This about that

[Stewart Corman]

I have been organizing thoughts and have made a few posts, but Rob asked me to open a new thread and present where I am at with HAWT (remember that this is a "work in progress"):

note: I am a mechanical engineer by trade and reliability testing, experimentation design was my expertise

basically, 3 bladed, stall regulated, simple blades based upon NACA4425 ...non-twist with high pitch angle

my "paradigm" is here:
http://www.geocities.com/scorman/turbine.html
_______________________________
I have pics and references at:
http://s145.photobucket.com/albums/r203/scorman1/

The "experiment" is a 12inch benchtop setup.
Artificial wind creation has been a problem , but I am making progress (photos soon to be posted in above link)
Replication by molding of blades is a another difficulty.
details: http://www.geocities.com/scorman/experiment.html

"Wind" is ref section with the orig Allison Pop Science 1980 article on patented helix and a few more bits of info

"Turbine" section is photos of assembly pieces/fabrication of 10+ft experimental turbine ..it will have single/multiple rotors and adjustable (not dynamic adjustable) pitch
__________________________

I have attached a "calculator" as an Excel file (sheet 1), so you can put in any WS, TSR, Diameter (replace the "?") and derive rpm, power avail, etc

blade calculations for diff stations for AOA and RE# are on sheet 2
__________________________________________
I will assemble a listing of reference links soon, categorized by subject matter.

Ultimate goal is a stall regulated 15 or 18foot multirotor helix turbine with three blades per rotor and two rotors capable of generating 2KW in low WS ie 10mph avg and 12.5mph rated design ...mechanical steering at 20mph.

All serious discussions encouraged (but not VAWT here!)

Stew Corman from sunny Endicott

[Rob Beckers]

Good stuff Stew!
Where is the scale experiment at right now? Did you built a multi-vane prop vs. a regular 2 or 3 bladed prop? What's the issue with producing 'wind'? If nothing else, you could mount it on a car.

-Rob-

[Stewart Corman]

there is an outdoor photo of the 12 inch model in my link showing three white wood carved blades ...the shaft has a locking collar, so that additional rotors/spacers can be easily added .. (new photos soon)
I need to replicate the blades more exactly if I need a minimum of 9 samples,
so I was thinking of casting them ...first attempt with plaster mold was marginal at best ...even tried lining it with sacrificial aluminum foil...I may try to roll form a SS sheet metal mold, rather than hollow out a hunk of aluminum. Want a smooth surface that can also be durable when molded parts are removed.

I just calibrated the 12v Pitman DC servo motor in the lathe with no load,10,20,40 ohms at each of four pulley speeds and calibrated the lathe speed with digital tach ..all readings voltage readings are very linear (as expected) ... I have already obtained rotation of the three blade rotor in excess of 2000rpm at 40mph

the problem with artificial wind source is uniformity across the field w/o vortexes cause by the propeller blade, and sufficiently wide to span the 12 inch models. I am using a Velometer 6000 with a 12 inch pitot tube to measure the wind at specific points in the plane of the rotor.
http://www.alnor.com/products/velometer.aspx

If nothing else, you could mount it on a car.

duh, NO ...it is winter here too and we are expecting 12->18 inches of snow
I need to work in my nice warm cozy basement, when I want, how I want

there was a Mythbusters program on TV that made a wind tunnel to bench test a high speed simulation of a train using a Lionel model. They created a diffuser by stacking soda straws in a fixture in front of a fan and got fairly laminar results.
I am duplicating that efforts and will try to incorporate a smoke trail.
Current setup will go up to about 30mph WS.

Once I get the outside experimental unit up and running, I will set the pitch at nominal numbers dictated by Allison ....but the indoor model will allow me to tweak the numbers for better power/stall-regulated parameters.

The model will NOT be determining projected efficiency,TSR, rpm, or power capability, BUT, it will compare all of the respective combos for those performance characteristics ...validation of experimental numbers can be easily verified by outdoor free air measurements on a stand on a windy day in the spring.

Remember, there are no shortcuts as the NIH crowd will try to jump on any conclusions that conflict with their paradigm.

