of lift at zero angle of attack (see Insect wing). Most insects control their wings by adjusting tilt, stiffness, and flapping frequency of the wings through tiny muscles in the thorax (below). Some insects evolved other wing features that are not advantageous for flight but play a role in something else like mating or protection.

Some insects, occupying the biological niches that they do, need to be incredibly maneuverable. They must find their food in tight spaces and be capable of escaping larger predators - or they may themselves be predators, and need to capture prey. Their maneuverability, from an aerodynamic viewpoint, is provided by high lift and thrust forces. Typical insect flyers can attain lift forces up to three times their weight and horizontal thrust forces up to five times their weight. There are two substantially different insect flight mechanisms, and each have their own advantages and disadvantages - just because odonates have a more primitive flight mechanism does not mean they are poorer fliers - they are, in certain ways, more agile than anything that has evolved afterwards.

Direct flight mechanism
Unlike most other insects, the wing muscles of mayflies and odonates (the two living orders traditionally classified as "Paleoptera") insert directly at the wing bases, which are hinged so that a small movement of the wing base downward lifts the wing itself upwards, very much like rowing through the air. In mayflies, the hind wings are reduced, sometimes absent, and play little role in their flight, which is not particularly agile or graceful. In contrast, even though dragonflies cannot hover in still air with this primitive mechanism (though, with careful use of wind currents, they can remain nearly stationary), damselflies can, and in both groups, the fore and hind wings are similar in shape and size, and operated independently, which gives a degree of fine control and mobility not seen in other flying insects, in terms of the abruptness with which they can change direction and speed. This is not surprising, given that odonates are all aerial predators, and they have been terrorizing other airborne insects since before the dinosaurs (if they couldn't outmaneuver their prey, they would have gone extinct long ago). This flight mechanism also gives the lie to one of the well-promoted "mosquito repeller" scams: the type where the claim is that the device replicates the wingbeat frequency of a dragonfly (which supposedly scares the mosquitoes away). Since dragonfly wing muscles insert directly, and the wings can beat independently, they do not have a constant wingbeat frequency - their wingbeat speed can vary at random, like a bird, and not necessarily even in a rhythmic pattern. No device can replicate this, nor do mosquitoes avoid dragonflies.



Indirect flight mechanism
Other than the two orders with direct flight muscles, all other living winged insects fly using a different mechanism, involving indirect flight muscles. This mechanism evolved once, and is the defining feature (synapomorphy) for the infraclass Neoptera; it corresponds, probably not coincidentally, with the appearance of a wing-folding mechanism, which allows Neopteran insects to fold the wings back over the abdomen when at rest (though this ability has been lost secondarily in some groups, such as all butterflies). In the higher groups with two functional pairs of wings, both pairs are mechanically linked together in various ways, and function as a single wing, though this is not true in the more primitive groups. What all Neoptera share, though, is the way the muscles in the thorax work: the muscles, rather than attaching to the wings, attach to the thorax and deform it; since the wings are extensions of the thoracic exoskeleton, the deformations of the thorax cause the wings to move, as well. A set of dorsal longitudinal muscles compress the thorax from front to back, causing the dorsal surface of the thorax (notum) to bow upwards, and making the wings flip down. A set of tergosternal muscles pull the notum downwards again, causing the wings to flip upwards. In a few groups, the downstroke is accomplished solely through the elastic recoil of the thorax when the tergosternal muscles are relaxed. Several small sclerites at the wing base have other, separate muscles attached, and these are used for fine control of the wing base in such a way as to allow various adjustments in the tilt and amplitude of the wing beats. One of the final refinements that has appeared in some of the higher Neoptera (Coleoptera, Diptera, and Hymenoptera) is a type of muscular/neural control system where a single nerve impulse causes a muscle fiber to contract multiple times; this allows the frequency of wingbeats to exceed the rate at which the nervous system can send impulses. This specialized form of muscle is termed asynchronous flight muscle, and is one of the physiological adaptations that cannot be easily replicated in artificial flying devices such as Micro air vehicles (see below). The overall effect is that many higher Neoptera can hover, fly backwards, and perform other feats involving a degree of fine control that insects with direct flight muscles cannot achieve.

Basic aerodynamics
There are two basic aerodynamic models of insect flight. Most insects use a method that creates a spiraling leading edge vortex. These flapping wings move through two basic half-strokes. The downstroke starts up and back and is plunged downward and forward. Then the wing is quickly flipped over, supination, so that the leading edge is pointed backwards. The upstroke then pushes the wing upward and backward. Then the wing is flipped again, pronation, and another downstroke can occur. The frequency range in insects with synchronous flight muscles is typically 5 to 200 hertz. In those with asynchronous flight muscles, wingbeat frequency can exceed 1000 Hz. When the insect is hovering, the two strokes take the same amount of time. A slower downstroke, however, provides thrust.

Identification of major forces is critical to understanding insect flight. The first attempts to understand flapping wings assumed a quasi-steady state. This means that the air flow over the wing at any given time was assumed to be the same as how the flow would be over a non-flapping, steady-state wing at the same angle of attack. By dividing the flapping wing into a large number of motionless positions and then analyzing each position, it would be possible to create a timeline of the instantaneous forces on the wing at every point in time. The calculated lift was too small by a factor of three, so researchers realized that must be unsteady phenomena providing aerodynamic forces. There were several developing analytical models attempting to approximate flow close to a flapping wing. Some researchers predicted force peaks at supination. With a dynamically scaled model of a fruit fly, these predicted forces were later confirmed. Others argued that the force peaks during supination and pronation are caused by an unknown rotational effect that is fundamentally different from the translational phenomena. There is some disagreement with this argument. Through computational fluid dynamics, some researchers argue that there is no rotational effect. They claim that the high forces are caused by an interaction with the wake shed by the previous stroke.

