Power Wing

by Pierre Benhaïem in Airborne Windenergy Forum · · 19 Replies · View last reply Respond
Pierre Benhaïem
Pierre Benhaïem

http://www.kitegen.com/en/2014/09/12/kitegen-power-wing/ The Power Wing is a modular structure with hinges. The structure of the C-shaped Power Wing allows to restrain drastically the bending moment encountered in a rigid flat wing tethered by its center. So, it gathers advantages of soft wings as lightness and of rigid wings as aerodynamical efficiency and long lifetime. It looks as a possible standard in airborne wind energy. Some adaptations as wings for a rotor for Daisy or Rotating Reel can be expected.

Roderick Read
Roderick Read

Yep, with really good control and a wide enough ring. something as massive as that wing could go around and pull a soft rope torsion set...As long as it behaved within limits. https://youtu.be/uG9OZ3yANc4

I'd be scared witless to fly three on a ring & I wouldn't want to stack them. I think more can be done with smaller wings before trying the big blade spinner mix.

I don't think Daisies need the cone for much.. They can fit more blades on a large diameter. Christof has shown they can suit more rigid wings. Expect changes, a lot. 

Pierre Benhaïem
Pierre Benhaïem

http://www.awesco.eu/esr3.html : study of bridled rigid wing.

In my opinion the most interesting feature of the Power Wing is not its size, but its C-shaped structure allowing a lesser bending moment. Other rigid wings deal with a strong cantilever effect. The question is if and how an appropriate bridle could allow to descrease this effect. 

Roderick Read
Roderick Read

Here's a good experiment.

Get a fairly large parafoil kite with a thin foil section. Hold the ends of the kite with say 2 foil shaped bars so the kite hangs upside down. 1 bar is lower down and a bit twisted forward. The kite takes a nice catenary with twist c shape in your workshop. Adjust your bridles, keep them up out of the way. Now the fun bit. Can we fill the kite with expanding foam? If we do it neatly do we get a rigid lightweight kite wing. It works for model planes. Spray the inside with a pva water mix, sparay the outside with silicon release agent. squirt the foam into the pockets. Wait till it's epanded into every space and cured. Neaten the leading edge. Test.

Pierre Benhaïem
Pierre Benhaïem

That said Power Wing is announced since more than 2 years ago. And there is not still any available data of flight.

Massimo Ippolito
Massimo Ippolito

The bridle system could improve the kite strength, but the bridles have significant drag, enough to change the angle of attack of the kite, that will fly permanently in relative dive wasting good wind.
More when the bridles, like kps, are around the aerodynamic barycenter of the wing adding drag to the wing itself reducing, even more, the AE.

Pierre Benhaïem
Pierre Benhaïem

Thank you for the indication Massimo. But is not the main advantage of the wing its biggest lightness resulting from the assembly of rigid elements forming a wing globally less voluminous thanks to its C structure? 

Massimo Ippolito
Massimo Ippolito

Pierre, definitively yes, the main primary advantages are:

the lowest achiveable drag, due to the minimal tickness

tensorstructural resistence of 1 hundred kN per meter of cord

No need of fan shaped bridle system that compromise AE and angles

derivided advantages:

highest aerodynamic efficency and highest speed

The lowest obtainabe wing weight

full traction breakage limit near 800kN  per 4m cord (obviusly the double of the tensostructural resistance)

Pierre Benhaïem
Pierre Benhaïem

Thanks Massimo. As Power Wing concept can have a positive impact on AWE R&D I would segment _ in Power Wing way_ my question, and you can reply yes or no if you want:

A wing composed of several (joined) rigid elements will be less voluminous, so less heavy than a wing in one piece, as cubic scaling law applies to each smaller rigid segment instead of the whole wing in one piece (by supposing that both are possible with an appropriate bridle system). Right. Wrong.

(Similarly) a C-shaped wing would allow the wing to be divided into segments, thus to be lighter (right, wrong), keeping rigidity of the profile in order to keep a high L/D ratio. Right. Wrong.

Resulting: As the segmented C-shaped wing is held by its two ends there is no cantilever effect. Right. Wrong. And as a wing in one piece is held by its center, there is a cantilever effect that is partially or fully eliminated by adding a fan shaped bridle system making indesired drag. Right. Wrong. And such a fan shaped bridle system does not prevent the cubic scale law making the heavier wing in one piece. Right. Wrong. Thanks. 

 

 

 

 

 

Pierre Benhaïem
Pierre Benhaïem

Alternatively to my precedent observations-questions:

Compared to a C-shaped segmented Power Wing for a same area, can a fan shaped bridle system allow the implementation of a so light wing, as the wing is in one piece and is held by its center with only one tether apart the bridle system?

