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Project : Ornithopter

ORNITHOPTERA machine designed to achieve flight by means of flapping wings.

Fatima Jinnah Women UniversityBachelors in Software Engineering

12/20/2014 ORNITHOPTER:

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An ornithopter (from Greek ornithos "bird" and pteron "wing") is an aircraft that flies by flapping its wings. Designers seek to imitate the flapping-wing flight of birds, bats, and insects. Though machines may differ in form, they

Submitted By:

ZAINAB EJAZBSE-2014-027

Submitted To:

MA’AM SUMAIRA SHAUKAT


Introduction:

Natural fliers like birds and insects have captivated the minds of human inventors through history. The ease and grace with which they take to the air vastly surpasses the state of the art in aircraft and their control systems. This is not to say that modern aircraft designs are ineffective, they are excellent in many respects. Propellers and turbines are very efficient methods of producing thrust and airfoils efficiently produce lift. A Boeing 747 achieves a dimensionless cost of transport (energy used divided by weight times distance) of 0.1, equivalent to a soaring albatross, and does it with amazing reliability, but it will never match the maneuverability of the albatross. The problem mirrors legged versus wheeled locomotion well. Wheels provide a stable, easy to analyze, and very efficient way of getting around with the sacrifice of a large amount of agility. Legs are notoriously difficult to control and current implementations are energy inefficient and flapping wing flight parallels this well. The unsteady fluid dynamics of flapping wings are poorly understood and it's difficult to get an ornithopter (the term used henceforth to refer to a flapping wing vehicle) to maneuver as desired.

Interest in the design and control of ornithopters has grown in recent years as interest has grown in the area of Micro Aerial Vehicles or MAVs. These small

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flying machines have struck the imaginations of many as ideal platforms for a variety of tasks including systems monitoring and surveillance where a swarm of tiny agents would be unobtrusive and have better access to confined areas than larger flying vehicles .

This thesis covers two years of work on the Phoenix ornithopter project, a 1.8 meter wingspan flapping wing flying robot, picking up from just after proof of concept work performed at the lab. From that point on two hardware revisions were produced of the Phoenix, one in summer 2007 and one in summer 2008. In the time between these summers flight testing and analysis was performed. Sustained steady level flight under computer control was finally achieved in August 2008.

The word "ornithopter" comes from the Greek words for "bird" and "wing." An ornithopter is a birdlike machine that generates lift and thrust by flapping its wings.

An ornithopter doesn't need to have feathers, though. The first ornithopters capable of flight were toys built in the late 19th century in France. Large-scale, piloted ornithopters were first developed in the early 20th century. Piloted ornithopters come in two basic categories:

engine powered human powered.

Most orithopters are about the size of small birds. Larger, man-carrying models have been attempted, but so far without proven success. Airplane-sized ornithopters have accelerated to takeoff speed on a runaway, but full takeoff has never really been successful. The ornithopter was popularized in Frank Herbert's Dune book series, as well as in the recent movie Sky Captain and the World of Tomorrow.

Bird-sized model ornithopters are cheaply available and used by hobbyists worldwide. The ornithopter was first designed by Leonardo da Vinci and drawn in some detail in his notebooks. In lieu of feathers, it used a membrane, showing that da Vinci had some basic understanding of the mechanism of flight. It was meant to be man-powered, but did not produce enough lift to take off. The da Vinci ornithopter was likely never built.

The closest we have come to a large ornithopter is a project run by the University of Toronto, called Project Ornithopter. The ornithopter resembles a propeller prop plane, but lacks a propeller and instead has flapping wings. As stated before, it has been accelerated at takeoff speed down a runaway, but has not yet attempted full flight.

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The first ornithopter was possibly built in Germany by Karl Friederich Meerwein in 1781 as a proof-of-concept for heavier-than-air flight. It is claimed to have flown, but more likely glided after being launched from a high place. When the aerodynamic principles of flight were elucidated mathematically in 1799 by George Cayley, it became obvious that gliders were more convenient than ornithopters, so much research in this direction was abandoned.

Some interesting projects have used chemically powered artificial muscles to flap the wings of small ornithopters. One day, robots like these might be used as imitation birds for surveillance. A benefit of chemically powered ornithopters is that they do not necessarily use combustion for power and thereby spare the sky from pollution.

A BRIEF HISTORY OF HUMAN POWERED AVIATION Human-Powered Aviation

It is no surprise that humanity’s first attempts at flight were in the form of birdlike, human-powered ornithopters. The great artist and engineer Leonardo Da Vinci is often credited as the first to propose a reasonable flying machine in 1490: a giant bat-shaped craft that uses both the pilot’s arms and legs to power the wings. Though the aircraft was never built, and we now know that it would not have flown, it was a remarkable achievement considering the knowledge of the day. At the turn of the 20th century, focus shifted both in the method of thrust production, from flapping wings to the propeller, and the method of power generation, from the human body to the internal combustion engine. With the aerodynamic problem greatly simplified, the impossibility of human flight was disproved by the Wright brother’s flight in 1903 and the stage was set for the boom of aircraft developments in the decades to come. Though work on human-powered aircraft was still carried on from time to time by several

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groups in various countries, it would be three-quarters of a century before anyone mastered the art of human-powered flight.

Paul MacCready's Albatross HPA in flight at NASA's Dryden Flight Research Center.

The first truly successful HPA came in 1977 when Paul MacCready’s Gossamer Condor flew a one-mile figure-of-eight course in 7 ½ minutes to capture the £50,000 Kremer Prize. What followed was breakneck development in the field, and a mere two years later the Gossamer Albatross flew 36 km across the English Channel, earning the team the second Kremer Prize. To date, the greatest HPA accomplishment was by M.I.T.’s Daedalus, which in 1988 flew 119 km from Crete to Santorini, an incredible feat worthy of the aircraft’s mythological name. These and many other HPA projects have pioneered methods of lightweight composite construction, power transmission, and multi-disciplinary aero-structural optimization, much of which has been published and made available to those eager to pursue the field.

MIT's Daedalus HPA in flight at NASA's Dryden Flight Research Center.

A BRIEF HISTORY OF THE ORNITHOPTER

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No practical ornithopter has yet been built big enough for people to fly in, although a team at the University of Toronto has been making progress since the 1970's. They have developed a fully functional engine powered scale model, and in 2006 made the first short flight of a full-size manned ornithopter. Other successful efforts have been made since the 1870's.

Leonardo Davinci's drawing from 1485 to the early 1490's were the first conceptualizations of practical winged mechanical flight. Although this design was never actually built, and the design is not really practical for a working device, Davinci's design for the flapping mechanism comes close to maximizing the efficient use of human power.

1490

All histories of ornithopters begin with Leonardo Da Vinci's human powered design. Although this was not capable of flight, it showed a great deal of careful thought and engineering. For example, the membrane wings clearly demonstrate Da Vinci's understanding that feathers are not required for successful flapping-wing flight. Also, the actuation mechanism comes close to optimizing the energy suppliable by the human engine.

1874

The first documented and witnessed flights of a mechanical flapping-wing aircraft were performed by Alphonse Penaud's rubber-powered model ornithopter in France. This established the template for subsequent model ornithopters, differing only in detail and materials.

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http://www.ornithopter.net/images/davinci-full.jpg

http://www.ornithopter.net/images/penaud-full.jpg


1929

The human-powered ornithopter by Alexander Lippisch was towed into the air and, upon release, would perform powered glides. This research was complemented by published papers describing Dr. Lippisch's theories for flapping-wing flight. This work took place in Germany.

1959

Emil Hartman also built a human-powered ornithopter that was towed into the air (by car), and then released to perform powered glides. This didn't have the same theoretical complement as Lippisch's work, but it was a very respectable effort in that it demonstrated the definition of a true ornithopter being birdlike, but not being a slavish copy of a bird. This work was done in England.

1960's

Percival Spencer, of the United States, developed a remarkable series of engine-powered free-flight ornithopter models. These were made in various sizes, with different engine sizes, and are clearly an original accomplishment. A modernized remotely-piloted version of this has been recently developed by Sean Kinkade of Florida.

1991

The Harris/DeLaurier engine-powered model demonstrated the technology required for a full-scale aircraft. This is recognized by the FAI as the first successful engine-powered remotely-piloted ornithopter.

1999

The Project Ornithopter engine-powered piloted aircraft, which is based on the technology of the Harris/DeLaurier model, self accelerated (flapping alone) on level pavement to lift-off speed.

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http://www.ornithopter.net/images/lippisch-full.jpg

http://www.ornithopter.net/images/hartman-full.jpg

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CONSTRUCTION AND MECHANISM OF FLIGHT:

How It WorksThe full-scale ornithopter is an engine powered aircraft that carries one pilot. All of the thrust and nearly all of the lift is created by the mechanical flapping of the ornithopter's wings. The two wings of the craft are joined by a centre section which is moved up and down by pylons connected to the drivetrain. The wings' thrust is due primarily to a low-pressure region around the leading edge, which integrates to provide a force known as "leading-edge suction". The wings also passively twist in response to the flapping. This is due to a structure that is torsionally compliant in just the right amount to allow efficient thrusting ("aeroelastic tailoring"). It should be noted, though, that twisting is required only to prevent flow separation on sections along the wing. It does not produce thrust in the same way as required by sharp-edged wings with little leading-edge suction . For a more in-depth description of the full-scale ornithopter's functioning please refer to "The Development and Testing of a Full-Scale Ornithopter " in the Research section. Don't miss our 3D interactive model of the ornithopter in the MultiMedia section!

