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Can a Human Fly Like a Bird? Page 1 OVERVIEW Is it possible to fly like a bird, with wings that fold into a backpack? This paper looks at the possibility of creating personal wings with sufficient performance to soar in most daytime weather conditions and concludes that this goal is indeed possible. I describe the approach I have taken towards realising the dream of wearable folding wings for soaring flight. For good penetration into headwinds and to fly between thermals, a lift to drag ratio of at least 30:1 is desirable. The threshold sinking speed for unlimited soaring ability is about 0.3 m/s and at least 0.4 m/s is achievable with wearable folding wings. LIMITATIONS OF EXISTING DESIGNS Hang-gliders and parafoils are the closest examples to wearable wings. Hang-gliders tend to suffer from poor lift distribution and high profile drag due to exposed wires and struts. Parafoils have very high profile drag due to numerous suspension lines and exposed pilot position. Increasing the performance of gliders demands drag reduction. For a sailplane to climb it has to have a sinking speed less than the speed of rising air currents. To fly long distances it must fly fast with low sinking speed. Combining the two requirements requires reduction in both induced and profile drags. Induced drag can be reduced through optimum lift distribution and increased wing span. Artificial wing tip feathers can also substantially reduce induced drag. Profile drag can be reduced by improved streamlining and surface area reduction. EXPERIMENTS DEMONSTRATING POSSIBILITIES A project in New Zealand involving Waikato University, the Waikato Technical Institute and Auckland University demonstrated a wing that met the above performance requirements. This was a membrane wing of 15 metre span and an aspect ratio of 27 and was mounted on a framework attached to a bus. Load cells measured lift, drag and pitching moment. Test results showed a best L/D of 50 at 18 m/s which translates to a sinking speed of 0.3 m/s. The sail was seamless and the internal kaugis did not touch the membrane, resulting in a very fair aerofoil contour for minimum drag. . STABILITY Finding performance possibilities has little future unless the resulting wings are stable in flight. This topic has long interested the author who has been fascinated by tailless aircraft designs. These were always limited by the need transfer tailplane function to the wing with consequent loss of efficiency. Thus the benefits of a tailless configuration were lost by the requirements for a larger wing. On the way to New Zealand from Germany to design a new competition sailplane for the New Zealand Gliding Federation I was able to observe albatrosses soaring behind the ship. It was clear from their relaxed flight style that the birds were very stable in spite of their near tailless configuration. I was determined to unlock nature’s stability riddle. I felt there must be a spring element involved yet experiments showed that there must be another factor. Flight muscle elasticity spring is the spring and to be effective there had to be a feedback element which compared wing lift with muscle tension. The missing aspect was discovered by Dr. A.D. Sneyd of Waikato University whose mathematical analysis showed that the spring had to have a reverse gradient. Looking again at bird anatomy, nature had achieved this through the deltoid crest on the humerus. The simple geometry of the deltoid crest provided the key to achieving pitch stability on an efficient cambered wing. All birds, bats and pterosaurs exhibit this geometry. This enables a mechanical feedback system that maintains a state of unstable equilibrium. I presented this theory as “The Principles of the Constant ‘g’ Stability System” at CEAS2007. WEARABLE FOLDING WINGS Combining all the successful elements makes wearable folding wings possible. Making membrane wings that fold is not so difficult. Creating folding wings that tension themselves via lift forces to achieve an efficient aerofoil shape is more of a challenge. Making them fold easily and compactly is difficult and allowing the flyer to extend and furl the wings at will is a further test of ingenuity. There is great potential to incorporate retractable artificial wing tip feathers. Once optimised, the maximum L/D could rise to 40 and minimum sink reduce to 0.35 m/s. Thanks to a donation of some carbon fibre windsurfer masts, the structural weight will be reduced to perhaps 10 kg, a low value for a complete aircraft high performance sailplane. CONCLUSIONS It is both possible and practical to design and build high performance wearable folding wings which enable a flyer to exploit microlift and thus enjoy extended soaring throughout the year. This will greatly increase the opportunities for enthusiasts to enjoy the sport of soaring at a much reduced coat with enhanced convenience. This was a very simple wing with no control system. While sweep forward and a flexible structure gave it static stability, it did not have the dynamic stability from a reverse gradient balance spring. Dereidactyl Specifications wing span 15 m wing area 8.3 m 2 aspect ratio 27 weight 18 kg gross weight 80 kg Testing a Dereidactyl Wing experiments at Hamilton Airport sponsored by NZ TV Can a Human Fly Like a Bird? D.Reid PO Box 143 Oneroa, Waiheke Island, AUCKLAND 1840, New Zealand Email: [email protected], www.wearablewings.ning.com