Stew Corman from sunny Endicott

[Rob Beckers]

:D I didn't say you have to hold that turbine out of your car window!
A nice roof mount. Install the whole thing in the garage, nice and warm, and you're off to the races...

You would need a whopper of a fan to make the wind speeds (and as you note, laminar flow, which means large cross-section) to do something useful for turbine testing. You can't very well duct the flow either, that distorts the results by forcing the air through the turbine. Car seems so much easier.

Spring is just around the corner (With -24C here today, look who's talking)...

I want to say though that I'm impressed with your very structured and scientific approach to doing these experiments. It'll be interesting to see the results!

-Rob-

[Stewart Corman]

http://s145.photobucket.com/albums/r203/scorman1/Experiment/

[Mark Parsons]

Greetings Stewart,

Looking forward to seeing some data that you collect with your benchtop model and a helix arangement of blades.

It would be very significant if Allison's concept and published power efficiencies can be verified.

Mark

[Stewart Corman]

In parallel with the indoors benchtop experiment, I am building a 10 1/2 foot "experimental" turbine w/ and w/o multiple rotors.

It is experimental because firstly it has no furling, so has to be brought up and down easily (check out thread on "tower design" which shows tiltover towers) and secondly each blade is mounted on a steel tubing spar mounted to hub with exhaust clamps , so that pitch can be set at will for 2 blade, 3 blade, 6 blade, 3blade x 2 rotor, 2blade x 3 rotor.
The benchtop is being tested to get ballpark parameters for all of the above under controlled variable WS simulation.

Note: although Allison was keen on his "helix" design, it is quite possible that a single rotor with six blades does the same job of increased torque, lower TSR, and lower WS peak efficiency (ie self furling as a drag limiting design)...data will tell the story whether multiple rotors are worth the extra effort. If it doesn't have dynamic pitch, by definition it is a drag limiting rotor with variable rpm generator requirements.
There is an argument that more than 3 blades is futile, because the extra blade "catches up" to the air from the previous blade adding no more power and increasing the drag.....BUT, if that is true at TSR=7, then a six blade single rotor at TSR of 3.5 would NOT violate that concept, especially if high pitch is chosen to be self furling and low speed rating.


Using templates for a NACA4425, I have carved the first blade:

http://i145.photobucket.com/albums/r203/scorman1/Turbine%20project/4327.jpg

here is the work in progress:
(note also the hardwood cherry leading and trailing edges with parallel grain)

http://i145.photobucket.com/albums/r203/scorman1/Turbine%20project/4322.jpg

these 4 foot blade sections are 9 inches wide at root, 3 inch cord at tip, and 2 1/2 inches max thick ...weigh about 7 pounds each w/o tubing

many pics of the construction are here:
http://s145.photobucket.com/albums/r203/scorman1/Turbine%20project/

OT -- if anyone wants specifics about any construction techniques/tools that I use and you see in my pics, we can open a new thread on that subject.

Stew Corman from sunny Endicott

[Mark Parsons]

Nice carving job Stew! :cool:

Must be nice to have the right tools. Wish I had a power hand planer...

I think your last post should be under a new DIY section thread along with some more details of how you chose, designed, made the templates and construction details for your blades.

With the increased use of Computational Fluid Dynamics (CFD) modelling these days, and the large amount of experimental work already complete, you'd think most of the arguments about wind mill blade design would be put to bed.

Mark

[Stewart Corman]

Mark,
When I have free time I may open a new thread on construction techniques.
Funny, but when I was editing the above post and said "hand plane" for picture title, it never even occurred to me that non electric ones are still being considered ...just like I wouldn't consider a "sanding block" a tool anymore, but on occasion I have to use it (manual labor, yuk!) ..then again in furniture/cabinet making, the easiest way to really screw something up at the very end is to use an electric sander!