Ornithopters and Flapping-Wing Flight

The problem of flapping-wing flight has been tackled by countless engineers and craftsmen, but until recently only moderate success had been achieved. The Subsonic Aerodynamics laboratory under Professor James DeLaurier at the University of Toronto has been a prolific contemporary contributor to the body of knowledge concerning flapping-wing flight, with successes in remote-controlled

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http://www.ornithopter.net/images/3view-full.jpg


ornithopters, flapping-wing micro air vehicles, and even a full-scale human-piloted engine powered ornithopter.

The University of Toronto's engine-powered ornithopter, "Big Flapper", during dawn flight testing at Downsview airport,

In 1991 the Professor DeLaurier and UTIAS were awarded the “Diplôme d’Honneur” by the FAI for having flown the world’s first engine-powered remotely-piloted ornithopter. Theoretical and experimental research intensified in subsequent years, culminating in the successful flight of a full-scale piloted ornithopter on July 8th, 2006. A patented wing-twisting mechanism and extensive research in aeroelastic tailoring has kept the University of Toronto at the forefront of ornithopter innovation for the last 20 years.

How Birds Fly When a bird is gliding, it flies the same way as an airplane. As the wings move through the air (blue lines), the special airfoil shape of the wings causes the air pressure above the wings to be lower than the pressure underneath. The difference in pressure is lift, a force that acts roughly perpendicular to the wing surface and keeps the bird from falling.

Flapping flight uses the same principle, but the movement of the wings is more complicated. There are three important motions in addition to the bird's forward motion:

1. Flapping 2. Twisting 3. Folding

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By flapping its wings down, together with the forward motion of the body, a bird can tilt the lift of its wings forward for propulsion. Why don't birds simply move their wings up and down, without

twisting and folding? Notice that the outer part of the wing moves down much farther than the inner part close to the body. Twisting allows each part of the wing to keep the necessary angle relative to the airflow. If part of the wing is angled lower than the airflow, there might not be enough lift. If part of the wing is pointed too high, there could be a lot of drag. The wings are flexible, so they twist automatically.

Wing folding isn't essential - ornithopters fly without it - but it helps birds fly with less effort. To see why it is helpful, think about what happens during the upstroke. Because the wing is going up, the lift vector points backward, especially in the outer portion of the wing. The upstroke actually slows the bird down! By folding its wings (decreasing the wingspan) a bird can reduce drag during the upstroke.

In addition to the three basic movements described here, birds can do a lot of other things with their wings to allow them to maneuver in the air. Instead of using their tails for flight control, they move their wings forward and backward for balance. To make a turn, they can twist the wings or apply more power on one side. For slow flight, birds can flap their wings almost forward and backward instead of vertically; the upstroke and downstroke produce lift without forward body motion.

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Since flapping wings are subject to unsteady flows - they not only move but accelerate through the air - they can produce more lift than fixed wings and are resistant to stalling.

ORNITHOPTER FLIGHT DYNAMICS AND CONTROL

Operating principle of the flapping wing At the wing downstroke the lift distribution is bigger altogether than when gliding and more shifted towards the wing tip. Thus, thrust is generated along the whole wing span during stroke motion. This works similar to a propeller blade with a very large pitch – only that the propeller torque force that has to be overcome, is here called lift.At the wing upstroke circumstances are reversed. Overall, the lift distribution is smaller and more shifted towards the wing root. Moving in the direction of the lift force, the flapping wing now acts as a wind turbine blade. If the lift force is big enough, it presses the wing upwards even without a mechanical drive. Thereby, the wing operates with the operating drag of a wind turbine against the flight direction.

Basic motion components of a bird wing In addition to the forward motionFlapping: motion at shoulder pointPulling: dragging of the outboard wing section during upstroke by the inboard wing sectionInclination: determined by the inclination of the stroke axisTwisting: increases toward wing tip mostly during upstroke in the direction of a positive angle of incidence and during the downstroke in the direction of a negative angle of incidenceTurning: (of the wing root) especially when flying with thrust and mostly in the direction of wing twistingSweep: of the outboard wing section during upstroke with the backward motion of the wing tip.The flapping motion of the wing is absolutely necessary for thrust generation. In general, also the wing twisting is necessary for aerodynamic reasons.In contrast, the turning and sweeping of the wing, as well as the pulling of the outboard wing section serve only to increase efficiency.

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Methods of wing twisting: a) Passive wing twistingb) Active wing twistingc) Aeroelastically wing twistingd) Aeroelastically controlled wing twistinge) Controllable wing twisting- radio controlled- automatically controlled

Design Stages1. Active articulated torsional drive(for the two principal motions)First, the wings beat up and down, whereby a lever mechanism causes the degree of deflection to increase from torso to the wing tip.Second,the wing twists in such a way that its leading edge is directed upwards during the upward stroke(positive angle of incidence). If the rotation were solely due to wing’s elasticity passive torsion would result. If the sequencing of the torsion and its magnitude are controlled by an actuator, the wing’s torsion is active.

2. The Wing:Lift and propulsion in birdsThe wing should consist of two-part arm wing spar with an axle bearing located on the torso, a trapezoidal joint, and a hand wing spar.The arm wing generates lift and the hand wing beyond the trapezoidal joint provides propulsion.Both spars of the inner and outer wing are torsional resistant. The active torsion is achieved by servomotor at the end of the outer wing which twists the wing against the spar via the outmost rib of the wing.

3. Partially linear kinematics for optimal thrustWhen the bird lifts its wings, the servomotor for active torsion twists the tips of the hand wings to a positive angle of attack, which is then changed to a negative angle a fraction of a wing beat period.The angle of torsion remains constant between these phases.Due to this sequence of motions, the airflow along the wing profile can be optimally used to generate thrust.

4. The torso (the fuselage)The battery, engine, transmission mechanism, and the control and regulation electronics are housed in the bird torso.By means of a two-stage helical transmission, the exterior rotor motor causes the

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wings to beat up and down with the required reduction ratio. This motor is fitted sensors that precisely registers the wing’s position.Both flapping and bending forces are conveyed from the transmission to the hand wing via flexible links.To make the bird agile and maneuverable, the opposing motion of the head and torso section is synchronized by means of a system of cables and servomotors. Thus the torso bends aerodynamically with simultaneous weight displacement.

5. The tail section an aid for lift and controlThe tail of the bird also produces lift; it functions as both pitch elevator and a yaw rudder.When the bird flies in a straight line, the V-position of its two flapping wings stabilizes it in a similar way as a conventional vertical stabilizer(Fin) of an aircraft.To initiate turn to the left or to the right, the tail is tilted; when it is rotated about the longitudinal axis, a yawing moment about the vertical axis is produced.

6. Control and regulation The on-board electronics allow precise and thus efficient control of wing torsion as a function of wing position.For this purpose, a powerful microcontroller calculates the optimal setting of the two servomotors which adjust the torsion of each wing.The flapping motion and torsion are synchronized by three sensors, which determine the absolute position of the motor for the flapping motion.Since the active joint torsion drive requires precise coordination between the flapping and twisting motions, it is subjected to continuous all round monitoring.

PROJECT ORNITHOPTERArticulated flapping wings

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Description of flapping wing constructions which have been developed together with the den EV-models

1. Requirements

The design layout demands on a technical flapping wing-among other things-results from the theoretical distribution of lift based on Robert T. Jones (USA 1950 and 1980) and the therefore necessary angle of incidence along the half span of the wing.

Here, for example, the relevant, extensively optimised functional distributions for a gently inclined flight of a rectangular flapping wing with the all-round airfoil CLARK-Y are shown. In this case, the angle of incidence at the wing root remains constant. The distributions of the downwash angle along the span are straight-lined in all three cases.

In the flapping wing design the moment of inertia round the stroke axis and the rotating axis of the flapping wing are also important.

2. Aeroelastic controlled articulated flapping wing

This isasic spar framework of a profiled flapping wing with an articulation for an additional flap motion of the hand wing spar. This is pulled down by a spring (spring device here not shown).

While gliding flight with its medium lift force, the hand wing takes over the stretched center position.

On downstroke in the hand wing area the lift forces distinct increase and the hand wing strokes up against the spring force.

If on upstroke the lift forces decrease the spring force pulls down the hand wing.

The small, on aerodynamic forces dependent and thereby aeroelastic stroke movement of the hand wing will be used by levers (brown for arm wing, green for hand wing) to control the twisting along the whole wing. In

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http://www.ornithopter.de/english/images/articulated/twist.gif

http://www.ornithopter.de/english/images/articulated/lift.gif

http://www.ornithopter.de/grafik/gelenk/gelenkflg.gif

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this way an articulated flapping wing with aeroelastic controlled twisting has been developed.

With the exception of the wing root rib all ribs are put freely rotatable on the spars. For the covering of the flapping wing the highly elastic polyurethane film Platilon U 04 is planed.

