Scaling Laws in AWES Design

Engineering Math & Physics
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I am saying more than Doug kindly allows, that in modern kite design the Morse Sled is classed as a fundamental ram-air tensairity case, given the “splint” (whisker). Further, the open tail vent is found obsolete and new kite designs are sealing off the rear. Finally, a ram-air valve is found (by kite folks like me) to be theoretically superior to a blower-system; by lower mass, lower cost, lower complexity, higher reliability, etc.; and therefore generally more scalable.

The prediction stands as first posed years ago on the Old Forum.

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This forum is as hard to follow in its own way as any other. I was specifically replying to Windy_Skies, regarding what was, upon closer inspection now, a 10-day-old post from Jan 5, but my reply appears as just another comment today, in “a topic”, rather than as a reply to that post, which was:

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Let anyone who wants think that the Morse Sled, one of the great kite designs of all time, is “unsubstantiated” compared to airborne-blower-dependent airbeams, which hardly exist.

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See about 1’56’’ a simple beam then a far more rigid tensairity beam comprising the beam + a strut as compression element + two spiralled cables. It is well explained on http://www.tensairitysolutions.com/.

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The image provided shows a basic design failure, that the wing root is clearly not stiff enough by its blown tensairity, with no cross bridling to correct the weakness.

The kite design shown was Roland Schmehl’s PhD effort suited to compare against the Darwinian winners of the power-kite design community.

Morse’s ram-air tensairity was more advanced 30 years prior. Note that even Morse’s tensairity is not as scalable as Culp’s SS OL design.

The ultimate kite scaling principle is earth surface only as the optimally stiff medium and air pressure is the rest. Whiskers and tubes fall short.

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A ram-air beam like this?

sledplan.gif1728×1248 43.7 KB

I get this when I try to search for Morse sled kites.

Dave claimed, I think, that was an application of tensairity. That was why I replied. I disagree.

I’ll refer to my earlier reply:

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Windy Skies, You found a Pocket Sled, that does not need any whiskers simply because at small scale cloth is stiff enough alone (note the applicable scaling law). However, at larger scales the whisker spars are standard. One of the reasons a “Morse Sled” search does not get good results is that few remember Morse for his innovation, even though so many see his kites at parks and beaches.

Make no mistake, the current trend in sled kite professional design is to seal the rear and refine the intake. The whisker spars persist. Pity on anyone who cannot see the ram-air tensairity principle at work. I have in fact been refining and testing New Tech Kites’ pocket sled along these lines, with a kite from my old friend Michael Lin, NTK Founder, even sneaking in a short splint at the key compression point. Let anyone disagree with the kite design world, no harm done.

Oh, another Old Forum discussion was tensairity scaled up by splinting itself made of inflated tubes, which appeared as a NASA Tech Brief in the 90s, but “tensairity” had not been coined (and claimed as a TM, but its becoming a term of art).

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Pity for me.
On http://www.tensairitysolutions.com/#concept/: “The main idea is to combine the lightness and simplicity of an airbeam with the load bearing capacity of a truss structure. The first developed Tensairity structure has the shape of a cylindrical beam, where the inflated hull is reinforced with a strut and two cables. The cables are spiralled around the hull and connected at both ends with the strut. The air-pressure in the hull pretensions the cables and stabilizes the strut against buckling enabling an optimal use of the materials in the beam.”

There is no “ram-air tensairity principle at work” in any known sled kite, even as it contains whisker spars.

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Rod, Go ahead and cite a commercial website instead of Wikipedia, which offers the airbeam “splint” criteria kPower invokes. In kite evolution, the truss nature of box kites really has led to the Morse airbeam. Good luck if you prefer blower-based airbeams, which have only fizzled in Swiss kite testing.

