Air Rotor Testing

[SamoaPower]

It seems that when aircraft propellers or fan blades are mentioned in the context of comparison to wind turbine rotors, there are many who jump on the occasion to cry - "they're different animals!" and are quick to point out the differences.

While indeed, there are differences, some of such differences are simply reciprocal. There are numerous examples in natural science of reciprocity, so that, in it self, it is not a valid reason to rule them out of consideration.

The reason I bring this up is that there is quite a bit of controversy as to what constitutes a "good" rotor. The main reason, in my opinion, that this issue exists, is that it is difficult to measure the relative performance of rotors. Few have access to wind tunnels to provide definitive answers and using the live wind is next to impossible. While some have sought to improvised a simulated wind environment by "moving the rotor" instead of "moving the air" (vehicle testing), the reported results have certainly been questionable at best.

While measurement of "air energy extractors" is difficult, measuring "air energy producers" is relatively straightforward. We can easily measure the input torque and RPM to get input power and measurement of thrust and/or air volume velocity of the output is doable.

It only remains to establish the relationship of these measurements to the question at hand. We really only need a relative basis on which to compare rotors.

Let's have some discussion.

Lets keep it civil, too, OK.


TomW

[TomW]

Samoa;

I really wish you had made this a Diary. Mostly because these blade / wind rotor "discussions" are the R.E. equivalent of discussing religion or politics. By that I mean very little logic is applied by some posters and faith is just too big a part of many opinions on this. Not to mention very few are capable of providing any real personal, reliable data and links to online resources that can be wrong and / or misinterpreted just adds to the disagreement.

I will stay out of it beyond that.

If it gets rowdy I will shut off the comments.

Cheers.

TomW

[southpaw]

My 2 cents worth.  A good rotor is one which turns your generator (alternator) fast enough to reach maximum safe output in the available wind, and is relatively easy to fabricate. A perfect blade which takes months to research and fabricate is in my opinion no better than one whittled out of a 2X8 in an afternoon as long as the juice is flowing into the batteries. The perfect blade may look better and may wiegh 1/2 as much but may be too complex for the average builder. No two turbines are identical and each blade set would have to be completely re-engineered. Sometimes good enough is good enough.

[SamoaPower]

Oh oh, looks like I caught you before your morning coffee Tom. I think you misread my intent in this post. What I'm looking for is a practical way to resolve some of the issues of this topic without resorting to black magic, smoke and mirrors. I'm not looking to start another theoretical fiasco that resolves nothing.

The rotor is such a significant part of our machines that it cries to be understood. The many claims made for this or that configuration simply confuse the issue for many, particularly the newbie. Without a way to measure and compare, it's bound to remain in your defined catagory of "religion and politics".

I'm looking for fact rather than fancy.

If you feel compelled to censor, please feel free.

[TomW]

Samoa;

Oh oh, looks like I caught you before your morning coffee Tom.

Not at all, quite the contrary. My  only Editors' concern is the nastiness that has accompanied other threads on this same basic subject.

I seriously doubt anyone can provide what you ask for but I suspect several will give "opinions". Personally, I find the subject interesting yet somewhat redundant. [See Southpaws' comment in this thread]. Or try this... 5 of the "experts" in the subject read the same scientific report, my bet is you will get at least 3 and maybe 5 interpretations of the same scientific report. Just human nature.

I assure you, sir, that I do not make editorial decisions based on opinions I personally hold on a specific subject. If I did all the theoretical "expert" opinions probably would not remain.

Anyway, enough off topic commenting here. As I said earlier I have no actual experience in this other than flying a few various units over the past 5 years or so. I am quite convinced, however, that improving performance by 5 or 10% can be more easily obtained by adding some length to the prop. That kind of makes over thinking the efficiency a bit redundant in my opinion. But carry on and I hope you find what you seek.

So much for staying out of it...

Cheers.

TomW

[TomW]

paw;

Well said!

