How Ornithopters Fly

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 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 Alexander Lippisch (ca. 1937) 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).
more information at related link 1

2.4 Active twisting by spar turning

This is a membrane flapping wing by Erich v. Holst / Karl Herzog (1963) with drive-controlled wing twisting in the arm wing section by spar turning. 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 stroke 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.

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 (h3) determines the magnitude of the wing twisting.

Both spars cross approximately in the center of the semi-span. At the cross point they are movably interlinked. For the torsion elastic spar (h3) not to pivote backward too much, a string or an elastic thread is apparently tightened between the tips of the spars.

During downstroke of the wings the lifting forces are increased. The spar h3 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 h3 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 (please look at Piskorsch Adolf (1975), Pressluft-Schwingenflugmodell Chalupsky) and the vidio of the related link 2

2.6 Flying wing ornithopter

2.7 In tandem

Ornithopter with two sets of flapping wings based on a Dragonfly, developed by E. v. Holst (194o) / K.Herzog (1963).

Here, for simplifying the mechanism both opposite halves of a wing are rigidly fixed to a unit. Thus the center of pressure 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. This is a model by Horst Händler (1988).

2.8 Thrust-Wing

By mechanisation of a Dragonfly's flight principle. Erich von Holst (1948) has developed his Thrust-Wing with two in the opposite direction rotating three-blade wings (please look at E. v. Holst with a video of the Thrust-Wing).

The flapping angle in one stroke direction constitutes 180° or 360° for a complete stroke 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 rate of advance of the Thrust-Wing - similar to a normal flapping wing - and fly with a small positive angle of attack of the Thrust-Wing axis (The rate of advance is the ratio of airspeed to circumferential velocity at the wing tip.).

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.

The Thrust-Wing model ENTOID (2007) by Velko T. Velkov has two parallel rubber powered rotors (please look at the related link 4).

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 lift force during the upward motion must be smaller than during downward motion. The bigger the difference, the better for the thrust (please look at the principle of flight/vector diagram). 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) related link 5.

In principle, the driveless Zeppelin Phoenix generates propulsion in the same way with its wings, by its own up and down motions. British engineers have developed it since 2016 (please look at related link 6). Also, hot-air balloons can become a little manoeuvrable with such wings during ascent and descent.

2.10 Rotating wings

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

The backward motion of the wing tip in the range of the lower end position is certainly disadvan­tageous for the lift.

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 by P. H. Spencer with a non-twistable arm wing section and passive twisting at hand wing section. From an aerodynamic point of view, however, there is still a kind of twisting in the area of the arm wing, due to the hand wing membrane connected at the rear. During downstroke it reduces and during upstroke it increases the lift on the arm wing. Eine Abwinklung des Handflügels nach unten hilft dabei. This indeed supports the lift shift, but requires negative lift in the whole hand wing area during wing upstroke.

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 (please look at the construction of the pinion feather by Alexander Lippisch).

This flapping wing design of the Seagull was developed by Percival H. Spencer (USA 1958) (please look at related link 7).

Today, this flapping wing design principle is widely used, also with inserted sail battens.

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. (please look at related link 8)

Even for splay and straddle movement of the feathers there are old design proposals. In contrast, with EV7b only with simple feather implementations experiments have been made.

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

A more recent experiment with artificial feathers is to be seen at the Birdman Georges Fraisé (France 2005, please look at related link 9).

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 turning or the twist axis is not standing vertical to the stroke axis.

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 the above 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 turning of relative non-twistable wing sections.

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

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.

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 (please look also at related link 10).

In my experience, the twisting method is only suitable for small spans (about 1 meter). Otherwise the adherence to a certain twisting is undetermined by the elasticity, tolerances and friction of the components.

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.

For details - including the principle of the drive mechanism - please look to the articles (in German) about the project by Paul MacCready and for further information via related link 11

Also at SmartBird by the company Festo the twisting of each wing side is controlled by a servo (2011). However, in this case only the twisting of the hand section of the wing is controllable. The inner arm section is not twistable. There, the lift during upstroke is therefore very small - despite bending of the hand wing.

The wingspan of the model is 2.0 meters, weight 0.450 kg, wing loading 9 N/sqm.

The wing covering is made of a 2 mm extruded Polyurethane foam plate which is formed to an airfoil (the cross section of the wing used at the model differs from the here shown wing ribs). Upper and lower side of this covering are not glued together at the rear end. So they can slide against each other and the wing is able to do twisting motion without folds (please look at the Shearflex principle in the next section).

On SmartBird it is noticeable that the wing mechanism imitates the bending between arm and hand section of bird wings. The accompanying images show a basic structure of the flapping wing mechanism during up and down stroke. You also can see an animation (2.1MB) of it.

For further details about the SmartBird project please look at related link 12

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. A video is available (please look at related link 13).

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:

• 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).
For more information about the Human-Powered Ornithopter (HPO) Project Snowbird please look at related link 14.

3.9 Shell wing

A wing with an active wing twisting by a drive controlled spar turning, developed by Albert Kempf (France 1998), please take a look at related link 15.

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.