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Page 1: Wearable Wings

Can a Human Fly Like a Bird? Page 1

OVERVIEWIs it possible to fly like a bird, with wings that fold into a backpack? This paper looks at the possibility of creating personal wings with sufficient performance to soar in most daytime weather conditions and concludes that this goal is indeed possible. I describe the approach I have taken towards realising the dream of wearable folding wings for soaring flight. For good penetration into headwinds and to fly between thermals, a lift to drag ratio of at least 30:1 is desirable. The threshold sinking speed for unlimited soaring ability is about 0.3 m/s and at least 0.4 m/s is achievable with wearable folding wings.

LIMITATIONS OF EXISTING DESIGNSHang-gliders and parafoils are the closest examples to wearable wings. Hang-gliders tend to suffer from poor lift distribution and high profile drag due to exposed wires and struts. Parafoils have very high profile drag due to numerous suspension lines and exposed pilot position.Increasing the performance of gliders demands drag reduction. For a sailplane to climb it has to have a sinking speed less than the speed of rising air currents. To fly long distances it must fly fast with low sinking speed. Combining the two requirements requires reduction in both induced and profile drags. Induced drag can be reduced through optimum lift distribution and increased wing span. Artificial wing tip feathers can also substantially reduce induced drag. Profile drag can be reduced by improved streamlining and surface area reduction.

EXPERIMENTS DEMONSTRATING POSSIBILITIESA project in New Zealand involving Waikato University, the Waikato Technical Institute and Auckland University demonstrated a wing that met the above performance requirements. This was a membrane wing of 15 metre span and an aspect ratio of 27 and was mounted on a framework attached to a bus. Load cells measured lift, drag and pitching moment. Test results showed a best L/D of 50 at 18 m/s which translates to a sinking speed of 0.3 m/s. The sail was seamless and the internal kaugis did not touch the membrane, resulting in a very fair aerofoil contour for minimum drag. .

STABILITYFinding performance possibilities has little future unless the resulting wings are stable in flight. This topic has long interested the author who has been fascinated by tailless aircraft designs. These were always limited by the need transfer tailplane function to the wing with consequent loss of efficiency. Thus the benefits of a tailless configuration were lost by the requirements for a larger wing.On the way to New Zealand from Germany to design a new competition sailplane for the New Zealand Gliding Federation I was able to observe albatrosses soaring behind the ship. It was clear from their relaxed flight style that the birds were very stable in spite of their near tailless configuration. I was determined to unlock nature’s stability riddle.I felt there must be a spring element involved yet experiments showed that there must be another factor. Flight muscle elasticity spring is the spring and to be effective there had to be a feedback element which compared wing lift with muscle tension. The missing aspect was discovered by Dr. A.D. Sneyd of Waikato University whose mathematical analysis showed that the spring had to have a reverse gradient. Looking again at bird anatomy, nature had achieved this through the deltoid crest on the humerus.The simple geometry of the deltoid crest provided the key to achieving pitch stability on an efficient cambered wing. All birds, bats and pterosaurs exhibit this geometry. This enables a mechanical feedback system that maintains a state of unstable equilibrium. I presented this theory as “The Principles of the Constant ‘g’ Stability System” at CEAS2007.

WEARABLE FOLDING WINGSCombining all the successful elements makes wearable folding wings possible. Making membrane wings that fold is not so difficult. Creating folding wings that tension themselves via lift forces to achieve an efficient aerofoil shape is more of a challenge. Making them fold easily and compactly is difficult and allowing the flyer to extend and furl the wings at will is a further test of ingenuity.There is great potential to incorporate retractable artificial wing tip feathers. Once optimised, the maximum L/D could rise to 40 and minimum sink reduce to 0.35 m/s. Thanks to a donation of some carbon fibre windsurfer masts, the structural weight will be reduced to perhaps 10 kg, a low value for a complete aircraft high performance sailplane.