The mode of operation of this articulated flapping wing, with its wing twisting by wing bending against the stroke direction, resembles a little to that of a bird's wing. But according to studies by Karl Herzog at birds the motion of the hand wing happens in another way. For them, the rotary motion of the hand wing is mechanically coupled with the forward and backward motion of the wing tip when spreading and pulling up the wings. This coupling is tighly in the stretched wing position and with increasing back-swept wing tips only loosely

Technical drawing of the wrist joint

Wrist joint of the wing in up- and downstroke position

Wrist joint with the torsion lever of the arm wing and the wire rope to lock the wing twisting in gliding Function of

the wrist joint of the wing. The yellow lever shows the rotation of the hand wing.

EV6 in flight, at wing upstroke

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http://www.ornithopter.de/grafik/gelenk/fluegelgelenk.gif

http://www.ornithopter.de/grafik/gelenk/handgelenk.jpg

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http://www.ornithopter.de/grafik/gelenk/armfluegel_torsionshebel.jpg

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3. With expanding wing twisting

This is the functional model of a stroke amplitude expanding wing spar in short form called shift spar. This mechanism can be used at main and at auxiliary spars of flapping wings.

Linkage above:

Shift linkage dismounted

Linkage below:

Shift linkage mounted

At a stroke moving of the middle shift linkage - here downward - the outer right spar section implements expanded stroke amplitude.

Technical drawing for a shift spar in a coaxil tube-in-tube construction Joints of the shift spar with different center distances for transmitting the flapping motion (1:1.5). Complete coaxial shift spars.

Adjacent one can see the spar framework of a profiled flapping wing for active wing twisting at the arm wing section by main spar rotation.

There is a stroke amplitude expanding auxiliary wing spar is used. In doing so the twisting at the hand wing section is increased. The spar shift linkage is hereby mounted coaxially.

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http://www.ornithopter.de/grafik/gelenk/schiebe.jpg

http://www.ornithopter.de/grafik/gelenk/schlagweite.jpg

http://www.ornithopter.de/grafik/gelenk/schiebeholm.gif

http://www.ornithopter.de/grafik/gelenk/holmgelenk.jpg

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The rib at the wrist is fixed firmly to the main spar and the auxiliary spar hinges fixed firmly to the wing root rib. All the other ribs are stuck on the spars rotating freely. The covering is done with an elastic foil.

For a passive or aeroelastic twisting the rib at wrist should to be pivoted on the main spar.

4. With adjustable twisting moment

By splitting the flapping and twisting tasks of a flapping wing on a main and an auxiliary spar its twisting moment can be designed adjustable.

Adjacent, the framework configuration of an aeroelastic twistable flapping wing with an adjustable twisting moment becomes obvious. The adjustment is affected by the torsion lever at the wing root.

The rest of the other ribs not shown here are fixed to the spars rotating freely. The covering is done with an elastic foil. Also shell and foam wings can be designed adjustable this way.

If you apply this system on a non-flapping aerofoil, a propeller blade or a wind turbine blade, their twisting can be controlled by the auxiliary spar.

Generally, the different types of wing systems can be combined together in many ways.

In the adjacent picture for example, the auxiliary spar of the arm wing with its torsional elastic force is used as a spring device for the small flap moving of the spar of the hand wing of an aeroelastic controlled articulated flapping wing.

The arm wing torsion linkage AT and the arm wing auxiliary spar AHi are hereby fixed firmly together. This way, the auxiliary spar-torsional moment will be transformed into a torque of the torsion linkage AT. This is pivoted at the front and presses down the spar of the hand wing in the indicated rotary direction. The pressure will be adjusted with the inlying lever InH at the wing root.

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http://www.ornithopter.de/grafik/gelenk/hilfsholm.gif

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The profiled arm part of this articulated flapping wing can also be combined with a membrane hand wing. This is possible a useful design for medial climbing and gliding flights.

Adjacent, the wing mechanism near the fuselage with the setting mechanism of the turning moment of the auxiliary spar, the damper of the wing twisting at the upper final wing stroke position and the servo to keep the wing twisting in glide position.

5. Aeroelastic controlled articulated flapping wing with adjustable twisting moment and expanding wing twisting at the wing tip

Wing framework of the model EV8 (2004), designed as an aeroelastic controlled articulated flapping wing, combined with a stroke amplitude expanding auxiliary spar of the hand wing at the wing tip section.

The twisting elasticity of this flapping wing can be adjusted by the torsion of the auxiliary spar of the arm wing at the wing root.

The downstroke twisting is slowed down by a dashpot.

The twisting when gliding can be fixed by a radio-controlled servo.

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The downstroke twisting here corresponds with the theoretical guidelines almost to the wing tip. But still disturbing is the high mass moment of inertia of this flapping wing round its axis of flapping and twisting.

In flight practice the articulated flapping wing has a big advantage. The bending of the hand wing in comparison to the arm wing depends on lift of the hand wing. At the same time it determines the distribution of the angle of incidence along the wing span. If the amplitude of the bending is estimable on flight pictures, the lift forces of the hand wing in comparison to the gliding flight can be estimated. Furthermore, the distribution of the angle of incidence the moment of the picture was taken can easily be suggested. With these tow information's selective adjustments of the twisting moment of the flapping wing, the driving power and the cycle time ratio are possible. Especially flight pictures taken approximately in the middle of up- and downstroke are informative.

6. Covering of flapping wings

As well as for the other EV-models a 0.050 mm thick elastic polyurethane-foil was used as cover for the aforesaid wing. Double-sided adhesive tape was used adhere the foil to the wing framework. Version of a trailing edge for flapping wings composed of a fishing line at the end of the airfoil wrapped with an adhesive tape.

MECHANISM OF FLIGHT:Flapping Flight

Now we take a look at ornithopters and birds and how they fly. An ornithopter is simply a bird-like flying machine. Rather than forcing air over an unmoving wing with a propeller, an ornithopter flaps like a bird to stay in the air. Flapping flight isn't much different than airplane flight. The wings have to generate lift and thrust to counter weight and drag just like a plane does. The flapping motion is what causes the problem for most people who wonder about birds.

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The reason flapping flight looks confusing:

When a bird flaps downward, everything seems to make sense. But when it flaps upward, it would seem that the result would cancel out the downstroke. The question is: Why doesn't flapping up push the bird down?

The best way to answer this would be to envision the bird's wing as though it were the wing of an airplane. The flapping motion in this mental picture would resemble a change in the angle of attack (angle of the wing relative to the airflow) of the wing. As long as the bird is moving forward, the wings are always generating lift.

It is important to remember that a bird's wing is not rigid. As it flaps, it changes the shape of its wings to get the best lift for each position. Also, the tail of the bird also generates a certain amount of lift and helps to stabilize the bird.

Energy Analysis

It is now time to examine how energy is utilized in an ornithopter. To do this we will perform a qualitative energy analysis and discuss how to find the actual power output of the ornithopter and its efficiency.

First, let's look at where the energy is stored and transferred. In my ornithopter, which is rubber-band powered, the process begins with the crank. As I wind the crank, the kinetic energy is transferred and stored in the wound up rubber-band. This is the first area where friction can increase the force required and therefore reduce the efficiency a bit. When the "Freebird" is raised up before flight, gravitaional potential energy is given to it. So, the total potential energy before flight can be expressed as follows:

Total PE = PErubber-band + PEgravitational

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The ornithopter is then released and begins to fly. This is a very complex motion in which the energy is transferred and transformed in many ways. First, as the rubber-band unwinds, its energy is transformed into kinetic energy causing the crank to turn. Again, note that friction is a factor in how much energy is wasted in this transfer. The crank moves the wing struts, another process in which friction could have an effect, and the struts cause the wings to pump. The moving wings give the ornithopter lift and thrust, propelling it forward and keeping it in the air, giving the entire ornithopter kinetic energy.

Power is the change in the amount of kinetic energy (in this case, of the ornithopter) for a unit of time. We can qualitatively show this like so:

Power = Change in KEornithopter / Time

Efficiency is the ratio of potential energy before the event to the total energy afterward. Qualitatively, this can be expressed as follows:

Efficiency = (KEused in flight + PEremaining) / PEtotal before

Now, with a knowledge of some physics equations, we can calculate the power output (in Watts) and the efficiency of the ornithopter numerically. First, the potential energy must be found. Before we stated this:

Total PE = PErubber-band + PEgravitational

The potential energy of the rubber-band can be measured with some creative devices, and the gravitational potential energy can be calculated as follows:

PEgrav = Mass x Height x Gravitational acceleration

The gravitational acceleration on the surface of the earth is approxomately 9.8 m/s2. We stated earlier the expression for power. In this case, the change in KE would simply be the KE at the end of the flight, and the time would be the time of the flight. So:

KE = 1/2 x Mass x Velocity2

so. . .

Power = (1/2 x Mass x Velocity2) / Time

And of course, the efficiency can be calculated using the expression from above. Cool.

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The principle of flight of ornithoptersThe basic operating principle of a flapping wing has already been discovered by Otto Lilienthal (1889). To help understanding an effective way of flying of big ornithopters his functional description is still trend-setting to the present day. But many details are still not understood.

Always there have been several different versions of the flapping flight theory. They all exist in parallel and their specifications are widely distributed. Calculating the balance of forces even of a straight and merely slowly flapping wing remained difficult to the present day. In general, it is only possible in a simplified way. Furthermore, the known drives mechanism and especially wing designs leave a lot to be desired.

