Hoe Ornithopters Vliegen

Flagge flag vlag pavillon


Beschijving van de slagvleugelconstructies
die gelijktijdig met de EV-modellen ontwikkeld werden

1. Eisen aan een slagvleugel


2. Aëro-elastisch gestuurde gewrichts-slagvleugel


3. Slagvleugel met instelbaar torsiemoment


4. Slagvleugel met toenemende vleugelverdraaiing nabij de vleugeltip


5. Aëro-elastisch gestuurde gewrichts-slagvleugel met instelbaar torsiemoment en verdraaiingstoename nabij de vleugeltips


Maximale geplande verdraaiing van de vleugel (de afbeelding aanklikken)

De gewricht-slagvleugel heeft in de vliegpraktijk een een groot voordeel. De afwikkeling van de handvleugel t.o.v. de armvleugel is afhankelijk van de lift van de handvleugel. Gelijktijdig bepaalt dit het instelverloop langs de spanwijdte. Als ook op de opnames van de vlucht de grootte van het verloop te herkennen is, dan is de grootte van de lift van de handvleugel t.o.v de glijvlucht d.m.v afschatten te bepalen. Bovendien kan eenvoudig op het instelverloop ten tijde van de opname gesloten worden (zie de afbeeldingen van de EV6 en de EV7). Met deze beide gegevens zijn de bedoelde instellingen van het slagvleugeldraaimoment, het aandrijfvermogen en van de slagtijdverhoudingen mogelijk. In het bijzonder zijn m.n. de afbeeldingen van de vlucht zo ongeveer in het midden van de vleugelslag zeer leerzaam.


In het volgende artikel vindt u nog wat meer details over de: Gewrichtsslagvleugel (in het Duits, PDF 1.3 MB).

Informations and suggestions for a further development you will also find in the article Lift during wing upstroke, versie 10.1, (Engelse versie, PDF 1,0 MB).


6. Vleugelbekleding van de slagvleugel

Hier volgt de beschrijving van de werkwijze voor

Het bespannen van een slagvleugel met een elastische folie

(in het Duits, PDF 360 KB)


7. Polsgewricht voor een sterke passieve buiging van de handvleugel
(Ik ken helaas geen Nederlands, dus in het Engels)

The downward bending of the hand wing during the upstroke in distance flight especially supports the

Aerodynamically, the bending of the hand wing is particularly advantageous if the lift is high in the arm wing area during upstroke, e.g. due to a turning of the wing root.


7.1 Mode of operation of the bending

The single effects of a strong bending of the hand wing during upstroke in distancevlucht can be described as follows.

  1. During upstroke the oncoming flow to the wing comes more from above. If it is able to give way downwards by the passive bending, its lift will no longer be as negative as with the stretched wing (please see the diagram lift distributions in distancevlucht). This makes the overall lift larger and the profile selection easier.
  2. At a wing there is always a flow along the wing due to different pressures along the wing. It causes the vortices behind the wing and thus the induced drag (please see diagram induced drag on the site Het Handboek ). This longitudinal flow is particularly strong when only the arm wing generates large positive lift. With its winglet effect, the bending decrease the longitudinal flow and thus reduces the induced drag. The longitudinal flow also becomes smaller because the lift in the hand wing area is no longer so strongly negative (please see point 1).
  3. From the large lift in the middle of the span, no more so much flows towards the wing tip. This helps with the concentration of lift in the middle of the span and thus with the generation of thrust.
  4. If the lift at the wrist is increased during the descent, e. g. as in the following image of the wrist, this also supports the concentration and enlargement of the lift in the centre of the span.
  5. At the bended hand wing, a small oblique blow from above occurs. Thereby the lift is shifted a little bit along the hand wing in blowing direction, thus towards the wing tip. Birds strengthen this effect by pivoting the hand wing backwards (wing sweepback).

    If there is positive lift at the wrist, negative lift is at least reduced by the oblique blowing, especially near the wing tip. This makes the profile selection easier. You can further enhance the effect by increasing the positive lift on the wrist during bending, e. g. as in the following image of the wrist.

    In birds, the sweepback of the hand wing is coupled with a sweepback of the forearm. The associated oblique blowing of the forearm supports the concentration of lift at mid-span.

    There is still a lot of experimentation to be done with the application of the oblique blowing. But the wing profiles in birds indicate, that there are rarely any negative angles of attack due to bending and pivoting of the hand wing (please see drawing of wing shapes and cross sections by Karl Herzhg).