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Another set of scaling laws in AWE are FARs (Federal Airspace Regulations) aviation regulatory constraints based on mass and velocity aloft. Higher mass and velocity imposes much stricter regulation, such that lower-mass lower-velocity architectures have an early advantage. It is foreseeable that higher-mass higher-velocity platforms do eventually become certified as airworthy, its just a longer harder path. Altitude is another regulatory dimension, and there are many other critical complexities lurking in the FARs with a scaling law effect.

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Further scaling laws inherent to AWE relate to the geographic dimensions of the atmosphere. Clearly AWES are altitude-limited by stratospheric constraints of lower air density (and low wind velocity above Jet Streams). On the other hand, AWES are free in principle to extend laterally to planetary scale, a far larger scaling dimension.

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Can the pressure inside of the tube become greater than the pressure outside of the tube? If so, how? What valves would enable that? The air needs to not be able to escape when the tube is squeezed.

I think only if that’s possible a tube like that becomes a more interesting building element, as that allows it to become stronger.

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5 posts were split to a new topic: Does torque transfer over a network of tensioned hoops scale better (better power to weight) than over a solid shaft?

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The only example of a rigid AWES of consistent dimensions is Makani M600. Let us evaluate its inherent risks, considering it is tethered and a tether can modify assessing the crash risk.

See also on Does torque transfer over a network of tensioned hoops scale better (better power to weight) than over a solid shaft? about scaling rigid elements.

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Answering the question of whether a ram-air tube develops higher pressure inside than surrounding pressure; the stagnation zone of the airfoil intake starts maximally compressed relative to the overall pressure field, and develops even higher pressure and stiffness with increasing velocity. Ram-air pressure is a far simpler, safer, cheaper, more scalable kite spar principle than any other.

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4:12: Basic Assumptions - Flight System
5:17: Scaling Factors
8:04: Specific Energy vs Linear Scale, Fly-Gen AWE Systems as a Function of
Mass-Scaling Coefficients
12:04: Evaluation

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Really liked the Andy Stough Windlift presentation @PierreB
Wh/kg being the fitness measure is so right.

Michiel Kruijff Ampyx was also saying they need to find their scale sweet spot

They will have to find new material or design tech to improve to larger scales
Although for their 3MW design they’re predicting a competitive LCOE already… Bonus!

Too large won’t fly in light airs, capacity factor goes down was similar to the message from
Mark Schelbergn Clustering Wind Profile Shapes to Estimate Airborne Wind Energy Production - TIB AV-Portal
&
Philip Bechtle Bonn Airborne Wind Energy Resource Analysis: From Wind Potential to Power Output - TIB AV-Portal
Who also said flying in Scotland is a great idea …so windy all the time especially with a bit of altitude.
Also had good financial arguments for day ahead spot price advantages for AWES which handle low cut in giving more yield at better energy price.

Kester Gunn from RWE Investigating Offshore Markets for AWE Techno-logies - TIB AV-Portal
had a bit of a warning for the Ampyx repowering towers business plan as it means knowing the fatigue loading history of each individual tower you build on… And the original engineer didn’t want to waste materials on a too heavy base. Also really like his advice on scaling floating systems that ones which handle tilt and can have the PTO low down are cheaper. Oh and have a safety fence on the platform too

Ciaran Frost BVG Global Prospects for Airborne Wind Onshore - TIB AV-Portal
was showing GIS data on how there were massive land areas with good economic potential for AWES exploitation in NE Europe , Russia, Kazakhstan, US … Even if you need loads of room to operate.

Thomas Hårklau, Kitemill: Past, Present and Future - TIB AV-Portal
was saying he’d love to work at home in Norway but going with economic scaling forces took them to China. Learning on small models was the key to scaling for all wind turbine manufacturers in history. And getting millions of operational hours was the scale key to getting cheaper finance.

Skysails are at 200kW containerised
KitePower nl are ~100kW containerised

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Two other videos also about scaling:

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Does the scaling potential depend solely on the square-cube law? Certainly, mass in flight is an obstacle to scalability for different reasons including safety and material resistance. But is mass an obstacle on its own, or in combination with other parameters?