Cheers.

TomW

[stop4stuff]

Surely 'moving the rotor' & 'moving the air' amount to the same thing...

Air has a particular density, so if the air is being moved (as in a wind tunnel) or the turbine being moved through the air (mounted on a vehicle) the energy extracted from the turbine in either situation would be very close.

One way to test the closeness between moving air & moving rotor results would be to build a small wind turbine (sans alt) & lift a weight by string wrapped around the turbine rotor spindle... the difference in results would be the difference in time it take to lift the weight the same distance at the same relative-air-to-rotor-airspeed in both situations. This way any arguments/misinformation/whatever will be settled b4 anyone kicks off.

paul

[vawtman]

Samoa

 Did you ever test the blades you made awhile back?

 I thought they were cool.http://www.otherpower.com/images/scimages/5176/Blades1B.jpg

[DanB]

It's tricky stuff..  Good data aquisition seems pretty expensive - both in time and equipment.  Very slight errors in measuring wind speed render data pretty much useless.  

I guess like anything there are lots of tradeoffs and compromises to be made.  In this case we consider... whats most efficient (not sure anyone even knows that for sure) - what's easy to make with given resources (which include time) - what starts well in low winds (which isn't likely to be the most efficient rotor at speed) - what can run fairly well over a range of TSR (because most of these machines do run over a range of TSR I think).

I think that here, we tend to take a pretty simplistic approach to it.  While I can't give really good data on the performance of the wooden rotors we make, I tend to think they're not too far off the mark when I look at the sort of power we're getting at known rpm.  

I also think that wind turbine blades are pretty forgiving.  

So there you have it... lots of opinions and no facts ;-)

[Spdlmt150]

Actual proof!!!!!!!!!

I completely agree with the above comment that "good enough is good enough"

But I can say without a doubt that my third rotor is much more efficient than my first. I don't get into using calipers on a wood carving project, but creating something that more resembled an airfoil certainly improved my output. Aside from that, I think that there are too many variables for anyone without A LOT of high dollar equipment to be able to test accurately.

If it spins up, it's good. If it doesn't, it's firewood.

[southpaw]

When I finally get down to building my first (hopefully not my only) wind turbine it will be because of the great bunch of pioneers and theoriticians on this board and thier successes, failures and great ideas. As long as there are those willing to spend the time on research, testing and trial and error there is nowhere to go but up. Thanks to all who contribute in their own way.

[Sparky01]

I guess I would have to ask if should we try to test and define an air foil or can we just let the big boys (comercial) do the expensive testing and get some independant testing proof on how they perform. There are many diferent types of blades that have been made for years and years on turbines from low to high RPM profiles. I agree that many of our blades are indeed a modified version of that, but unless we can find an accurate way to test, what would seem to be small variances will end up being unuseable data. Indeed yes, I believe that a turbine used to induce an air current in an area that will not have atmospheric errors, and that is carried out by one person could possibly have some real facts. Is there someone that has the available time and resources? Is there enough effeciency to be gained? IMHO I believe that not matter what type of airfoil we are running MPPT (once all the bugs are worked out)will give us all the boost that we all are looking for.

I do believe that all of us working together toward the same goal will achieve greatness.  I do wish there was a place on this site to meet like minded individual in our area.

That is my take anyway-----------------Free RE for Me--------

[ghurd]

From the 1 meter and under peanut gallery...

I make some blades.  They work or they don't.

If they are not fast enough, I cut the ends off.

If they are not powerful enough, I make longer blades.

If I need good blades, I buy some from Rich H.

(and I don't cut the ends off of those)

G-

[TomW]

Sparky;

 I do wish there was a place on this site to meet like minded individual in our area.

In a way thats what the IRC does. Lets you talk one on one or to a group and you can quickly sort out who is close to who. Currently we have regulars from Texas, Colorado, Iowa, Missouri, New Hampshire, Oregon and Illinois for sure. As well as regulars from Australia, The U.K., Scotland [is that U.K. too?] The Netherlands, Canada, Nova Scotia [is that Canada?] and New Zealand.

