During the pull-out maneuver, Peregrine falcons were observed to adopt a succession of specific flight configurations which are thought to offer an aerodynamic advantage over aerial prey. Analysis of the flight trajectory of a falcon in a controlled environment shows it experiencing load factors up to 3 and further predictions suggest this could be increased up to almost 10g during high-speed pull-out. This can be attributed to the high maneuverability promoted by lift-generating vortical structures over the wing. Wind-tunnel experiments on life-sized models in the different configurations together with high fidelity CFD Simulations (LES) show that deploying the hand-wing in a pull-out creates extra vortex-lift, similar to that of combat aircraft with delta wings. The aerodynamic forces and the position of aerodynamic center were calculated from the Simulations of the flow around the different configurations. This allowed for an analysis of the longitudinal static stability in the early pull-out phase, confirming that the falcon is flying unstably in pitch with a positive slope in the pitching moment and a trim angle of attack of about 5 • , possibly to maximize responsiveness. The hand-wings/primaries were seen to contribute to the augmented stability acting as 'elevons' would on a tailless blended-wing-body aircraft.

A novel approach for sensing and characterising the flow over an aerofoil is introduced. Arrays of flexible wind-hair-like sensors distributed over an aerofoil, which are tracked remotely using high-speed imaging and processing, acting as “digital tufts”, are used to provide real-time readings of local flow information with high temporal resolution. The use case presented in this paper has the sensors embedded within the suction side of a NACA0012 aerofoil and tested in a wind tunnel for varying angles of attack in static and dynamic tests. The time-averaged signals were able to provide information pertaining to the free-stream velocity and instantaneous angle of attack. The capability of the sensor type to provide temporal flow information is also explored. The sensors were used to detect low-frequency oscillations, which are pre-cursory to stall. These are hypothesised to be linked to breathing modes of the laminar separation bubble, causing a shear-layer flapping observed on the sensors. Such low-frequency oscillations were also detected shortly before separation in the ramp-up studies.

Purpose Covert feathers on avian wings can show dynamic pop-up behaviour in rapid succession as a reaction to turbulent gusts. The purpose of this paper is to understand the possible flow control mechanism induced during such dynamic motion cycles. A model aerofoil is designed with suction side spanwise control of rows of bio-inspired flaplets. Design/methodology/approach A NACA 0012 aerofoil is equipped with a spanwise row of eight flaplets at 80% chord, connected to pneumatic actuators and can be deployed to max 15° in a prescribed open–hold–close manner. The model is placed in a water tunnel and flow measurements are done in the wake of the flaps during a cycle using particle image velocimetry. Findings During opening, boundary layer flow is sucked into the void space between the wing surface and the flaplet, which induces backflow underneath the flaplet and traps the fluid inside. This fluid is expelled downstream during closure, which generates a forward directed jet as seen by the formation of a vortex-ring like structure with higher axial momentum. The entrainment of the jet leads to the re-energising of the boundary layer flow further upstream. Originality/value This paper presents a furtherment of understanding of the action of pop-up feathers for separation control. The actuation of the bio-inspired flaplets shows a flow vectorising effect which can be used for active separation and gust control. In the case of incipient separation, flaplet action can act to re-attach the flow because of the jet entrainment effect.

A novel approach for detecting characteristic flow signatures precursory to stall along aerofoils is introduced. It uses arrays of flexible wind-hair like sensors distributed around the aerofoil which are tracked remotely using high-speed imaging and processing. The sensors act as “digital tufts" providing real-time readings of local velocity information with a high temporal resolution. Such sensors are integrated into a NACA0012 aerofoil and tested in a wind-tunnel for varying angles of attack in static tests and dynamically in a ramp-up test. For the static tests, the mean values of the sensor signals provide information on local free-stream velocity and angle of incidence. The fluctuating part of the signals show that at angles approaching separation prominent low frequency oscillations are detected, the magnitudes of which scale with the angle of incidence. These are hypothesised to be linked to breathing modes of the Laminar Separation Bubble causing a shear-layer flapping observed on the sensors. Such low-frequency oscillations were also detected short before separation in the ramp-up studies. As the high-speed cameras are mounted in a simulated “on-board" position, the sensing method could be used for early stall warnings in small-scale UAVs with integrated on-board object tracking cameras.