Flying Insects: How are they Airborne?
Flying mechanisms given by nature are the most complicated airborne systems mankind has seen. This article attempts to unravel the details and degroup the types of flight made by different sizes of birds and insects.
When a butterfly passes by while going on a nature hike (as long as flowers are present), we can stop a moment to appreciate a very simple fact: a butterfly flies, and its size is rather small. If you leave some cut fruit until it is moldy, we find, even the smaller sized fruit flies. As a child, you must have tried to build small model airplanes, or if you even only compare them to paper airplanes (probably the smallest man-made flying thing you ever saw) you start to get a feeling for how well evolution optimized insects are.
Compared to paper planes, insects have engines, flapping wings, sensors, navigation systems, gyroscopic stabilizers, landing gear and of course all the features required for life i.e. reproduction and metabolism, built into an incredibly small volume. Evolution really is nature's excellent engineering team. The most incredible flayers, such as the common house fly (Musca domestica), can change its direction of flight within a range of 30ms only, using the stabilizers that nature has built by reshaping the original second pair of wings. Human engineers are getting more and more interested in the technical solutions evolution has chosen and are trying to achieve the same miniaturization.
How does an insect such as a fruit fly (Drosophila melanogaster) navigate through the three axes? The lift generated by a fixed wing follows an empirical relation like
mg= f * A * v* v *row
where 'A' is the surface of the wing, 'v' is the speed of the wing in the fluid of density 'row'. The factor 'f' is a pure number, usually with a value between 0.2 and 0.4, which depends on the angle of the wing and its shape; here we use the average value 0.3. For a Boeing 747 , the surface is 511 square meter, the top speed at sea level is 250 m/s; at an altitude of 12 km the density of air is only a quarter of that on the ground, thus only 0.31 kg/meter cube. We deduce that a Boeing 747 has a mass of about 300 ton. For bumblebees with a speed of 3 m/s and a wing surface of 1 square cm, the calculated lifted mass is about 35 mg, much less than the weight of the bee, which is about 1 g.
The mismatch is even larger for fruit flies. In other words, an insect cannot fly if it keeps its wings fixed, therefore, insects and small birds must move their wings, in contrast to airplanes, not only to take to or to gain height, but also to simply remain airborne in horizontal flight. In contrast, airplanes generate enough lift with fixed wings. So, if we look at some flying animals, such as hummingbirds and dragonflies, we note that the larger they are, the less they need to move their wings particularly at cruising speed.
The formula also partly explains why human powered airplanes must be so large. But how do insects, small birds, flying fish or bats have to move their wings? In fact, the answer is just being uncovered by modern research. The main point is that insect wings move in a way to produce eddies at the front edge which in turn thrust the insect upwards. The aerodynamic studies of butterflies and the studies of enlarged insect models moving in oil instead of in air are exploring the way insects make use of vortices. Researchers try to understand how vortices allow controlled flight at small dimensions. At the same time, more and more mechanical birds and model 'airplanes' that use flapping wings for their propulsion are being built around the world. The field is literally in full swing with the aim to reduce the size of flying machines. However, none of the human-built systems is yet small enough that it actually requires wing motion to fly, as is the case for insects.
The above formula also shows what is necessary for a flying creature for take off and landing. The lift of wings decreases for smaller speeds. Thus, both animals and airplanes increase their wing surface in these occasions. But even strongly flapping enlarged wings often are not sufficient at take-off. Many flying animals, such as swallows, therefore avoid landing completely. For flying animals which do take off from the ground, nature most commonly makes them hit the wings against each other, over their back, so that when the wings separate again, the low pressure between them provides the first lift. This method is used by insects and many birds, such as pheasants. As every hunter knows, pheasants make a loud 'clap' when they take off.
Both wing use and wing construction depend on size of the creature. There are four types of wings in nature. First of all, all large flying objects, such airplanes and large birds, fly using fixed wings, except during take-off and landing. Second, common size birds use flapping wings. (Hummingbirds can have over 50 wing beats per second.) At smaller dimensions, a third wing type appears, as seen in dragonflies and other insects. The fourth type of wings is found at the smallest possible dimensions, for insects smaller than one millimeter; their wings are not membranes at all. Typical are the cases of thrips and of parasitic wasps, which can be as small as 0.3 mm. All these small insects have wings which consist of a central stalk surrounded by hair.