I probably missed some there but thats just my memory.

Pop on over sometime and check it out. It is Pot Luck on what may be going on but we usually have some good discussions in between the gunfire.

Thats a joke we use knives.

Anyway, everyone is welcome and most of the guys are pretty good folks.

You can just click on the link in my .sig or use an IRC client to go to irc.otherpower.com port 6667 channel #otherpower.

Cheers.

TomW

[kenputer]

Nova Scotia is the wind capital of Canada!!

 Lately there is not a day goes by that the news does not have something to do with more large wind farms looking to set up in our area to get away from the coal powered generating plants.

ken

[SamoaPower]

Well people, out of the 14 comments received so far, only one barely touched on the presented question. I must have phrased it badly or perhaps, Tom's admonishments scared off the thinkers.

To the "good enough is good enough" crowd:

I, for one, am very thankful for the rise in technology and to those responsible for it. I really don't want to go back to driving my car of 30 years ago or to my computer of the same era. If you're content with your "horse and buggy" wind turbine, please continue on and I will continue trying to improve mine. Cp=0.2 indeed!

To those that trivalize the potential improvement:

I believe that 20-30% in rotor output power improvement is realizable for many. If you're not interested in that, please continue on as you were.

To those that think that testing is not affordable:

Exclusive of the prime mover, I can set up instrumentation to measure thrust and RPM for less than US$50.

To those of you that didn't understand the question:

What I'm trying to determine is whether rotor testing as an "air mover"(propellor) is translatable to wind turbine performance.

Thanks to all for at least responding.

[TomW]

Samoa;

or perhaps, Tom's admonishments scared off the thinkers.

I am not all that scary but blame me if it makes you feel better. Apparently you didn't want to hear what one man who has  built and installed many of them thought because I saw a comment from DanB that seemed to also support the "good enough is good enough" philosophy. The comment on accurate testing being nearly impossible for all but a very few of the users, if any, seemed to be lost on you, also.

Anyway. I got nothing to add to the rotor discussion but I can't stand by and let you blame

me for the shortcomings of your thread's responses. I just stated that uncivilized postings like seen on other recent threads on wind rotors would not be tolerated.

No sense in beating a dead horse. I think this one is well and truly beaten by now. At least when it comes to garage builders, most of these folks just need something that works and do not have the resources to pursue experimental testing.

Much money has been spent on this stuff and after those millions you still cannot find a model that everyone agrees on. What makes you think you can do better is my question?

If it trips your trigger I say go for it and hopefully share it and show me I am wrong. Like I said a Diary would have been a much better place for this.

Cheers.

TomW

[wdyasq]

Samoa,

When I get to the point of testing alternators and blades - I'll think about rigging something up. I guess the initial thinking should be how to hang and drive such a thing.

In your opinion, could one learn anything by building a scale set of blades and run the Reynolds numbers up to near actual? Or, will this testing need to be full size blades and full size power?

I also think there is about 25-35% power left 'on the table' due to poor blade design . Much of this has to occur at the low end of the wind range where the power is most needed. It is a shame no one is experimenting with the airfoils paid for by the US taxpayer.

Ron

[Countryboy]

I'm old school - if it ain't broke, don't fix it.  If you're getting usable power from less than perfect blades, keep using them.

I think if someone really wanted to test different airfoils, one of the easiest ways would be to make several sets of 4 foot blades for an Ametek, and then compare performance data.

[kamikaze]

Hi Samoa,

I know its been tried before, and that its less than perfect, but I intend to test my latest blade setup by careful mounting on the back of a ute, hopefully within a week.

My new cnc mill is due to arrive at the end of this month, and I will attempt to carve a number of blades of varying shapes based on my own best guess. For me achieving good efficiency and superquiet running are the goals. I would like to try a turned up tip, winglet style as well, to see how it performs.

