Author Topic: How to determine how good blades are (Read 5395 times)

willib · « **on:** July 06, 2006, 01:18:05 AM »

i was thinking of ways to test out my blades.

What parameters should i use to test them

what i have to work with is

a 16" fan

a mini gen

a 0 to 20 N scale

scope to measure rpm

multimeters

i was thinking of a no load test vs. a loaded test.

but i dont want the measly gen output to detract from the focus of the experiment.

the epoxy glass combo added about 1N weight to the blades

they weigh about 1.15 N without the epoxy glass and about 2.15N with the epoxy glass

http://www.digitaldutch.com/unitconverter/

sorry about the weird weight scale but thats all i have to measure weight.

1.15N converts to 0.26 lbs i think.

so each blade weighs a 1/4 lb , before epoxy glass, 0.48lbs after , and that includes the wooden hub mount, four 1/4 20 nuts about three inches of 1/4 20 threaded rod , and four lock washers and four flat washers.

getting back to testing

since i only have one epoxy glassed i had to add weight to the other two.

so i found two metal objects about 1N in weight and got them balanced.

how should i test them.

i think , that a 200mA load is the same , no matter what the alternator parameters are?

when testing these before (pre glass) i got about 1.5A out of them , the tips were bent back a couple inches , which is one reason i went and made the shells for them.

willib · « **Reply #1 on:** July 05, 2006, 09:15:15 PM »

one of those weights just flew off .lol

Murlin · « **Reply #2 on:** July 06, 2006, 05:40:32 AM »

Thats a real cool machine you have there.....

I been following your blade design, nice job.

Variable pitch?

Murlin

SmoggyTurnip · « **Reply #3 on:** July 06, 2006, 06:59:50 AM »

I think you should do a set of power curves for them.

Connect a pot to each phase of the generator.

Use the pots to change the rpm by adjusting the load.

Plot power verses rpm for each wind speed.

.

kitno455 · « **Reply #4 on:** July 06, 2006, 08:16:33 AM »

with a spring scale and rpm measure, a home built prony brake can tell you the power, if you can calibrate it.

allan

SamoaPower · « **Reply #5 on:** July 06, 2006, 05:11:30 PM »

Well willib, I think you have a number of problems. Is it just the picture or my eyes, but do all three blades have a different planform? Why would you want to test a rotor with three different blades? The results wouldn't mean much and vibration would be an issue.

"i was thinking of a no load test vs. a loaded test."

I don't think an unloaded test tells you anything usefull other than it rotates. My grandson's pin wheel does that.

"a 16" fan"

You can't use a small fan on a rotor that's much larger in diameter and get anything usefull. Short of a wind tunnel or a much larger fan (questionable because of swirl), I'm afraid you're stuck with the wind.

"i think , that a 200mA load is the same , no matter what the alternator parameters are?" Wrong. Measuring power out of an alternator requires knowledge of the alternator efficiency vs. output to extract the rotor power.

You can't put balancing weights on the airfoil surface. It will alter the airfoil lift.

Kitno455 has the right idea. Set up a Prony brake to measure the rotor torque. Apply a variable load to the alternator and record torque, RPM and windspeed (you DO need an anemometer in any case). From that you can extract the rotor performance.

In my opinion, you're in for a disappointment. Blades with those tremendous roots are going to have loads of drag. I'd be very surprised if you exceed 20% rotor efficiency and a TSR of two.

I think you've gone wrong from the start by taking the results from a blade calculator and then only using a section of that result. Don't think you can do that. You keep quoting a TSR of 6 which is probably what you plugged into the calculator but by only using a piece of that design, the TSR goes out the window.

Did you ever come up with the performance data for that airfoil?

willib · « **Reply #6 on:** July 06, 2006, 07:46:39 PM »

this is what i got.

http://www.otherpower.com/images/scimages/2965/line_299DC3A8.jpeg

this is with a Y connected load of three 1 ohm resistors.

its all i had.

i could maybe scrounge up some 1/2 ohm resistors .

i varied the wind as seen by the blades

i'm not sure what this means , but i think the blades are stronger than the alt, because when i short out all three phases together , it still turns.

SamoaPower · « **Reply #7 on:** July 06, 2006, 09:52:45 PM »

I thought this post was about measuring rotor performance.

Your graph only says something about the alternator since you didn't supply wind speed.

As a reference point, an area of 9.6 sq ft (3.5 ft diameter) has an available wind power of 49 watts at 10 mph.

willib · « **Reply #8 on:** July 06, 2006, 11:06:00 PM »

Wind speed ... i am using a fan.

i do not have a wind gauge.

