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October 30, 2020, post by Dave Santos

Dutch-Roll with Dynamic-Stall and Vortex-Lift components are proposed identified as ideal megascale kite flight modes for AWE.

A few "kite notes" that apply as well to aircraft, insect, and bird flight-

"Push-Turn"- very effective low-actuation-force turning by releasing tension on an inside wing (or wing tip), rather than pulling more tension on an outside wing (or tip).

"Pop"- pulling (or pushing) a sudden AoA increase for Dynamic Stall, ie. for kitesurfing jumping. The most energetic aerodynamic mode.

"Dance"- spontaneous Free-Energy oscillating motions of a kite (Dutch Roll), increasing in amplitude as wind velocity does. This motion can tapped as a damping factor.

These are substantially scale-invariant effects given constant "most probable wind velocity". They will manifest with simple km-scale AWE sail-wings.

More than mathematical-physics benchtop curiosities, these aerodynamic principles could power the world.
November 1, 2020, post by Dave Santos
Enhanced Dynamic Stall by Load-Motion Shock

When a skydiver pops open her parachute, there is an "opening shock." Similarly, when a kitesurfer jumps, she is "powering up" the kite (increasing AoA) while moving crosswind as fast as possible, away from the kite.

These are special cases of Dynamic-Stall, where fast opposed Load-Motion momentarily boosts dynamic stall. For the purposes of an optimal pumping AWES, momentarily-opposed load-motion can greatly increase cyclic dynamic-stall force amplitude.

In the context of power kite inscribing a figure-8 (lemiscate) on a spherical surface ("kite window"), the moment to maximize dynamic stall is after right crossing the mid-point in the dive/side-slip phase and progressively swinging forward, more into the wind, for a compounded dynamic-soaring (DS) dynamic-stall boost. Weaker flight phases at the tops of the 8 are high L/D low Cl/Cd. The high amplitude difference in force between these kite flight modes is maximally energetic. Many cases of biological flight are dynamically similar.

Specific aerodynamic-factor scale-invariance to mega-scale challenges mathematical physics research starting from benchtop scale experiments and numbers. There are no formal go or no-go predictions yet. The heuristic predictions posed here remain open.

Note that mega-scale air-volumes are non-dimensionally less viscous than micro-scale Re assumption, and non-dimensionally less inertial than meso-scale high velocity. These are key mega-scale AWES aero-engineering factors.
Dynamic Stall

" ...it is well known that most hovering insects employ angles of attack much higher than the stalled angle of an airfoil. Typical values during the translational phase are about 300 − 500 (Ellington 1984). Dragonflies and butterflies employ even higher angles of attack. At these ‘stalled’ angles, the wing can generate higher transient lift coefficients compared to the steady state value, a phenomenon called dynamic stall. Recent discussions have mainly focused on the role of dynamic stall on lift enhancement (Dickinson & GĻotz 1993; Ellington et al. 1996; Wang 2000b). A side effect of dynamic stall is the increase of drag. In fact at such high angle of attack, it is no longer most convenient to separate lift and drag in the traditional sense, which was appropriate for an unstalled airfoil. Wang recently argued that insects might use both lift and drag to maneuver in air (Wang 2003). In particular, a wing executing idealized kinematics similar to those used by dragonflies uses mostly pressure drag to generate the vertical force to hover (Wang 2003). Classical steady and unsteady airfoil theories, however, were designed to treat the regime of small angle of attack where the flow is attached at the leading edge; they do not predict pressure drag. To extend these theories to a full range of angle of attack, it is necessary to include both the leading and trailing edge vortex. The theory presented here is a second-order unsteady theory for a stalled airfoil. In §5.1 it was shown that the combination of vortex- and attached-flow added mass forces produces a maximum lift coefficient at an angle between 45o and 52.2o, in fair agreement with observation."

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Jane Wang | Department of Physics Cornell Arts & Sciences 

"Research

"I am fascinated by the physics of living organisms, with a focus on understanding insect flight. How does an insect fly, why does it fly so well, and how can we infer its ‘thoughts’ from its flight dynamics? The movement of an insect is not only dictated by the laws of physics, but also by its response to the external world.

We have been seeking mechanistic explanations of the complex movement of insect flight. Starting from the Navier-Stokes equations governing the unsteady aerodynamics of flapping flight, we worked to build a theoretical framework for interpreting and predicting the functions of an insect’s internal machinery for flight. In this approach, the physics of flight informs us about the internal computing scheme for a specific behavior. 

Our most recent work makes new connections to neural science. We build physical models for quantitative analyses of flight reflexes, and relate our findings to the underlying neural feedback circuitries for flight."