Aluminium rigid wings? + Blade Materials & Erosion

Hi. Why does not any of the rigid wing kites use aluminium for construction? Or even on traditional wind turbine blades? Sure it will be a little heavier than fiberglass or carbon fiber composites, but i imagine it will be cheaper to make, you will not get long term erosion problems, and it is 100% recyclable at the end of the wings lifetime unlike carbon and glass composites.

Wing tip erosion on traditional wind turbine blades:


Given the scale of airframe most AWES teams are aiming for, any weight penalty requires a faster takeoff / higher cut-in / lower capacity factor and is less productive in any given wind.
The energy density of wind doesn’t scale with wing size.

However, you’re right, the established standards of aluminium wing manufacture (apart from the rivets over the skin… eugh) has a lot of positives. If that slight drag penalty is compensated by other performance and cost factors maybe aluminium wings could be used where they are in network arrays of small wing units… Anyone spotting a theme in what I say?

If anyone is in Edinburgh any time soon it may be worth finding out if you can knit a wing from fibre composites instead … https://www.nccuk.com/events/icmac-2020/
The NCCUK focus on wind energy devices is Light-Weighting
https://www.nccuk.com/sectors/energy/

If weight is so critical why are many of the AWE companies putting generators on the wings adding weight and drag to the flying mass?

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Aluminum was used earlier for windmills but had issues with fatigue. Just hearsay, you may search the web for a story from someone knowledgeable.

With regards to placing the generators on the wing, there are two main ways to extract energy from AWE. These are lift and drag mode. Drag mode is only feasible by using an onboard generator or having some sort of torque transfer. You dont have many choices for drag mode AWE. If you opt for lift mode AWE you must make other compromises.

So - rather turn the question back to you, how would you do drag mode awe without the generator?

Strange. It does not seem to be a problem on commercial airliners. They have wings made of aluminium, and they flex up and down all the time. Does windturbine blades have significantly different loads compared to commercial airliner wings? If anything i would think that a windturbine blade has less flexing back and forth than an airplane going trough landing and takeoff cycles and going trough turbulence. While a windturbine blade has a more constant load in one direction bending it “backwards” and centripetal force that to some extent helps to lessen the bend.

Does drag mode make more power than lift mode AWE?

This is typical of the decline of finding accurate information on the internet, Ever notice how, in the last few years, you try to find out some fact and all you get is “Well I would think…” (usually from the same 3 or 4 websites, no matter what your topic!!!)

A major factor holding back AWE seems to be the lack of basic knowledge of wind energy by the wannabe AWE practitioners. Just because they are new to wind energy, they think wind energy is new, and whatever they “would think” must be true.

No, wind energy is not as simple as people think. It is much more of a challenge. That’s why we had a man on the moon before we had a windfarm. Aviation is easier than wind energy.

In the early days of trying to build windfarm-size turbines, of course the first thing tried was using airliner technology to build wind turbines. “Hey let;s get Boeing to build a turbine!” The fact is, wind energy beats blades to death. Just look at the pictures of blade erosion shown in this thread. This happens to all wind turbine blades. I’ll include a picture of some here at our facility after ~10 years of service.Bergey Leading Edge-2
Now think about it - have you ever seen this sort of wear on an airplane wing?
Anyway, while I would not categorically say you could never achieve success with aluminum blades, so far nobody has had much luck. Aluminum, as a metal, is known for fatigue.

What people underestimate is the amount of brutal punishment wind turbine blades undergo, spinning most of the time 24/7/365, with few breaks, usually well over 100 MPH, sometimes over 200 MPH. It is turbulence that really beats them up - think of driving down a road at 160 MPH and hitting bumps - it would rip your car apart. Turbulence rips wind turbines apart. They explode around here on a regular basis. Our small turbines around here have a 22-foot diameter and weigh over half-a ton. (just the turbine, not the tower which weighs a few tons itself). The 11-foot blades weigh 50 lbs each. One near here suddenly has only one blade. Sometimes you see them missing all three blades. Wind turbines don’t just wear out, they are beaten to death. The main factors required in wind energy are reliability, and durability.

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For wind turbines see this: Materials for Wind Turbine Blades: An Overview

Kites do not experience I think this (high and) cyclic (gravitational) loading so perhaps you could do a separate analysis on using metals there, if perhaps only as a covering. I like the idea if it doesn’t add too much weight.