That random orbital PorterCable 5 inch disc sander is unbelieveable ..with 80 grit it will cut faster than a belt sander and with much more control...nice vaccuum attachment also besides the variable speeds ...the grinder with 7 inch 40 grit flat flap wheel is just an animal at 6000rpm ...great for removing scale on rusted tower C channel too

Thinking of spraying epoxy paint for these exterior blades, but right now the only color I have on the shelf is candy apple red ... have to check out the local surplus store

Stew Corman from sunny Endicott

[Mark Parsons]

My background is Electronics with a bunch of industrial machinery and equipment design and applications. I am not an aircraft or avionics engineer. However, I consider myself an avid student of physics and have applied physics successfully in developing many new technologies for manufacturing and test equipment. That said, who can answer the question:

Why don't wind turbine blades more resemble aircraft propellers?

The physics would appear at first glance to be bi-directional in power in / out similar to motor / generator, or pump / turbine analogues.

A propeller has a twist to apply a constant AoA along its length for the apparent wind resulting from the angular velocity. A propeller varies the airfoil shape along its length to again compensate for the angular velocity. A couple of references:

A nice visual showing this twist and airfoil change:
http://www.woodenpropeller.com/Blade_outline.html

Betz and Prandl propeller design theory:
http://www.mh-aerotools.de/company/paper_1/epplerhepperle.htm

I assume the same Betz that theorized the maximum power collection ratio that all wind turbine designers have come to respect.

Educate me...:)

Mark

[Joe Blake]

I know even less about errordi - aeraddy - - wings'n'thingz, but I would have thought that the aeroplane propeller is designed to handle a lot more power (per unit of prop area) at much higher air speed, and that in designing the aeroprop, one knows, almost to watt, how much power is going to be applied, whereas the wind generator has such a wide range of conditions to handle that the two designs would have to be different.

An interesting question though.

Joe

[Rob Beckers]

My understanding is that wind turbine blades work physically exactly the same as airplane propeller blades (one extracting energy from the air vs. the other energizing the airflow). In fact, my fluid dynamics book analyzes both types of props at the same time with the same equations, getting to the Betz limit for wind turbines that extract torque from their rotation. So, IMO the best wind turbine blades would follow propeller blades, using appropriate and known NACA profiles for example, while taking the specific working conditions of turbines into account (lower relative wind speeds, larger variability of speeds, usually fixed pitch etc.).

I can give a call to a friend of mine who does computational fluid dynamics for a living. He models/analyzes oscillations in jet turbine propeller blades. If anyone knows he should...

-Rob-

An update: I just called up my CFD buddy. He confirms what I said above. Both types of props are the same, with the same underlying physics. Just optimized for different working conditions.

[Mark Parsons]

Thanks for the answer Rob.

It begs another couple of questions. If Wind Turbine blades should look more like airplane propellers for best efficiency, why don't more of them?

Hugh Piggot's design is carved from a plank without much (if any) twist to my knowledge. OK, DIY'ers can't get there easily.

Certainly, commercial ventures should be striving for best efficiency. How many commercially available Wind Turbines utilize a blade that resembles a propeller with twist and varying airfoil along its length from root to tip?

Too many questions, too little time.:(

If I was considering a business venture that manufactured fiberglass blades, I'd want to be very sure that they were the best geometry possible....

Mark

[Rob Beckers]

Good point...
I don't really know much about the large wind turbines, but I will bet you a bottle of Grolsch that they do have proper twist and taper to be optimized for performance. I also bet that all the newer ones use CFD to find out ahead of time how the turbine is going to behave, and to optimize it.

My take is that there is quite a disconnect between the small wind turbines and the large ones. Two different worlds. The small turbines come from hobbyists that carved their own blades, built their own rigs, and then made a business out of it. The trial-and-error method. It looks like only recently the small turbine manufacturers started to use CFD to design windmills. That new Southwest Windpower genny seems to have been run through a simulator before building the first blades. I know Eoltec uses CFD to design theirs (and the Scirocco blades do have taper and twist to them).

If you look at efficiency curves for small vs. large wind turbines there's quite a gap. The small stuff is generally around 20 - 25% max., while the big guys do 40% and more. I don't know if this is inherent to the size (ie. maybe big turbines are always more efficient).

