CoolIP index           

Unsteady Aerodynamics as a Basis for AWE


Unsteady aerodynamics is the messy real-world reality of all kite flight. As oscillation fundamentals are understood & mastered, the conceptual basis for AWE greatly expands.


Mechanical power typically involves a drive-train of rolling stress-wave wave-guides called wheels, with only a fraction of embodied structure actively loaded. Just as a cyclist pedals a bicycle, oscillating power converts to continuous rotation by cranking levers. A lever does the work of a far larger wheel with just a fraction of full wheel structure, but over a limited, reversing, rotation range. Ancient lever variants confer design flexibility, for example, a bell-crank re-vectors force orthogonally.

Levers make lower fundamental harmonic operating modes practical, without giant wheels. In bio-locomotion, this means flapping wings driven by tendons. In AWE, wind-driven bio-mimetic foils, string, & levers can drive generators.

The standard wind turbine is an expensive & massive object. To fly far fancier specialized turbines at larger & larger scales means weight & cost soon become critical, then hopeless. Flygens hardly help. Desperate chasing of high-performance by the forced trading-away of inherent-stability grows dependence on active flight-automation.*

Like basic turbines, practical oscillating wings are passive-controlled, self-regulated by tuned coupled aero, inertial, & elastic forces. The challenge is to optimize every phase of oscillation. Special lift mechanisms in unsteady aerodynamics are a plus, offsetting discontinuous trade-offs.

All in all, tensile mechanical-advantage driven by short-period kite-wing oscillation is a major AWE scaling & cost solution.

Case Study: The Wing-Mill

Long reeling-cycles are a popular oscillation mode in many pioneering AWE concepts. The recovery phase gap in generation is awkward. Short-period loops & figure-of-eights, with brief embedded recovery phases at the upper turns, are an effective alternative.

Self-oscillation is simple for a flapping-wing membrane wing-mill. A tip-hung kite-wing is naturally sensitive to disturbance & tends to self-oscillate in wind like a flag by a mix of pendulum & unsteady-aerodynamic forces. Dutch-Roll oscillation, a major short-period dynamic mode for conventional kites, traces frontal figure-of-eights. Suspended wing-mill fly oblique eights. Characteristic motion of various wings can range from a tight waggle suitable for close formations to wide-swept lazy-eights.

A wingmill's flapping cycle outputs strong sinusoidal or sawtooth power pulses. A high-amplitude-spike occurs by an air-hammer effect, as a low-stretch wingmill "pops" on each tack. Optimal pre-tension of hung wingmill is low static tension, with no slack in the relaxation/recovery phase.

Current membrane wingmills are not too high in aspect ratio or the flapping breaks up into subharmonics; there is easy usable power in a broad working wing of L/D of ~5-15.

The right sort of tail on a hung wing-mill acts is a tuned oscillation promoter & regulator. Flag motion is mostly unsteady-aerodynamic, with some inertial action. Whip-lashing is the inertial component. [see links below] Of all the world's country flags, Nepal's is uniquely shape-optimized to flap in thin air & survive storms.

The critical speed for reliable flapping onset is set by the design. Early onset can be triggered by a control nudge. Flapping frequency is dependent on the wing's characteristic-length harmonics (span & chord), windspeed, and line tension. An added kite-tail hosts a parade of aero-inertial transverse waves, with back-reflected longitudinal waves promoting regular self-tacking of the forebody wingmill. Quality flapping is a resonance between a fundamental oscillation mode of the leading-edge wing & coupled oscillations of the tail-flag.

Useful power is fundamental mode vibration; higher modes are parasitic. Local short-period harmonics are damped out of the forewing by battens or membrane stiffness, but tolerated toward the tapered tailend as a minor stability cost.

Unsteady-Aero Operational Constraints:

Long-period reeling-cycles wear on kiteline. Short period cycling can avoid reeling wear altogether or rely on small sections of ruggedized line at pulleys or capstans.

Flogging, as destructive sail flapping is called, is a manageable wingmill design issue. The British Admiralty's vast experience flogging flags offers lessons: Re-hem flags as they fray; premature fraying is mostly high-wind damage; a flag brought down when gale threatens lasts many years. Passive flogging mitigation is workable: Ease halyard tension or add a "cut-out" mechanism like the simple self-furling of an elastic lower corner attachment. KiteLab has flown membrane wingmills for thousands of hours from trees, thru many storms, & long hours under kites, without wear as a problem.

High-wind flags are made smaller & thicker. This characterizes scaling limits to membrane wing-mills, which with current materials can grow to nicely to 1000m scale, but not much more; after which populous arrays offer ultimate scalability.

In sailing, non-prepreg carbon fiber sandwiched between Mylar membrane is reputed flog-damage resistant. Cuben fiber, based on UHMWPE, is also considered robust. Its practical to make fairly large "solid" wing-mills of foams like EPP, but even low-tech materials like bamboo & cardboard can comprise an unsteady wing to power home or village.

There is an art to designing and tuning unsteady short-period systems for utmost performance across wide wind ranges.


Unsteady aero-to-mechanical dynamics is a fundamental basis for AWE. Optimized unsteady aerodynamic performance is a key to success.


* Toy-kite drag-stability and aerospace-UAV actuation-overhead are roughly comparable control performance versus cost trades. Its as if a bulk communications-theoretic thermodynamic burden is placed on intensive aircraft control of any kind. Still, the kite trumps by safety, simplicity, and scalability.

Previous work on flag physics is lead by-
Zhang Lab

Whip physics as studied by another lab-
Shape of a Cracking Whip by Alain Goriely and Tyler McMillen      2002

CoolIP                       ~Dave Santos             ,22Oct2010        M2369

Comment and development of this topic will be occurring here.       
All, send notes, drawings, and photographs!

Terms and aspects:   

Related links:

Commentary is welcome:

  • v