 If the ute mounting is not suitable, then I have enquired about a wind tunnel of suitable size, costing around $2000 for a day.  I sure hope the ute works out.

Cheers,

[hvirtane]

Hi,

in a way I'm thinking that this is a good subject. I'm one of those who are thinking that there might be quite a lot room for improvements.

I'm as well thinking that there might be something quite fundamentally wrong with the idea of using simple Betz maximum theoretical efficiency at each stage of the blade and to build the blade according to that calculation.

- Hannu

[SamoaPower]

Paul,

I think the problem with vehicle testing is that the air flow is effected by the vehicle itself, even if the rotor is mounted on the front. We tend to forget that a body in an air flow effects the flow some distance in front of and to the rear.

Other issues are accurate speed measurement and local winds. Also, to measure the output of a rotor alone would need some sort of Prony brake. Measureing the output of an alternator would require accurate characterization of the alternator to determine the rotor parameters.

I'm looking for a simpler way.

[SamoaPower]

Ron,

Yes, the test set-up certainly deserves consideration. I notice that quite a few set up a short test tower which, of course, could be used.

The issue of the prime mover isn't much different than what's needed for alternator testing.

Certainly testing of scale models in wind tunnels is routinely done for airfoils and aircraft. I think the correction factors are more involved than simply accounting for Reynolds number but I have no detailed knowledge. Personally, I'd feel more confident in full scale testing.

[SamoaPower]

kamikaze,

I'm not sure what an ute is but assume it's some sort of vehicle. See my comment above on the problems with vehicle testing.

Yes, this is the problem. Vehicle testing has its problems and wind tunnels are beyond the reach of most of us.

That's why I'm looking for another way.

[SamoaPower]

vawtman,

Yes, that rotor was tested some 20 odd years ago when it was made. Cp=0.42.

[bob golding]

my take on this,based on very little experiance, is that the dynamic range we are expecting our turbines to work in is so great that it is almost impossable to make a blade that will be effcient over the whole range. i think, but dont know for certain, that the large utilitity turbines are run at slow speed even when there is no wind to keep them effciecient at ideal wind speeds. that might work for large turbines but for the sort we are making it is better to gather the data on wind speed and direction over a long period of time and then design the turbine for that location. in my own case i could do with longer blades in the summer than the winter. i might build some longer ones and try them over the summer. but there is always the danger that a summer storm might come up and wreck them. i think having a larger alternator and having some electronic control would prove more fruitfull. thats the route i am thinking of going anyway. for now my blades get better every time i have to make another set. i rely on my turbine and the longer it is down the more gas i have to buy,so i tend to make my blades quickly. i am on my third set so far and they work fine as long as i dont do somehing stupid like getting them caught in the guys,twice!@!@! hit by the tail,once,and forgetting to put the back rope on when lifting the tower,once and never again. had i hit reverse hard while watching the whole thing falling towards the truck. all these senarios are in hughs book i noticed...afterwards.

have fun

bob golding

[scorman]

I will respond to your specific initial questions.

Firstly, when driving a turbine rotor with a motor, the dynamics are completely different. As with any "fan blade", the rotor will produce swirling vortexes and the wind thus generated will not be uniform in cross-section (big hole in middle) and will spread out do to pressure difference vs the adjacent atmosphereic conditions. The air behind the rotor is dead still and you are now accelerating it to full speed by inputting a known amount of electrical energy. This is NOT to say that different blade geometries ie taper, twist, profile, cord dimension won't require different electrical juice, just that to correlate the data is unlikely to determine which one works the best in reverse with the wind blowing to generate electric power.

Back to running a rotor in a wind, some of the key features of any blade configuration has to do with air that is wasted by throwing it off the tips and also creating vortexes behind and the friction caused by drag at the highest velocities which detract from the lift which produces power.