49 Watts !! on what planet?

try 20

SamoaPower · « **Reply #9 on:** July 07, 2006, 02:39:42 AM »

"Wind speed ... i am using a fan."

Doesn't work, period!

"i do not have a wind gauge."

There is no meaningful measurement you can make on rotor performance without knowing wind velocity.

"49 Watts !! on what planet?

try 20"

Wind Power (Watts)= 0.5 x rho x V^3 x A x K

where:

rho = air density in slugs/ft^3 = .002378 (nominal)

V = wind velocity in mph = 10

A = swept area in ft^2 = 9.6

K = units factor = 4.31

Pw = 0.5 x .002378 x 1000 x 9.6 x 4.31 = 49.2 watts

This is the power in the wind. Not the power out of the rotor or out of an alternator. This is what you have to start with. For a Cp=0.2 (overall efficiency), you would see about 10 watts.

Willib, do you really want help?

Flux · « **Reply #10 on:** July 07, 2006, 03:27:38 AM »

I was tempted to keep out of this, but I have to agree with Samoapower.

Unless you have a large wind tunnel or have a decent anemometer with data aquisition system and a means of taking 1 min or 10 min averages of power over a long period and sorting the results into bins, you might as well not bother.

The only thing that you could do is to try direct comparisons between blades with identical alternators as loads, in close proximity at the same time.

From the pictures it looks as though your chord gives you a tsr of about 3. I have no idea of what it is form the angles as I don't know what they are.

The prony brake is fine in a wind tunnel, but no use in real life wind. You need a lot of skill and knowledge to make any sense from a simple fan. At least try to make some form of wind tunnel out of it before wasting effort.

Your trailing edge looks to be thick but that may be a photographic problem.

There is little that most people can do about blade design and testing with the facilities available. Some useful work has been done on blade profiles for wind power, mostly for large machines and not relevant to small ones. Some design has been done on profiles for small blades and if you can get hold of those profiles and use them as intended they should be fine. Use them not as intended and all the benefit goes out of the window.

Many promising sections that have worked well for aircraft fail to show benefit for wind power.

Unless you are quite skilled in this field or have exceptional facilities you will not do any better than the type of blades that DanB or Hugh Piggott use unless you are extremely lucky and the result could be much worse.

Your ideas on alternator design are odd but are easily verified. By all means keep posting your blade ideas, but I would advise others to ignore it until we have genuine proof of results.

I try not to be negative in my comments but there are times when it seems to be necessary to prevent others with limited experience thinking that they are reading something useful.

Flux

Flux · « **Reply #11 on:** July 07, 2006, 06:04:13 AM »

Normally when I write something like this I have a cup of tea, read it again and delete it. The kettle wasn't boiling this time.

My comments were a little harsh, but when I find someone trying to determine blade output with no means of measuring wind speed it causes rash thinking.

Probably the only useful way for most people to even attempt something like this is with vehicle testing. It always leads to optimistic results but done carefully with a small machine in front and high up it should give reasonable comparative results.

It should be possible to calibrate a decent pm dc motor so you have a good idea of power in from power out. A decent size dual rotor alternator should also be reasonably easy to calibrate. Failing that an induction motor with dc applied to one winding could be calibrated as an eddy current dynamometer.

This method has to be done on calm days and they are rare, make sure you put up another wind generator to keep the wind away for a week.

If you are prepared to take this seriously then we could all benefit, but these are the most difficult tests you are likely to make with wind power and unless the results at the end can be trusted then it is a no go area.

Sorry for the rant earlier.

Flux

willib · « **Reply #12 on:** July 07, 2006, 07:41:42 AM »

thats ok rant away , if you like .

it is a relief from mr. negativity from samoa.

ghurd · « **Reply #13 on:** July 07, 2006, 08:13:44 AM »

I have a wind meter, and can say a fan never worked for me.

Off the top of my head (meaning probably wrong), the meter reads 16MPH in front of the fan. The blades turned 60RPM. I gave up on those blades. Lucky I was to lazy to dismantle the assembly.

The wind picked up a couple days later. Enough to register on the wind meter, so 6MPH or more. Maybe 6 to 8 MPH.

Took the blades outside.

They worked quite well... Outside, in real wind.

Sounds like you need a free NIB RatShak hand held wind meter. At Yahoo mail me, ghurd1.

