question on blade angles

[kitestrings]

Hi Folks,

I've run some of the base inputs for a 15'-er thru Hugh & Altons blade calculators, with much the same results.  I was also re-reading various articles that I'd printed or book-marked on blade design.

This one defines the 'blade angle (beta)' as the "angle between the chord line and the plane in which the rotor spins":

http://www.scoraigwind.com/download/windrotord.pdf

I have always understood that the 'drop' and 'woodwidth' as in Hugh's were intended to yield an angle representing the flat (front/upwind side) of the blade for carving purposes.

http://www.scoraigwind.com/sheets/bladedesign.xls

If I take a given station this appears to be the case.  Example, (my basic math/sanity check) at a given section:

=DEGREES(SIN(Drop/Wood Width))  = "angle AB", or "Blade Angle in Degrees" looking at Alton's

This falls off a little bit at the stations closest to the root, where it probably doesn't matter much, but I just wanted to make sure that I hadn't misunderstood this angle to be something it wasn't (that was common understanding here).  I was quite upset as a kid when to learn that "spatula" didn't have an "r" at the end.

One other question, we're planning to use a MPPT (Classic) controller(s), so if the rotor speed is say 3x cut-in at furling, should we be taking that into account by adjusting the TSR a bit higher?

thanks for any help,

~kitestrings

[Flux]

Yes the drop and chord width give you the angle beta near enough. The angle is based on the line joining the radius of the leading edge to the trailing edge. For hand carved blades this is near enough the flat face, few people do much about rounding on the leading edge.

With conventional loading the tsr falls constantly from cut in to full load, it is only near design tsr near 10mph. With the classic the loading will hold the speed up so you don't need to do anything to the blades, they will just run faster and produce more power when you keep the loading near the ideal.

Flux

[kitestrings]

Thanks Flux,

I suppose the proflile selected would have some effect - one of the fatter, rounder airfoils for example.  In the case of the a more standard 4412 profile it appears the largest discrepancy is nearest the hub where it would have the least impact (and the theoretical angle becomes a compromise of other things - depth of stock, compexity, etc.)  This one is labeled 4415, but I believe they intended "4412" as described in the write-up (unless they're the same?).


Still we can make the templates, and layout the drop, to account for this with no addtional work really.

There are a couple of the definitions that are not particularly clear on Alton's calculator.  The first I think you've answered:

Blade Angle In Degrees: angle of (front) face of prop

Blade Slope: derived from Blade Angle In Degrees for you

The latter appears to be just the Drop over the Chord Width expressed as a decimal, or rise/run if you will, but it doesn't appear to be in degrees in any event.

I'm assuming a TSR input of (95 rpm * 15' * 3.14) / 7.8 mph * 88 fpm/mph) = 6.5 currently unless there's a reason to do otherwise.  Thanks again,

~kitestrings

[SparWeb]

I'll have to go read Hugh's write-up again to remember the difference you're asking about.

Basically, some angles are more useful for understanding what the air is doing, others are useful for marking the lines on the boards before cutting...  something like that.
The red line in the diagram is the chord line. 

The NACA 4415 is just a fatter version of the 4412.  The numbers mean something, too:
Camber is 4% of the chord,
Center of the camber is 40% of the chord,
Thickness maximum is 12% of the chord.

So the 4415 is just 15% thick.  Thickness adds weight, but in return you get more strength and a very slightly better characteristic in stall (but the difference is negligible in WT blades).

Aerodynamicists love playing with these numbers.  I was once shown how you can use the 1930's NACA airfoil numbers to create the "modern" Wortmann FX 63-137 (within 1% or something).
I wonder if I could crank out a NACA number that matches the GOE222 airfoil shape that's got everyone all steamed these days.

[kitestrings]

SparW,

This is interesting.  I learned something new - which is why I keep coming back.  Oddly, I think the 4415 may be a typo.  Throughout the article they refer to a 4412 airfoil.

You've captured what I'm was asking - whether the angle in the common calculators (Hugh's, Alton's, Warock's) is indicating the angle between the chord line and the plane in which the rotor spins, or the face of the blade and the plane in which the rotor spins.  I've understood Flux to say it was the former, and further that it didn't matter much bases in part on how most people carve them.

My thinking is it is like cut a board.  Knowing which end we're trying to keep determines which side of the line that I want my saw kerf.  It may have minimal impact, but in the layout why introduce an unnecessary error?

I've read some of your responses on different airfoils.  What is your current favorite, or is there one?

I'm sure glad there are folks like yourself who put some of this theory into palatable form for the rest of us.  I confess, I had an brief mental image of an Aerodynamicist being a trapeze artist playing some sort of percussion intstrument mid-flight.

~kitestrings

[opo]

I wonder if I could crank out a NACA number that matches the GOE222 airfoil shape that's got everyone all steamed these days.

What about naca6616

opo

[SparWeb]

NACA 6616...  something like that.  Lots of camber, centered toward the middle.  There is also a thickness profile to look at, and the basic "teardrop" of the 4-digit NACA airfoils doesn't always suit.  If it doesn't work, the laminar series (also NACA) may do it.

I may still have the old airfoil co-ordinate generator programs that I used in school.  Back in the days when computers were coal-fired.

[opo]

NACA 6616...  something like that.  Lots of camber, centered toward the middle.  There is also a thickness profile to look at, and the basic "teardrop" of the 4-digit NACA airfoils doesn't always suit.  If it doesn't work, the laminar series (also NACA) may do it.

I may still have the old airfoil co-ordinate generator programs that I used in school.  Back in the days when computers were coal-fired.

It is no only the shape that matters, but also their polars. NACA6616 vs GOE222 may look a little bit different profile-wise, but their respective polar graphs are very close. It will be nice to have the naca exact equivalent though.

Cheers