I am in the middle of an experiment using a 12inch bench model and an artificial wind source ( not perfected yet) capable of 30mph WS. I have created about a dozen different blade designs and configurations and have not drawn any conclusions yet, other than thick blades at the tip don't work.

Hugh Piggot claims in his tutorial that flat, untwisted, and non-tapered blades may not be "ultimate designs", but there is surprisingly little to gain for fixed pitch, stall regulated, variable rotation spedd turbines.

You only have to check out the Bergey XL-1  to reach that same conclusion:

from an older thread "This is the current Bergey XL.1 airfoil. No twist. No taper....

but they work!

"Constant pitch with a little bit of the trailing tip cut off."

http://www.otherpower.com/images/scimages/110/xl1_airfoil_profile.JPG

That said, it is difficult to model a design and then blow it up to full size.

One reason is simple geometry. The airfoil analysis programs like JavaFoil show performance as a function of RE#, but RE# linearly depends upon WS and cord width.

TSR determines AOA and is independant of WS, so if you have a TSR=6, then a 12 inch rotor tip will spin at the same speed in mph as a 12 foot rotor (but 12x rpm), BUT the cord width is only 1/12 to scale it properly.

So, if in a 15mph WS and the cord of a real rotor is 5 inches (like Bergey), RE# is 360,000 , but the 12" model only has RE# = 30,000 ( and it goes down from there at lower stations!).

BTW, here is an interesting link to model testing/modeling being done at Clarkson Univ upstate NY:

http://www.clarkson.edu/honors/research/papers/Rector-M.%20Curtis.doc

Their interest is similar to mine in that higher pitch angle, higher solidity resulting in lower TSR, can result in higher efficiency for low WS conditions such as 12mph avg WS. I can note that Hugh indicated this premise in his charts and equations, but drew an different conclusion for his ultimate design.

I am almost ready to fly a 10+ footer with two rotors and 6 blades. I will update my progress when I have some data to publish.

I have some photos listed in my profile including most recently the finished blades

Stew Corman from sunny Endicott

[finnsawyer]

The maximum efficiency of an air plane propeller is supposedly 83% while the best efficiency that a windmill blade can provide is 59%.  That's quite a difference.  Doesn't that say that there is a fundamental difference in the physics that govern the two situations?  Well, it's fairly obvious that for the propeller nearly all of the lift force is in the desired direction while for the windmill blade only a small fraction of the lift is in the direction of rotation.  So, the lift to drag ratio of the propeller will always be better than that of the wind mill, meaning that it can operate closer to its ultimate limit (100%?) than the wind mill blade (59%).

Another factor is that propellers rotate faster than wind mills.  But in dimensions they can be similar.  You need to be sure that the air flow in both cases acts in the same fashion, namely laminar.  It seems that laminar flow is possible in a wind mill blade as wind tunnel tests are supposed to have shown that for small dimension air foils the flow was laminar, whereas in the case of full scale airplane wings the flow was laminar for only about six inches, becoming turbulent after that.

If the air flow across a wind mill blade is laminar there are a couple of things we can say about it.  The first is that the forces due to pressure that accelerate the air will be in the direction of the air streams just as in the case of flows obeying Bernoulli's Equation.  The forces acting on the flow that take energy or power from the air stream will be at right angles to the air stream.  That is, tending to either squeeze or expand the air streams.  This comes from the geometry of the blade or air foil.  Another interesting thing we can say about the flow is that when two identical stream lines separate at the leading edge of a windmill blade they will travel a shorter distance to reach the trailing edge than in the case of a blade at rest, due to the motion of the blade.  That's assuming the flow doesn't separate from the blade along the windward side.  I don't know whether these observations would be of any use, but they are interesting.