(sorry guys, last one)

G-

willib · « **Reply #14 on:** July 07, 2006, 08:34:52 AM »

"From the pictures it looks as though your chord gives you a tsr of about 3. I have no idea of what it is from the angles as I don't know what they are."

station # , angle , chord length

, 42 , 13"
, 23 , 10"
, 14 . 7.2"
, 9.5 , 6.2"
, 6.5 , 5.3"
, 4.5 , 4.5"
, 3 , 4"
, 1.9 , 3.8"
, 1 , 3.3"
, .3 , 2.6"

i took some liberties in my designs as most people do.

what i did was i took the chord lengths from a two blade design and incorporated it into a three blade design, this gives me a stout blade , with more airfoil surface area

i also neglected the blade calculator's choice of a root section , 20" was a little too extreme , even for me. so i made it a more reasonable 13"

WXYZCIENCE · « **Reply #15 on:** July 07, 2006, 10:54:05 AM »

Willib, looking at the first picture I finally figured where I have seen it before. Looking out the window of a Boeing 737. Should be able to do about 600 mph. Joe

willib · « **Reply #16 on:** July 07, 2006, 01:07:33 PM »

I sent you an email , maybe we could trade

Flux · « **Reply #17 on:** July 07, 2006, 02:06:10 PM »

Interesting.

Not sure what angle of attack and CL you chose but from the angle on the outer part where it matters you are nearer tsr 8 or a bit higher.

If you used the chord for 2 blades they you should be 50% high on the values for tsr6 if that is what you aimed for. Your actual figures are 2 to 3 times what I would expect.

Flux

willib · « **Reply #18 on:** July 07, 2006, 02:41:08 PM »

Really?

thats not what warlock blade calc says

those angle numbers are for a standard TSR 6 blade,i believe

let me check the other blade calculator, i've looked at altons before and got similar angle numbers from both, i'll check again

willib · « **Reply #19 on:** July 07, 2006, 02:46:55 PM »

they match perfectly if you choose an AOA of 6 degrees in altons ..

wdyasq · « **Reply #20 on:** July 07, 2006, 02:50:50 PM »

Flux,

I have studied rotor dynamics for years. I'll agree with you. I personally don't think enough reality is shown to folks trying to reinvent the wheel. Some have little knowledge of structures. Many have no knowledge of aerodynamics. Most have knowledge of neither, don't know how to go about getting useful results and only want to beat their own chest and tell others how to do stuff.

I will bet there are tens of thousands of pages of information on small wind turbine blade design on the internet. I know I have read through thousands of pages of reports on just HVAT stuff.

The problems with the VAWT and the inability for some of the best and very well funded scientists in the world to solve the prolems of the HVAT over many tries and years convinced me I don't need to be doing research there. Although I will admit the Gorlov style shows some promise.

Well, good luck willib - and let us know when you figure out the engineering dynamics of glass-fiber/foam blades.

Ron

willib · « **Reply #21 on:** July 07, 2006, 02:57:16 PM »

Flux , thats why i asked this question back in May .

http://www.fieldlines.com/story/2006/5/27/162956/613

Flux · « **Reply #22 on:** July 07, 2006, 03:43:27 PM »

Yes you are right if you use 6 deg as the angle of attack. I used 4 deg.

six deg should still be perfectly acceptable, I never tried 6 deg.

Still disagree with your chord by a large amount, I think you must be using a very low value of CL even allowing for your choosing the values for 2 blade.

Your chord seems far too much at the tip. It may look worse because you have gone to the extremes at the root that the calculators imply.

I admit that my blades always want to run below the design tsr in higher winds, but seem happy at about 12 mph. I always assumed that the drag was increasing at the higher speeds but perhaps using AOA of 5 or 6 deg would be better.

I have also found that you can increase the chord a significant amount without trouble, but I think you have gone too far.

If you can manage meaningful tests then it would certainly be a good start.

Flux

SamoaPower · « **Reply #23 on:** July 07, 2006, 05:43:04 PM »

Flux,

I concur about his chords.

I disagree however, about how to select the AOA. I believe willib used 6 degrees because that's the default in the calculator rather than the data for his airfoil. For best efficiency, the AOA should be that which occurs at maximum L/D for the chosen airfoil. For a particular airfoil I use, a change in AOA from 4 to 6 degrees results in a CL change of 0.94 to 1.18, which is more than 25%.

There is also a AOA correction factor based on the blade aspect ratio that hopefully, the calculator takes into account. Like you pointed out before, the calculators are just a starting point.

I don't believe that the AOA should be chosen arbitrarily, even over a narrow range of 2 degrees.

wdyasq · « **Reply #24 on:** July 07, 2006, 07:02:00 PM »

airfoils vary in proper AOA....

Ron

SamoaPower · « **Reply #25 on:** July 07, 2006, 10:16:39 PM »

Agreed, Ron. That's why having the polar plots for an airfoil is necessary.