What I also don't know is how much difference a crappy airfoil vs. a really good one makes. Bergey's fixed width, fixed pitch blades seem to work OK. The question is how much is there to be gained if you bolted on a set of really good blades of the same span. I'm sure there's a difference, just not how much.

-Rob-

[Stewart Corman]

OK, I'll chime in here with some armchair observations.

The propeller does not want to waste any energy applied by the motor in contrast to the wind turbine whose "stall regulated" design does exactly that.
Don't compare to large commercial units that uses sophisticated dynamic pitch control to result in a fixed rpm, hand tailored to the electricity conversion mechanism. These large ones are similar in that respect to the propeller, since with a known operating rpm, many parameters can be optimized for fluid flow analysis.

Back to the stall-regulated variable speed HAWT comparison. Propellers make horendous noise ...turbines cannot be acceptable with even moderate noise in populated areas. So, why waste air energy? BECAUSE if a unit was designed to produce 2KW at 17.5mph ie 15ft diameter @250rpm, then at 35mph it would be spinning at 8x rpm and 8x power or 16KW which is unexceptable with tip speeds of 245mph. All of the units rated at 26->30mph WS are functioning around 10% efficiency whereby the tip is doing all the work and lower parts are drag dominated or stalled . It is at slow WS that their efficiency is around 30+%.
If the entire blade were providing equal lift, then the root would have to be enormous to withstand the loads. Also, in order to get a 3 degree pitch tip to function at high power, the root has to be twisted or the thing would NEVER start spinning ...AH, but twisting the root causes it to be even more drag than than if a non-twist as Hugh Piggot claims. That is why he stated on on page 7 at bottom "straight, untapered, untwisted blades ...surprisingly little loss of efficiency in making these compromises"

http://users.aber.ac.uk/iri/WIND/TECH/WPcourse/page7.html

Look at his chart there as well ...the lower the TSR, more pitch angle can be used in a flat blade, so that a twisted blade with 4 degee tip and 15 degree root (typically limited by thickness of blade blank) may not really perform better than a 12 degree flat profile optimized for those parameters. In order to produce any useable power at low WS, great compromises have to be made in profile designs which result in something that is far from optimum except at perhaps one specific WS.

At lower TSR and higher efficiency, such as Claus Nybroe's Windflower, it functions mostly in the laminar region, whereas the high TSR >7 are mostly in the turbulent region. The aerodynamics are quite different. Low TSR are lift dominated ie wide blades, while high TSR are drag dominated ie thin blades.
An 18 footer in a 17.5mph WS in a low TSR design produces 350ft-lbs of torque at 70rpm ... that's 3KW ...ideally, if it losses efficiency faster than WS gains power ..then it could produce that power w/o increasing rotation speed and be truely drag limited ...go ahead ..waste that power!


Stew Corman from sunny Endicott

[Mark Parsons]

Thanks Stew.

To paraphrase your response (correct me if wrong):
"Propellers are designed for best efficiency under very turbulent conditons and turbine blades are for more laminar conditions."

I understand that aircraft propellers make as much noise due to the fact that tip speeds exceed speed of sound. The power from the engine feeding into the airstream also generates many watts of volume.

I know Sandia labs did many tests in the 70's on VAWT designs. Have they done same for HAWT designs?

Mark

[Rob Beckers]

Just for fun and giggles, a few comments on airplane propellers. As it happens I hold a US pilot license and instrument flying rating, and until moving to Canada flew quite a bit in fixed as well as variable pitch prop planes.

First, the majority of props are fixed pitch. There are many more small planes with fixed than variable pitch props (Prop governors are expensive! It also takes an extra endorsement on top of the regular pilot's license to fly them, they are considered "complex airplanes" by the FAA). So let's take the airplane I've spent most of my hours in, a Piper Cherokee PA-28.

