Fly like a Bird

IOP science logo Bioinspiration & Biomimetics Lab Talk Article

Excerpted from http://iopscience.iop.org/1748-3190/labtalk-article/45972

Fly like a bird

The ability to change wing dihedral is built into birds in the form of their ability to flap their articulated wings for propulsion. Researchers at the University of Illinois at Urbana-Champaign have investigated how birds effectively maintain their manoeuvrability without a vertical tail from a dynamics and control standpoint. This research has resulted in a unique way to control a tailless ornithopter in gliding and perching flight.

Figure 1. Parkzone ® Ember modified with articulated wings

The use of wing dihedral was of particular interest. The use of wing twist for control is a well-established concept; wing sweep may be effective, but not very intuitive. On the other hand, a few recent papers did propose the idea of using canted winglets for yaw control. The authors evolved the concept of wing articulation independently, and brought it to the point of experimental realisation (figure 1) to mimic a flyer like the barn swallow (figure 2).

The founding premise for this control is the observation that, when the centre of gravity of the aircraft is placed behind the wings with an asymmetric dihedral angle, an asymmetric lift distribution leads to a yawing moment that is in the same directional sense as the rolling moment and hence counters the adverse yaw generated in the absence of a vertical tail. This observation suggests the possibility of controlling the wing dihedral actively to control the yawing motion of the aircraft (figure 3). Figure 3 also explains the use of wing dihedral for controlling, in conjunction with the horizontal tail, two of the four longitudinal flight parameters, speed, angle of attack, pitch angle and the glide angle, independently.

Figure 2. Barn swallow

Another benefit of the proposed control scheme is that it can be extended readily to flexible wings. Flexible wings deform naturally to produce a dihedral effect, and furthermore, the dihedral angle is enhanced with increasing wing twist.

These benefits come at a price. Placing the aircraft centre of gravity behind the wing aerodynamic centre compromises the lateral stability of the aircraft. Additionally, wing camber causes the control effectiveness of the mechanism to depend very strongly on the angle of attack. In fact, the control effectiveness reverses its sign at certain angles of attack, depending on the manoeuvre. The dependence of control effectiveness on cambering may also suggest why birds change their wing camber actively during flight. Increasing the camber improves the lift, but degrades the effectiveness of a vital control mechanism. These facts mandate that an articulated wing flyer be equipped with a sophisticated automatic control system, and one such system capable of handling modelling uncertainties and control reversal is being developed by the authors.

Figure 3. Dihedral for flight control

About the author

Aditya Paranjape is a doctoral student in Aerospace Engineering at the University of Illinois at Urbana-Champaign . He is writing his doctoral thesis under the supervision of Professor Soon-Jo Chung. Professor Chung specializes in bioinspired robotics and nonlinear and decentralized control. Professor Michael Selig, also with the Department of Aerospace Engineering, is a renowned expert in low Reynolds number aerodynamics.

Fly like a Bird

Fly like a bird

About the author

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