Blade profiles, which do you use?

[madlabs]

Hi All,

I'm working on my next set of blades. This time I laminated some ceder so as to have the stock wider at the root. I'm using Hugh Piggot's spreadsheet and sticking to the chord numbers all the way to station  3.

Anyway, I'm looking at profiles. I'm leaning to a NACA2412/ I'm a little bummed that I already cut the flat on the back according to the spreadsheet, I can see that I can't quite stick to the NACA2412 profile all the way to station 3, but it will make it to 4. Anyway, anyone know what profile Hugh uses? I poked around on his site but didn't see it. What profile are y'all using? Something with a perfectly flat underside would be cool. But I haven't gone so far with this set that I can't start another.

Thanks!

Jonathan

[Flux]

I suspect Hugh uses the same profile as I do, " Ersatz"

If you are not aiming for complicated high lift profiles then make the front face flat and for the back make the thickest part about 1/3 the way from the leading edge. Run the trailing edge to a point and round the front edge to look nice. If you care to radius the bit near the leading edge it usually turns out very similar to ClarkY.

Your NACA profile will not be much different but the front face won't be flat, you can go to lots of trouble making templates and make a splendid job of it and you will be very proud. Unless you have quite sophisticated data logging equipment you won't see any difference from my guessed profile but you may need to alter the alternator matching a bit to get the best match.

If you aim for one of the high lift profiles you will need to take infinite care and you probably won't get it near enough with templates unless you take forever, you probably need a CNC router. You will also need to get an excellent surface finish and keep it that way during its life if you want to keep it optimum.

It will certainly behave very differently and the settings and matching will be drastically different from the simple ClarkY shape. It will perform better if you accidentally get the alternator matching right for it but if you just substitute it for the simple profile it won't necessarily work as well or better.

Once you have got to mppt and can match your load for any profile it may be a better propposition but for normal use it may be a labour of love for no improvement at all.

Flux

[madlabs]

Flux,

I'm not going to go crazy trying to get it perfect. But looking at my last set of ersatz blades, the profile is very different from "real" airfoils. So my thought was to get a profile to shoot for, something to go by but not get all hung up on perfection. I get that one gets quickly diminishing returns seeking perfect airfoils, I just wanna know I'm in the correct ballpark. That way I won't keep wondering if my blades would do better with a "real" profile and I can stop making new blades.

Looks like a NACA4412 profile will work with what I have already cut on the blades. I found an airfoil print program and making a template takes a about 5-10 minutes a station so my plan is to do a simple go/no-go for the profile. I really don't think it will slow me down much.

Any thoughts on the NACA4412 profile?

Jonathan

[Flux]

4412 will be fine. The thinner sections seem to work better so 4412 is your best option.

I agree with your philosophy, I probably have some natural instinct for making something that looks like an aerofoil. ClarkY is the one that is nearest flat on the front face but 4412 is not that much different. If you had something far from the mark before it may make a worthwhile difference but more often changing the pitch angle will get much the same result.

Similarly I have never found it worth worrying about width or angle in the inner 1/3 of the blade so just get your outer stations right.

Taper and twist also affect the matching but if you can match things right a parallel non twisted blade works very well on air gap machines where start up is easy.

Let us know how it turns out.

Flux

[Seekscore]

The hand carved blades are probably most like the Clark Y profile as the front is for the most part flat. The NACA4412 profile seems to have had more wind turbine research regarding performance. Looking at the comparison of the shape of the airfoil, I really can't see much difference.

Here is an overlay of the two profiles.



Mike

[madlabs]

Hehe, I can see that there must be a HUGE difference between the blades.

Thanks for the help, the blade blanks have been marked and my bro in law is hacking away as we speak. As soon as I clean up the template making mess I'll go join him. The templates came out very well. It did take me longer than I thought, but I think it will be time well spent. If nothing else it helped burn the profile into my mind's eye.

The hub is done and the blades are well on the way. I'll be doing a good pickup truck drive test to see if I get a significant performance boost. I'll post picks of my new babies as soon as they are presentable.

Jonathan

[SparWeb]

I was juggling 4415, 4412, 3415 around when doing my blades the last time, then I realized that at the scale of the wood, the difference in profile would equate to about 1/64".  That's like one shave of the planer on the wood.

I stopped worrying about it after that.  Just get the angle of attack right and get the leading edge round.

[madlabs]

Spar,

Yeah, I can see that. Most of the reason for the templates is so that the little voice in my head will 'go away'. The lil  voice kept asking about "real" profiles and wondering about what difference that would make. I've learned to give in and answer those questions or it keeps bugging me.

Now you know I am stark raving mad.

Jonathan

PS The blades are coming out REALLY nice looking. But I bet they work about the same as the last set.

==
rephrased

[freejuice]

 This might be useful for planning a set of blades, maybe not....
As a side note to blade profiles I use the standard Dan's profile and it makes good power too,  but I left the trailing edge a bit thicker...its about 1/4 of an inch and the blades run fairly quite too....unless they run unloaded...
 So in the end the blades are a bit more robust profile....however I'm glad they are...we had some strong winds over a period of about 1-1/2 days  and I had a fuse blow which "unloaded" the turbine,  well it stayed together without self-destructing, in this state for about 8 hours and the winds gusting at times to 45 mph.... this was on a 17 footer.

The wife didnt shut it down but she did say she heard it "howl" a few times and looked outside at the turbine; she said the only thing she could see was the center of the hub ..everything else was a blur....groan.....so I again showed her where the shutdown switch was and asked her to train here eyes and ears for anything out of the ordinary.