They have a fixed pitch, 73 inch 2-bladed prop. That's 1.85 meter diameter. At takeoff it turns at around 2500 rpm, so each tip moves at 242 m/s. That makes 873 km/h. Not quite the speed of sound, that is around 1280 km/h at sea level. In fact, my understanding is that normal props should really not get that close to the speed of sound. Besides making a lot of noise, their efficiency would go down the tubes as well due to compression effects. So, a prop tip going through the sound barrier would mean something is going wrong (dive, prop overspeed etc.).

For sure, much of the noise an airplane makes comes from the propeller. 800+ km/h is quite a speed. It's not just the speed though, but noise means turbulent air. So it has also to do with the shape of the blade and how much turbulence it produces. The other noisy part of an airplane is the exhaust system, or lack thereof. Most small planes don't really have a muffler. That Piper in the example has an expansion chamber, followed by a very short pipe. Not much muffling going on there. Even with the prop spinning at low RPM these things are noisy! My understanding is that newer airplanes are addressing noise much more seriously.

So how about tip speeds of small wind turbines? A Whisper 500 clocks in at 430 km/h. Quite a speed! A Bergey Excel does 390 km/h. An Eoltec Scirocco does 260 km/h at its tips at rated power. The amount of noise it produces is strongly correlated to tip speed (there's more to it, blade profile and how much turbulence it produces matters too).

Stall for wind turbines should ideally not come into play until reaching rated speed. Any part of the blade stalling before that means loss of efficiency. Efficiency doesn't matter beyond rated output and wind speed, in fact as noted one would want to shed energy by increasing drag and stalling the blade.

Another $0.013 cents...

-Rob-

[Mark Parsons]

Rob,

We're learning more tidbits about you. I couldn't afford too many instruction hours past my ground school at the time. Someday ..... :)

Piper Cherokee - nice machine. I'm trying to recall my Cessna 152 and 172 familiarizations. Thinking take off RPM was considerably more than 2500. Of course, the prop diameter was less than 73 inches. Also recalling my Ultralight experiences in my friend's Bushmaster. That Rotax was really revving up...:eek:

I gotta think that ducted fan turbines running up to the 10,000 RPM range have tip speeds well in excess of speed of sound. Further consideration - due to compression of the air through each blade pass, they are probably designed to keep under the compressed media speed of sound inside the turbine housing. I'm sure, as you stated, to prevent heavy efficiency losses from compression waves.

Back to stall regulated turbine blades. Any empirical testing done with published results to your knwoledge?

Thanks.

Mark

[Rob Beckers]

Back to stall regulated turbine blades. Any empirical testing done with published results to your knwoledge?


What exactly is the question Mark?
Stall regulated turbines work just fine. The one I peddle (Scirocco from Eoltec) uses just that: Stall pitch regulation. Keeps RPM at 245 max at all wind speeds.

-Rob-

[Stewart Corman]

Rob,
Stall for wind turbines should ideally not come into play until reaching rated speed. Any part of the blade stalling before that means loss of efficiency. Efficiency doesn't matter beyond rated output and wind speed, in fact as noted one would want to shed energy by increasing drag and stalling the blade.

Well phrased and I am in total agreement, and my point was to properly design a turbine whose rated peak efficiency is only 15mph and I'd be tickled if it ran at same rpm if/when WS goes higher (up until the mechanical furling takes over).

Stew Corman from sunny Endicott

BTW, for those considering carving HAWT blades, I updated my Excel calculator to include twist. It adds twist degrees to the tip (considered zero) and proportions it uniformly to the root

I saved it with two scenarios ...20mph, 15P fixed pitch, TSR=3.5 and twisted 15 degrees starting at 4 degrees blade pitch, TSR=7

noted that high TSR = high RE# = turbulence = drag dominated, low TSR = low RE# = laminar flow = lift dominated

[Mark Parsons]

Hi Rob,

This document linked below is an example of published empirical testing data. I can find lots that Sandia did back in the late 70's and early 80's on VAWT's but can't find much on HAWT testing.

http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/1980/802469.pdf


Hi Stew,

Great work in the spreadsheet. It is always good to model things mathematically prior to construction. Constraining the variables is also always the challenge. If you can find a little time I would appreciate a short explanation of the source of some of your formula constants. I am wondering where they derive and what assumptions are assumed.

