Flying fish aerodynamics

Put flying fish in a wind tunnel, and they’re as aerodynamically polished as most birds.

Earlier analyses of their bodies suggested as much, but the calculations were hypothetical.

“We directly measure the aerodynamic forces,” wrote aerospace engineers Hyungmin Park and Haecheon Choi of Seoul National University in a Sept. 10 Journal of Experimental Biology paper. “The gliding performance of flying fish is comparable to those of bird wings such as the hawk, petrel and wood duck.”

Park and Choi’s specimens were caught in the Sea of Japan and formally belonged to Cypselurus, a flying fish genus with a cylindrical body, exceptionally broad pectoral fins and unusually developed pelvic fins near its tail. Another genus, Exocoetus, has narrower pectoral and smaller pelvic fins; researchers liken them to biplanes and monoplanes (.pdf).

Propelled by a tail-motor action on the surface of waves, the fish regularly make gliding flights of more than a hundred feet, at speeds above 30 mph. (In a widely circulated video shot from a Japanese ferry, a fish stayed aloft for 45 seconds.) Flying helps the plankton-eating fish escape from predators.

With the fish dry-mounted and filled with urethane to maintain shape (.pdf) in wind tunnel tests, Park and Choi were able to measure the fishes’ lift-to-drag ratios, how far they moved horizontally per unit of vertical fall, and other aerodynamical measures.

At their precise angle of exit from the water, the fish achieve their greatest lift. The fin arrangement pushes air down the fishes’ bodies towards their tails, like a jet. And when the fish are just above the water’s surface, they benefit from a ground effect, in which air pressure underneath their fins creates lift.

The researchers are now curious about the potential role of texture differences between the top and bottom of flying fish pectoral fins.

The pair are also designing a flying fish-inspired plane. In an e-mail, Choi gave few details, but it would appear to be quite low-flying.

“The ground effect for reducing the drag force is very important in the design,”

aerodynamics of insect flight

Bird, bat and insect wings are complex structures that are moved in stereotypical ways to generate lift and thrust.  It was once thought that animal flight could simply be understood by assuming that animals were no different from aeroplanes.  The claim that "bumblebees can't fly" is based on this assumption.  Clearly bumblebees can fly. The truth is that bats, birds, and especially insects, use unconventional aerodynamic mechanisms for generating the forces necessary for flight.  We have recently begun to visualize and understand the aerodynamic tricks that these animals use to generate lift and thrust.   This research is valuable not only in terms of our understanding of animal flight mechanics, but also for the development of new technologies, such as micro-air vehicles and improved propeller designs, which have significant engineering applications.

In this essay, we will briefly explain how animal flight is different from aeroplane flight, how animal flight is typically studied, and present some of the emerging theories and applications of this work.  The complexities of biological wings and wing motions present many technical challenges for studying flight.  Here, we use the term "flight" broadly and note that it applies to many behaviours including gliding, soaring, hovering, parachuting, manoeuvring, and even take- off and landing.  This essay is not limited to the work of our own research group, but hopefully will convince the reader why it is valuable, and necessary, to look to animals for aerodynamic insight.

Conventional aerodynamic theory

Let us begin with aeroplane wings and a basic understanding of how they generate lift.  Structurally, aeroplane wings are rounded at the leading edge, sharp at the trailing edge and are often cambered, meaning they have a slight curvature when viewed in cross section.  An aeroplane wing generates lift when the airflow becomes separated at the leading edge, and the air moves faster over the upper wing surface than along the lower surface.  This causes a pressure difference to develop between the upper and lower wing surfaces because, in accordance with Bernoulli's principle, fast-moving fluid has a lower pressure than slow-moving fluid.  It is the pressure difference above and below the wing that causes lift.

The amount lift produced by aeroplane wings depends on many factors.  These include the relative air velocity, the camber of the wing, the area of the wing, and the angle of attack relative to the direction of the oncoming air.  Aeroplanes manoeuver by manipulating some of these parameters during flight with control surfaces such as rudders and ailerons.  To turn, the pilot adjusts the ailerons on the wing surfaces, changing the lift produced by the left and right wings and causing the plane to roll, and therefore turn.  During steering, the pilot will also adjust the angle of attack of the rudder for yaw control and elevators on the tail for pitch control. Lastly, an example of changing the wing area and camber occurs when a passenger jet lands.  The engines are throttled back as the plane approaches the runway, and the slower speeds would decrease the lift produced by the wings. However, the pilot extends the trailing edges (flaps down) increasing the wing area and camber.  This preserves the lift at the slow speeds necessary for a smooth touch-down.