SamoaPower · « **Reply #26 on:** July 08, 2006, 04:06:10 PM »

Flux,

What do you see as problems with using a Prony brake in real wind? I haven't tried it yet but was considering doing so with my 16 ft rotor.

I planned to use a strain gauge for the torque data and sampling wind speed and torque at a few points per second rate with the load ramping at a slower rate, perhaps 10 seconds. The computer can then sort it out and apply averaging as needed.

Am I missing something?

Flux · « **Reply #27 on:** July 09, 2006, 01:14:38 AM »

I thought you were dealing with the conventional form of prony brake where you have to read spring balances (scales I think you call them). If you use strain sensors to measure the torque you could make it respond fast enough. It will still need a lot of cunning to be able to remotely alter the load.

If you can make it work remotely and you can sample load with the wind speed at short intervals it will work. Not sure how you are intending to provide the load, by a friction clamp or a rope round a drum. The rope schemes need 2 sensors for tension.

Any drag device to produce load will work. If you have a strain gage to measure the torque, some means of measuring shaft speed and wind speed at the same instant you can do it.

Are you familiar with strain gages, it is another field that is in theory simple but unless you know the traps you can have a lot of trouble. Commercial force transducers take the variables out but at a high cost.

Flux

willib · « **Reply #28 on:** July 09, 2006, 02:02:34 AM »

how about this , i was thinking about making the stator movable , between two set points. this way the drag on the rotors would depend on the electrical load on the stator , and not physically touching the rotors at all.

and i could use a lever arm with my 20 Newton spring gauge

Flux · « **Reply #29 on:** July 09, 2006, 01:33:20 PM »

Can't quite follow your description, are you still on about the prony brake or using your alternator,

If you mean what I think and you are measuring the torque reaction on your stator then yes that is fine, that is the standard dynamometer scheme and from torque and speed you measure power. If you are vehicle testing then you can use your spring balance.

It will not work in live wind conditions, the thing will never settle, wind is too wild. you would need a fast responding torque measurement that will not oscillate at a low frequency like a mass spring system. I suspect that you have under estimated the instantaneous changes in wind speed. It may work with a damper such as a dashpot.

Flux

willib · « **Reply #30 on:** July 09, 2006, 03:56:41 PM »

"If you mean what I think and you are measuring the torque reaction on your stator then yes that is fine, that is the standard dynamometer scheme"

Yes Flux , that is what i meant.

would the stator have to be able stop the blades , when shorted , to make this test more meaningful?

because it doesnt ..

and since i've changed the AOA to its proper value , its ability to stop the blades has dimished.

they used to run at 17 RPM with all three phases shorted

now they run at 33 RPM with all three phases shorted

this is with the fan on high

SamoaPower · « **Reply #31 on:** July 09, 2006, 04:16:01 PM »

I agree that spring balances would not be useful in live wind.

Perhaps I didn't use the correct terminology in saying strain gage and should have said load cell, although many load cells are made with strain gages.

I've considered two ways to apply controlled variable load. One is using a trailer electric brake which allows you to smoothly apply braking force electrically and remotely. This would substitute for the hub assembly in the typical axial flux design. Reaction torque could then be measured from the backing plate with a load cell. It could be built into the machine permanently and provide an additional brake.

The second way is the same as what you have done for alternator measurements. Allowing the stator a little freedom in rotation (very little is needed for the load cell) and measure reaction torque there with the alternator load being varied remotely. I think this could be quite useful in setting up a pitch control algorithm as well.

Data collection of torque, RPM, wind speed and load can be done with an inexpensive data logger. A four channel Hobo is about $100 and various sampling rates can be set. I'm also considering setting this up permanently.

An inexpensive source for the load cell and electronics are the electronic bathroom scales widely available, many for under $50. Accuracy of a few percent is reasonable.

Flux · « **Reply #32 on:** July 10, 2006, 02:23:24 AM »

Can I answer Samoa and willib in the same post.

Willib, your alternator should be able to at least load the blades down well into stall, it need not be more powerful than that as you are looking for performance at normal power, the stall characteristics are of little use below the reasonably flat peak.

Samoa, yes I agree with the use of domestic scales for the force transducer as long as you keep the load in exactly the same place and direction. Commercial force transducers are rediculously expensive for a few tests. It is likely the use of silicon strain sensors and mass production that make the domestic things available at a reasonable price, they seem reasonably accurate but the accuracy may fall if the force is not applied in the same way. Would be easy to calibrate it with the final assembly.

Not familiar with the trailer brake, if it is consistent and can be asdusted easily it should work. Most electrical brakes I have met are on or off.

Flux

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Author Topic: How to determine how good blades are (Read 5395 times)