In every respect ornithopters are still standing at the beginning of their design development. But powerful drives make very beautiful flights already possible.

2. Operating principle of the flapping wing

Diagram 1

Optimized lift distributions for a gently inclined climb flight with limited wingspan

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http://www.ornithopter.de/english/images/principle/diagram_1.gif


Diagram 2

Optimized lift distributions for a gently inclined climb flight with unlimited wingspan On a stretched flapping wing lift is generated similar to an inflexible airfoil flown against from the front. But during the wing upstroke the air flow hits the wing rather from above and in the downstroke rather from bottom. These modifications are small in the area of the wing root and gets bigger towards the wing tip. With permanent changing twisting the flapping wing must adapt to these alternating incoming flow directions. But in the interest of thrust generation the lift distribution must not be constant along the wing span over the flapping cycle.

During the wing downstroke the lift distribution is bigger altogether than when gliding and more shifted towards the wing tip. It is easy to imagine that thrust is generated along the whole wing span during stroke motion. This works similar to a propeller blade with a very large pitch - only that the propeller torque force that has to be overcome is here called lift and is also used like that.

At the wing upstroke circumstances are reversed. Overall, the lift distribution is smaller and more shifted towards the wing root. With the stroke movement in the direction of the lift force the flapping wing now acts as a wind turbine blade. If the lift force is big enough it presses the wing upward even without a mechanical drive. Thereby, the wing operates with the operating drag or working drag, of a wind turbine against the flight direction (please takes a look at the vector diagram).

Forces at the stork wing in the upstroke

At the same time, the outboard wing areas are flown against rather from above. There indeed is generated negative lift but similar to a propeller also thrust (Please look at the vector diagram).

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Whether in the upstroke the wind turbine or the propeller function dominates depends on the wing twisting and on the shape of the lift distribution (for more details, please see following chapter).

Comparison of aerodynamic machines

The adjacent picture clarifies that the comparison does not apply in all respects to a propeller or to a wind turbine. The velocity proportions at the flapping wing are completely different. But the rotating machines are not designed for simultaneous lift generation. Furthermore, at the flapping wing the lift force at mid-span of the wing is never zero - as like at the rotating machines.

A flapping wing is an aerodynamic machine with two strokes, the upstroke and the downstroke. In unaccelerated horizontal flight of a flying wing ornithopter the degree of efficiency of this machine is equal to zero. It only moves itself but emits no power.

But if you add a fuselage and a tail unit to the flying wing ornithopter, the flapping wing must apply power to overcome the parasitic drag. Now the flapping wing renders output. Now, paradoxically - with an otherwise unchanged flight attitude - the efficiency factor becomes bigger than before (bigger than zero). For example the efficiency factor of the flapping wing increases with the size of the tail unit while keeping the balance of forces. So the parameter efficiency factor is relatively inapplicable for evaluating flapping wing. The total thrust gets bigger the more the lift distributions of the up- and downstroke are different from each other - especially at the outboard wing area where the most working will be performed. If the difference equals zero working drag and thrust have the same size and cancel out each other. The total thrust equals zero, then. At an existing lift difference the thrust is also increased with increasing flapping frequency and flapping amplitude.

The size of lift is also specified by the angle of incidence at the wing root. With the above-mentioned lift distributions the angle of incidence during the flapping motion of the wing is always kept constant. The pictured differences of lift at the wing root results only from different induced downwind angles.

To equalize the smaller lift at the wing tip during the upstroke, at least partly the lift should be increased at the wing root at the same time. Here, no researches are known about the angle of incidence at the wing root of birds. To balance the total lift during the flapping motion, E. v. Holst (1943) suggests a turning of the wing root synchronic to the wing twisting. There the angle of incidence should get larger in the upstroke and smaller in the down stroke. But this is not to be seen on birds in cruising flight or only in a minor way.

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Another way to generate a constant total lift force shows a swan. It increases considerably the angle of attack at the wing root in the lower wing position. But then it reduced the angle once again during the upstroke.

Also, it may be birds increases the angle of attack and/or the airfoil camber in the area of the elbow in the upstroke. Also by this way will be supported the shifting of the lift in the direction to the wing root and the lift force in the upstroke gets larger und the total lift more constant, respectively.

For a steady flight, all forces - more precisely, force impulses - affecting the ornithopter during a complete wing beat cycle must be in balance. The propeller effect must not only balance the wind turbine effect but also all remaining drags of the wing and the aircraft. At the same time, the positive part of the lift must outbalance the negative to an extent, that it can carry the weight of the aircraft.

Flapping wing properties during flight1. Gently inclined climb flight

At the wing upstroke the aerodynamic forces along the wing can be adjusted by suitable wing twisting so that the torsional moments round the wing hinge balanced themself (Please look at the following diagram 3). Here, the wing area close to the fuselage acting as a wind turbine directly powers the outboard wing area acting as a propeller. This is the 1st possibility to use the wind turbine energy.

There is no energy consumption or transfer at this upstroke configuration. The wing can virtually be flapped up by the drive without effort. Propeller and wind turbine effects cancel out each other. The overall effect of the upstroke in the thrust direction is thus equal to zero.

Diagram 3

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A special lift distribution at the upstroke when the stroke momentum of the inner and outer wing section balanced each other exactly. Thus, the wing can be moved upwards without an external force.

Due to the lever action of the wing at this upstroke setting the positive lift close to the fuselage must be bigger than the negative lift at the wing tip. In total, there still remains some positive upstroke lift. The wing down stroke with its generally strong generation of lift and thrust can ensure the balance of the remaining forces during the whole flapping cycle.

Would one do without lift in favour of thrust generation in the upstroke the following should be considered. To generate the complete lift impetus only in the downstroke - in virtually half of the available time - the lift force and consequently the wing area, too, would have to be almost doubled. This and the corresponding lift fluctuations are only appropriate in exceptions.

As to be seen in the diagram 1 shown lift distributions the average lift of both working cycles are different in size. At least at low flapping frequency, this will result in an obvious pendulous movement of the fuselage. But due to thereby generated variations of the angle of incidence it deadens itself quite effectively. These variations are not included in the diagrams.

Naturally, other settings are possible in the area close to the preceding lift distribution. They are well suited for gently inclined climb flights with a moderate flapping frequency. My EV-ornithopters have been built for this way of flying.

2. Cruising flight * Starting from the previously described flight scenario for the horizontal cruising flight it is more advantageous to increase the total lift during the upstroke and shift it a little more towards the wing tip. There, only a little bit of negative lift is generated - if any at all (Please look at the force vectors of the following picture). But by this way, the wind turbine effect and its working drag are increased.Forces at the wing of a stork during up- and down stroke by Otto Lilienthal

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That this should be beneficial is amazing at first. The wind turbine effect now can no longer be used for generating thrust in the area of the wing tip. Would it not be better simply to increase the flapping frequency?

According to a proposal by Otto Lilienthal the wind turbine or the wing upstroke energy may also be used again in a2nd possibility. At first, the working drag slows down the flying ornithopter. Thereby detracted kinetic energy of the model can be accumulated in a spring. This spring must be positioned in a fashion that it is tensioned at the upstroke. It relaxes in the downstroke, supports thereby the flapping movement, generates thrust and transfers wing upstroke energy back to the kinetic energy of the model.

A 3rd possibility for using the wind turbine force lies in the acceleration of the wing mass in upstroke direction. If the wings are then slowed down at the upper final wing position by a spring and accelerated in downstroke direction, retransfer of the upstroke energy is also affected in this way. Thereby, the acceleration of the wing must not be limited to the initial stage of the upstroke.

In the upstroke, a mechanical drive of the flapping wing is not necessary in these cases. The wing even releases energy to the above-mentioned springs. Anyway, the wind turbine motion must act against any force otherwise no lift can be developed on a freely movable wing.

The wing upstroke energy output normally is relatively small. It will be adjusted bigger the more flow-favorable the aircraft is built

A good way to decrease the wind turbine effect in spite of strong lift generation is the pulling or the dragging of the outboard wing section during the upstroke of the inboard wing section. Thereby the outboard section of the wing becomes a winglet to the inboard section of the wing.

- This mainly has a bisecting effect on the effective wind turbine span.

- At the same time, it reduces with its winglet effect the induced dragof the inboard wing section.

- Furthermore, it reduces problems of wing inertia especially in the area of the upper final wing position.

To enable at the upstroke strong lift at the inboard wing section it will be equipped with large airfoil camber.

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Swan in cruising flight

Close to the water surface a Mute Swan during cruising flight. The downstroke twisting is negligibly low.

Here, for a lift generated upstroke of the inboard wing section the angle of incidence of the wing root will be temporarily decreased at the beginning.

In the upstroke increase and decrease of the angle of incidence move like a wave from the wing root to wing tip. Therefore wing twisting changes the direction in the upstroke.

Due to the pressure difference on the upper and lower side of inboard wing section the turnable outboard wing section intends to stroke up. But it is probably prevented by the negative lift at the wing tip section.

The leading inboard wing section stoped at the upper position until the outboard wing section reached the top of its stroke.