Unconventional aerodynamics and the flapping wing

Bird, bat and insect wings are quite different from aeroplane wings.  Most biological wings tend to have sharp leading edges and textured surfaces.  The surface properties of biological wings can include ridges (for example bones in bat wings and veins in insect wings), folds, gaps, and other microscopic features on their surfaces.  Biological wings often change shape and are frequently deformed during flapping.  Birds and bats shorten their wing span by drawing them toward the body during the upstroke and extend their wings during the downstroke.  Similarly, soft wing membranes can deform like the material in a sail, creating a dynamically changing surface.

The other consideration in our comparison to aeroplane wings is in the flapping motion of biological wings.  An animal wing is constantly changing velocity as it flaps, slowing down and stopping at the ends of the downstroke and upstroke, and then accelerating into the next halfstroke. Furthermore, the wing base will always be moving slower than the wing tip, meaning that the wing velocity increases from base to tip.

Approaches to studying biological flight

Typically, studies begin by observing an animal in flight and quantifying the wing and body motion, or kinematics.  About 20-30 pictures per wingbeat are needed to see the flapping wing motion clearly, so high-speed (digital) cinematography is needed for the rapid wingbeats of insects.  Videography (using conventional video recorders) is limited to 25 or 30 frames per second and hence is suitable only for large, slow flapping birds and bats.  Accurately measuring the wing movements is best accomplished by using two cameras, which allows the 3-D coordinates of objects (e.g. wing tips, other wing features, body position) to be calculated using stereophotogrammetric techniques.  From accurate position data and the temporal resolution of the measuring system, velocities (and accelerations) can be determined from the captured sequences.  The wings and body are also usually photographed in isolation to accurately determine wing shape, body shape, and the animal's centre of mass.  Isolated wings or wing models are placed in a wind tunnel and used to measure lift and drag, which characterizes the animals’ capacity for flight.

Mathematical models can then be constructed using the measured kinematic and morphological data.  The blade-element theory, for example, divides the wings into a number of strips, each with its own velocity and area. The forces acting on each 'blade-elements' are calculated, and these are summed to give the total forces for the flapping wings. The blade-element analysis can be greatly simplified by invoking the quasi-steady assumption: that the aerodynamic forces on an element are the same as those acting on a fixed wing at the same instantaneous  velocity and angle of attack.  This assumption is probably reasonable for the slowly flapping wings of large birds, but it seriously underestimates the lift produced at the high wingbeat frequencies of insects. An inappropriate use of the quasi-steady assumption is certainly one way to ‘prove’ that bumblebees cannot fly.

To understand the origin of the fluid forces we can visualize the airflow around the moving wings.  Flow visualization is important for observing vortices, turbulence and other disturbances to the flow of air.  All flow visualization techniques rely on "seeding" the air with small particles that can be photographed, filmed or observed directly during the wingstroke.  Smoke has traditionally been used and can be easily generated by vaporizing oil on a hot wire placed in front of an insect or vertebrate beating its wings in a wind tunnel.  Modern techniques use lasers to illuminate particular areas of the flow (planes) and digital cameras to capture images during flapping.  Software is then used to determine the movement of the particles.  This technique is called particle image velocimetry (PIV) or particle tracking velocimetry (PTV). Visualizing the flow of air over the surface of insect wings has provided great insight into the mechanisms that insects use to generate lift.

Using mechanical models of insects and animals

Those who have attempted to work with animals in a laboratory know that, despite great efforts to design and organize experiments and control all extraneous variables, the study animal will usually do whatever it damn well pleases.  Furthermore, real animals can be difficult to work with because of their size. Simply put, insects are stubborn, small and beat their wings very quickly.  However, as long as the proper relationship between flapping frequency, fluid viscosity, and wing size is conserved, we can use a more cooperative and larger mechanical model; it's a simple matter of physics that the flow patterns over the wings will be the same.  In other words, the airflow over a real hawkmoth wing is the same as that flowing over a mechanical hawkmoth ten times as large flapping its wings at 1/100th the frequency of the real insect.  This scaling offers a powerful tool for studying the aerodynamics of flapping flight provided that the challenge can be met of building mechanical/robotic versions of real animals.  It was with such a mechanical model, the ‘Flapper’, that we discovered in 1996 how insect wings produce much more lift than could be explained by conventional aerodynamics.

