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Executive Summary

If we take time to look towards the sky and observe the way birds fly, we will discover a revolution in the world of aerodynamics and flight mechanics. Birds and insects have high degree of maneuverability and agility. They are able to morph their wings significantly to optimize their flight performance at various flight stages and allow for different mission roles during a single flight (i.e. perching, loitering, landing, hovering and taking off), an ability that will greatly enhance the performance of unmanned flight vehicles, especially orniothopters. Most birds will have their wings fully extended during the downstroke to maximize the production of lift, but adduct their wings during the upstroke to decrease drag penalties. Not only do birds change their wing span during the flapping cycle but they also change the wing’s incidence and angle of attack to maximize the production of thrust mainly during the downstroke. Figure 1 shows the lateral and dorsal views of the path of the wingtip and wrist of a characteristic wing beat in a pigeon flying at various speeds. From the figure, it is clear that the wing beat cycle of birds is far more complex than the straight up and down motion used by the current ornithopters.

Passively morphing the wings during flight can achieve such complex profile without suffering any energy penalties. A first step towards achieving efficient flapping flight with increased lift and agility in general is to optimize the wings performance in steady level flight at low Reynolds number. It is hypothesized that this optimized performance can be achieved through three steps:

  1. Increasing the torsional stiffness of the wing in the chord wise direction at the tip during the downstroke to maintain efficient thrust production.
  2. Decreasing the wing reference area during the upstroke to decrease the drag penalties.
  3. Passively implementing Continuous Vortex Gait; a set of kinematics used by birds to produce lift and thrust at high speed steady level flight. Figure 1 shows the frontal, lateral and dorsal views of both wing tip and wrist paths for this gait.

The objective of this research is to model, design and fabricate a new spar for the current ornithopter wing that will passively achieve the desired wing tip kinematics with an ultimate goal to optimize steady level flight performance for the wings at the low Reynolds number regime. Again this is a first step towards achieving an optimized flapping flight with increased lift and agility.

Continuous Vortex Gait 3-view
Figure 1: Frontal, Lateral and Dorsal Views Showing the Wing Kinematics of the Continuous Vortex Gait.

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