It can be summarized that active flying is only possible through shape change. Only two types of shape changes are possible for active flying: that of propellers (or turbines) and that of wings. Engineers are studying with intensity how these shape changes have to take place in order to make flying most effective.
Compared to paper planes, insects have engines, flapping wings, sensors, navigation systems, gyroscopic stabilizers, landing gear and of course all the features required for life i.e. reproduction and metabolism, built into an incredibly small volume. Evolution really is nature's excellent engineering team. The most incredible flayers, such as the common house fly (Musca domestica), can change its direction of flight within a range of 30ms only, using the stabilizers that nature has built by reshaping the original second pair of wings. Human engineers are getting more and more interested in the technical solutions evolution has chosen and are trying to achieve the same miniaturization.
How does an insect such as a fruit fly (Drosophila melanogaster) navigate through the three axes? The lift generated by a fixed wing follows an empirical relation like
mg= f * A * v* v *row
where 'A' is the surface of the wing, 'v' is the speed of the wing in the fluid of density 'row'. The factor 'f' is a pure number, usually with a value between 0.2 and 0.4, which depends on the angle of the wing and its shape; here we use the average value 0.3. For a Boeing 747 , the surface is 511 square meter, the top speed at sea level is 250 m/s; at an altitude of 12 km the density of air is only a quarter of that on the ground, thus only 0.31 kg/meter cube. We deduce that a Boeing 747 has a mass of about 300 ton. For bumblebees with a speed of 3 m/s and a wing surface of 1 square cm, the calculated lifted mass is about 35 mg, much less than the weight of the bee, which is about 1 g.
The mismatch is even larger for fruit flies. In other words, an insect cannot fly if it keeps its wings fixed, therefore, insects and small birds must move their wings, in contrast to airplanes, not only to take to or to gain height, but also to simply remain airborne in horizontal flight. In contrast, airplanes generate enough lift with fixed wings. So, if we look at some flying animals, such as hummingbirds and dragonflies, we note that the larger they are, the less they need to move their wings particularly at cruising speed.
The formula also partly explains why human powered airplanes must be so large. But how do insects, small birds, flying fish or bats have to move their wings? In fact, the answer is just being uncovered by modern research. The main point is that insect wings move in a way to produce eddies at the front edge which in turn thrust the insect upwards. The aerodynamic studies of butterflies and the studies of enlarged insect models moving in oil instead of in air are exploring the way insects make use of vortices. Researchers try to understand how vortices allow controlled flight at small dimensions. At the same time, more and more mechanical birds and model 'airplanes' that use flapping wings for their propulsion are being built around the world. The field is literally in full swing with the aim to reduce the size of flying machines. However, none of the human-built systems is yet small enough that it actually requires wing motion to fly, as is the case for insects.
The above formula also shows what is necessary for a flying creature for take off and landing. The lift of wings decreases for smaller speeds. Thus, both animals and airplanes increase their wing surface in these occasions. But even strongly flapping enlarged wings often are not sufficient at take-off. Many flying animals, such as swallows, therefore avoid landing completely. For flying animals which do take off from the ground, nature most commonly makes them hit the wings against each other, over their back, so that when the wings separate again, the low pressure between them provides the first lift. This method is used by insects and many birds, such as pheasants. As every hunter knows, pheasants make a loud 'clap' when they take off.
Both wing use and wing construction depend on size of the creature. There are four types of wings in nature. First of all, all large flying objects, such airplanes and large birds, fly using fixed wings, except during take-off and landing. Second, common size birds use flapping wings. (Hummingbirds can have over 50 wing beats per second.) At smaller dimensions, a third wing type appears, as seen in dragonflies and other insects. The fourth type of wings is found at the smallest possible dimensions, for insects smaller than one millimeter; their wings are not membranes at all. Typical are the cases of thrips and of parasitic wasps, which can be as small as 0.3 mm. All these small insects have wings which consist of a central stalk surrounded by hair.
It can be summarized that active flying is only possible through shape change. Only two types of shape changes are possible for active flying: that of propellers (or turbines) and that of wings. Engineers are studying with intensity how these shape changes have to take place in order to make flying most effective.
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