High lift coefficient and biplane kite

Florian Bauer (https://www.researchgate.net/profile/Florian_Bauer10) has published as co-author “Power Curve and Design Optimization of Drag Power Kites” (https://repository.tudelft.nl/islandora/object/uuid:c40f14fc-b4ba-498a-84c4-f2b745b4417b?collection=research) and “Drag power kite with very high lift coefficient” (https://www.eal.ei.tum.de/fileadmin/tueieal/www/theses/Bauer/HighLiftCoefficient_20170831.pdf).
Both papers investigate the biplane configuration and very high lift coefficient.
The results are impressive:
“Power Curve and Design Optimization of Drag Power Kites” mentions: “…a 40m wing span biplane kite with a wing area of 80m2, a lift coefficient of 4 and a tether length of 370m achieves a nominal electrical power of 7 MW…”.
Some high lift elements like slots or flaps are shown on https://www.youtube.com/watch?v=q_eMQvDoDWk&t=281s.
Perhaps that also would be suitable for the pumping mode.
I would like to know your advice. Thanks.

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I do find the paper very interesting. It points at some possible optimizations that could gain the industry as a whole.

I believe some AWE actors are already using multi foil wings, look at Twingtec at 0:50 in this video, Im sure there may be more examples. https://www.youtube.com/results?search_query=twingtec

Now to criticise the paper a bit, I think the numbers are optimistic, and I dont think its a good thing to create too much hype at this point, until at least one actor has proven reliable production of electricity with AWE. I’d rather make conservative estimates early on and then rather deliver to exceed those later (which is probably also quite difficult as the initial estimated were not conservative enough).

First, a practical issue: If the plane is 40 meter wingspan, I’d say the loop should be at least 100 meter radius for a single wing AWE system. This is probably even too tight, as we want to avoid too much roll and also different windspeeds at either side of the wing. With a 400 meter tether you are flying at >40 m/s and just 40 m above ground (20 degree elevation stated). You can add more tether (370m to 1000 m?), but then the power ratings will probably decline a bit due to tether drag.

Next, when rating power, there are certain absolute limits that must be accounted for. I think the most apparent are winch power soaking ability, and tether force. If the wing should create 7 MW power that would be some really hefty tether/winch. Also, the generated power is not entirely constant due to gravity and the fact that you are sometimes flying towards and away from the wind. My point is that if the kite is able to hold 7 MW peak at 10 m/s wind, the average production will be far less.

In short I don’t buy the 7 MW number.

If you had instead stated 1 MW average power at 10 m/s would that not already be quite impressive according to the current state of AWE?

The illustration of the abstract provides as lift and drag coefficients (CL and CD) respectively 4.6 and 0.1.
Should not the drag coefficient be far higher in regard to the lift coefficient, growing with the square of the CL?

The main idea is “…the power increases cubically with CL and decreases
only quadratically with CD…”.

I believe a G (L/D) value of 46 is extremely high. I would expect a high lift coefficient kite like this to have less G than a single airfoil.

The concept of high lift does have merit in the sense that increasing Cl, even if you decrease G in the process, will increase the G value of the AWE kite if you also take tether drag into account.

You might as easily just use a Cl of 1.5 (normal wing) and increase the wing area by a factor of 3 for the same effect.

http://www.dept.aoe.vt.edu/~mason/Mason_f/HiLiftPresPt1.pdf and some other documents seem give a maximum CL value about 3. The aerodynamic devices are well described.
Florian’s paper indicates 4.6 for a biplane kite. In some way the biplane configuration is also seen as a lift device: the 80 m² area concerns only one surface, not the two surfaces.
Such a wing or similar could be particularly suitable for my rotating reel conversion system (https://link.springer.com/chapter/10.1007/978-981-10-1947-0_22, see also Rotating AWE systems topic), the crosswind wings rotor or joined blades rotor becoming both powerful and not too fast to go with the ring with hydro turbines.

So the 4.6 Cl value is actually just 2.3 on each wing?

Given the inherent risk of AWE itself I would not consider adding more risk by introducing this kind og wing now. That is, unless the AWE is not feasible without it

I realize that power is Cl^3 and Cd^2, BUT:

Cl and Cd are connected. Increasing Cd will most likely also increase Cd. Cd will probably increase relatively more in the process. Look at that last foil in the series of four: the lift is pointing very much backwards.

Next, we realize that increasing Cl above 1.5 is not really that easy. You need multi profile or other tricks, and with these tricks your wing may not fly as well as the simpler normal wing profiles (eg stalling and nonlinearity issues, as a software guy I wouldn’t know the details?).

If we assume that Cl/Cd is fixed, the power is only proportional to the Cl. So if you can increase the Cl from 1.5 to 4.5, you will get 3x the power. In reality I expect for these profiles, Cd will scale relatively faster than Cl, and in effect we are perhaps only getting 2x the power, and a wing that is difficult to produce and control in addition.

