The Flight of the Fly: Wings, Hovering, and Balance
Notes on how a fly beats its oblique wings to hover, with sketches of the insect
These notes study the flight of the fly ('mosca'), observing that its lower wings are more oblique than its upper ones in both length and breadth, and describing how, in hovering, it beats its wings with great speed and sound, raising them obliquely to strike the air edgewise and lowering them to strike it full on. A geometric analysis resolves the insect's balance into oblique lines e f and a b c d meeting at right angles, so that the power of descent equals the power of rising, with the hind legs acting as a rudder. Several pen sketches of the fly and a force diagram labelled a b c d e f appear in the right margin.
On this page
The fly's lower and upper wings
The note observes that a fly's lower wings are more oblique than its upper ones, both in length and in breadth. This asymmetry of the wing pairs is presented as a starting point for the analysis of its flight.
How the hovering fly beats its wings
Holding itself still in the air, the fly beats its wings with great speed and sound, raising them by the length of the wing. On the upstroke it carries the wing forward obliquely so it almost cuts the air edgewise; on the downstroke it strikes the air full on; and it would rise but for the counterweight of its own oblique posture.
Geometry of the wing-beat and the fly's balance
The obliquity of the poised fly runs along line e f, and the up-and-down motion of the wings along lines a b c d, which cross e f at right angles. The power of descent along e f is set equal to the power of rising along the wing motion a b c d, keeping the insect stationary; the hind legs serve as a rudder, and to flee it lowers its wings as far as it can.