As of now even 7 days of continuous operation is a milestone. I think perhaps they’ll try to get to that and other milestones before worrying about what happens after years of continuous operation without maintenance.

No reason to start with something known to not work even having modest goals like 7 days production. Carbon/composite /sailcloth should be the goto materials for AWE, stuff like aluminum only after careful consideration

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On http://www.magnigyro.it/fr/autogire :

Les pales, réalisées en matériaux composites, sont très robustes et ont une durée de vie quasi illimitée.

Translation: “The blades, made of composite materials, are very robust and have an almost unlimited lifespan.”

As usually an autogyro has aluminium blades (for example http://www.buildagyrocopter.com/sport-rotors-gyroplane-rotorblades/), this company expects an advantage of durability by using composite materials.

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Is the erosion of the leading edge of traditional wind turbine blades because of sand and other abrasive particles in the air? And if so will the higher altitude of AWE minimize or eliminate the problem? Or what about moving it offshore? Will then salt cause erosion? Or will the erosion then be eliminated because of less abrasive particles in the air?

You dont see these erosion problems on the leading edges of commercial airliners, or transport planes even if they are in almost constant use, and at much higher speed than wind turbine blades. Is that because of aluminium being used, or because there is less abrasive particles at higher altitudes?

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Here are some results from trying to find the right terms for it on Wikipedia:

Here you can explore particulates in the atmosphere, as well as other things: https://earth.nullschool.net/ In the menu choose Particulates.

Is it possible to anodize (hard coat) the leading edge of aluminum turbine blades to make them more resistant to erosion? Has anyone done this?

Aluminium kinda anodises itself by forming an external aluminium oxide skin naturally.

From anodising.com

CHARACTERISTICS

The anodic oxide structure originates from the aluminium substrate and is composed entirely of aluminium oxide. This aluminium oxide is not applied to the surface like paint or plating, but is fully integrated with the underlying aluminium substrate, so it cannot chip or peel. It has a highly ordered, porous structure that allows for secondary processes such as colouring and sealing.

HOW IS IT DONE?

Anodising is accomplished by immersing the aluminium into an acid electrolyte solution and passing a current through the aluminium. A cathode is mounted to the inside of the anodising tank; the aluminium acts as an anode, so that oxygen ions are released from the electrolyte to combine with the aluminium atoms at the surface of the part being anodised. Anodising is, therefore, a matter of highly controlled oxidation - the enhancement of a naturally occurring phenomenon.

It can be very pretty
But you will need a big tank of acid

Some search results:

Although the composite material technologies employed boast many advantageous characteristics, they also have some inherent weaknesses and drawbacks, such as performing poorly under transverse impact (i.e. perpendicular to the reinforcement direction) and being sensitive to environmental factors such as heat, moisture, salinity and UV; as will be discussed. To address these weaknesses and environmental sensitivities, a great deal of effort is invested by blade manufacturers and blade material manufacturers in creating effective protective surface coatings [8] [9] [10].

Weigel [27] discussed the importance of utilising an effective leading edge erosion protection system on helicopter rotor devices as well as describing the creation of an new advanced protection system. In order to select an appropriate leading edge protection material, the study evaluates the protection characteristics of a wide range of materials in relation to parameters such as rain and sand erosion resistance (using the Rain Erosion Test Facility at the University of Dayton Research Institute [26]) as well as performance under hydrolysis, impact, UV exposure and salt fog exposure. Weigel [27] identifies that elastomeric materials, such as polyurethanes, can provide superior resistance to solid particle erosion (such as sand) in comparison to metals, and are only outperformed with regards to rain erosion by metals; as a result of poorer polyurethane performance at direct impact angles

On rain erosion: https://www.researchgate.net/profile/Tayfun_Tezduyar/publication/306923225_Computational_analysis_of_wind-turbine_blade_rain_erosion/links/59dd04f1aca27218a760512e/Computational-analysis-of-wind-turbine-blade-rain-erosion.pdf

Talk about ice, insects, and sand: A review of surface engineering issues critical to wind turbine performance

ROTOR BLADE LEADING EDGE EROSION – REAL LIFE EXPERIENCES
When leading edge erosion is not repaired or is repaired incorrectly, the health of the blade and the turbine is jeopardized.

