How Birds Soar Against the Wind Without Beating Their Wings
Gaining and spending impetus in glides; the wind lifts the bird as a wedge lifts a weight
A dense study of soaring flight opens with the note that a bird rises most easily by first taking a slight descent to gain impetus. Leonardo analyses flight against the wind along an oblique downward line, arguing that the wing meets less air at its edge-on shoulders than beneath its full breadth, so it advances where the air resists least. To climb, the bird enters above the wind, lowers tail and wing-elbows and raises its shoulders, and rides its stored impetus upward as a wedge is driven under a weight; as the impetus is spent it dips again to renew it, alternating descent and reflected rise. A concluding line on the facing folio states the principle that moving air against a still bird is equivalent to moving the bird through still air. Marginal sketches show a gliding bird, a horizon line and a small mountain peak.
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A bird rises most easily by first descending to gain impetus
That bird will rise into flight with greater ease which takes the impetus of its motion with somewhat of a descent at the beginning of its flight. The stored speed of the dip is then spent to lift the body, so that ascent and descent alternate as the acquired impetus is gained and consumed.
Flying into the wind: wings cut the air edgewise
When the bird flies against the wind it must gain ground along an oblique line toward the earth, entering under the wind. It advances against the air that resists least, which touches only the shoulders in the thickness of the wings, and never along the line of their breadth; so the wing is presented edgewise to the oncoming air where it strikes.
The wind lifts the soaring bird as a wedge lifts a weight
Once above the wind the bird rises without beating its wings, because the wind passing beneath it raises it as the wedge raises the weight. This would throw it backward, but the ascent slows and consumes the impetus; when spent, the bird would be overturned unless it at once lowers the rudders of its wing-shoulders, enters under the wind and lowers its tail, whereupon, declining, it regains the lost impetus and rises again by the reflected motion.
Moving air against a still bird equals moving the bird through still air
It is as much to move the air against the bird, itself immobile, as to move the bird against the still air. Therefore, Leonardo says, invoking the first proposition, the two cases may be treated as equivalent — a statement of the reciprocity of relative motion that the sentence breaks off before completing.
Sketch of a gliding bird above a horizon
At the head of the left folio a bird is drawn with outstretched wings beside a vertical reference line and a rectangular frame, and a long horizontal line carries small bird figures over a low mountain peak. The drawings set the wind-flight analysis against a schematic landscape horizon; no transcription accompanies them beyond the heading of the figure.