Spiral Leading-Edge Vortex

As air passes around the sharp leading edge of an insect wing, it breaks away from the wing and rolls up into a leading-edge vortex (LEV). You can see this effect yourself by moving a spoon broadside through a cup of coffee, and watching the swirling motion from the edges. In both cases the fluid moves along a circular path, demonstrating a lower pressure at the centre of the vortex. (To swing a weight around on a string you have to exert a centripetal force by pulling it towards the centre, and the fluid is similarly sucked towards the centre by the low pressure.)  So the LEV is a region of low pressure above the wing, and this provides an extra suction that increases the lift.  The only problem is that the flow continues to feed into the LEV.  This would normally cause the vortex to grow so large that it breaks away from the wing, ruining the lift and stalling the wing.  However, we discovered that the flapping motion causes the LEV to spiral out to the wingtip, siphoning off the vortex and delaying stall. The augmented lift, coupled with the delayed stall, is the principle mechanism that insects use for generating lift.  Spiral LEVs are not new to aerodynamics, and indeed they keep delta-winged aircraft like Concorde up in the air.  However, those spiral LEVs are generated passively by the swept leading edge of the wing. What is unexpected and interesting about insect flight is that the spiral LEVs are created and stabilised by the flapping motion itself.

Rotational lift

When an insect reaches the end of the upstroke, it must rotate its wings to place them at the correct angle of attack for the start of the downstroke. Similarly, the wings must flip over between the downstroke and upstroke. Ellington first suggested in 1984 that these rapid rotations could produce extra lift, drawing on some experimental and theoretical results for aeroplane wings with rapidly increasing angles of attack. Michael Dickinson's group, working at Berkeley with a mechanical model of a fruit fly (Robofly), clearly demonstrated this effect in 1999. Not only is this lift important for weight support, but it is also a potent mechanism for flight control;Dickinson speculates that fruit flies generate steering torques by carefully adjusting the timing of wing rotation at the stroke transitions.

Wake recapture

Insects generate lift by producing and shedding vortices from their wings. These vortices move with the wake as spiralling masses of air that slowly decay and disappear, rather like the tip vortices of aeroplanes. For insects with high wingbeat frequencies, such as flies, the vortices move only a short distance before the wing returns in the cycle, and they can use this as a point of leverage for generating additional lift.  This process of ‘wake recapture’, described by Dickinson's group in 1999, is another mechanism that fruit flies use for generating extra lift.  This mechanism, unlike the LEV, might not be a widespread phenomenon because it needs a relatively high wing beat frequency.  But it does suggest that other mechanisms whereby vortices interact might be useful for generating lift or torques for steering. Current research is investigating insects with two pairs of wings (forewings and hindwings) such as locusts and dragonflies.  The forewings produce and shed vortices; how do they interact with the flapping hindwings and the vortices that they are creating?  

University of Cambridge

courtesy >>>

biological flapping wing aerial vehicle put to use in early days

The 9/11 masterminds needed air support-- total situational awareness --eyes in the sky with more mobility and more ability than satellites, people, or conventional UAVs and helicopters could provide. These UAVs had to be designed such that they could be captured on video, because they would appear frequently. (Later in the day, however, at least one obvious UAV was used.) This is where the biologically inspired design of the bird comes in. Spy pigeons have been used since World War 1. Remote control ornithopters have been around since at least the 60s. They have evolved into machines with telecommunication electronics, weapons and sophisticated sensors for all types of vision.



This vision was needed more than anything. First, there's the obvious purpose of reconnaissance and real-time mission status awareness. The area of Manhattan had to be under tight surveillance, if only to ensure the stage was properly set for the main events. What if one of somebody who had learned of the plot decided to end it by taking a few shots at the plane before it struck its main target? The World Trade Center "collapses" were the most important events of the day... that and the planes hitting on target. 

nano ornithopter uncontrolled take off

counter phase mechanism - dragongly ornithopter

courtesy ornithopter pilot

Ballooning With the Stegodyphus Spider

There are 21 different species of the Stegodyphus. Most are found in Africa, Europe and Asia. There are a few species that have made their way to Brazil. Not all species of Stegodyphus use ballooning as a form of dispersal.