At 160-200 MPH, rain, sleet, and snow cause blade erosion. Dust is another big factor. As you go higher there is less dust, more precipitation and more freezing water. Metal fares better than fiberglass. Aluminum might be made to work, but there have been many attempts at aluminum wind turbine blades, none operating today that I know of. I’ve always thought a metal leading edge strip would be nice. The old Winchargers for operating a radio and maybe a light bulb, used copper foil stapled on to the blades. I think one of the biggest factors regarding metal leading edge strips is securing them to the blade permanently so they do not come loose. When leading edge tape is used, it gets abraded and starts to peel off after a few years. One loose edge at 200 mph sounds like a freight train / huge helicopter trying to land in your neighborhood!

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This conversation made me curious about leading edge erosion of aluminum wings. As pointed out, the speed of jetliners is so much faster than most wind turbine blades, we might expect more erosion. Well, I did find this link to an article about the degradation of leading-edge inflatable neoprene wing de-icing boots:

https://www.aviationpros.com/aircraft/business-general-aviation/article/12341137/deice-boot-maintenance-101-the-why-and-how-of-pneumatic-deice-boot-care-and-repair

Turns out ozone and static electricity buildup are the main culprits.

Regarding erosion from dust: I’ve had hang-glider and paraglider pilots come down from a mile or two in the air in a notorious dust-devil area called Lucerne Valley in California where a series of nonstop huge dust-devils blows through like a freight train during 100-degree+ sunny days in summer. Dry lake beds provide the dust. The dust-devils are also thermals that the pilots can ride upward. They come down with dirt in their teeth and complain about sand and gravel in the air way up there.

With regard to erosion of aluminum leading edge surfaces, I’m not specifically aware of a problem with that per se, although it may exist since aluminum is a rather soft metal. The reason aluminum has never worked out for wind turbine blades, from what I’ve heard anyway. is metal fatigue, in a structural sense.

I know there have been some very large early experimental wind turbines using aircraft-technology-based aluminum blades, but that choice of blade materials did not catch on, with wood/fiberglass becoming the material of choice. Not sure why there is not more in the way of metal leading edges, but there are several factors that may be relevant. One could be a conflict with the coefficient of thermal expansion of various materials - if the metal expands more than fiberglass, it could cause detachment on hot days for example. Also, any imbalance in the rotor from a detached component could damage the turbine. How to attach such a protective leading edge sheath might also be a problem - if water gets behind it and freezes, it might cause issues. Weight might be a factor. Maybe aerodynamic performance could be degraded by the interface between composite and metal.

I do know of one brand that did add metal strips for turbines on islands in the North Sea where the leading edge tape (soft) would start detaching after just a couple of months. The metal strips are screwed on and are now an option with additional cost. That is the same 10 kW model we have at our facility. It is the most well-known 10 kW turbine, but the engineering is lacking, made up for by building it way stronger than it should need to be.

One of many problems with this model is inadequate overspeed protection. They are supposed to furl sideways to control output in strong winds, but instead they tend to first go into a very noisy overspeed phase. Next, the inverter is forced to “let go” of the turbine, which is at that point producing more power than the inverter can handle. So then the turbine is running unloaded and therefore spins even faster, often sounding like a huge helicopter or freight train. At that point, running way faster than it should, it finally develops enough thrust force to furl sideways. But it furls too far, going all the way sideways. At that point the rotor is no longer aimed toward the oncoming wind at all, so it slows to a crawl. The turbine comes almost to a stop, so then, with ~zero thrust loading, it swings back into the wind (facing the wind). But the grid regulations dictate that the inverter go through a five-minute sequence to reconnect to the grid, so the turbine immediately accelerates back up to overspeed, with the blades traveling over 200 MPH, still unloaded. This cycle endlessly repeats until either the storm passes, or the turbine explodes, throwing a blade. We often see this model with missing blades. In one case a blade was thrown over 700 feet. We have one with just a single blade left about a mile from here right now.

There is a manual furling cable that allows you to manually furl the turbine from the base of the tower by turning a hand-winch. But this feature was only added to make some certifying agency happy, and the cables always break, due to a very poor choice of attachment point on the tail, whereby the cable has insufficient leverage compared to its insufficient strength.

So on the one hand, dealers recommend manually furling the turbine when a storm approaches, but on the other hand, you can only do this a few times before the cable breaks. They all break eventually, and the tech support will tell you to just forget about the cable. They can sell you a new one, but it will break just like the last one - a waste of time and money to bother replacing it. Even just fixing a loose edge on leading edge tape costs well over $1000 since the installers need to call in a crane. To fix a piece of tape…

This is the “best” small wind turbine in the world and it is a piece of crap with so many interdependent failure modes that it is only a matter of time til it fails, yet all the standards are written around its flaws because without this under-engineered yet over-built model, there is pretty much no choice out there. The certifying bureaucrats had to allow one model to be certified, or they would be out of a job. It is that simple.

