12/28/2023 0 Comments Lift drag thrustWhile the velocities of fixed wings and rotor blades remain oriented primarily horizontally in aircraft, this is not generally true for the wings of slow-flying animals. The notion that lift acts vertically and drag acts horizontally in level flight may become inaccurate during slow, flapping flight. Note that 2D perch forces ( F takeoff and F landing) are simply plotted along a straight horizontal line at the same rate as the aerodynamic forces. d We synchronize our AFP measurements with high-speed kinematics to show how the net 2D aerodynamic force vector varies in magnitude and orientation along the bird’s trajectory (the tracked eye) during a representative flight. Vertical lines denote takeoff and landing, and gray-shaded regions show downstrokes. These wing velocities are governed primarily by flapping kinematics, rather than the bird’s body velocity (gray). During a representative flight between two instrumented perches in the AFP (bird avatar enlarged 2× for clarity), each wing has an effective vertical velocity V z, horizontal velocity V x, and lateral velocity V y ( c) at the wingtip (black) and at r 2 (radius of gyration light blue). b The net force from both wings can be decomposed into net horizontal F x and vertical F z components, both of which are directly measured in a new aerodynamic force platform (AFP). Together, lift and drag make up the total force generated by the wing, F wing. L and D effectively act at each wing’s radius of gyration r 2, so we base v wing on the wing’s velocity at r 2 (see the Methods section). However, during slow flapping flight, the total lift L (solid blue arrow) and total drag D (solid red arrow) vectors generated by an individual wing are directed differently, because wing velocity v wing does not align with body velocity v body, as shown for a bird’s first downstroke after takeoff. a During steady forward flight, total lift L (dashed blue arrow) counters bodyweight W (solid green arrow) in the vertical direction, and total drag D (dashed red arrow) is countered by net thrust T (solid dark gray arrow) in the horizontal direction of body velocity (yellow). The aerodynamic force platform enables direct measurements of lift and drag components. Although this body-centric aerodynamic force analysis has been proven particularly successful in aeronautical optimization, it is unclear how informative it is for understanding animal flight because of how their flapping wings move with respect to their body. Consequently, aerodynamic research across engineering and biology has traditionally focused on how lift 3 is generated and can be maximized and how drag 4 can be minimized 1, 2. However, lift generation induces net drag on the body, which does require aerodynamic power (drag × speed) to overcome, because drag opposes flight velocity. In the body frame, the external work exerted on the air to generate net lift is zero, because lift acts perpendicular to the average body flight velocity and, therefore, does not oppose flight. The net lift force on the body counters weight in the vertical direction, while net thrust counters net drag in the horizontal direction of body velocity 1, 2 (Fig. Like other flying animals that propel themselves, birds sustain level flight by generating net aerodynamic forces with their flapping wings that balance gravity and body drag. Such low ratios are within range of proto-wings, showing how avian precursors may have relied on drag to take off with flapping wings. The parrotlets repurpose lift and drag during these flights with lift-to-drag ratios below two. Wingbeat power requirements are dominated by downstrokes, while relatively inactive upstrokes cost almost no aerodynamic power. Upon landing, lift is oriented backward to contribute a quarter of the braking force, which reduces the aerodynamic power required to land. At takeoff they incline their wing stroke plane, which orients lift forward to accelerate and drag upward to support nearly half of their bodyweight. To determine how birds use lift and drag, here we report aerodynamic forces and kinematics of Pacific parrotlets ( Forpus coelestis) during short, foraging flights. The lift that animal wings generate to fly is typically considered a vertical force that supports weight, while drag is considered a horizontal force that opposes thrust.
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