CONCLUSIONSIt is both possible and practical to design and build high performance wearable folding wings which enable a flyer to exploit microlift and thus enjoy extended soaring throughout the year. This will greatly increase the opportunities for enthusiasts to enjoy the sport of soaring at a much reduced coat with enhanced convenience.

This was a very simple wing with no control system. While sweep forward and a flexible structure gave it static stability, it did not have the dynamic stability from a reverse gradient balance spring.

Dereidactyl Specificationswing span 15 mwing area 8.3 m2

aspect ratio 27weight 18 kggross weight 80 kg

Testing a Dereidactyl Wingexperiments at Hamilton Airport sponsored by NZ TV

Can a Human Fly Like a Bird?D.Reid

PO Box 143 Oneroa, Waiheke Island, AUCKLAND 1840, New ZealandEmail: [email protected], www.wearablewings.ning.com

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MINIMUM PERFORMANCE REQUIREMENTSGliders design to utilise microlift are able to soar in weak lift and so extend opportunities for reliable flight. Can a wearable wing glider be designed and built with this kind of performance?To further the wearable folding wing project I developed a spreadsheet to explore the range of possibilities. As described in my CEAS2007 paper, “The Principles of the Constant ‘g’ Stability System”, the spreadsheet can receive a wide variety of inputs and calculate stability, stresses and performance.MICROLIFT SOARING ABILITY PREDICTEDThe calculations, using drag coefficients dereived from the Dereidactyl experments, demonstrate the potential to create folding wearable wings able to soar in microlift conditions.

The above graph compares sinking speeds of a peregrine falcon, paraglider, hang glider, Archaeopteryx glider and Wearable Wings. The calculated performance of wearable wings with tip feathers extended is almost identical to the Archaeopteryx. With tip feathers retracted, the weratble wings show much better penetration.

INTRODUCTION TO PERFORMANCEIn considering the possibility of a human flying like a bird, it is useful to consider what sort of gliding ability is necessary for reliable soaring. Humans are not strong enough to sustain flight through physical effort hence the ability to extract energy from the atmosphere is needed. There is always some movement in the atmosphere. A glider gains energy from gravity, sliding down a glide-path. To avoid reaching the ground it must find air rising faster than its descent speed. Thus the lower the descent speed the greater the chance to stay aloft.Hang-gliders and parafoils can only fly in conditions of strong lift. The same applies to most soaring birds where the larger species can live only where these conditions prevail. Recently gliders have been designed to utilise weak lift and are called microlift gliders. On the chart below, a falcon, parafoil,

hang-glider, microlift glider, Dereidactyl wing, and Wearable Wing are compared. My Dereidactyl wing shows the best performance. This wing never reached a man carrying state yet its measured performance shows the potential for development.The design of wearable folding wings demanded a size that could be folded into a backpack. Thus the long, narrow Dereidactyl configuration could not be considered. Nature uses finger-like wing tip feathers to disperse wing tip vortices in three dimensions. Emulating these was aided by excellent research by the Swiss engineer, LaRoche and his WingGrid. It is not only performance that is important; pilot safety is vital. The most dangerous phase of flying is landing. For a pilot to land on his feet, his approach speed must be slow resulting in the danger of an abrupt wing tip stall and injury. The wing tip feathers greatly reduce this hazard.

CROSS COUNTRY FLYINGFlying accross country involves two basic elements. The first is circling in thermals to gain altitude. The second is straight line flying between thermals.

During cIrcling flight, the wing operates at a high lift coefficient and at g > 1. A typical circling radius involver g ~ 1.2.

During this phase the ideal wing will have high camber and no twist. The wing-tip feathers will be fully extended and fanned.

For high speed inter-thermal flying, the wing-tip feathers will be retracted and membrane tensioned held as high as possible. Here, minimum profile drag and surface want to be at a minimum. A streamlined pilot enclosure is important for this high speed flight with minimum loss of altitude.

