theoretical blade shape vs compromised shape

[imsmooth]

I am just about done with my blades carved from 2.5" thick wood.  The angle continues to increase in pitch until I drop the full 2.5" at the root.  Fiberglass blades have the ability to increase the pitch more by dropping the trailing edge and even raising the leading edge.  To do this with wood probably would not have the proper support, and if it did there would be an increase in weight.

Has anyone done measurements to see if there is any significant improvement in startup and power generation between the two?  I can say from the first set I made they spun up in low winds and really got moving.

[bob g]

its been a number of years now since i did a bunch of research but here is what

i remember...

there can be a huge difference as you start to add camber to a profile, done correctly the low pressure region moves forward as the high pressure region moves further off the trailing edge of the blade, this increases power.

the problem iirc is a couple of things

1. cambered blades are hard to carve, concave surfaces are harder to carve than convex surfaces using common methods.
   
2. cambered blades can be rather peaky, in that they really come into their own at a rather narrow tsr, and fall off rather rapidly on either side of that limit.
   
   

without some sort of power point converter which uses feedback to keep the blades running at a speed which keeps them within their peak range.

so far it appears most folks have perfected the more common profiles and found them to work over a fairly broad range without resorting to more complicated electronics.

although i really like seeing more folks working toward mppt for windgens, i suspect

in the near future those converters will be worked out and rather common place, much like the early progression went from either no dump load controller or very crude controllers to what is now some very good ones.

soon as someone perfects a 1 or 2 kwatt buck converter, getting control and feedback over it will be fairly simple with a microcontroller,, once that is accomplished i would expect folks starting to work with more advanced profiles.

in my opinion an advanced highly cambered profile likely can attain 50% more power than a simple profile, perhaps more than a crudely carved clark Y profile.

my opinion only, yours and other may very well vary

bob g

[Flux]

I am not sure what this question was about. He has asked something similar before and I agree entirely with your answer on your take on the question.

My take on this one was not on different profiles but on the effect of compromising an existing profile by running out of wood at the centre compared with using fibreglass to produce the crazy wide sections with huge pitch angles that calculators predict.

I can't quote any scientific measurements and that is why I didn't reply. All I can say is from what I have observed but never scientifically measured.

I find that the inner 1/3 of the swept area does so little that you can virtually do what you want in that region and it has little effect. My own findings are that the very wide chord and huge pitch may help start up but I have never needed it except for an early iron cored pmg with belt drive. All reasonable direct drive machines ( including most good designed iron cored ones) will start in lower winds than they can produce in.

At best I think the theoretical profile at the centre makes the thing more critical to tsr and I am inclined to think that for a conventionally matched machine the end result is worse. I suspect the wind doesn't go where the calculators assume it does, they are based on very simple theory. I am inclined to suspect that if you can mount the blades without significant drag at the centre you can leave out the inner third of the blade with no obvious loss.

I just use bits of wood big enough to give the strength needed at the root ( I am too lazy to laminate things). The inner third has to take what the timber will allow and although I have never used fibreglass I would use the same approach unless the large section at the root was necessary for strength ( which it probably isn't as you have to fit it to a hub somehow)

There is little point in keep asking these very theoretical questions as the complexity of making even simple power measurements is so complex and tricky that nobody will be in a position to answer them. If I hadn't been making blades 40 years before I met the first calculator I may have been asking similar questions but all the improvements I have seen over the years have come from removal of iron and field excitation loss to gain low wind performance. Once these things are accepted the common machine matched to low winds does quite well with simple blades. The high wind performance of these things optimised for low winds is way below what we obtained with older methods but the overall energy capture is more consistent on the average site.

If you want it at both ends then you have to track the peak of the prop power curve and the gain is so spectacular that you will be happy with simple blades for some time to come. When you have found accurate ways to measure power curves then you may be able to extend this even further with better blade profiles.

Flux

[imsmooth]

I thank you both for your answers.  I think the question has been answered.  I guess the only thing missing is a true scientific measurement, but this is probably unnecessary.  If the end power is not affected, and the startup is easy, regardless of the design, then going crazy with the deep pitch and wide root is not worth it.

thanks again for your time.

jk

[Curtis Shipp]

I found this pitch calculator and it can be used to get the angles for a blade for a given wind speed. I am working on a constant speed hub that will work to maintain a set RPM regardless of wind speed. So I want my blades to be the right twist because I need the braking at high wind speed of the root pitch. The specs are 10' dia./ 250/300 rpm to produce a constant wattage and not overspeed in high wind. This will be done without any electronic controls. It will feather just like a prop on a fullsize aircraft. No gears are involved and it is adjustable for other rpm's.  Pictures will be posted soon.

[Curtis Shipp]

I forget the website http://www.nclra.org/Programs/PropCalculator.html sorry about that.