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How Ornithopters Fly

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Erich von Holst and Karl Herzog

1. Prof. Dr. Erich von Holst

Erich von Holst mit seinem Modell
Erich von Holst
with his model

Erich v. Holst (1908-1962) researched on biological and aerodynamic problems of animal flight. In interplay between creative flying models, many experiments and consequentially thoughts he reached new perceptions about the flapping flight. Thereby a main focus of his work was on the determination of force distributions along the wing on up and downstroke.

These conceptions of the aerodynamics and flight mechanics of flapping flight were the basis for his new wing designs with the corresponding drive mecha­nisms. Each of his flapping wing models was equipped with an especially for this designed rubber motor.

Erich von Holst explained the propulsion or thrust generation of birds in cruising flight by the shifting of lift along the wing. Correspondingly he wrote in his paper about artificial birds as a means of studying the flight of birds (1943):

functioning of lift and thrust forces, how do birds fly
Basic principle of lift and thrust generation in the flight of birds
This is the original image with unequal cycle times. An image with equal cycle times, see The principle of flight / How birds fly

It would be the ideal situation for the bird when during the downstroke of the wing the arm section and during the upstroke the hand section was missing. You can make it plausible if you imagine, the wing area would be reduced and could slip along the longitudinal axis of the wing. It would then be shifted to the wing tip on the top of the stroke and to the wing root on the bottom point. In this way, seen over a whole flapping period, while maintaining the transverse force Q (or the lift) the thrust S gets larger than the backward directed force R. This ingenious trick of nature solves the mystery of bird flight.

That you can even use the energy loss associated with the rearward directed drive R and can recover again (see The principle of flight) was not yet known at that time.

This view of the flapping flight by E. v. Holst - with his combination of theory and practice - has strongly influenced the development of my EV ornithopter models shown here.

picture from the video by Erich von Holst
720 x 576 mp4 (3.6 MB)
480 x 360 mp4 (2.1 MB)
320 x 240 mp4 (1.6MB)

There is film footage with sequences in slow motion of some flapping wing models by E. v. Holst in the 1940's. It can be seen the large model Swan and the model of a Brimstone Butterfly in flight. Furthermore there is shown his Thrust-Wing with its rotating flapping wings - an inspiration from the flight of Dragonfly. The video is an excerpt from the film Wings Over the World.

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1.1 Publications by Erich von Holst

Erich v. Holst, Tierflug und Modellflug.
Luftfahrt und Schule, 6. Jahrgang, Dez. 1940, Heft 3, Seite 24-26 Berlin-Charlottenburg 2

PDF

(in German)
1.6 MB

Animated pictures of a slow motion film from this article. One can see the rise off ground of a rubber-powered ornithopter and particularly the changing of the angle of incidence at the wing root recommended by Erich von Holst.

animated gifs of a slow motion film
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Erich v. Holst and Dietrich Küchemann,
Biologische und aerodynamische Probleme des Tierflugs. Naturwissenschaften 13. Juni 1941, Volume 29, Issue 24-25, pp 348-362 Springer-Verlag, Berlin
or
Holst, E. von and Küchemann
Biological and Aerodynamical Problems of Animal Flight. Royal Aeronautical Society Journal, vol. 46

PDF

(in German)
2.2 MB

Erich v. Holst, Gesammelte Abhandlungen

look at
reference literature

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1.2 Crank drive for generating the stroke and
twisting movement of a flapping wing

The rubber powered crank drive with a filament gear, developed by Prof. E. v. Holst, generates the stroke and the twisting movement of the wing at the same time. Thereto, the linkage coming from the crank pin is pivot-mounted only in stroke direction at the spar of the flapping wing. But in rotating direction it is not pivotable.

Thereby, the up and down movement of the crank pin is used to drive the stroke movement of the spar. At the same time the back and forward movement of the crank pin is used for rotating the spar. Furthermore, the spar is inflexible connected with only one of the outer ribs (Ri). Thus, the crank mechanism actively effects the twisting of the flapping wing. Stroke and rotating movement of the wing spars are phase-delayed by 90 degrees.

You can find descriptions of several rubber motors in the articles by Karl Herzog (in German) of March und April 1963 below.

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2. Karl Herzog

A great admirer of E. v. Holst was the academic painter and university painter Karl Herzog from Tübingen. In general he has done a very great service to the documentation of Erich von Holst's models. Mr. Herzog also has drawn and built his own bird models according to the models of E. v. Holst and has been busy intensively with the flight of birds. Karl Herzog has been my pen friend over many years.

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2.1 Flapping wing flight in nature and science

In the year 1963/64 the modelling magazine Mechanikus published a series of articles by Karl Herzog with the title Der Schwingenflug in der Natur und in der Technik

Among other things he drew and described there how the models by E. v. Holst can be built. Below, these articles are available separately in PDF-format (all in German). But there is also a compilation (PDF 9.0 MB) of all articles.

Of these series there is also an old abbreviated English version (PDF 2.4 MB) with partly different pictures. It was also translated in the Netherlands (PDF 1.2 MB) by Louis van Telgen.

Efforts in the history of flying

PDF 1.5 MB

Biophysics of the bird flight

PDF 1.4 MB

Flapping wing models by E. v. Holst

PDF 1.9 MB

Flapping wing models by E. v. Holst

PDF 0.7 MB

5. Sequence

PDF 1.0 MB

6. Sequence

PDF 0.7 MB

7. Sequence

PDF 0.4 MB

8. Sequence

PDF 0.7 MB

Pathways to the technical flapping flight

PDF 0.7 MB

Today, the Aue-Verlag, Möckmühl, Germany, owns the copyright of this series by Karl Herzog. Text and drawings may not be published elsewhere.

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2.2 Further works by Karl Herzog

Later Karl Herzog has written the literature book Anatomie und Flugbiologie der Vögel, published 1968. At the pages 136 to 168 it contains the chapter Demonstration of the animal flight on the basis of bird models and flapping flight gadgets. Regrettably the book is not available today.

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2.3 Deutsches Museum

Herzog has also played an important part in the design of the permanent exhibition Flight in nature in the Deutsches Museum in Munich.