Similar to the rotational effect mentioned above, the phenomena associated with flapping wings are not completely understood or agreed upon. Because every model is an approximation, different models leave out effects that are assumed to be negligible. For example, the Wagner effect says that circulation rises slowly to its steady-state due to viscosity when an inclined wing is accelerated from rest. This phenomenon would explain a lift value that is less than what is predicted. Typically, the case has been to find sources for the added lift. It has been argued that this effect is negligible for flow with a Reynolds number that is typical of insect flight. The Wagner effect was consciously ignored in at least one recent model.

One of the most important phenomena that occurs during insect flight is leading edge suction. This force is significant to the calculation of efficiency. The concept of leading edge suction was first put forth to describe vortex lift on sharp edged delta wings. At high angles of attack, the flow separates over the leading edge but reattaches before reaching the trailing edge. Within this bubble of separated flow is a vortex. Because the angle of attack is so high, there is a lot of momentum transferred downward into the flow. These two features create a large amount of lift force as well as some additional drag. The important feature, however, is the lift. Because the flow has separated, yet it still provides large amounts of lift, this phenomenon is called delayed stall. This effect was observed in flapping insect flight, and it was proven to be capable of providing enough lift to account for the deficiency in the quasi-steady state models. This effect is used canoeists in a sculling draw stroke.

All of the effects on a flapping wing can be reduced to three major sources of aerodynamic phenomena: the leading edge vortex, the steady-state aerodynamic forces on the wing, and the wing’s contact with its wake from previous strokes.

The size of flying insects ranges from about 20 micrograms to about 3 grams. As body mass increases, wing area increases and wing beat frequency decreases. For larger insects, the Reynolds number (Re) can be as high as 10000. For smaller insects, it can be as low as 10. This means that viscous effects are much more important to the smaller insects, although the flow is still laminar, even in the largest flyers.

Another interesting feature of insect flight is the body tilt. As flight speed increases, the body tends to tilt nose-down and more horizontal. This reduces the frontal area and therefore the body drag. Since drag also increases as forward velocity increases, the insect is making its flight more efficient as this efficiency becomes more necessary. Additionally, by changing the geometric angle of attack on the downstroke, the insect is able to keep its flight at an optimal efficiency through as many maneuvers as possible.

The development of general thrust is relatively small compared with lift forces. Lift forces can be more than three times the insect’s weight, while thrust at even the highest speeds can be as low as 20% of the weight. This force is developed primarily through the less powerful upstroke of the flapping motion.

The second method of flight, fling and clap, functions differently. In this process, the wings clap together above the insect’s body and then fling apart. As they fling open, the air gets sucked in and creates a vortex over each wing. This bound vortex then moves across the wing and, in the clap, acts as the starting vortex for the other wing. By this method, circulation and thus lift are increased to the extent of being higher, in most cases, than the typical leading edge vortex method. One of the reasons this method is not employed by more insects is the expected damage and wear to the wings caused by the repeated clapping. It is prevalent in insects that are very small, and experience low Reynolds numbers.

Biochemistry
The biochemistry of insect flight has been a focus of considerable study. While many insects use carbohydrates and lipids as the energy source for flight, many beetles and flies preferentially use the amino acid, proline as their energy source. Some species also use a combination of sources and moths such as Manduca sexta preferentially use carbohydrates for pre-flight warm-up.

Current research
Scientists study insect flight for a variety of reasons: biological development and understanding of the animals, a purely scientific interest in unsteady aerodynamics, or the engineering interest to develop Micro Air Vehicles (MAVs) or similar devices. The most obvious and, arguably the most useful application is Micro Air Vehicles. Based on the size of the MAV, different flight methods make more sense. Currently, most MAVs are larger than insects and fly at Reynolds numbers closer to bird flight. For this reason, they are generally rotorcraft or use fixed wings and propellers. For a smaller MAV flying at a smaller Reynolds number, the flight mechanics of insects become attractive. Additionally, MAVs that are the size of insects can accomplish a number of tasks that larger vehicles cannot.

In 1993 the RAND Corporation determined that the development of insect size flying and crawling systems were possible and could give the United States a significant military advantage. In 1996, DARPA funded research into MAVs through the Small Business Innovation Research program. At this time, it was concluded that a six inch MAV was feasible and capable of performing extremely useful missions. The history of this field of research is very suggestive of its future applications. A successful MAV could be used for search and rescue, military or law enforcement surveillance, or chemical or biological agent detection. The primary use, however, would probably be reconnaissance in confined spaces.

The potential benefits of MAVs are extremely promising. Possible uses include detection of poisons and drugs or search and rescue in burning buildings or after natural disasters. These uses have no foreseeable negative consequences for anyone but the individuals whose jobs these machines could replace. Some things that an MAV could do are obviously destructive. Small explosives or chemical/biological agents could be delivered to a precise location for assassination attempts or any number of missions that would be dangerous for a soldier. The existence of MAVs means that soldiers do not need to risk their lives. At the same time, however, these missions directly result in the deaths or casualties of enemies. Other military applications are reconnaissance and surveillance. With the changing nature of warfare, precise urban tactics require reliable intelligence to minimize civilian casualties and property damage. In this sense, more and better information can also save lives.