In wind tunnel tests, air speeds across air foils are measured from pressure differentials using multistage manometers.  These devices are available commercially, but one could make their own if they are into glass blowing and working with mercury.  I did that sort of thing once during a summer job, but I have no interest in fooling with mercury today.  Water could used, but its density may be too low.  If the wind speeds don't change too fast these devices could be used in the field.  The main problem, however, is that one should determine pressure profiles across the wind mill blades when rotating.  Needless to say that would be an engineering challenge.      

[tecker]

It is all an Rpm issue You can"t move enough air to propell 1000 lbs or better with a wide foil or long blade like a low speed wind rotor uses with all the stress . So you go into the 8000 to 12000 rpm range and use foils that shear off large chunks of air to achieve thrust and compression. These smaller foils just idle along in when air moves across them at 10 to 60 mph .  

[Seaspray0]

It seems to me that the propeller design has a much simpler problem to solve.  The engineer knows that he wants a certain thrust for a given propeller speed (operating speed) and designs it for that "ideal" condition.  On the other hand, we can't make the wind blow a specific speed so we don't have an "ideal" condition to design towards.  The prop on a wind generator has to work well in variable conditions... one size fits all.  You can't get the same efficiency.  Just look at the america's cup boats.  They have multiple sets of sails for multiple wind conditions.  One set is great until the wind speed changes.  What are you going to do?  Change your props everytime the weather changes?  I'll place my bets on the prop designs Tom and the others have.  If you can't be perfect, atleast keep it simple, cheap, and easy to make.  Remember, the whole idea is to have the best cost per kwatt, and Tom and the others here are the leaders in that field.

[SamoaPower]

Many thanks Stew. This is the type of discussion I hoped to provoke rather than the cries of the non-techies saying that we shouldn't bother.

I first became interested in this question when perusing a report from Florida State on improved ceiling fan blade design.

http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1059-99/index.htm

One thing that immediately struck me was the similarity of their design to contemporary wind turbine blade design. This suggested that, perhaps we could work the problem backwards, at least on a relative basis if not absolute. If their testing of fans resulted in a design that is also suitable for a wind rotor, couldn't we also use a fan testing protocol, or something similar, to compare rotor designs? I know this a bit of a stretch, but I think it's worth pursuing since conventional rotor testing is so difficult.

I certainly agree that the dynamics of the two cases are very different. However, if the intent is relative comparison of the result of rotor/blade parameter changes, and the sign of the tested result of those changes is the same for both testing protocols, then I think we may have something useful. Are you aware of any particular parameter that this would not hold true for?

I find the rest of your comment quite fascinating and commend your efforts.

[Jerry]

Hi SamoaPower.

You wanted an active discusion on this subject and it looks like you got it.

I see by the above discusion here many folks here have some very good book lernin.

There are some very large words and scientest type being mentioned I've never heard of.

So as you can tell this places me at the bottom of the wind generater propeler food chain.

I've been threatening to build a wind tunnel for several years now. But so far its just been talk.

However, I did get started a few days ago going into the hunting and gathering mode for the wind tunnel parts.

I will build this thing, I will test this thing, then the experts can tell what I'm doing wrong.

My first mistake may be that its actuly not a tunnel at all. Not yet any way.

I'll be using 2 each 2 hp 1725 rpm variable speed DC motor to drive 4 each 24" 4 blade air mover blades.

I'm building all this into that 24"X48" garden wagon Lowes has on special now at $67.98.

I need to have this thing movable. While testing I'll be using it under my large covered patio deck area. My wfe dose not want it to be a permenate part of the deck. So I'm makeing it mobil.

I'll support it at one end of the deck and station the blade/genny 24 ft away at the other end.

I know there'll be some type of turbulance or vortisies but my mane goal here is to evaluate several diferant styles of 4ft blades. So they'll all be exposed to the same moveing air, good or bad.

I'll do some picture and a report in another post later.

I was kinda draging my feet on this project but you've got me hyped up again.

Thanks.  This thing may be primitive but it will be intresting even if the info is not quite accurate.

                           JK TAS Jerry

[elvin1949]

TomW

 Well said.

later

elvin