Thanks
Mark

[Stewart Corman]

had to ask an open ended question????

Hugh's tutorial:
http://users.aber.ac.uk/iri/WIND/TECH/WPcourse/index.html

BTW, most formulas came from Piggots tutorial, but also here and off these charts you can see some typical numbers:
http://www.eng.fsu.edu/~kroth/eml4450fall04/eml4450L21.pdf
Manwell is at UMass and I had him on the phone ..I will talk to him again when I have data

If one is molding blades ...I like this high tech profile:
http://www.eng.miami.edu/~acfdlab/CFJ_webpage/CFJ_web_dir/introduction_CFJ.html

basics by Claus Nybroe (page 5->10):
http://www.windmission.dk/workshop/BonusTurbine.pdf

blade evolution:
http://www.nrel.gov/docs/fy00osti/28410.pdf

Arvel Gentry's paper on jib/mainsail:
http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf


High lift aerodynamics:
http://www.aoe.vt.edu/~mason/Mason_f/ConfigAeroHiLift.pdf
http://www.aoe.vt.edu/~mason/Mason_f/HiLiftPresPt1.pdf

NREL stall regulated HAWT:
http://www.nrel.gov/docs/fy99osti/26091.pdf

NREL blade families:
http://wind.nrel.gov/designcodes/papers/NREL%20Airfoil%20Families%20for%20HAWTs.pdf

Airfoil primer (click on link inside):
http://www.dreesecode.com/

Newest high tech design:
http://www.mh-aerotools.de/airfoils/windmill.htm

This should keep you busy for a few spare minutes!!!;)

Stew Corman from sunny Endicott

[Stewart Corman]

The last attachment for my Excel calculator had an error for adding twist,
I used a "+" instead of a "-" :rolleyes:
I have attached the supposedly corrected version

note the following for :

TSR = 7, tip at 4 degrees and twist = +12 degrees
station wind a AOA
64 8.13 4.13
52 9.97 2.97
40 12.88 2.88
28 18.08 5.08
16 29.74 13.74


note: I see now that taper becomes crucial in calcs ..there was a chart imbedded in my other thread that shows a twist of 73 degrees to produce a uniform 4 degree AOA along entire blade ...BUT that is probably for a very long thinly tapered blade profile ?? ..note that my stations have a 1:3 taper, but that rotation speed = 1:4 for those stations ...ie non linear AOA.

In above, root is at 16 degrees whereby my fixed pitch would have to be at 12 degrees to get similar AOA of 4 at tip for efficient tip lift ie maximize TSR

This could be the justification for twist that I hadn't seen before.
More of the blade in non stall mode could mean that runaway is still possible tho ..I want opposite effect ie 1/2 the blade to be dragging in high wind ...testing will tell ...numbers don't lie

Stew Corman from sunny Endicott

[Stewart Corman]

I found this as a pdf file while looking for something else on my HD, and I really don't remember where it came from ,
but after some Googling I found the link for this gem (sort of):

http://www.uce-uu.nl/swd.htm
in particular - rotor design:
http://www.copernicus.uu.nl/uce-uu/downloads/SWD/04Rotordesign.pdf

I have the original pdf, which is from a few years earlier circa 1977, but is too large to attach here and starts with some more basic designs:

http://i145.photobucket.com/albums/r203/scorman1/Wind/bladetypes.jpg

It probably should have been at the top of the long list as a "first read"

BTW, this shows that for an NACA44 series profile, that the optimum tip angle is set so that the AOA is 4 degrees so that the lift to drag ratio is the greatest. Go to the calculator and choose a design TSR (since that determines the apparent wind angle) and then subtract 4 degrees to get the blade tip set angle. In above case of TSR =7, those numbers were 8.13 - 4 = 4.13 degree set angle. Change the design to a TSR = 3.5, then we have 15.95 - 4 = 11.95 degrees ...this one will self start at low WS for a non-twist, the higher TSR cannot.

