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Der Vogelflug

(Reprints of the book are available in German and English)

>>> online version of the German edition



I. Introduction
II. The Fundamental Principle of Free Flight
III. The Art of Flight and Dynamics
IV. The Force which lifts the Bird in Flight
V. General Remarks on Air Resistance
VI. The Wings considered as Levers
VII. The Energy required for Wing Motion
VIII. The Actual Path of the Wings and the Sensible Wing Velocity
IX. Apparent Effort of Birds
X. The Over - estimation if the Energy requisite for Flight
XI The Work required for Various Kinds of Flight
XII. The Foundation of Flight Technique
XIII. The Air Pressure on a Plane Surface moved Perpendicularly and Uniformly
XIV. Air Pressure on a Plane Rotating Surface

The Center of Pressure on the Wing during the Down-stroke

XVI. Increasing the Air Resistance by Beating Movements
XVII. Economy in Energy due to Accelerated Wing Lift
XVIII. The Expenditure of Energy for Flight without Locomotion (Hovering)
XIX. The Resistance to the Oblique Movement of a Plane Surface
XX. The Energy required in Forward Flight with Plane Wings
XXI. The Superiority of Natural Wings over Plane Wing Surface
XXII. The Determination of the Wing Shapes
XXIII. The most favorable Wing Section
XXIV. The Advantages of Curved Wings over Plane Surfaces
XXV. The Difference between Plane and Curved Wings over Plane Surfaces as Regards Air Resistance
XXVI. The Influence of Wing Outlines
XXVII. The Determination of the Air Pressure in Birds' Wings Surfaces
XXVIII. The Air Pressure on Birds' Wings determination in Rotating Surfaces
XXIX. Comparison of the Direction of the Air Pressures
XXX. The Work necessary for Forward Flight with Curved Wings
XXXI. Birds and Wind
XXXII. The Air Pressure on a Bird's Wing measured in the Wind
XXXIII. The Increase of Lifting Effect due to Wind
XXXIV. Air Pressure on the Bird's Wing in Calm Air, deduced from Measurements in Wind
XXXV. The Energy required for Flight in Calm Air as deducted from the Wind Experiments
XXXVI. Surprising Phenomena observed when experimenting with Curved Surfaces In the Wind
XXXVII. The Possibility of Sailing Flight
XXXVIII. The Bird as our Model
XXXIX. The Balloon as an Obstacle
XL. Calculation of the Work required for Flight
XLI. The Construction of Flying Apparatus
XLII. Concluding Remarks

Plate I: Air Pressure on Plane, inclined Surface
Plate II: Air Pressure on Plane, inclined Surface - Curved Surfaces rotating in Still Air
Plate III: Air Pressure on Plane, inclined Surface - Curved Surfaces rotating in Still Air
Plate IV: Air Pressure on Plane, inclined Surface - Curved Surfaces rotating in Still Air
Plate V: Air Pressure on Plane, inclined Surface - Curved Surfaces in the Wind
Plate VI: Air Pressure on Plane, inclined Surface - Curved Surfaces in the Wind
Plate VII: Air Pressure on Plane, inclined Surface - Inclined and Normal Surfaces
Plate VIII: Wing Details of Stork



Twenty years have passed since a German Engineer, Otto Lilienthal, published a systematic account of his researches in what, at that time, seemed to be a barren field : the conditions governing mechanical flight as demonstrated by birds. Until that time, and indeed even up to a comparatively recent date, the most varied and frequently impossible speculations were ripe as to the causes which enabled birds to fly. Differences in specific gravity, special construction of the skeleton, anatomical specific properties, and the like, were invented to account for the ability of birds to sustain and alter their position in the air and in some cases even the fiat went forth that man should not attempt a feat which nature had evidently denied to him, and that he should not tempt providence.

It was therefore essentially the merit of Lilienthal, by applying his mechanical training to the problem of birdflight, and by a series of systematic investigations, to evolve some order from chaos, and to finally reduce the great mystery to a purely mechanical or dynamical proposition which, although incomplete in many respects, yet gave tangible shape to the whole, making it possible for future workers to fill the various gaps and to rectify some of the earlier assumptions which had. necessarily to be made.

That, in the light of later experiments, some of Lilienthal's coefficients and equations had to be superseded, cannot detract from his merits. Such facilities as wind tunnels and Eiffel towers, in and from which to test models, were not at his disposal ; he had to originate and construct, almost single-handed, every piece of apparatus used in his investigations. By the gleam of the lantern lit by his work it became first possible to trace a pathway in the darkness, whereon subsequent investigators, equipped with the more powerful searchlights of modern science, were able to open up an unexplored domain.

We are justified in calling Otto Lilienthal the Father of Gliding Experiments. Though his work was not followed up in his own country, Germany, yet it bore ample fruit in other countries, as witness the work of Chanute, Pilcher, Wright Brothers, Ferber and others. Indeed, the modern aeroplane, with all its astounding records of altitude and distance, is the lineal descendant of that ill-fated motor-glider which caused the death of Lilienthal, and unfortunately the death-roll which he headed is stretching out and ever growing as the number of those who "ride the wind" on aeroplanes is increasing, and their feats becoming more important.

But this very inheritance of disaster gives food for reflection, and it is permissible to speculate whether the development of dynamical flight would not have proceeded on different lines had Lilienthal been spared some more years of useful life.

There can be no doubt in the mind of any reader of Lilienthal's work that he was aiming at "economy" in the work required for flight. He is constantly calculating and holding up to our admiration and emulation the wonderful economy of nature as applied to the flight of the larger birds, and it is safe to assume that his gliding experiments were but a means to an end, viz. to familiarize us with the wave motion and eddies of the air ocean around us: to develop or train a special sense for these constant changes in wind pressure and direction. That wonderful mechanical creation, the wing of a bird, with its supporting and propelling properties, was his constant, though apparently unattainable, ideal. With an engineering equivalent to it, with partly beating wing, did he expect to ultimately solve the problem of mechanical flight. His successors were content to perfect his gliders, they stopped short at the tangible success of supporting planes and relegated the propelling function to the aerial screw propeller, admittedly an inefficient mechanical device, which can only be retained so long as we are content to move with the speed of an express train and content to risk the consequences. The few isolated attempts to construct machines with flapping wings were too crude to prove - as is assumed - the hopelessness of this principle, and more and more public attention crystallizes an the sensational achievements of mono and biplanes-wonderful and gratifying no doubt , but woefully neglectful of the demands of efficiency or economy.

Motors of from 26 h.p. to 200 h.p. are requisitioned to enable one or two men to "fly."
Had it not been for the marvellous possibilities of the explosion motor, we should have been forced to be more economical with our available motive-power, and in all probability should, by now, have opened a new direction of investigation along the lines so frequently urged by Lilienthal, namely, "Bird Flight."

Nature, though prolific, is ever economical, and it behoves us to strive likewise after economy in the dynamics of flight, by trying to emulate the great model, so constantly exhibited to us by nature, viz. the bird.

It is in the hope of stimulating afresh some careful experimenters to a renewed attack upon the wing problem, with its wonderful detail of structure, that the translator has urged the publishers to place Lilienthal's work before the English-speaking world.

A. W. I.

(A. W. Isenthal, 1911 (Translator))