Now there is another option that would also give us the same effect. Increase the wing area by 2x. I think if you are seriously considering going for a wing like this, you might consider that option as well. Perhaps even the wing with twice the area will weigh less, after all structural weight has been added

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Its odd to see so many Northern EU university-incubated ventures competing with similar but unproven kiteplane platforms for a market that has not yet developed. The technical claims seem like wildly speculative marketing, rather than the caution academia normally shows.

Note that high Lift Coefficient is a lower velocity flight mode where power kites excel. It won’t make much difference whether a kiteplane is a biplane or not, with a tail or not, and so on, if careful comparative testing is destined to favor the power kite.

It all may boil down which wing can crash, not kill anyone, pop right back up, and does not have an RF com link that can be jammed, plus the highest power-to-weight to-boot.

Good luck to all players who find themselves with the wrong architectural down-select, that they may somehow migrate to the winning architecture. This would be natural if everyone was cooperating more rather than competing for a quick return on a shakey investment.

Biplane kites are old hat in classic kite design, but monoplane kites dominate all high-performance kiting, for well known reasons. A bare biplane advantage over a monoplane is a slower landing velocity and stronger airframe, to not crack up quite as soon as faster kiteplanes.

Pray for third-party testing to settle the fog of AWE product claims.

That increases also the cantilever effect, while the two wings of the biplane are held.

These values are extracted from the abstract connected by the second link, the third link toward the complete paper indicating 4.1 MW.

Parasite drag increases exponentially with speed. Induced drag decreases exponentially with speed. So, again, its a give and take depending on your mission statement.

Biplanes, because of their lighter wing loading, have very low induced drag. But because of the increased surface area, they need to travel very slowly to prevent parasite drag from taking over.

Which is why biplanes are not typically used anymore (outside of nostalgia) because the more powerful engines with today’s technology would increase parasite drag too much.


Because AWE of the single wing type inherently struggles with tether drag, flying slower with a biplane could be more appropriate for AWE relative to regulat flight.

I think the biggest advantage of the biplane may be saving weight. When the wingspan is reduced this also rduces the weight of a beam in the wing.

The AWE biplane in question though has extremely high wing loading. Goes to show that airplane knowledge is not directly transferable to AWE

http://www.energykitesystems.net/FAA/FAAfromMakani.pdf page 3, the rated power is 5 MW for the M5, the wing mass being 9,900 kg.

page 14, the utility-scale system is 11,836 kg and is rated at 4.1 MW (see page 17).

It looks that the power/mass ratio is better with the monoplane in very high scale.

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I would like to found the equivalent of flaps and slots for flexible wings. I think about a segmented wing comprising two or three sections with different angles of attack and bridles, allowing the air to cross.
As a result the crosswind kites would fly slower in smaller figures by keeping a similar power.

Applying D-line input (“brakes”) across the power-kite TE is the flap-equivalent to rigid aircraft. For slotted-wing effects, networked kites can emulate the same topological and geometric relations.


(co-author Florian Bauer @floba )

As an alternative to conventional wind turbines, this study considered kites with onboard wind turbines driven by a high airspeed due to crosswind flight (“drag power”). The Hypothesis of this study was, that if the kite’s lift coefficient is maximized, then the power, energy yield, allowed costs and profit margin are also maximized. This hypothesis was confirmed based on a kite power system model extended from Loyd’s model. The performance of small-scale and utility-scale kites in monoplane and biplane configurations were examined for increasing lift coefficients. Moreover, several parameters of the utility-scale system were optimized with a genetic algorithm. With an optimal lift coefficient of 4.5, the biplane outperformed the monoplane. A 40 m wing span kite was expected to achieve a rated power of about 4.1 MW with a power density of about 52 kW/m2. A parameter sensitivity analysis of the optimized design was performed. Moreover, to demonstrate the feasibility of very high lift coefficients and the validity of a utilized simplified airfoil polar model, CFDs of a proposed high-lift multi-element airfoil were performed and the airfoil polars were recorded. Finally, a planform design of a biplane kite was proposed.

However it is a quote from @floba’s publication, page 2.

The more complete quote is:

Because the power increases cubically with CL and decreases only quadratically with CD,eq, moreover whose major contributor usually is the tether drag, the following Hypothesis was made in this study:
Hypothesis. Maximizing the main wing’s airfoil lift coefficient to or close to its physically feasible maximum, also maximizes the power and energy yield as well as the allowed costs and profit margin of a drag power kite.

I think this is true at least because in the simplified Loyd’s formula : [P = 2/27 aD A w³ CL (CL/CD)² ] , the coefficient of lift (CL) works one factor more.

In practice tether drag is a limiting factor, so that a very high L/D ratio for the kite alone is not an essential parameter. So by taking account of tether drag a very high lift coefficient could lead to a higher efficiency (beside it, a theoretical example of Magnus cylinder in crosswind maneuvers provides a very high value of the lift coefficient, and a relatively low value of the squared glide number).

I think the higher aspect ratio of the biplane configuration leads to a still higher lift coefficient. But I am not sure this feature is fully optimized in this design. I think a balancing between the aspect ratio and the area / span ratio would be to find.