Titelbild

Hoe Ornithopters Vliegen

Flagge flag vlag pavillon

Gewrichts-slagvleugel

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

1. Eisen aan een slagvleugel

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2. Aëro-elastisch gestuurde gewrichts-slagvleugel

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3. Slagvleugel met instelbaar torsiemoment

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4. Slagvleugel met toenemende vleugelverdraaiing nabij de vleugeltip

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

Schlagflügel-Verwindung

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.

Verwijzing

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).

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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)

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7. Polsgewricht voor een sterke passieve buiging van de handvleugel (in het Engels)

The displacement of lift along the wing which is required for flapping flight takes place mainly in the initial and final phase of wing upstroke (please see distances between the centres of lift). It is advantageous to support it by a sequence of suitable changes in the angle of attack of the various wing sections. A good way to do this is to bend the hand wing downwards. The constructional effort for a strong bending of the hand wing however, is only worthwhile, if a high climbing performance is aimed for the model. In this case, the difference in lift between the arm and hand wings is particularly large during upstroke. As much lift as possible must be concentrated in the middle of the span. The bending of the hand wings with their end plate effect helps here. But even without this functional background, the bending in flight of an ornithopter looks good.

To put it somewhat simply, the bending of the hand wing begins and ends in birds with the beginning and end of the upstroke (please see Lift during wing upstroke, version 10.1, Engelse versie, PDF 1,0 MB).

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. Birds may use a similar operating mode of their wrists.

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. 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.

Wrist for strong bend of the hand wing
Concept voor één vleugel polsgewricht

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 bend, although not so far as in birds (tip-reversal upstroke).

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. My earlier idea, that also the inclination of the wrist axis front downward causes an automatic increase of the angle of incidence of the hand wing was wrong.

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 displacement of lift.

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

When reaches the wing the lower final stroke position, decrease the lift force in the outer wing section. 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 rotation of the wing root in the lower final stroke position. But only with a rotation 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 the lift displacement 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.