Sears Wind Machine

[wdyasq]

I went by the local windmill farm.  The owner put pieces of an old Sears Windmachine in my truck and asked if I would build a new blade.

A few measurements and some pictures will help to start analyzing the project.  It appears the blade was near 6' long originally.  My dial calipers read the hub thickness at ~.73 -.74" so it was probably cut from a ¾" blank and has eroded a bit. Paint on the hub makes it appear the hub was ~4.25" in diameter. There isn't enough of the center of the blade left to ascertain true width but I will use 4-1/2 inches as my best guess. The wood in the break looks similar to good spruce or cedar.  There is no odor left I can detect but it tastes more like cedar than spruce.

Airfoil thickness at 12" is ~.51" and it was close to 2.6" wide I would guess- at one time.51/12/3.7/1.25; .33/18/.46/1.07;  .32/24/.395/?; 3/30/.36/1.04; 2.7/36/.4/? - This is thickness/"station"/width/peak.  These numbers will be entered in a spreadsheet to compare them and see if there is a pattern or, if they make enough sense.

Pictures were taken so I the twist and possibly the airfoil might be modeled.  I'll do it in a CAD program but it can be done by hand or, possibly by eye.

General views:

Twist:

Airfoils:

Screen shots of modeling the twist - CAD program is RHINO:

The airfoil may be the "Clark Y".  It was fairly common and used on many things in the era this machine was built.  It looks similar and the thickness is in the range of the measurements, the peak is near the right spot and the model looks close.  By moving only two of maybe 10 points, it is a close match. This airfoil will be modified to the 15% version and used.

I shall put as much twist as possible in the root area and use the approximate twists the original shows.  The twists will be "smoothed" by making the trailing edge a fair curve in the CAD program.  This will be a close approximation of the original yet still give as much performance as one could hope to have with this platform, In My Opinion.

Table of measurements:

station    width    depth     peak    %airfoil    %peak    twist

12.     3.7    0.51    1.25    0.137838    0.337838    8.28
    
18.     3.3    0.46    1.07    0.139394    0.324242    10.17
    
24.     3.2    0.395        0.123438                 8.66
    
30.     3    0.36    1.04    0.12            0.346667    4.06
    
36.     2.7    0.4        0.148148                3.38
    
    

The next installment will tackle the modeling and, if the time I available, the actual CNC cutting of the blade.

Ron

[johnlm]

Wow, You are really doing this up right. I commend your ability to try to exactly duplicate the original prop. I think I would have just ran the genny with some external motor drive of some kind and figgured out where the cutin rpm was and went from there to determine the TSR requirement and then just carved a new 6 ft prop.

It would be most interesting to see a few pic of the generator.  Do you have any idea of the vintage of this machine or who produced it for Sears?

Johnlm

[rotornuts]

Just my thoughts.

I'd be tempted to carve all new blades for this thing as it could prove difficult to balance the new blade with the old. The profile looks something like an NACA 4412 with an 8-10 degree twist tip to root and the wood looks like either cedar or larch(tamarack if your east of the rockies). It's really hard to say what the profile actually is which is going to make it even harder to match. Perhaps you could take measurements of the other blades as well and average the info although as I said before I'd be awfully tempted to replace them all. If that one is rotted out the others aren't likely to be far behind. Looks like a fun project.

NACA 4412

Camber - 4% cord length

Max. camber located at 4/10ths cord length or 40%

Max. thickness - 12% cord length

When determining camber your cord line must run from the very tip of the rounded leading edge and through the profile to the end of the trailing edge in a straight line. This will almost always place some profile above and below the cord line. Next you draw the mean cord line which is the line created when you place an equal amount of profile both above and below the line. The difference in % of the cord length between the cord line and the mean cord line at its maximum point of deviation is your camber. The position of maximum deviation is the max. camber location. Pitch angle is determined using the cord line as the reference point.

Have fun!

[rotornuts]

It may be wiser for me to say the angle of attack is determined using the cord line. Someone may try and call me on what pitch really is.

[wdyasq]

You may be right on the airfoil.  Although the takeoff I did indicated it had a truely flat bottom and an ~14% airfoil including the erosion. I feel lucky I don't have to "match" anything. What is seen is only a little less than half of the blade.  These machines had a single one piece two-bladed prop as did the Windcharger.

I doubt this machine will ever make another Watt of power as this is part of a display of over 30 windmills. I'll take a few picturs of the mill if I can remember to take my camera with me next time I go there.

As far a s drawing airfoils goes, I put the ordinates in my CAD program and let it do the work.  My old boatbuilding habits come back and I require myself to 'fair' the curves before I convert them to solid matter.  As I remember, there are some rules for the nose radius if one cares to exactly follow the old rules for drawing NACA airfoils.

Ron

[JW]

wdyasq,

 Really nice writeup, ya its pretty much amazing what one used to be able to purchase at Sears.

JW

[hiker]

just a shot of  a couple props..

steep angle at the hub then tappered off to the tip...

no numbers---just made a line from hub to tip..and went from there..

with a airfoil on backside...

[hiker]

[newguy]

you probably already know this but a lot of the machines during that era used props carved out of sitka spruce for its straight light strong grain

[hiker]

ive heard of bush pilots thats have cracked up in the bush and broke there props...

then carving a blade from some dried out deadfall..musta took some real guts to crank that baby up and take off..............

[wdyasq]

Your other option was to stay there and probably die.  

Ron