I do not yet have a specific answer to this question, so clarification would be welcome. I will only give two examples:

  • I had mentioned the problem of negative kinetic energy for Makani M600 during the downward part of the trajectory. Of course, the kinetic energy depends on the mass, but the flattening (if possible) of the trajectory would make it possible to avoid having to undergo it (in addition to the weight).

  • A second example came from a simple experiment I just done, and who will make someone laugh with a little experience in physics, which I am not. I easily held a carbon tube 6 mm in diameter and 2 m high by holding it vertically at its base with my thumb and forefinger. On the other hand, the more the bar was tilted the more difficult it was: it is indeed normal because instead of the vertical bar being focused on a point on the ground, the mass of the tilted bar is distributed over several points, my poor thumb and index undergoing leverage of its weight. As a result a tilted (from the bottom to the top) SuperTurbine™ (Serpentine ™) could benefit from a lesser weight as it scales thanks to the numerous smaller rotors; but however it would undergo a higher weight constraint due to leverage, likely leading to a shear effect towards its base. Such a negative effect is mitigated as well as the consequences when the SuperTurbine ™ is maintained by its middle by a mast.

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Scaling

The experience of most framed kite makers is that kites don’t scale. A design that flies well with a 1m wingspan, will not fly well, or at all, if scaled down to 100mm or up to 10m. But this is because the weight /area ratio of framed kites increases with size, and changes in weight/area profoundly affect stability. Larger framed kites also flex, bend and distort proportionally a lot more than small ones, which effects flying behaviour. Above about 10m wingspan, even with carbon fibre, framed kites become either too heavy to fly in a useful wind range, or very fragile.

My experience is that single line kites do generally scale and over a very wide range- provided their weight/area ratio stays relatively constant.

The smallest ram air flag kite is 0.75 m span, 0.35sq.m, 0.1kg. The largest is 45m span, 1250 sq.m, 360kg. The mini’s weight/area is around 0.28kg/sq.m, the mega’s is 0.29kg/sq.m. They both fly well, and very similarly, with good recovery and no weaving. The mega is more than 3,500 x’s the mini by area.

Single skin kites can also be scaled up until fabric and corded reinforcements reach their strength limit without any significant increases in weight/area- but in very large sizes would have far too much pull to be practical for kite event flying.

Apart from weight/area, there are some other properties of kites that don’t scale:

As referred to above, there is a non-scaling effect in deflections- that fabric and structures are subject to higher overall loads in larger kites, so will deflect/distort/bend proportionally more. This is significant for rigid and framed kites. but isn’t generally noticeable for ram air or single skin kites within fabric and cording strength limits.

For ram air kites, the mass of enclosed air increases with the cube of dimension while their area and weight scale with the square. Air doesn’t have weight, but it has mass. To mitigate this, in very large sizes, ram air kites are made with no internal partitions so that their enclosed air mass ( > 5 tonnes for a 1250 sq.m flag kite) can rotate somewhat independently when the kite is correcting - otherwise its inertia would slow correction to a problematical extent.

Reynolds number (Re) effects. Re is an indicator of the onset of turbulent flow, which definitely doesn’t scale- but is also only significant for very tiny kites in our case.

Surface roughness doesn’t scale (a 20mm bump is significant for a small kite but less relevant on a larger one) - and has some effect I expect- has been posited as the reason that very large leading-edge inflatable (LEI) kite surfing kites fly in lighter winds than smaller ones.

And entrained air mass? When a kite moves it carries some mass of air with it (not just the boundary layer) and this mass is a cube function of dimension, not square - how significant is this for scaling?

This review is encouraging about perhaps almost no limit for scaling for a (flexible) single skin kite which does not undergo the mass of enclosed air in inflatable cells of a ram air kite. The limits are the resistance of material.

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