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How compact should the the folded wings be? I wanted wings that could be carried on a bus, worn as a backpack on a bicycle and easily permit walking through doorways. Limiting joints to three per side and having retractable wing tip feathers permits a full span of 8 metres with a folded height of 1.2 metres when worn as a backpack. WING HINGE CONSTRAINTSCompact folding wings require joints that fold through almost 180 degrees so that spar tubes lie parallel when the wing is folded. The hinges must resist both bending and torsion.

WING FOLDING

Folding wings are the norm in nature. Creating high performance soaring wings that fold into a backpack is a challenge not yet realised. In this section will look at attempts to create a practical configuration that is light, compact, workable and reliable.

THREE DIMENSIONAL GEOMETRYIn flight the wing spar is curved in three dimensions yet when folded need to lie compactly. This is achieved by adjusting hinge axes.

TENSIONING THE WINGA membrane or sail wing has to be under tension to function. On boat sails the wind force inflates the sail. This ceases to be effective at low angles of attack; the sail begins to luff and loses its designed shape.This is particularly serious for glider wings since at high speed they fly at low angles of attack. Two solutions to this problem include ribs or battens to retain sail shape or tension applied to the fabric. The 15 metre span Dereidactyl wing had 7 internal struts called “kaugis” plus a trailing edge wire tensioned to 30kg.

FOLDED SIZEWith a folding wing, the spar stiffness cannot be used to brace the wing. Instead lift forces can be used combined with a leading edge tendon also found in nature. This pteroid tendon supports the wing leading edge and provides the counter-force to balance membrane tension.Further sail tension comes from internal kaugis. Since sail tension needs to increase with speed, these kaugis must have adjustable length. This length is set by the pilot’s speed control.The wing tip kaugis is designed to take bending loads, unlike the simpler Dereidactyl wing. This avoids high torsional loads on the hand spar with its wide bracket making room for the three sets of kaugi. STRUCTURAL STRENGTHEach spar element must have sufficient strength to sustain bending and torsion loads over the complete flight envelope with an adequate safety margin. Thanks to inherent bending relief and the feedback stability system, flight loads are limited to pilot manoeuvres. The stability system limits imposed loads to defined values. Thus neither gusts nor piloting can induce excessive flight loads.

MAJOR STRUCTURAL COMPONENTS

The photos of the folded wing are of an early version of the wing without feathered tips, Jan. 2009. The drawings show my latest ideas for kaugi positions.

nylon feather sheath

carbon hand spar

carbon forearm spar

carbon upper arm

CARBON WING, FEATHERS RETRACTED

backpack frame

FOLDED WING

The wing plan shown above is the configuration of 1 september 2009. Here the wing spars are made from windsurfer masts. The wing tip feathers are formed from polypropylene foam with bamboo shafts. The kaugis are foam with carbon fibre caps.

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STRUCTUREFor my first attempt, I decided to emulate nature and fashion the wing spar in terms of upper arm, forearm, hand and fingers. The shoulder joint is pin jointed and able to transfer bending loads accross the structure.

FOLDED WING AS A BACK PACKThe photo on the left shows the complete wing folded on to an aluminium pack frame. For flight the pilot is supported by the frame by a series of straps, not shown here. This was the focus of the project, to have the backpack wing unfold into a high performance sailplane enabling a person to fly like a bird. To become operational, the pilot needs a streamlined envelope and helmet.SHOULDER JOINT & PTEROID

The shoulder joint is pin jounted, the upper arm connecting a centre tube. This tube can be pilot rotated for speed adjustment. The pteroid tendon geometry is the basis for pitch stability.

ELBOW PIN JOINTThe elbow joint is offset to permit the spar tubes to fold against each other. The hinge pin is angled so that the ex-tended spars form an

arch matching the wing trailing edge shape. This hinge has to support both bending and torsion and the present arrangement is more mock-up than engineering.

WRIST JOINT, PTEROID STRUT & PTEROID TENDON

feather sheath

forearmhand

PACKFRAME, WING STRUCTURE AND SAIL

WING STRUCTURE PARTLY FOLDED

WING FROM PORT AFT QUARTER, JANUARY 2009

SHOULDER FROM UNDERSIDE SHOWING REVERSE GRADIENT SPRING GEOMETRY

angle of attack

dragliftresultant

centre spar

shoulder pivot

balance momentMb = Mb * tan[angle]

The shoulder pin transfers most of the flying loads to the centre spar. The remainder is resisted by the stability balance spring. This proportion varies with the angle of attack of the wing.