After the outboard wing section was pulled up hanging on its hinge, also its turning in the upper stroke position occurs only with the lift.

If in total lift results in the upstroke, the flapping wing permanently acts indeed as an aerodynamic two-stroke machine in lift direction, but as seen in flight direction alternatingly reversed. Nevertheless, via wind turbine energy utilisation the whole drive energy will be used for thrust generation - of course with the usual losses of the profile drag and of the induced drag. But these also always accrue for the lift generation.

In spite of changing acceleration direction, flight velocity should be kept constant. Thereby are definitely advantageous a high stroke frequency and a large model mass.

During such cruising flight configurations of the upstroke, its lift increase more than during the gently inclined flight. Apart from thrust generation also lift generation during downstroke can be released. Therefore, its lift distribution will be shifted rather towards the wing root and concurrently be adjusted smaller.

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Diagram 4

Distributions of lift for a cruising flight with unlimited wingspan. Also the distribution of the upstroke is optimized in relation to the induced drag.

Altogether, in cruising flight the lift distributions of both of the work cycles have been approximated to those of gliding. One approximates them the more flow-favourable the aircraft is built. Less thrust is then necessary. Furthermore, the induced drag of the downstroke decreases noticeably this way.

Perhaps it might be enough to shift the lift only a little along the span, without changing its size - in the upstroke towards the wing root, in the downstroke towards the wing tip. However, a twisting of the wing root is necessary for that.

Seagull in cruising flight

During the whole downstroke and at the beginning of the upstroke the angle of attack is increased in the center of the half span.

The advantages of the flapping wing working in opposite directions during wing up- and downstroke lies especially in the relative even lift generation. The perpendicular movement of the fuselage disappears almost completely in horizontal flight.

Altogether, a very effective steady cruising flight can be achieved with thrust only directed forward and not upward. Thereby, the flapping frequency is obviously lower than during the following way of flying.

3.3 Strong inclined climb and hovering flight

Precedingly, flight situations are described during which lift is directed upward and thrust forward. The all up weight is thereby carried by the wing lift. In short, this can be called Flying with lift.

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But similar to a helicopter, during flapping flight the weight force can be balanced by a slipstream directed downward or by a thrust force directed upward. This is Flying with thrust. Thereby, the wing upstroke practically affected only with the drive. At least in steady flight, the thrust force is always perpendicular to the wing-stroke plane and can be adjusted according to their inclination.

Small bird on approach

If the thrust force points exactly in flight direction, there is either pure flying with thrust (perpendicular climb flight) or pure flying with lift (horizontal flight). In settings between these extremes and during a horizontal motion not too slow, the balance of all up weight is affected both by thrust and by lift directly generated at the wing. These mixed configurations are also assigned to flying with thrust.

The taking off of an ornithopter, hovering on the spot, strong inclined climbing flight and slow horizontal flight are only possible according to the method flying with thrust.

In contrast, moderate fast horizontal flights can be conducted with both ways of flying - with quite a different demand for power, though. Relatively fast horizontal flights or cruising flight can be achieved only by flying with lift.

In flight praxis, especially the inclination of the stroke plane acts as identification criteria for ways of flying. In horizontal flight it is vertical to the flight direction. If it differs considerably (more than about 10 degrees) it is flying with thrust. Furthermore, a big upstroke wing twisting in a passive wing twisting is an indication for this way of flying - at least at high Reynolds numbers. Also, a relatively high power consumption relating to the horizontal velocity points to a flying with thrust.

Furthermore, the legs of birds, at least of the larger ones, are not fully stretched backwards when flying with thrust and their body still is not fully directed in flight direction. But in publications of bird flight research it is only rarely pointed out to these both unequal ways of horizontal flight. The high power consumption during slow flight is commonly only ascribed to the thereby increasing induced drag.

Flying with thrust can be carried out in technical model making since the beginnings of aviation. But in horizontal flight of large and weightily ornithopters this way of flying demands considerably more energy than flying only with lift.

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http://www.ornithopter.de/english/images/principle/approach.gif

http://www.ornithopter.de/english/images/principle/penaud.gif


Otto Lilienthal had already distinguished clearly between these two ways of powered flying among birds and has pointed out the enormous flight energy during slow flight.

How does an ornithopter create thrust and lift - despite of alternating flapping directions?

The answer can be found in the Handbook (handbook How Ornithopters Fly only in German) based on well-known results of research. Apart from the aerodynamics of up- and downstroke, the dynamics of the flapping wing is also taken into consideration. The correlations are described with equations and diagrams. Your own calculations are made possible, which may be helpful for developing specific ornithopter models. Furthermore, you will find useful tips for ornithopter models in practice.

The relatively simple equations for changing circulation distributions make it possible to vary the lift distribution and to determine the appropriate wing twisting.

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Vortex system behind an ornithopter

The ornithopter subject also extends to the field of bionics. It is an attempt to develop better ornithopters by understanding the biological design principles of birds.

The handbook was translated in French by Jean-Louis Solignac. With his knowledge as an aerodynamics expert and with his experience he has contributed a lot to the improvement of the handbook.

Jean-Louis Solignac, Maître de Recherche, acted as deputy head of the departmentPrinciples of Aerodynamics in the directorate of aerodynamics of the national French research institute O.N.E.R.A. (Office National d'Études et de Recherches Aérospatiales).

1. The photos of the handbook

2. Calculating flapping wings under the precondition of quasi-stationary conditions First, the flapping wing is theoretically devided into stripes with a very small span. Then, for each of these wing sections the aerodynamic forces are calculated under stationary or constant oncoming airflow conditions. Their sum results from a numerical integration over the whole wing span.

Configuration of the forces

This way, you get the total forces of lift and propulsion of the flapping wing at a fixed moment of time of the flapping

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cycle. The corresponding wing twisting, the profile- and induced drag can be determined in the course of this calculating scheme, too.

Locations for calculation

This process is repeated in equal time segments of the wing stroke motion. Thereby, the changed factors as for instance the distribution of circulation, conditions of oncoming airflow or the dihedral of the wing form the basis. At the same time, stationary conditions are postulated. It is therefore presumed that the airflow does not change during the time span of calculating. Furthermore, unsteady airflow behaviour is not considered.

That way - thus by stringing together different steady conditions - time force progression under quasi-steady conditions results.

The force of a whole stroke motion can be obtained by numeric integration of the force progression over the considered time span. Thereby, up- and downstroke of the wing are advisably considered separately. Finally, the summary of up- and downstroke forces leads to the total forces of a whole flapping cycle.

Frequency of wing beats and the weight of birdsBut according to Erich von Holst this quasi-steady method only leads to useful results during a fast forward flight with relatively low flapping frequencies (large birds). Otherwise, the influences of unsteady airstream behavior become too strong. Later publications verify these constraints. As an example also the following analysis by M. Neef.

3. Result of the latest research

Dr.-Ing. Matthias F. Neef has examined in his dissertation Analysis of the flapping flight by numeric flow design engineering the unsteady flow at a moved wing. Thereby, he reached a similar vorticity system as aforesaid. However, his picture with a sinusoidal flapping motion-sequence is more specified and more detailed.

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Isolines of the circulation along the flight path

4. The tip vortex of the flapping wing

The isolines of circulation of a flapping wing shown above also can be visualized as single vortex filaments.

Vortex filaments runing parallel and with a similar direction of circulation, twist themselves to a single vortex in their shared center at the wake of the wing.

Wingtip vortices of the flapping wing

This way, the majority of the vortex filaments combined build up the wing tip vortex. During the flapping cycle its starting point is moving back and forth along the trailing edge of the wing - especially in the upstroke. Therefore, the vortex trail behind the flapping wing in plan view shows lateral contractions in regular intervals.

Also at birds, which are flying in cruise flight the lateral movement of the starting point of the vortex along the trailing edge of the wing has already been observed. This continuous-vortex gait is contrary to the vortex-ring gait when birds Flying with thrust.

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Helical wingtip vortices or slipstreams of a bird in continuous-vortex gait during cruise flight

When we imagine the wing tip vortex in the adjacent picture in three-dimensions be aware a surprising view.

The starting point of the vortex of one wing side not only moves back and forth along the trailing edge of the wing. It also follows the flapping motion. Seen in flight direction these both movements together resulted in an approximately circular path line. If now also include the forward motion of the flapping wing one sees the helical shape of the wing tip vortex spreading backwards.

Also the tip vortices of a propeller are arranged in a helical shape. They wrapped the propeller slip stream and are an essential part of it. In opposite to the propeller at the flapping wing simply the windings of the tip vortices are pulled more apart. Hence, in the three-dimensional view of this vortex picture will be visible a slipstream at each side of the flapping wing.

An according vortex structure is desirable also at ornithopters in cruise flight. Therefore also in the upstroke, a large lift must exist - maybe larger than indicated here - and the transition between the lift distributions of up- and downstroke must be smooth. In the movie recording of a flying swan for example, you see that the increase and decrease of the angle of incidence moves like a wave from the wing root to wing tip.

In order to generate large thrust at an ornithopter, the cross-section of the slipstream is to make as large as possible. Shifting of the spanwise distribution of lift is a dominant factor here. At downstroke the lift should be shifted as far as possible towards to the wing tip and at upstroke towards to the wing root. Furthermore the stroke angle of the wing should be chosen relatively large without, however, losing sight of thereby decreasing lift.

