pvc blade profile

[pepa]

i have been experimenting with diferent designs for blades made from pvc pipe and fiberglass utility poles. the type of blades that worked best so far have a cloth cover epoxyed over the entire blade giving a flat, twisted front to the wind and a round downwind side. someone on the board noted that pvc blades did not have a true air foil shape so i have been testing a set of 10' blades made from pvc that comes real close to the carved blade profile, i used hugh piggott's break drum plans blade profile. i was surprised that the new design has improved the preformance of the blades a lot. i will post numbers for all three types of profile as soon as i finish this last test, using the same danb type generator for all measurements. the genn has 1x2x1/2 neo #50 magnets duel rotor 12/9. below are some ruff drawings that will give some idea of how they are built. pepa







comments please pepa

here are links to the missing pictures. i could not put them in the post becouse they are over 100kb so i made them links.
kurt

pic 1

pic 2

pic 3

[jondecker76]

I'll be watching this thread with interest. I myself have also been wondering how these desgns compare. Personally, i would do PVC blades even if they are within 10% of the efficiency of carved blades, given that they can be constructed in no time at all.

Looking forward to your results

[Flux]

Blades made with a flat front face will behave much more like the wooden ones and will match a machine intended for wooden blades much better.

If you keep the front face angles similar to Hugh's then performance will be similar.

The back profile being part of a circle is not a true aerofoil but will work quite well. The old " Freelite" blades were of this profile and no doubt were made on a spindle moulder, they worked quite well.

The characteristics of pipe blades without the flat front surface will be totally different and you will need totally different angles to suit a machine intended for wooden blades. The mean line of that single surface style of aerofoil is completely different. They mainly suit low speed low tsr and even then they are not very good.

I think it is a personal issue, I would find any form of pvc blades far more difficult to make than wooden ones and the end result is not likely to be good, even if it performs I don't think it will have a long life.  If it is easier for you then fine, some finding making wooden blades a big issue, I don't find it so, maybe because I started young and nothing is a problem to a kid.

Flux

[QUAZ HOLT]

Pepa , your picts are to big can you resize pleasessss.... Bill

[gotwind]

HI Pepa.

Looks a very interesting solution - however the first 3 images are not appearing.

Probably too big and they've been 'kindly'deleted.

Try and resize too max 600 pixels on longest side and keep file size to under 50 kb each

Thanks

Gotwind

[SamoaPower]

Pepa,

I'm glad to see that you are interested in and paying attention to airfoils, the heart of a blade.

The real issue here concerning rotor efficiency is airfoil Lift to Drag ratio (L/D).

As you can see, almost anything works to a degree if you want to operate at a TSR of 2 or 3. However, if your alternator design requires a TSR of 7, you better have a high L/D if you want reasonable efficiency. Windstuff Ed once pointed out that even flat boards as blades can be 25% efficient. Rightly so, if you're willing to accept a low TSR.

Unfortunately, blade L/D is not easy to measure or predict. It takes a wind tunnel and special equipment. A generalized airfoil shape will have an unknown L/D as do probably most of the blades produced here.

I think we can say that using a section of a circular pipe as a blade will not have a very high L/D. Your example shows that the root area has something more approaching a generalized airfoil shape compared to the tip. Unfortunately, the outer 25% of the blade radius is where most of the work is being done. THIS is where it's more important to have a higher L/D.

Jondecker says that he is willing to give up 10% in rotor efficiency to have simpler construction. You may be giving up more than you think. An example:

Say you have wind conditions that produce 100 watts in the wind. With a well designed rotor of 45% efficiency, you extract 45 watts from that wind. A rotor of 35% efficiency obviously will extract 35 watts. This is 22% less power than than the better rotor. Efficiency counts!

What's the answer? Start your rotor design with a published airfoil with known parameters. It'll pay off.

[dinges]

Makes sense to use the highest L/D-ratio possible (in gliders, this would give the best gliding distance). IIRC, in gliders the maximum L/D-ratio is nowadays about 60 (without resorting to boundary layer suction).

Got any info on good profiles for windmills? I.e. with high L/D-ratio, but still DIY-able to build? As in 'with a flat bottom, instead of a hollow one)?

Peter.

[SamoaPower]

Ah yes Peter, the forever quest for the highest performance/simplicity ratio. I still think, there is no free lunch. If you want it, you work (and pay) for it.

The kicker in your question, of course, is "DIY-able". I basically don't see why DIYers couldn't reproduce virtually any airfoil if the right materials and techniques are used.

Good flat-bottom airfoils? No, I don't know of any but it's been a few years since I've looked. The best I recall was an L/D of about 50 and a CL of 0.8. In comparison, the Wortmann FX60-126 used on my 16 foot rotor has an L/D of 100 and CL of 1.08 at 5 degrees Angle of Attack and running at a TSR of 4.5. This is highly under-cambered and is all aluminum using monocoque construction (rivited sheet, tubular spar and ribs). I think in the future, I want to explore composites.

There's more to airfoil selection than just L/D. Stall characteristics, for one. This could be significant if you want a stall limited machine.