SHOULDER JOINT GEOMETRY

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STABILITYIn order to achieve maximum performance, the wearable wings’s efficiency and surface area needs to be minimised. Using Nature’s Constant ‘g’ Stability System allows stable flight without a tail. Constant ‘g’ Stability is a mechanical feedback system that maintains a state of unstable equilibrium by balancing wing lift against spring tension.To ensure longitudinal stability, basic principles must be followed. The system balances spring tension against a lift dependent factor. Only in this way can the system detect changes in ‘g’ forces. In other words, pitch stability is based on measuring vertical acceleration changes. THE NECESSITY OF A NEGATIVE SPRING RATEA factor that is perhaps the hardest to grasp is the need for the balance spring to have a negative spring rate. The system is in equilibrium when spring tension balances a defined lift moment. If the wing loses altitude then extra lift is required to regain lost altitude; however, a lift decrease results in reduced spring tension. This contrdiction is resolved through a negative spring rate.HOW GEOMETRY MUST CHANGE WITH LIFTLift changes must be accompanied by changes in wing twist and sweep that produce a restoring moment, a negative feedback mechanism. These effects must be compatable with wing folding and must resolve inherent conflicts.SYSTEM BALANCEFor the system to work, spring tension extends the wing while lift forces fold the wing. At equilibrium, balance occurs when lift equals weight. The pilot’s manoeuvre control adjusts this balance setting.CONFIGURATION IN NATUREIn nature, two mechanisms operate together: 1. The flight muscle acts between the deltoid crest on the humerus and the breastbone. The elasticity of the flight muscle is the balance spring. Lifting loads cause the humerus to lift up and rotate, the to a lower incidence. A lift decrease causes dihedral to decrease and incidence to increase.2. When flying, lifting forces have to keep the wing extended so that the creature does not have to waste energy. This is achieved by angling hinge axes so that lift forces extend the wing.

SUMMARY OF STABILITY PRINCIPLES1. Balance spring acts between humerus deltoid crest and body.2. Spring gradient is reversed by humerus geometry. (effective only through a narrow angular range)3. Degree of stability depends on the restoring moment per g.

STABILITY AND THE WEARABLE FOLDING WINGUnlike nature, I designed the upper arm soar to have a pin joint instead of a ball joint. This eliminates the option of spar twist with lift change and so creating a restoring moment requires a different mechanism. A lift increase causes the upper arm to increase its sweep angle. This sweeps the wing back and also reduces spanwise tension producing a correcting pitch down moment.INERTIA AND THE SPEED OF CORRECTIONThe stability system maintains a state of unstable equilibrium and low pitching inertia and quick correction are important aspects. The first component to move is the humerus since it is directly connected to the spring. For example, with a lift reduction, the humerus will swing forward forcing the forarm out, tensioning the wing, increasing tension

and restoring lost lift. Compensating wing twist is helpful to maintain lift if angle of attack is reduced without involving high inertia pilot rotation. The wing must actually develop more lift than weight since the initial lift loss means some altitude is lost as well. This is why to reverse gradient spring is essential to achieve dynamic stability.

HOW THE SYSTEM SEEKS EQUILIBRIUMConstant ‘g’ stability is an active feedback system which maintains a state of unstable equilibrium. It is a mechanical equivalent to modern ‘fly by wire’ used in military aircraft. The stability system involves two unstable elements which combine to form a stable one. The first one is an unstable cambered wing and the second is a reverse gradient balance spring.An unstable wing cannot be made stable by itself unless there is some means to create an aerodynamic correcting moment. This can take the form of inducing twist, sweep and or camber change. The reverse gradient spring has no stable balance point. If the wing alters its pitching moment with lift changes to produce a restoring moment, the reverse gradient spring allows

ACHIEVING THEORY IN PRACTISEIn creating wearable folding wings it is important to keep theory constantly in mind since it is all too easy to design non-viable mechanisms. The trickiest aspect has been the reverse gradient spring mechanism.

balance spring

Before flying, the spring moment is at a maximum and spring tension is lowest. There is only one point where balance is achieved.

pitch down

pitch up

L = W

sprin

g

pivot

FINDING EQUILIBRIUMwith a wing reverse gradient spring system

trim

There is only one point where net moments are zero and this is the lift about which the system is stable.