In case of very great demand of thrust, the shifting of the span wise distribution of lift in upstroke can be supported by a strong downward bending and/or backward bending of the hand wing. At the same time birds are using the shortening of the arm wing.

5. Formation flight of birds

Downwash distributions at the wing of an ornithopter in cruise flight.

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V-shaped staggered flight formations result in a measurable energy conservation for each single individual. This is particularly the result of aerodynamic influences. With the aid of the ornithopter theory conclusions can be drawn about the mode of functioning concerning the energy savings.

In connection with its lift the leading bird necessarily generates a wing tip vortex at both wing tips. For it this implies a loss of energy. It is relatively big for birds with high wing loadings and short, tapered wing shapes. But the following bird can try to tap the energy content of one of both wing tip vortices to make its own flight work easier.

Drag reduction at the formation flight of birds

Well known is the hypothesis that the following bird uses a field of uplift of its leading bird. It is generated by the tip vortex spreading backwards at the outer edge of the flight formation. This up wind enables the following bird to increase its own thrust without performing additional flight energy. But it is more effective to use the angular momentum of the incoming vortex to reduce the wing tip vortex of its own wing.

Flock of birds in flight

The problem for the following bird is the optimal adjustment of all distances in the three-dimensional space behind the leading bird. It must try to adjust the distances to the flapping wings of its leading bird in a way that the proper part of the leading bird's vortex passes it in a suitable moment and at the optimal position. It can surely feel the best flight position, but thereby it must also make compromises.

Other flapping wing designsIn designing of ornithopter models there are mainly two major tasks, the development of the drive technology and the development of the flapping wing. In general, the wide interest lies in the drive systems and components. But the main problem of the development of such aircrafts are the flapping-wings. In this field of design desire differs very widely from reality.

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Below, the attempt is being made, to give a rough overview about the physical characteristics of known flapping wings. But this collection doesn't claim to be complete.

1. The bird wing, the ideal

Naturally, the great archetype for technical flapping wings is the living bird wing. His great effectiveness due to his manifold possibilities to move purposeful and to change the shape will certainly be unobtainable in aero modelling for a long time. This is also true for his weight distribution and his sensor technology.

In this drawing by K. Herzog the anatomic subdivision of the bird's wing in arm- and hand section is pictured. It can also to be used advantageously when describing technical flapping wings. The longitudinal parts of these wing sections are rather different depending on bird species.

2. Membrane flapping wingsApplication range

Membrane flapping wings especially are changing the direction of chamber in the hand wing section according to the flapping direction. This way, they can produce much thrust and achieve steep climbing flights (flying with Thrust). But up to now they are less suited for gliding flights and for flying with lift.

2.1 The sail as archetype

A sail - though in other circumstances - has about the same function as a flapping wing. It shall generate as much thrust as possible under changing approach flow directions.

By material selection, layout, division into parts, sail trim and rig tuning the sail characteristics can vary in wide ranges. Battens give the sail more stability and an

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http://www.ornithopter.de/english/images/wings/bird_wing.gif

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optimal shape. A lot of descriptions with sophisticated tips about the fabrication of the sail and its practical use can be found.

Indeed, a lot of systems of membrane flapping wings have been developed, but detailed information about them is barely available

2.2 Simple membrane flapping wings

The pinion feather by obviously was optimized for thrust generation. Therefore, he increased the chord in the outer wing area. But this pinion feather was not intended for generating lift at the same time. It is merely a propeller for changing the direction of rotation.

Tim was the first in mass-produced rubber powered flapping wing model - with simple membrane flapping wings - invented by Albertini Prosper and de Ruymbeke Gérard (France 1969).

The membrane printing of Tim in the marginal picture was drafted by K. Herzog.Under the designation Tim Bird this model is available in trade till today.

2.3 Simple membrane flapping wing with battens

Here a famous Membrane Flapping Wing, equipped with small battens for stabilisation of the membrane, developed by A. Pénaud (France 1872)

2.4 Active twisting by spar rotation

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This is a membrane flapping wing by Erich v. Holst(1943) with drive-controlled wing twisting in the arm wing section by spar rotation. Only the rib at the end of the arm wing (number 9) is fixed to the spar. It is linked with a crank drive which effects the stroke movement as well as the rotary movement of the spar.

The twisting in the hand wing section happens largely passively. In addition, a transition from cross to longitudinal battens can be seen. In spite of alternating profile chamber direction during a flapping cycle a relatively purposeful increase of wing twisting tip wards is made possible.

The bird models by K. Herzog (1963) follow this scheme, too.

2.5 Aeroelastic twisting by spar torsion

The flapping wing model of the Czech Cenek Chalupsky (1934) was flying steadily without a tail unit. Its achieved climb power is still considered remarkable today.

Weight 3.1 kg

wing span 2 m

cane spar 10-15m

covering [109 oz]

ceiling [79 in]

linen [394-590 in]

Each flapping wing of this ornithopter has two spars. The straight, bending resistant spar (H1) transmits the power of the stroke motion. The bended torsion elastic spar (H2) determines the magnitude of the wing twisting.

Both spars cross approximately in the center of the half span. At the cross point they are movably interlinked. For the torsion elastic spar (H2) not to bend backward too much a string or an elastic thread is apparently tightened between the tips of the spars.

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During downstroke of the wings the lifting forces are increased. The spar H2 and the wing are twisting. The magnitude of the twisting acts in accordance with the magnitude of the lift force and the stiffness of the spar. It therefore happens aeroelastically.

Additionally to the twisting the tip of the spar H2 bends upwards in the downstroke. As a reaction it bends downwards at the other side of the cross point - thus, in the section of the arm wing. Thereby, the camber of the airfoil is increased a little. Thereby, an adaptation to the requirements of an effective stroke motion takes place.

2.6 Flying wing ornithopter

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Here an ornithopter without a tail unit, developed by Jean-Louis Solignac (France, 2000).

The flapping wing model has a very simple and light driving mechanism and is powered by a rubber motor. With a wing span of 15 cm (5.9 in) it has a weight of only 0.6 grams (0.021 oz [US]). The airplane performances are amazingly good. The particular about this flapping wing is the down cambered airfoil shaped by battens. Thereby it flies in a stable attitude without a tail unit. This can theoretically be explained with the shifting of the pressure point of thin airfoils. It can be tested in the adjacent experiment with a paper airplane. The cross-section of this paper airplane equates to a down chambered airfoil.

If you keep the center of gravity at the same distance dform the leading edge like at the paper airplane, also a lightweight flying wing glider made of balsa is flying perfectly.

2.7 In tandem

Ornithopter with two sets of flapping wings based on a Dragonfly, developed by Erich von Holst (1943).

Here, for simplifying the mechanism both opposite halves of a wing are rigidly fixed to a unit. This way, the pressure point of the model is fixed between the two wing units.

In such tandem arrangements with wings flapping in opposite directions the vertical pendulousness of the fuselage should be avoided. This, however, bears the disadvantage that the backmost flapping wing is in the turbulence wake of the front one. Only for very small wings and at very small Reynolds numbers this may be beneficial.

2.8 Thrust-Wing

By mechanisation of a Dragonfly's flight principle Erich von Holst has developed his Thrust-Wing (1940s) with two in the opposite direction rotating three-blade wings.

[Zainab Ejaz]

http://www.ornithopter.de/grafik/fluegel/holst_libelle.gif

http://www.ornithopter.de/grafik/fluegel/tandem.jpg


The flapping angle in one stroke direction constitutes 180° or 360° for a complete flapping cycle. Three instead of two wing blades per rotor offer a constant supporting force.

In contrast to a propeller at the Trust-Wing also can be generated a considerable lift force perpendicular to the direction of flight. One must only increase the Thrust-Wing advance ratio - similar to a normal flapping wing - and fly with a small positive angle of attack of the Thrust-Wing axis. (The advance ratio is the ratio between the distance moving forward one revolution and the diameter of the Thrust-Wing.)

This is a fine example for an innovative transfer of biological principles of a flapping wing in engineering. But the specialism bionics did not exist at that time.

2.9 Oscillating stretched wing

Thrust also can be produced by raising and lowering a stretched wing in flight. But thereto the lift or the transverse force during the upward motion must be smaller than during downward motion. The bigger the difference, the better for the thrust. Furthermore, a continual alignment of the angle of incidence is normally necessary.

Here a strikingly simple generation of an accordant oscillating motion of the wing by using an eccentrically pivoted rotating mass consisting of the mainspring and the gear. In this case the wing is aeroelastic twistable. The idea was coined by W. B. Mituritscha (probably from Russia, 1953).

Unfortunately, a forward and backward motion of the wing occurs along the way. However, this can be avoided by a second counter rotating mass.

There are diverse proposals to generate an oscillation motion of the wing by a pilot who is flying in a hang-glider or another ultralight aircraft - for example by fast press-ups or knee-bends.

Entirely different model experiments with oscillating wings shows Karl-Heinz-Helling with his Double flapping wing airplane (2008).

2.10 Rotating wings

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http://www.ornithopter.de/grafik/fluegel/oszillierend.gif


To avoid the accelerating forces at the final stroke positions flapping wings rotating on a cone-shaped shell where sometimes built whose apex lies at the wing root.