Any other brand is even worse, in some other way. So the state of “small wind” these days is almost a lost cause in light of solar having become so cheap. I see a great opportunity for someone to develop a reliable, affordable small wind product, but so far, nobody has. A decent turbine will be fine in 20 mph winds, but when you get up to 30 mph and above, they all tend to fail long before blade erosion becomes an issue.

One huge problem is the fact that any model requires years of continuous operation in many challenging high-wind locations, before the problems become identifiable. And the recent thrust toward “certification” makes it worse, since it requires a defined model that does not change, and the certification must be paid for so the manufacturer is locked in to whatever bad design they started with.

The whole thing has became a joke, once enough bureaucracies became involved. It is not a happy story. But it could an endless open opportunity if anyone could sufficiently improve the technology to make it as cheap and reliable as solar…

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How about a complete change in methodology? Instead of weathering the high winds, why not avoid them by automatically retracting the system to shield the turbines from high winds. If this can be reliably achieved then this method will be a critical advantage of AWE over conventional wind turbines. Automatically launching and landing should not be difficult to develop considering we have developed self driving vehicles.

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Hi Gordon:
Windfarms do shut down turbines, feather the blades, etc. when extreme winds are imminent. Not just strong winds, but a major windstorm. Small turbines theoretically could be shut down if they had the physical means, assuming a furling cable hadn’t broken for example (which they always do), and you had either some person monitoring weather in real time, ready to shut things down, or some automated system that watched the weather predictions. I furled a 10-kW turbine here a few times when extreme winds were predicted, before the furling cable broke. Still, you end up wasting productive winds if they turn out not to be extreme, then when extreme winds do arrive, your furling cable is already broken. This is the “best” 10 kW small turbine. The manufacturer tells you to just forget about fixing the cable. But the machines do not furl properly on their own, but instead cycle between overspeeding and almost stooping when sideways. Ridiculous really. But I’ve found really extreme wind events are not always predicted, but instead just productive winds are predicted, then you wake up in the morning to see your tower blown across the yard, or maybe the blades thrown a few hundred feet. And when extreme winds are predicted, they may turn out to be just productive winds instead. They don’t always get the predictions exactly right. And extrem winds can be highly localized, almost just random events. I related recently watching a tree blown down by a gust-nado in a residential neighborhood. No other tree was touched. It happened on a very calm day! We stood there in spookily calm air, looking at this very large tree just blown down and split into three pieces right in front of us, with all surrounding trees not distrubed!.
Now a windfarm does have personnel in place to shut things down if needed for some major predicted storm, which is pretty rare since the utility-scale turbines can adjust to very high winds and still keep operating. Then again in the middle of the night, for an unpredicted wind event, maybe nobody is there or able to respond quickly enough.
For small turbines, saying you will shut them down in high winds is usually just a symptom of a newbie who is unaware that your best production occurs right at the edge of when you would want to shut things down, that the actual severity of the wind may not be exactly predicted, and that being in a high wind is not the best time to try to shut down a system.
I know, I know, we’ll just shut it down if things look dangerous. Sure these “easy answers” almost always emanate from the minds of newbies who have never tried to shut down a wind energy system during a high-wind event, or don’t consider that maybe nobody will be around when extreme winds hit - vacation?. Usually it is in response to someone pointing out that they have no real knowledge of wind energy and have no means of overspeed protection in their system. I’ve heard it so many times in the last couple of decades, I’ve lost count. I’ve certainly heard it regarding AWE many times. Everything sounds easy when you don’t really have to do it, just talk about it. Picture this: Your system is making great power, but then you get a little power spike, so you shut things down. But then the wind seems to die down a little so you think “we’re wasting all this power” so you start it up again. But then the wind gets strong again so you shut it down. Over and over you keep taking off then landing, in high winds? Assuming anyone ever develops a worthwhile AWE system, it will probably need a way to respond to overspeed in real time, like ever other wind energy system…
Meanwhile, leading edge erosion happens at all wind speeds, not just in storms. Shutting down everything whenever it gets windy would really not be a good way to address leading edge erosion. It is used for protection of the whole system when major wind events are predicted, but does not work for general overspeed protection in productive winds that contain brief excursions to excessive wind speeds.

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