Note also that more crude profiles like those formed by a PVC pipe are not really that bad and some have great lift, which is how the original farm water pump units functioned.

Stew Corman from sunny Endicott

[Stewart Corman]

I was provided simulation data for NACA4421 produced by XFOIL, which is a design and analysis program that calculates lift and drag for various RE#s vs AOA after you specify the blade profile.

RE# simplified is Reynold's # = 9360 x Speed (mph) x Cord width (ft)

So, if you want higher RE# at a fixed WS/TSR to get into a better performance range, then you need a wider cord

The key functions of how any airfoil will perform is lift coefficient Cl which produces the torque, and the ratio of lift to drag which shows where the energy is lost.

http://i145.photobucket.com/albums/r203/scorman1/Experiment/data%20charts/NACA4421Cl.jpg

http://i145.photobucket.com/albums/r203/scorman1/Experiment/data%20charts/NACA4421lDratio.jpg

Note that unless you match a twist to maintain uniform AOA for a specific TSR, that as AOA changes along the blade stations, the efficiency changes.
However, TSR is never a constant vs WS in a real turbine, so there will be an optimum performance at some WS, since AOA will change dramatically as the staions approach the root.
Trick is to design slightly left of peak for tip, so that lower staions perform better as you go over the hump, and to make the blade tapered, so that the root has a much larger cord than the tip. Note also that many designs and tutorials talk about AOA = 4 degrees , which is NOT supported by above graphs.

Stew Corman from sunny Endicott

BTW ..note that a ratio of 100:1 is never achieved as was in previous chart posting of 1977 article

[Stewart Corman]

Found a few bugs, changed the lift and drag calcs, added Lift/Drag ratio, and added a third sheet for a progressive twisted design found at:
http://www.otherpower.com/images/scimages/4937/Resin_Transfer_Molding.pdf

Stew Corman from sunny Endicott

[Doug Dobbyn]

Hi; I'm pretty new here, just signed on, (in other words you can write this off as blather)and have been perusing threads and thoroughly enjoying the reading and information. This thread caught my attention when airplane propellers were mentioned. Nope, not a pilot, or engineer, but I do play with model airplanes, and have carved a few props, and had many discussions about what makes for a good one with fellow prop carvers. So for what it's worth, here goes- Typical props for models powered by a small gas or electric motor would be very similar to those found on the real thing, and as has been discussed may or may not have much relevance to wind generators. However! I carve props for rubber powered models which run at a much lower speed, some indoor ones you can actually count the revolutions as they fly, so this may have a bit more relevance since the speeds would be closer, and the blade shape, pitch, and AOA are very different. A higher pitch, twisted, fatter blade generally is the rule to take advantage of torque rather than speed, which leads me to wonder if they wouldn't work to the same advantage on a generator? Time to do some testing with mini props. Does this make sense to anyone?
Doug

[Stewart Corman]

Doug,

A higher pitch, twisted, fatter blade generally is the rule to take advantage of torque rather than speed, which leads me to wonder if they wouldn't work to the same advantage on a generator?

close ..if you mean by "fatter" you mean wider, then yes we agree..it improves "lift" ...if you mean the actual tickness of the blade, then no, thinner not thicker blade to reduce drag component

most recent work on this was done by Visser at Clarkson:
http://72.14.205.104/search?q=cache:3UhNhaOaGGIJ:www.clarkson.edu/honors/research/papers/Rector-M.%2520Curtis.doc+%22Solidity+and+blade+number%22+ Clarkson&hl=en&ct=clnk&cd=2&gl=us&lr=lang_en
click on HTML version to get photos/graphs

Stew Corman from sunny Endicott

[Doug Dobbyn]

Yes, sorry for the goof, I meant wider, narrowing to the tip as in theory that with the tips moving faster the blade needs to be narrower to get a constant flow of air over the blade from hub to core. Sort of. I'm not real good at all the aerodynamic theory, so bare with the poor explanations. Thicker would create a different airfoil shape, probably not as efficient at slow speeds, with more drag, and of course heavier and harder to spin. Thanks for replying Steve.
Doug