Examples: The Rotor Dragonfly (1944 and 1989) by Adolf Piskorsch

and the flight model by Horst Händler (1989).

Both ends of the driveshaft are bended in Horst Händler's model. Thereon, the wings are attached freely twistable. The angle of incidence is guided by the upward pointing levers on the wings.

2.11 With non-twistable arm wing section

This is a membrane flapping wing with a non-twistable arm wing section and passive twisting at hand wing section.

The arm wing is triangle shaped and has a large wing depth at the wing root. Arm- and hand wing membrane overlap in wing span direction. Obviously, the hand wing spar could make a little flap movement at the wrist. Later the hand wing depth was enlarged.

This flapping wing design of the Seagull was developed by Percival H. Spencer (USA 1958)

Today, this design principle of flapping wings with inserted battens is widely-used.

3. Profiled flapping wingsApplication range

Profiled flapping wings or double-sided covered wings may work with a very high efficiency. With their mostly relatively low flapping frequency and the small operating range of lift coefficient of a simple airfoil not much thrust can be

[Zainab Ejaz]

http://www.ornithopter.de/grafik/fluegel/rotorlibelle.jpg

http://www.ornithopter.de/grafik/fluegel/rotierend.jpg

http://www.ornithopter.de/grafik/fluegel/spencer_seagull.jpg

http://www.ornithopter.de/grafik/fluegel/segellatten.jpg


produced. Not, at least, if the full lift must be generated concurrently (flying with lift). Therefore, profiled flapping wings are suited especially for a level flight, the gently inclined climbing flight and of course for changing to gliding flight.

3.1 With artificial feathers

To ease the twisting, the closed airfoil can be fanning-out. So far, this is particularly used for large manned ornithopters.

Adjacent, a flapping wing with staggered wing tips of the manned Schwan 1, developed by Walther Filter (1955, at the Hannover fair 1958). The angle of incidence deflection of the feathers designed as several wings was controllable.

Even for splay and straddle movement of the feathersthere are old design proposals. In contrast, with EV7bonly with simple feather implementations experiments have been made.

A further example for artificial feathers is the Icarus by Emiel Hartman (England 1959).

More recent experiments with artificial feathers are to be seen at the

Birdman Georges Fraisé (France 2005) and

Ornithopter Project by Ryszard Szczepañski (Poland 2002).

3.2 With inclined hinge of the hand wing

A special version of a flapping wing derives from K. Herzog (1963). With this wing, the rotation or the twist axis is not standing vertical to the stroke axis.

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http://www.ornithopter.de/grafik/fluegel/w_filter.gif

http://www.ornithopter.de/grafik/fluegel/gestaffelt.jpg

http://www.ornithopter.de/grafik/fluegel/federn.gif

http://www.ornithopter.de/grafik/fluegel/schlagfluegel.gif


The arm wing should perform a flapping motion and a twisting motion at the shoulder joint. With rubber threads between arm- and hand wing the latter was pulled down a little (aeroelastic wing).

This is also an early suggestion for an articulated flapping wing with an additional flap movement of the hand wing.

The kink of the profile between the arm and the hand wing lies approximately at the same location as on theabove-mentioned membrane wing by P. H. Spencer.

3.3 Twisting by tilting the leading edge of the wing

The feature of the pitch propeller by John Drake lies in the twisting of the leading edge, not the trailing edge of the flapping wing (England, flight tests in 1978).

3.4 With stepped twisting

An approximate wing twisting can also be achieved by a stepped rotation of relative non-twistable wing sections.

The model EV4 (1979) was also equipped with such a rotation of single wing sections. But in this case, the rotation was controlled by the wing drive.

A typical representative of a passive stepped twisted wing is the Step-Twister with his foam wings (Depron) by Karel Pustka (2004). The developing gap between the wing sections is covered with a membrane.

[Zainab Ejaz]

http://www.ornithopter.de/english/images/wings/cycle.gif

http://www.ornithopter.de/grafik/fluegel/stufig_ev4.jpg

http://www.ornithopter.de/grafik/fluegel/stepped.jpg


3.5 Twisting by stroke movement of the auxiliary spar

Here, the wing twisting is generated by a phase-delayed stroke movement of the main and auxiliary spar - developed by Emile Räuber (France 1909).

This technology was also used at the EV2 (1976). In the margin, the wings with their two spars powered separately are to be seen.

The function is similar to the wing of a Dragonfly. Here, too, the phase-delayed flapping movement of the main and auxiliary spar determines the amount of the wing twisting.

Dragonfly picture

Furthermore, the Dragonfly obviously works with a strong spar at the leading edge. With the phase-delayed flapping movement of three spars the camber of the airfoil can be influenced, too.

The supports or linkages of the three spars at the body are clearly recognizable as dark, partly crossing structures at the back of the Dragonfly.

3.6 Servo controlled wing twisting

This is a lifelike and airworthy half-size replica of pterosaurs - a Quetzalcoatlus Northropi (QN). The aerodynamics of this ornithopter should equate the original as far as possible. The idea comes from the creative genius Paul MacCready Ph.D. (USA 1985).

The twisting of the wings was controlled by servos and the flight attitude was stabilized by backward and forward motions of the wing tips and nodding motions of the head.

[Zainab Ejaz]

http://www.ornithopter.de/grafik/fluegel/phasenverschoben.gif

http://www.ornithopter.de/grafik/fluegel/hilfsholm.jpg

http://www.ornithopter.de/english/images/wings/dragonfly.jpg

http://www.ornithopter.de/grafik/fluegel/maccready.jpg


3.7 Shearflex principle

Here an aeroelastic twistable profiled flapping wing according to the Shearflex Principle. This system makes a relatively inelastic covering applicable. If the twisting along the wing is constant and not to excessive, the airfoil contour accuracy is therefore very good.

Here, the twist elasticity will mainly be determinate by the spar designed as wing leading edge.This system was invented by Professor James D. DeLaurier and Jeremy M. Harris (Canada 1994).

The ornithopter with its tripartition of the flapping wing is interesting, too. Jeremy M. Harris 1977 has applied it for patent.

On the adjacent photo James D. DeLaurier and Jeremy M. Harris can be seen with their remote-controlled model, 3 m in span and with combustion motor. A sustained flight was achieved 1991.

Here, a corresponding replica with an electrical drive system by Horst Händler (1994).

3.8 Oscillating wing tips

The main spar of the Snowbird has no hinge, and instead flexes to produce the desired flapping motion. By this way the wing tips perform an oscillating motion. Thereby the wing twists passively under aerodynamic loads.

The principle of a wire powered wing oscillation has some marked advantages particularly for man powered ornithopters:

[Zainab Ejaz]

http://www.ornithopter.de/grafik/fluegel/harris.gif

http://www.ornithopter.de/english/images/wings/shearflex.gif

http://www.ornithopter.de/grafik/fluegel/williams.gif

http://www.ornithopter.de/grafik/fluegel/delaurier_harris.jpg

http://www.ornithopter.de/grafik/fluegel/haendler_del06.jpg

http://www.ornithopter.de/grafik/fluegel/haendler_del07.jpg

http://www.ornithopter.de/grafik/fluegel/hpo_airborne.jpg

http://www.ornithopter.de/grafik/fluegel/snowbird.jpg


fail-safe wing position for gliding

applicable for large wing spans with its accordingly low induced drag

very few moving parts

Todd Reichert has played an important role in developing of the Snowbird and he also has flown it successfully. It was the world`s first officially certifieded short flight of a human-powered ornithopter (Canada 2010).

3.9 Shell wing

A wing with an active wing twisting by a drive controlled spar rotation, developed by Albert Kempf (France 1998, Apparently, the upper side of the wing consists of a cambered hard shell, which is shaped with foam on the lower side to a profiled airfoil wing.

A long thin plate with a cambered cross section may be twisted easily and wrinkle-free. Also the aforesaid Shearflex principle is using this property. This flapping wing category here is called shell wing.

Such equipped Truefly is to be seen in the adjacent picture - an ornithopter with a wonderful flying sight. It also was the first ornithopter which achieved strong climbing flights with profiled flapping wings.

Ornithopter models EV1 to EV4

1. Ornithopter model EV1

first flight 1975

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http://www.ornithopter.de/grafik/fluegel/hpo_sequence.jpg

http://www.ornithopter.de/grafik/fluegel/aktive.gif

http://www.ornithopter.de/grafik/fluegel/truefly.jpg

http://www.ornithopter.de/grafik/bilder1/ev1_modell.jpg


wing span 2.90 m

Weight 5.4 kg

wing loading 5.3 kg/sqm

max wing chord 0.44 m

flapping cycle 0.8 s

flapping angle 60 deg

EV1 in powered flight

EV1 in gliding flight

EV1 with its wings in the upper final stroke position

The twisting along the whole span unfortunately was too small for powered flight. It was made harder by the large rigid trailing edge.

[Zainab Ejaz]

http://www.ornithopter.de/grafik/bilder1/ornithopter_im_anflug.jpg

http://www.ornithopter.de/grafik/bilder1/ev1_start.jpg

http://www.ornithopter.de/grafik/bilder1/ev1_gleitflug.jpg

http://www.ornithopter.de/grafik/bilder1/ev1_oben.jpg


Wing twisting in the downstrokeBecause the high wing loading the model flies relatively fast. The advantage of this is that the wing twisting must not be big.

