Basic properties of the flapping wing and features of the flapping flight

The principles of the flapping flight of all flying creatures realized either by birds or by insects are just the same. THE FLAPPING WING HAS AN ELASTICITY BOTH ALONG THE SPAN AND THE CHORD. The elasticity along the span conduces to the smoothness and high effectiveness of the flapping flight. The adaptive elasticity of the wing along the chord make it possible to achieve a maximum traction in a wide range of frequencies and amplitudes of the flaps. THE TRACTION IS CREATED BY THE OUTER PART OF THE WING WHILE THE INNER ONE GENERATES THE MAIN PART OF THE LIFT. When the inner part of the wing is moving upward the outer part lags and bends downward in relation to the inner part because of the air drag and inertial forces. In this way the drag of the wing as a whole diminishes. When the inner part is nearing the up position the elastic forces raising the outer part upward and latter continues to create traction while the reaction aids the inner part and the wing as a whole to pass across the up position and to begin moving downward.THE WING ELASTICITY ALONG THE CHORD IS SUCH THAT THE WING PRACTICALY DO NOT BEND IN GLIDING-EQUILIBRIUM POSITION.

The direction of the wing flapping motions is not perpendicular to the flight direction but with declination backwards which decrease by increasing of the flight velocity. The wing behavior and the aerodynamic forces action during a flapping motion are shown on fig.1.(The lines of the paths at the scheme are projections of the solid lines.)

FLAPPING WING AERODYNAMIC FORCES V - flight velocity U - flapping motion velocity W - total wing velocity X - drag P - TRACTION ( U > V ), LIFT ( U < V ) R - total aerodynamic force

It is evident from fig.1 that during both the downward and upward motion the outer part of the wing is creating the traction, while the inner part does the lift.In the course of the flapping motions the wing behavior automatically results from the inherent elasticity of the wing.The inner part of the wing produce lift during the upstroke too, because the flapping motion velocity U is little in comparison with the flight velocity V and the inner part is twisted downwards in equilibrum position.

THE FORCES ARE DIFFERENT(SIZE AND DIRECTION)IN EACH SECTION OF WING AND IN EVRY MOMENT OF CYCLE OF THE FLAPPING FLIGHT.

THE AIR STREAM OF THE FLAPPING WING IS FAN-SHAPED AND IS IN DEPENDANCE FROM CORRELATION FLIGHT VELOCITY/FLAPPING MOTION VELOCITY FOR THE DIFFERENT SECTIONS.

The above statement clarifies the flapping flight mechanism. It is important to point out that the mass distribution and the mass/elasticity ratio are of great significance for a maximum flapping wing efficiency. THE MASS / ELASTICITY RATIO IS DISTRIBUTED OVER THE WING IN A WAY ALLOWING THE WING - WHEN BENDED AND THAN RELEASET - TO COME BACK TO THE EQILIBRIUM POSITION WITHOUT VIBRATION

As result the wing bend along the chord only under air resistance but not under inertial forces. This property determinate the way of bending of the wing at the flapping movements, that decide the problems of the mechanics and aerodynamics of the flapping flight, pitching and hering.

These conclusions illustrated by the wing behavior at level flight but the described properties and relationships give the opportunity to explain the other types of modes of the flapping flight. 

                                                              May 1994

                           

September 2011