Young, James J

English

Birds fly with apparent ease, remaining in control during manoeuvres despite large variations in wind conditions. They do this reliably, repeatably and efficiently. This thesis looks at the flight control of two species of bird, the lesser black-backed gull (Larus fuscus) and the red kite (Milvus milvus). Rotational stereo videography, a 3D multi-point tracking technique, is employed in the field to study the flight control of wild members of these species in free flight. The motion of the bird’s wings and tail was related to the measured flight path to estimate flight control parameters, giving an indication of the control strategies being implemented by the birds. By examining the equivalent control laws for fixed wing aircraft this work highlights the features of the birds’ flight control strategy which might enhance the performance of a gliding bird when compared to a traditional fixed wing aircraft with discreet trailing edge control surfaces.Looking firstly at longitudinal control of flight path angle in gliding flight, it was found that the gulls used fore and aft movement, adjusting the effective sweep of their wings to control their longitudinal flight path angle when in steady glide. This result is interesting as it highlights the decoupling of pitch angle from longitudinal flight path control as is typically found in rigid body fixed wing vehicles. Secondly when making gliding turns the gulls kept their tails furled and used bank angle to adjust their turning radius much in the same way that rigid body fixed wing aircraft do. It was found that the equations of motion for rigid body fixed wing turning mechanics can be applied to model the gulls turning performance as in this case the tail played little roll in controlling turning flight. Conversely a very different result was seen in the turning performance of the red kites. These birds make active use of a widely spread and comparatively large set of tail feather to enhance their control in turning flight. In a straight glide the tail was seen to twist up to ± 30º to help the bird to maintain its desired ground track and in steeper turns the tail was pro-versely deflected into the direction of the turn to enhance the effective amount of pro-turn force and to increase turn rate and reduce turn radius for a given bank angle. Turns at a lower bank angle made more use of this tail twist than turns at higher bank angles where a small change in wing angle has a larger effect.The study of the birds inspired three novel control strategies: wing sweep for pitch control, wing twist for direct lift control, and wing rise/flap for variable lateral-directional stability. These three control schemes are implemented on a representative small un[1]manned aerial vehicle focusing on control about the principal axes using an articulating main wing, with freedom to rotate about the wing root. Wind tunnel testing and computational modelling using a vortex lattice method were used to study the flight dynamics and control potential of these strategies.Wing sweep for longitudinal flight path control was found to be highly effective, particularly at higher angles of attack. This effective weight shift changes the moment arm between the centre of pressure and the centre of mass and can generate large pitching moments without making large changes to the angle of attack, as such dynamic manoeuvres such as end over end tumbles are achievable. Wing twist for roll control was found to be no more effective than properly sized ailerons, with the upgoing wing being pushed close to its critical angle and in extreme cases stalling and inducing control reversal. Symmetric wing twist for direct lift control has some transient benefits but effectively changes the longitudinal trim position of the fuselage as the wing adopts a new setting angle, its effect was of limited benefit. Finally, being able to dynamically vary the wings dihedral / anhedral angle in flight can profoundly affect the lateral direction stability of the vehicle. High dihedral angles stabilise the spiral mode and excite the Dutch roll mode and might be beneficial in environments where the vehicle would be subject to strong and variable lateral gusts. To conclude, birds fly quite differently from three-axis, rigid body fixed wing vehicles and some elements of directly controlling the main wing may be beneficial for small unmanned aerial vehicles that demand high levels of agility performance in unsteady flow fields. Having articulated wings and tails extends the flight envelope of ‘fixed wing’ vehicles, traditional design rules become less applicable and new performance metrics and control concepts are required to fully exploit the benefits over a fixed wing design.</p

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Flight control and performance estimation of wild free flying birds and implications for small-unmanned air vehicles

https://research-information.bris.ac.uk/files/369713980/Final_Copy_2023_03_24_Young_J_J_PhD.pdf

Flight control and performance estimation of wild free flying birds and implications for small-unmanned air vehicles

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