These creatures are very small with an average length of only 10mm, and an average weight of only 85 to 150mg. This small mass and body size are the reason that it is able to be carried away by the wind. This type of transportation obviously has a very small mass limit. The spider lets out 3 to 4 strands of thread to induce this floating transportation, each a length of about 60 to 80 centimeters for an overall length of 1.8 to 3.2 meters. The original study states that they can reach altitudes of over 1000 meters above the ground.

The most recent study has shown that the Stegodyphus is able to gain altitude after releasing many strands of silk. These fan out to create a large surface area, which enables larger or more massive spiders to disperse as well. Although they have no ability to maneuver while in the air it is impressive that they have found such a unique way to disperse themselves. This technique obviously works well, because they have spanned to four different continents.

 In the first one you can see how they raise their tails to begin releasing the thread. In the second photo if you look closely behind its tail you can see some of the thread.

The new study points out that the 20-year-old model for spider "ballooning"—which assumed that spider silk is rigid and straight and spiders just hang at the bottom—was flawed when applied in moving, turbulent air.

When a spider wants to travel long distances, it simply casts out a strand of silk, captures the breeze and "flies" away. They are known to travel hundreds of miles, even ending up on islands in the middle of the ocean. 

Researchers at Rothamsted Research redesigned the model to allow for elasticity and flexibility in the spider's dragline, its most sturdy line of silk used for moving about and snagging prey. When the dragline is caught in a turbulent breeze, it becomes highly contorted, catching air like an open parachute and sending the spider on an unknown journey.

Now scientists have figured out how this mode of transportation works. They also discovered that spiders have very little influence where they're flown when caught in a stiff wind. 

The spider has virtually no control of where or how far it travels by this means, said Andy Reynolds, a Rothamsted Research scientist. This is how a "ballooning" spider can end up in the ocean hundreds of miles from shore.

In more calm breezes, though, spiders can drift just a few yards to invade new territory orsurprise prey.

Although the new model better illustrates how spiders "fly," there is still room for refinement, and the team plans to observe spiders in turbulent airflow inside wind tunnels to improve the model. Better understanding how spiders travel long distances could help scientists control farmland pests.

"Spiders are key predators of insects and can alleviate the need for farmers to spray large quantities of pesticide," Reynolds said. "But they can only perform this function in the ecosystem if they arrive at the right time. With our mathematical model we can start to examine how human activity, such as farming, affects the dispersal of spider populations."

Gliding Vine Seeds inspires aerodynamicists

Alsomitra macrocarpa, known as the Alsomitra vine or the Javan cucumber is “a type of climbing gourd.”

The vine is found mainly in the forests of Java, Indonesia.  Where it grows truly remarkable seeds by the hundreds in “football-sized pods.”

What makes these seeds so remarkable is the fact that they are virtually aerodynamically perfect gliders.  Each seed has a set of “paper-thin” wings that can support the seeds minimal weight with the slightest breeze.  The wings allow the seeds to travel hundreds of meters through the forest and once the seed lands, the wings rot away.

This remarkable evolutionary adaptation allow the seeds to disperse all throughout the Indonesian forest.  Each seed now has a greater chance to develop into a fully grown vine because they aren’t competing with other vines for nutrients and sunlight.

The excellent aerodynamic properties of these seeds invoked two Japanese engineers, Akira Azuma and Yoshinori Okuno, to study the plant over 20 years ago.

The engineers discovered that the seeds glided with an angle of 12 degrees.  This means that the seeds fall only 0.4 meters per second.  These seeds truly have the best aerodynamic capabilities of any winged seed.

Decades before the two Japanese engineers calculated these figures, the excellent aerodynamic properties of the seeds were noticed by a man named Igo Etrich. Igo Etrich was an Austrian born flight pioneer who specialized in fixed-wing aircrafts around the turn of the century. Igo was inspired by the seeds, not of the Alsomitra vine, but of a very similar plant called Zanonia macrocarpathat possessed very similar seeds. In 1903, Igo developed some of the worlds first gliders using a shape very similar to the Alsomitra seed.