  6. The possibility to use the wind turbine energy of the arm wing for thrust generation in the outer wing area is strongly limited. The hand wing reduces its stroke path by bending and changes its direction of action. Furthermore, the negative lift is very small. Whether the acceleration of the wing mass upwards, and there an end position spring, can absorb the whole wind turbine energy, must be checked. Otherwise, a compensation spring is appropriate.
  7. With the bending, the sequence of wing twists in the individual wing sections can be coupled mechanically with the stroke motion near the stroke end positions. This supports the displacement of lift along the wing and thus the generation of thrust (please see chapter 7.3). In addition, it can be used to increase the lift during wing upstroke near the final stroke positions of the wing, thus without much working drag.
  8. The division of the flapping wing into two sections combined with the bending reduces the problems with mass inertia during the flapping motion in the area of the flapping end positions (please see Lift during wing upstroke, version 10.1, PDF 1.0 MB, chapter 7.3).
  9. The transition from the bent to the outstretched wing position begins, before the arm wing reaches its upper final wing position. When the arm wing arrives at the top, it should wait there, until the hand wing has reached the outstretched wing position. Therefore, the drive should pause there for so long (please see above paper Lift during wing upstroke, chapter 6.3, Fig. 20.).
    The upstroke of the arm wing consists of a relatively fast upstroke motion, followed by a short waiting time in the upper stroke end position.
  10. During take-off and landing with stretched wings a possible stroke movement of the wingtips must be considered.
  11. Even without a functional background, the bending during the flight of an ornithopter looks good. It increases the naturalness and liveliness of the stroke motion. It appears more harmonious.
  12. The calculations made here for lift, thrust, wing twisting etc. are no longer valid. One is again dependent on conjectures and on the method of trial and error, probably without measurements and comparison of intermediate results.
  13. With the bending of the hand wing described here, you can certainly achieve higher thrust values than without.

I only became aware now (2021) that the passive bending of the hand wing offers so many aerodynamic advantages. The engineering effort and the experimental requirements for the bending of the hand wing of an ornithopter are large. It has to be examined whether the various tasks can be solved in another way or can be done without it (e. g . by restricting to kruisvlucht).


7.2 Pivoting of the wrist axis

wrist axis pivoted inwards on the rear
Pivot angle κ (Kappa) of the wrist axis for an automatic increase of the angle of incidence of the hand wing by the bending

If one pivoted the wrist axis rear inward one achieves an automatic increase of the angle of incidence when bending the hand wing downwards (please see adjacent drawing). In this way, the angle of incidence changes self-acting in the desired direction at the beginning and end of the upstroke. This influence on the angle of incidence can approximately replace or at least complement the twisting of the hand wing during upstroke.

During the bending of the hand wing, its angle of incidence or angle of attack increases. Due to increasing lift, the bending comes to a standstill already after a short upstroke motion of the arm wing. Depending on the upstroke speed, this is the case with a different bending. In this way, the maximum size of the bending automatically adapts to the beat frequency. The higher the beat frequency, the greater is the bending.

In birds, a corresponding, but only small change in the angle of attack of the hand wing is steered by the arm wing. When the stretched position of the arm wing is reached, the angle of attack of the whole hand wing becomes somewhat smaller and relatively inflexible (glide position). When the arm wing leaves its stretched position, the angle of attack of the whole hand wing becomes a little larger and more flexible again. K. Herzog 1968 has not written anything about a change in the angle of attack of the hand wing during the bending movement.

The paper bird wing by Karl Herzog shows a feather fan extending from the wrist to the back. When angled down, this fan obviously overlaps the different setting angles of the adjacent wing areas.

Wrist for strong bent of the hand wing
Concept voor één slagvleugel polsgewricht met een sterke hoek van de handvleugel,
voor een distancevlucht

If one inclines the wrist axis front downward (please see adjacent drawing), the tip of the hand wing is slightly pivoted backwards when the hand wing will be angled downward, although not so far as in birds (tip-reversal upstroke). However, the benefit of such a small pivoting motion backwards is not clearly visible. By the drag resp. thrust near wingtip, probably only the force for bending downwards is increased resp. reduced.

An example of an inclination and a pivoting angle of the wrist axis is shown in the concept drawing. The optimal size of these angles and their possible combinations must be determined by experiments. Unfortunately, there are no empirical values for this yet.

zwenkhoek van de handvleugel buigingsas
Waarschijnlijke zwenkhoek van de handvleugel buigingsas bij de licht gestrekte vogelvleugel.