2. Ornithopter model EV2 first flight 1976

wing span 2.96 m

weight 5.4 kg


max wing chord 0.42 m

The active twisting of the wings resulted from the separate controlling of the main and auxiliary spars in stroke direction.

For covering the wing the elastic foil type Platilon U 04was used for the first time, thickness 0.03 and 0.05 mm.

EV1 in comparison with EV2, top view

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http://www.ornithopter.de/grafik/bilder1/ev1_abschlag.jpg

http://www.ornithopter.de/grafik/bilder1/ornithopter_ev2.jpg

http://www.ornithopter.de/grafik/bilder1/schlagfluegelmodell_ev2.jpg

http://www.ornithopter.de/grafik/bilder1/ornithopter_ev1_ev2.jpg


3. Ornithopter model EV3 The EV3 did not get past the phase of design. Only plans exist (terminated 1978).


first flight 1980

wing span 3.06 m

Weight 4.8 kg


aspect ratio 10

Airfoil [120 in]

The arm and hand wing sections were actively pitch controlled separately from the drive at the relevant section root. The arm wings were rigid. The hand wings should be twisted aeroelastic. The twisting of the hand wing was appropriate for gliding.

EV4 in flight, in wing down stroke

In the area of the hand wing you can see a small aeroelastic twisting.


first flight 1981

wing span 2.70 m

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http://www.ornithopter.de/grafik/bilder1/ornithopter_im_flug.jpg



weight 5.2 kg



first flight 1983

wing span 2.90 m

Weight 5.2 kg


aspect ratio 7.4

EV6 in the upstroke

with a relatively constant twisting along the whole wing. Thus, the angle of incidence grows linear towards the wing tip.

EV6 in the downstroke

There is also a constant twisting along the whole wing.

[Zainab Ejaz]

http://www.ornithopter.de/grafik/bilder2/ev6_ornithopter.jpg

http://www.ornithopter.de/grafik/bilder2/ev6_im_flug.jpg

http://www.ornithopter.de/grafik/bilder2/ev6_fluegelaufschlag.jpg

http://www.ornithopter.de/grafik/bilder2/ev6_aufschlag.jpg

http://www.ornithopter.de/grafik/bilder2/ev6_fluegelabschlag.jpg

http://www.ornithopter.de/grafik/bilder2/ev6_abschlag.jpg


7. Ornithopter model EV7a first flight 1989

wing span 3.14 m

weight 4.6 kg


aspect ratio 10.5

Wing twisting

At the EV7a again was used aeroelastic twisting along the whole wing. At rest the wing takes the twist position of the upstroke (look also at the preceding picture).

While gliding

Wing root fairing

No inclination of the stroke plane applied. Therefore, a bend results in the trailing edge at the transition between the rotational symmetric wing adapter roller and the airfoil of the wing.

[Zainab Ejaz]

http://www.ornithopter.de/grafik/bilder3/konstrukteur.jpg

http://www.ornithopter.de/grafik/bilder3/ornithopter_ev7a.jpg

http://www.ornithopter.de/grafik/bilder3/ev7_gleitflug.jpg

http://www.ornithopter.de/grafik/bilder3/uebergang.jpg


Ornithopter model EV7b first flight 1992

wing span 3.20 m

weight 4.8 kg

aspect ratio 10

airfoil NACA 4412

Wing with feathers

To enlarge the wing twisting near the wing tips experiments with feathers have been made. The twisting of each wing feather should be significantly larger than in the case of the continuous airfoil - especially during the downstroke.

The feathers were not designed to decrease the induced drag, because that would be too difficult.

EV7b in gliding flight

The connection line of the feather tips corresponded to the ideas of the designer. The feather tips continued the camber and twisting of the hand wing.

In powered flight (in direction of down stroke) the bending of the feathers was often quite variable as seen in the following fly-by of an ornithopter.Usually it is quite useful in gliding.

[Zainab Ejaz]

http://www.ornithopter.de/grafik/bilder3/ornithopter_ev7b.jpg

http://www.ornithopter.de/grafik/bilder3/feathers.jpg

http://www.ornithopter.de/grafik/bilder3/federn.jpg

http://www.ornithopter.de/grafik/bilder3/schwungfedern.jpg

http://www.ornithopter.de/grafik/bilder3/staffelung.jpg


Ornithopter model EV7c first flight 1994

wing span 2.94 m

weight 4.7 kg


aspect ratio 11.3

airfoil of arm wing S1020

airfoil at the wing tip E203

The new wing mechanism which replaced the auxiliary spar of the hand wing was not successful.

8. Ornithopter model EV8 first gliding flight in 2000

wing span 2.8 m

Weight 3.6 kg

wing loading 5.9 kg/qm

aspect ratio (wing) 11.7

airfoil of arm wing S1020

Today, ornithopter designs are still on the minds, as well as the drawing boards, of adventurous and imaginative people determined to achieve the old dream of flight with flapping wings. Some success has been achieved recently toward a practical ornithopter by James DeLaurier and his team at the University of Toronto Institute for Aerospace Studies. With the help of four research students, test pilot Jack Sanderson and a few other volunteers, DeLaurier achieved his lifelong dream

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http://www.ornithopter.de/grafik/bilder3/absturz.jpg

http://www.ornithopter.de/grafik/modell8a/ev8.jpg

http://www.ornithopter.de/grafik/modell8a/flg_oben.jpg


of flying a full-scale ornithopter on Jul. 8, 2006, at Downsview Park, Toronto. Equipped with a 24-horsepower engine and a model airplane turbo booster, the Project Ornithopter vehicle flew for 14 seconds at an average speed of 88 km/h, in the process traveling a third of a kilometer.

RC ornithopters are a relatively new addition to the world of radio control flying, and they're completely different to any other type of aircraft.

Avitron 2 ornithopter:

Future Applications of the Ornithopter

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The ornithopter, though firmly rooted in the psyche of many aerospace designers as ancient technology, has much to offer as a means of advancing the science of aerial vehicles. As scientists like Dr. John McMasters have observed, "a door is fmally opening to the realization that there may be much more to learn in further, truly multidisciplinary investigations of the biomechanics of flight as it may relate to a wide range of practical aircraft types" (McMasters & Cummings, 2004, p. 3).

In the burgeoning field of unmanned micro air vehicles, ornithopters have found a niche for which they are especially well suited.

Unmanned Aerial Systems

One of the principal advantages of an ornithopter as an unmanned aerial system, especially one used for surveillance purposes, is the low aeroacoustic signature of such an aircraft (DeLaurier & Harris, 1993).

Compared with fmed-wing, propeller-driven aircraft, and even those with high-efficiency, slowly rotating propellers, omithopters have the potential to be the quietest of any aircraft design (DeLaurier & Harris, 1993).

In addition to low audio profiles, ornithopters are inherently well suited to small scale aerial vehicles because of their aerodynamic properties.

A flapping-wing aircraft has considerable advantages in small-scale designs because of efficient operations at low Reynolds numbers combined with the ability to fly by thrust alone (Shkarayev & Silin, 2009).

Though the small size means that micro ornithopters are more susceptible to wind gusts than larger vehicles, the potential for enhanced maneuverability, including hovering, and even backward flight, makes flapping wing designs a subject of keen interest (Shyy et al., 2010).

Micro Aerial Vehicles (MAVs) currently in development, such as the Microbat, are proof that ornithopter technology, though still in the early stages of development as a technology, has much to contribute to the science of aerial remote sensing (Singh & Chopra, 2008).

VSTOL Applications

Flapping wing designs have the potential to not only revolutionize the field of micro-sized aircraft, but to drastically change how Very Short Take Off and Landing (VSTOL) aircraft are utilized.

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Though most research in ornithopter flight has dealt with the dynamics of cruise flight, it is not unreasonable to speculate that flapping wing technology contains the elements necessary to support not only VSTOL capabilities, but also the possibility of high speed subsonic flight (DeLaurier & Harris, 1993).

Unquestionably, myriad other applications for flapping wing technology exist that have yet to be identified; and even some that have, such as continuously variable-span wings based on those of gliding birds, have yet to be solved (McMasters & Curnrnings, 2004). The prospect of quiet, efficient, and maneuverable aircraft technology capable of applications of any size is reasonable assurance that ornithopter development will continue well into the future.

ConclusionInspired by nature, ornithopters were long considered the only viable means by which man could achieve the freedom of flight that proved so elusive. As lighter-than-air and eventually fixed-wing machines overshadowed the development of flapping wing aircraft, ornithopters were marginalized and often discounted as a futureless technology. Examining the past for inspiration and developmental breakthroughs in ornithopter design shows that flapping-wing flight is far from the pipe dream it has been often characterized to be. Instead, ornithopters occupy the rare position of being critical to the formative stages of aviation as well as to its future. As technology unfailingly advances and aerospace designers continually seek new ideas, the ornithopter once again has emerged as a kind of technological chimera, combining seemingly incongruous elements of aviation's distant past and inspiration from nature with revolutionary technology to create an adaptable solution to the needs of the ever-developing science of aviation.

[Zainab Ejaz]


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[Zainab Ejaz]

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