Massimo Ippolito
Massimo Ippolito

the answer is more complex because among wings that needs fan shaped bridles we have kiteplanes and flexifoils to take in account. Flexifoils are a brilliant and remarcable idea to engulf air pressure in the wing compartiment in order to realise a discretised longeron/cantilever, exploiting the tesorstructural resistance of the fabric. Flexifoils limit the number of fan shaped bridles and limit the weight because the air itself become structural. Kiteplanes are more rigid with relatively higher in efficiency, but they needs a full sized longeron dressed with an aerodynamic shape. A 800kN kiteplane single rope require a construction that weight around 50tons. With two thick bridles in the middle of each wing span the weght coud be reduced to 30 tons, I cite those data that was an analisis FEM i've done and could be verified in the real realisations, changing materials the result could improve a little, adding more bridles we could observe a non linear decrease of the weight and of the diametre of the bridles itself to withstand such force. leading to an insatisfactory result with a lot of added drag and high weight yet. Makani chosen the two bridles system for the kiteplane after educated evaluation, Makani as everybody else, needs to develop very high lift forces, in order to exploit the fligth speed with the propellers, during the production propellers turns in turbines developing a negative thrust vector that is coincident with the wing drag vector thus killing the good AE of the kiteplane. 

   

Pierre Benhaïem
Pierre Benhaïem

Thank for the detailed answer Massimo. On http://euanmearns.com/the-eroei-of-high-altitude-wind-power/  you indicate "the Wing weight is 250kg@130sqm and the suitable ropes full extended are 1000kg@2500m.(500kg each rope) The developed wing traction is by design 500kN (250kN each rope)". The difference is huge with a kiteplane even if it comprises an appropriate bridle system.

Concerning Makani, as the turbines have a "good" drag resulting of energy production, do you mean that turbines "bad" drag would be too high? Some papers assume that appropriate turbines drag add 50% of the wing drag, wing+turbines speed becoming 2/3 wing speed. If it is the case for Makani, how much supplementary (bad) drag is added, according to estimations you could have? In a presentation of the last AWE conference 2015, measures of power/m² highly favored Makani (rigid wing with turbines aloft as you know) over Ampyx (also rigid wing, but with reel-out/in or yo-yo system, generator at ground), perhaps due to its reel-in phase (expense of energy).   

Pierre Benhaïem
Pierre Benhaïem

https://collegerama.tudelft.nl/Mediasite/Play/1065c6e340d84dc491c15da533ee1a671d is the link for M. Diehl's presentation showing Makani (right, with 25% potential) and Ampyx (left, with 2.5% potential) wings, in 3'08" from the beginning.

Massimo Ippolito
Massimo Ippolito

Pierre,

It doesn't exist a "good" drag in kinetic wind exploitation.

I long time I had no occasion to explain to Moritz the HAWP in general, he is stuck to 7 years ago concepts so he isn't anymore a reference IMO. The double kite was one of my initial idea, patented by my company. But I drop it because it is a wrong and inefficient concept. Also the ill-famed Diehl's conjectures are mostly wrong, I would be happy to invite here Moritz to provide him a deep update, in order to avoid noise, inconsistencies and poor figure but it seems unreachable, or he changed the again the e-mail.  Pay attention that I never impose my view, however we gained both a strong scientific and technological knowledge on the topic and the needed creativity that is the most valuable asset in new product developments, and we could share it in a self-evident manner.    

Pierre Benhaïem
Pierre Benhaïem

Massimo,

There are several different points in your answer.

I think you refer in "RotoKite" as "double kite", and SEQUOIA IT S.R.L as your "company". Here is some data of Gianni Vergnano's patent: https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=WO&NR=2007000788A1&KC=A1&FT=D&ND=3&date=20070104&DB=&locale=fr_EP# . An immediate finding is the high number of citations about relevant prior art (X or Y in the preliminary search report). But in fact these citations concern rotating wings, not precisely rotating wings and reel-out/in method as "RotoKite" is. IMO these citations by the preliminary search report of prior art would maybe have been able to be avoided, for example by joining claim.6 to claim.1 (maybe some changes have be made as an answer is required when X or Y occur). Besides these considerations IMO RotoKite is an interesting device, allowing the tether being not laterally mobile, avoiding an excessive drag, allowing a simpler control, allowing also a better maximization of space and land use (although I prefer a steady-state device). "RotoKite" could be improved using two Power Wings.

I think spaced "dancing" kites by MoritzD's method is also a possibility to maximize the space in spite of the _not too important_ lost space in the middle. The full wings go fast, sweeping more. But perhaps a rotating device like RotoKite is easier to control, thereby compensating that only the tip parts of the wings go fast. IMO a rotating wing is also more scalable and easier to implement.

"Good" drag is a wrong or right expression I use as the wind going across the turbine is slowed down, the turbine converting its energy.

In my last message I quoted MoritzD quoting data from Makani and Ampyx.   

Pierre Benhaïem
Pierre Benhaïem

To resume my previous message IMO a crosswind (single) kite system is more efficient than a rotary (being also crosswind) kite system in regard to wing area, but less efficient in regard to space (and land) used.