When determining the pivot angle of the bending axis of the hand wing, one can orientate on the pivot angle of bi birds with slightly stretched wings. When the wing is slightly stretched, as shown in the adjacent picture, I assume that the bones of the hand are approximately perpendicular to the bending axis. Because there are barely existing muscles for twisting or turning of the hand wing (please see Anatomie van een vogel-vleugel en Literatuur Karl Herzog, p. 46). However, the alignment of the axis changes when stretching and pivoting the hand wing and possibly also when bending strongly.

Due to the pivot angle, there occurs a small turning of the hand wing around its longitudinal axis when it is bent down. This makes the angle of attack large near the wrist. However, it still remains negative at the wingtip. This is exactly what is desired for the self-actuated bending. In addition, seen in the longitudinal direction of the wing, the large lift at the wrist supports the large lift at the arm wing. On the hand wing, therefore, its rotation is aerodynamically more advantageous than its twisting.

On upstroke with a barely recognizable bending, the elastic range at the beginning of the bending is already sufficient for changing the setting angle. There, the twisting of the hand wing is determined in particular by the widely together grown bones of the hand and by tendons.

With the articulated flapping wings used so far, it has proven to be advantageous also to control the twisting of the arm wing by the passive bending of the hand wing. In the above wrist-concept drawing, therefore, the fuselage near part of the wrist was approximately taken over from the above Aëro-elastisch gestuurde gewrichts-slagvleugel (according to Literatuur Karl Herzog, the wrist of birds is also divided into two partial joints, a proximal ulnacarpal joint and a distal mediocarpal joint). The necessary twist of the arm wing during up- and downstroke is thereby already completely carried out during a small bending of the hand wingj, in the close range of the stretched wing position (zie de click-afbeelding boven an de Vlucht images van de EV7a). If the hand wing bends further, the twisting of the arm wing remains unchanged. This behaviour of the arm wing has advantages in shifting of lift.


7.3 Volgorde van de bewegingen van arm- en handvleugel met verplaatsing van de lift (in het Engels)

When the wing approaches the lower stroke end position at the end of the downstroke, the lift force in the outer wing section decreases. Also, the inertia of the hand wing now supports the downward movement of the spring rod. The spring rod increases the angle of attack of the arm wing, at least close to the wrist. In this way, the arm wing takes over the lift of the hand wing. In the lower final stroke position, thus there is a strong lift for a short time. But also the angle of attack of the hand wing has increased during this time.

Also with birds, not only the angle of attack of the arm wing, but with it also that of the hand wing is increased with turning of the wing root in the lower final stroke position. But only with a turning of the wing root the arm wing alone can absorb the whole lift for the upstroke. And only with a large lift of the arm wing are fully exploited the advantages of a wing bending. The large lift near the lower final stroke position supports the reversal of the flap motion of the whole wing.

The angle of incidence of the hand wing, which has increased in the lower end position, becomes even larger in the course of its bending during upstroke. Due to the changing direction of its oncoming flow, however, this does not apply to its angles of attack. Along the hand wing will be occur a balance between positive lift close to the wrist and negative lift in the area near the wing tip, until the maximum angle of bending is reached (for example, like the course of the lift at the stretched wing tijdens een opgaande slag). Close to the wrist, however, its angle of attack is still increased. Together with the winglet effect of the hand wing, certainly a larger lift can be held together in the arm wing and concentrated in the centre of the span, than without a strong bending.

When the arm wing reaches the upper end position, its angle of incidence at the wrist initially remains large due to the spring rod. Thus, without upstroke motion increases its angle of attack. After the upstroke motion, perhaps with rotatie van de slagvleugel and thus increased lift in the span centre, the lift is shifted a little to the outside. In the following waiting time, until the hand wing is stretched out, the arm wing has a very large lift near the wrist (compare with the animation of the zwaan). This is supported by the winglet or end plate effect of the still angled hand wing.

The advantage of the large lift in the opper stroke end position, is the missing wind turbine function with its negative thrust. Surely birds have such a possibility.

Subsequently also the hand wing reached, without significant power generation, the upper final stroke position respectively the extended wing position. There, because of the increased angle of attack at the wrist, grow its lift and thus its force upwards. As the spring rod now gives way, the angle of incidence on the wrist becomes smaller at the same time. Thereby the hand wing takes over parts of the lift from the arm wing. This continues because the angle of attack of the hand wing increases further at the beginning of the downstroke.

The motion sequence of this wrist thus supports the process of lift shifting between the arm and the hand wings. In addition, the motion sequence of the two wing sections reduces the mechanical stress on the spar and the drive mechanism in the final stroke positions. The moment of inertia of the single wing sections is clearly smaller than that of the whole wing. But a good possibility to control the curve of the model or to influence the twisting or rotating of the hand wing is still missing (Servo in the area of the hand wing).