0

+

M

-

stable stabilityslope

effect of wing camber

lift effect

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CONTROLConstant ‘g’ stability is a mechanical feedback system. Pilot control is via adjusting the parameters of the system rather than direct control movement. For example, the system is stable about a single airspeed for a given g setting. To fly at a different speed, the pilot changes the wing camber. To manoeuvre and fly at a different g setting, the pilot adjusts the effective balance spring tension. Directional and lateral control involve differential adjustment.A person wearing backpack wings has only two hands for control. For optimum streamlining, the pilots arms and hands must be contained within the wing’s contour except for protruding head and feet.I concluded that the best position would be with hands by hips, the control levers extending from the backpack towards the pilot’s front. Control options include levers moving fore and aft, side to side, twist grips and brake type levers. In longitudinal flight, the pilot needs to control manoeuvre and speed. With my wearable wings, the shoulder hinge pins resist most of the wing bending loads. Spring tension resists the residual loads. For manoeuvre, rotating the centre spar alters the ratio of air loads between hinge pin and balance spring and so alter’s the wing’s equilibrium point. TRIM SPEEDFlight speed is determined by adjusting another equilibrium point. Feedback always seeks a zero moment balance point where lift moments balance camber induced pitching moments. In level flight, lift moments are constant and camber moments vary with speed, there is only one speed at which the wing is in equilibrium. Thus, the pilot adjusts flying speed with a camber control.Camber can be adjusted in two ways. The first is by varying the angle of the frame mounted centre kaugis. For slower flight, camber is increased. The second is by varying membrane tension. This can be achieved through upper arm angle and wing kaugi adjustment to change both spanwise and chordwise tension. This tension needs to increase with airspeed. GIVING THE GLIDER INSTRUCTIONSWith constant ‘g’ stability, the pilot does not control the wing directly, but gives it instructions. Wing stability is such that it seeks equilibrium airspeed and lift. This means that the wing flies itself until new instructions are received. Thus, a landing approach in gusty air requires no pilot input to maintain a constant descent angle and airspeed. A consequence of this is the option to have control locks such that control input is only possible when the lock is released such as with a bicycle brake lever. This can result in more relaxed pilot control since no input is required unless the pilot wants to a change, the wing being largely immune to air turbulence.CIRCLING FLIGHTCircling requires flight at more than one g to compensate for centrifugal force. The pilot must adjust both manoeuvre and speed to circle at particular circle radius such as in a thermal. The most convenient controls for the pilot could be twist grips for differential wing twist plus adjustment of manoeuvre and speed with arm levers.On a glider the elevator controls both manoeuvre and speed.

DIRECTIONAL STABILITYNeither birds, bats or pterosaurs have vertical tails. Directional stability relies on wing planform. The picture on the left shows

three bird planforms which achieve directional stability in different ways.The frigate bird at the top has its inner wing swept forward with dihedral and its outer wing swept back with anhedral. The pelican in the middle has a similar planform except for splayed wing tip feathers. The eagle below has a slightly swept forward wing with dihedral. Below, the condor and pelican illustrate the consistancy within nature.

LATERAL STABILITYSimilar factors affect lateral or sideslip stability. The membrane wing requires a vertical spar bow to match the trailing edge deflection with lift. This means dihedral on the inner wing and anhedral on the outer wing.

WEARABLE WING PLANFORM with tip feathers retracted. It relies on sweep and dihedral for lateral stability.

The royal albatross shows the typical sea bird planform with its swept back wing tips

Gull

The gull and fruit bats have similar wing shaped that give directional and lateral stability.

The precise degree of directional and lateral stability can be determined mathematically. As observed in nature, there is wide tolerance in the choice of wing shape. The essence is an inner wing with sweep forward and dihedral with an outer wing having sweep back and anhedral.

Large pterosaurs had long heads at the end of an extended neck. This arrangement should be impossibly unstable in yaw. It worked because it was part of a dynamic system whereby head inertia played an essential part.