It is also believed that the shape and aerodynamic properties of the Alsomitra seed also inspired the Horten Brothers to develop the first “flying wing” aircraft, known as the Ho 229 for Nazi Germany in 1944. It was the first tailless fixed-wing aircraft to be powered by a jet engine. The shape of the seed lent itself very well to the design of this aircraft because it cut down on parasitic drag, or the drag caused by moving an object through a gaseous or fluid medium. It cut down on drag because it eliminated the need for a tail and it had a very shallow wing design.

This is a case where nature greatly inspired pioneers of human flight and a few little seeds helped change the world. >>>>> courtesy Professor Lorena A Barba

Festro bird mechanism

Remant Micro-inspired drone of insects

Onera has been studying the feasibility of a 2002 micro-theft device which is inspired by that of insects. As part of the project Remant, research was conducted in key disciplines (aerodynamics, mechanics, flight dynamics) to observe, understand and represent the phenomena involved

New models have been developed and simulation tools have been developed to define control laws. Also, thoughts on the actuation system led to the development and implementation of a mechanism for setting a pair of flapping wings to reduce energy costs.

The aim is eventually to be able to carry out missions of observation and intelligence may be confined urban (indoor), with shots from onboard sensors.

Microdrone a swing-wing, about fifteen inches of span, is expected to have adequate flight performance (fast forward flight, hover stabilized), with greater autonomy from a helicopter of similar size.

 

The actuation system is inspired by the wings of the thorax of insects. It consists of a curved structure shaped pitch, linked to a 
named tergum preload plate, on which are fixed wings.

A layer of electro-strictifs materials can bend the pitch by two branches of the bimetallic effect. The tergum is then subjected to a buckling that leads him to oscillate between two positions (high and low). The wings are flapping and set at a frequency directly dependent on the electrical signal sent to the electrostrictive materials.

The whole behaves as a resonant cavity with its natural frequency. Thus, it is possible to obtain large beat amplitudes at high frequencies close to resonance, then the movement can be maintained from a small amount of energy.

Control of aerodynamic phenomena is essential to understand and implement the flapping flight. Several experimental campaigns of measurements and flow visualization were conducted for several years to establish empirical models. Numerical models are also being considered.

A complete simulation appointed OSCAB was also développé.Les kinematics of the wing were able to be optimized to generate the aerodynamic forces required for propulsion and levitation of microdrone.

The search for control laws using innovative methods (genetic algorithms, neural networks) is expected to soon reach the expected performance (driving speed, hovering stable, maneuverability).

----->>>> courtesy  ONERA,France.

xflr simulator - tutorials

how do flies fly?

How do flies fly?

Not like birds and airplanes, says Dickinson.

Birds and airplanes stay airborne on wings whose shape and angle create lower pressure above the wing, which helps lift them.

Their flight is explained by a theory called "steady state aerodynamics."

But flies' wings are constantly flapping -- nearly 200 times a second -- and the wings move mostly side to side, not up and down.

To understand the aerodynamic forces generated by flies, Dickinson built a huge model of the wings of a fruit fly, Drosophila melanogaster.

Dubbed "Robofly," the contraption mimics the atmospheric effects of a fruit fly's one-millimeter-long wings flapping in air.

They built a 25-centimeter (15 inch) robotic wing, which flaps and rotates at one-hundredth a fly's speed in a two-ton tank of mineral oil. Three motors move the robotic wing back and forth in precise motions determined by a computer. Bubbles pumped into the tank show the aerodynamic patterns. Sensors measure the forces on the wings during each phase of the stroke.

 ---courtesy dickison lab

Aerodynamics of flapping flight

 Sample data illustrating the motion kinematics and the measured yaw torque on a dynamically scaled robot of a fly. The wing kinematics in this example generate a desired torque about the yaw axis of the body frame. [right] Color-coded description of the angles describing the motion of the wings.

Using a dynamically scaled model insect we measure the forces produced by a flapping insect wing. By translating and rotating the wings at a range of angular velocities, the resulting aerodynamic forces are measured using sensors attached to the base of the wing. This allows the identification of the various mechanisms by which insect wings generate aerodynamic forces.   ---- courtesy The Dickinson Lab 

CFD flapping simulation

ornithopter gearbox plans