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. Generally the bionics of the bird wing is very interesting (please take a look at external link 1).
2. Membrane flapping wings
Application range
Membrane flapping wings especially are changing the chamber direction 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 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 membrane flapping wing systems 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's merely a propeller for changing rotation directions.
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 membran, developed by A. Pénaud (France 1872)
(more informations at external link 2).
2.4 Active twisting by spar rotation
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 tipwards is made possible.
The bird models by K. Herzog (1963) follow this scheme, too.
2.5 Aeroelastically 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 toda
.weight
.wing span
.cane spar
.covering
.ceiling
.3.1 kg
.2 m
.
.linen
.10-15m
.[109 oz]
.[79 in]
.
.[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 crosspoint they are movably interlinked. For the torsion elastic spar (H2) not to bent 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 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 during downstroke. As a reaction it bends downwards at the other side of the crosspoint - 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
external link 3.
2.6 Flying wing ornithopter
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 drive. With a wing span of 15 cm (5.9 in) it has a weight of only 0.6 gramms (0.021 oz [US]). The airplane performances are amazingly good (for the construction of the flapping wing model please also take a look at external link 4.)
The particular about this flapping wings 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 d form 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 Reynold's numbers this may be beneficial.
Model by Horst H?ndler (1988).
2.8 Thrust-wing
By mechanisation of a dragonfly's flight principle
Erich von Holst has developed his thrust-wing model with two in the opposite direction rotating three-blade wings (1940). The flapping angle in one stroke direction constitutes 180° or 360° for a complete flapping cycle (please take a look to the video at external link 5).
Three instead of two wing blades per rotor offer a constant supporting force (see also configuration of the rubber powered model ENTOID by Velko T. Velkov (2007) external link 6).
In contrast to a propeller a lift force perpendicular to the thrust is generated at the thrust-wing, too. One must only increase the thrust-wing advance ratio (v/u) - similar to a flapping wing - and fly with a positive angle of attack of the thrust-wing axis.
This is a fine example for an innovative transfer 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 (please take a look to 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 aeroelastically 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 counterrotating mass.
There are diverse proposals to generate an oscillation motion of the wing by a pilot who is flying in a hang-glider or an other 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) external link 7.
2.10 Rotating wings
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 wingroot.
Examples:
The Rotor Dragonfly (1944 and 1989) by
Adolf Piskorsch
and
the flight model by Horst H?ndler (1989).
Both ends of the driveshaft are bended in Horst H?ndler's model. Thereon, the wings are attached freely twistable. The angles of incidence is guided by the upward pointing levers on the wings.
2.11 With non-twistable arm wing section
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 (Please also take a look at the construction of the pinion feather by Alexander Lippisch).
This daedalean flapping wing design of the Seagull was developed by Percival H. Spencer (USA 1958)
(please take a look at external link 8).
Today, this design principle of flapping wings with inserted battens is widely-used.
3. Profiled flapping wings
Application 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 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 faned 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 also take a look at external link 9).
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 Ikarus by Emiel Hartman (England 1959).
More recent experiments with artificial feathers are to be seen
- at gliders with out-faned wing tips
by Johannes Huser,
- at the
Birdman Georges Fraisé (France 2005) and
- at the Ornithopter Project by Ryszard Szczepa?ski (Poland 2002).
(please look at external links 10, 11 and 12)
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.
The arm wing should perform a flapping motion and a twisting motion at the shoulder joint. With rubber threads between arm- and handwing the latter was pulled down a little (aeroelastically 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 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 rotations 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 R?uber (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: 300 KB)
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 also take a look to external link 13 and 14).
3.6 Servo controlled wing twisting
This is a lifelike and airworthy replica of a pterosaurs - a Quetzalcoatlus Northropi (QN). The aerodynamics of this ornithopter should fully equate the original. The idea come from the creative genius Dr. Paul MacCready (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 take a look to the articles (in German) about the project by Paul MacCready and for further informations via external link 15.
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. Its wing span is 2.0 meter.
The wing covering is made of a 2 mm extruded Polyurethan 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 take a look at the Shearflex principle in the next section).
At 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 und down stroke. You also can seen an animation (2.1 MB) of it.
For further details about the SmartBird project please take a look at external link 16.
3.7 Shearflex principle
Here an aeroelastically 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 determinated 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 take a look at external link 17).
Here, a corresponding replica with an electrical drive system by Horst H?ndler (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 (Canada 2010).
For more informations about the Human-Powered Ornithopter (HPO) Project "Snowbird" please take a look at external link 18.
3.9 Shell wing
with active wing twisting by a drive controlled spar rotation, developed by Albert Kempf (France 1998, please take a look at external link 19).
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 creaselessly. Also the aforesaid shearflexed wing is using this property. This flapping wing category here is called shell wing .
The 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.
In the essay Flapping Wing Designs (in German, PDF 1.8 MB, version 2.3, 2008) additional information about these flapping wing designs can be found.
In conjunction with the EV-models developed flapping wings are to find on site: Articulated flapping wings
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