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AEROFOILThe wearable folding wing uses a double surfaced tensioned membrane to create its aerodynamic shape. This is combined with five artificial wing tip feathers that can retract into the membrane for high speed flight and also for wing folding.A membrane derives its aerofoil shape from air pressure. Tension is maintained by internal struts called “kaugis”. These to not touch the membrane and insure a very fair profile provided tension is equalised to prevent creases.

ARTIFICIAL WING TIP FEATHERSEmulating nature’s wing tip feathers confers a number of aerodynamic advantages. 1. substantial reduction in induced drag 2. resistance to wing tip stallingBoth help to make wearable folding wings a practical propo-sition.

The above photo shows dramatically the prominent wing tip feathers which serve to diffuse the wing tip vortex in three dimensions. This reduces losses due to induced drag by a factor of two at high lift coefficients, greatly enhancing soaring ability in narrow thermals.I have made my artificial wing tip feathers from polypropolyne foam, cut with a hot wire. this material is light, tough and easily formed. The feather shafts are bamboo which are supported by an aluminium frame which slides on a curtain track for retraction.

ARTIFICIAL FEATHER CHALLENGESNot surprisingly, creating and deploying artificial feathers presents a number of challenging problems. This is especially true when the feathers must also be retractable.

FEATHER FLEXIBILITYVery apparent in nature is the way the feathers flex with air loads. I feel this is to help diffuse the tip vortexes in three dimensions while at the same time facilitate wing folding. The folded feathers have to slide over each other.The polypropolyne foam is very flexible while the bamboo feather shafts can be shaved to optimum thickness for correct deflection under air loads. Untimately carbon fibre shafts could replace bamboo.

RETRACTIONFor both wing folding and high speed flight, the feathers can be retracted inwards within the double surfaced membrane.

When the feather fan is closed, it can slide on a track within a nylon sheath which protects the feathers.With feathers retracted, both wing span and area are decreased, reducing drag and increasing high speed performance.

EFFECT OF CENTRE KAUGI DEFLECTIONIncreasing centre kaugis deflection alters wing camber of the inner wing, increasing wing lift and inducing a pitch up moment. This will be an important element for speed control. My spreadsheet works out the effect for different speeds.ADVANTAGE OF A MATHEMATICAL MODELA folding wing is necessarily complex and a computer model is a great help. My spreadsheet has 168 named variables. This gives an idea of the difficulty of predicting flight behaviour. Thanks to inherent feedback, stability and control is not as daunting as it might seem.

PILOT SUPPORTThe flyer need to be firmly attached to the wing. I have chosen a combination of two methods, a series of straps to the backpack frame and a streamlined envelope integrated into the sail. During take-off and landing, the pilot is supported solely by the straps with his legs free for running. Once airbourne the pilot slides his legs inside the envelope and zips himself in.While not strictly necessary, a soft, foldable streamlined helmet integrating with the envelope will enhance performance and increase safety. Since the wings are folding and wearable, it is important that the pilot can easily wear or carry the folded wings while walking and travelling. To this end, the backframe has conventional shoulder straps.

CROSS-SECTION AT THE WINGTIP showing how the feathers angle to suit the local air flow. They form a cascade which makes them very resistant to stalling.

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BACKGROUNDThe aerodynamics of gliding have facinated me for a long time. I first worked for the glider builders Glasflűgel because of wanting to learn more about fiberglass construction but naturally became fascinated by their sleek products.

In the 1070’s, I worked with Dr Alfred Sneyd, a mathematics professor at Waikato University, Hamilton New Zealand. We wanted to discover how the pterosaurs balanced without tails, and we did!

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After moving from Ham-ilton to Waiheke Island, I started experiment-ing with new ways of achieving constant g stability.

I did not feel able to emulate nature and so searched for alternate mechanisms.

A friend was able to interest the Australian TV science program Be-yond 2000 to film some experiments with some model wings.

DEREIDACTYL PROJECT

FILMING STABILITY EXPERIMENTS BY THE AUSTRALIAN TV PROGRAM “BEYOND 2000”

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Can a Human Fly Like a Bird? Page 10

WILL HUMANS BE ABLE TO FLY LIKE BIRDS?This is the culmination of my life’s work. The search has not been continuous yet like nature it is evolutionary. Building wearable folding wings is work in progress and input from others if success is to be realised.

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