Propeller construction

General:

The following describes the steps required to construct a pine core, composite propeller. This prop has four spars per blade, two each side. The basic design has many flight hours on canard aircraft with no known failures. A canard is a severe application, another strong indication this is a valid design. My first prop took only 3 hours to accurately carve each blade. Total cost $120 (plus cost of Bates prop program). It’s flown 110 hours with no problems.

Propeller construction is intimidating to most builders. The builder can’t imagine how they can accurately control these surfaces that change in all 3 axis. The high rpm’s suggest extreme forces. I hope these instructions are detailed enough to dispel these fears. If not, you can always use this information to make a display propeller.

My prop extension has a .625 inch pilot diameter. Conventional extensions have 2 1/4 inch diameter. If you have a conventional prop extension, it will be necessary to to machine a short (1 inch or so) adapter to convert the pilot diameter to .625. The small pilot diameter is needed to allow the spars to wrap around the hub center without interfering with the mounting bolts. There may be alternate solutions, I have not pursued this.

I didn’t originate the design. I’ve seen credit given to a number of people, but I obtained all the details through Nigel Fields.

 

 

Fixture for propeller laminating. One fully cured blank in foreground.

 

2) Creating Cad template:

The output from the computer program includes a table of coordinates which describe the blade shape at each of nine locations (stations). Just enter the x and y values into the cad program. Actually you enter the supplied x value and then the YU value. Then you enter the x value with the YL value. Then rotate the image to the Blade angle specified for that station. You really don’t have to be a cad expert to accomplish this. You can download free cad programs off the internet. Send the cad image to your printer, then glue the paper to poster board. A cereal box works great for your template material. I use a 3M spray adhesive. You want to avoid using a glue that will cause the paper to expand.

After you print out the image, use a pair of calipers or accurate tape measure to verify that your printer is accurate in both the x and y axis. Some printers stretch the images in one axis, thus introducing a substantial error in your template. 

 

3) Preparing to cut the leading and trailing edges:

a)      At what position do I place the smallest template? I put the leading edge of the template on the centerline of the prop blank. This will result in the propeller having a back sweep. I use the cad program to measure the distance between the leading and trailing edge of that template. I then mark those two points on the prop blank. See pic.

b)      Where to put the largest template? Your best bet is to place the center of the template on the center of the prop blank. That's what I did on my first prop. However, you will notice in these pictures that I offset this template toward the TE on this prop.  I measured the distance between the two edges using the cad program. I multiplied that value times .72 and then place the trailing edge at that distance from the blank centerline. Then I marked both the leading and trailing edge on the prop blank. If you have any doubt about where to place these two templates, you could alternately just place the center of the two templates on the center of the prop blank. This would eliminate the sweep back and provide a more conventional appearing prop. In fact, Bates recommends this simpler approach.

 

c)      Connect the two Trailing Edge (TE) points with a straight line. All of the templates will butt that line. Knowing the distance between the LE and TE of all the templates now allows you to mark both of those points on the prop blank. (Check out the 1.742 dimension on the cad image below). Connect the dots and you are now ready to cut out the entire LE and TE.

4) Mark the hub area for cutout:

Using cad program, draw the hub area of the prop. Print it out and use a pin to mark the blanks at the joint locations. See pic.

Note: you need to make sure you are not building the prop backwards. My prop rotates clockwise, so leading edge goes to right side of blank when viewed from the rear of aircraft. Also, the flat side of the prop is always to the rear of the craft.

Double check!!!

Use a band saw to define both the trailing edge and the leading edge. Don't leave excess material, just go for it!

5) Remove excess thickness:

Using the cad program, I calculate the maximum blade thickness at each of 9 stations. These are marked at the TE and LE for each station. Then just take a hand saw and cut down to your pencil marks. I use a draw knife to remove material between each station. This only takes 45 minutes per blade

6) Remove excess material from the flat side of the blank:

I use the cad program to determine where the flat intersects the leading edge.  For station 34 (34 inches from prop center) this dimension is .739 inches. See drawing below. So I place a pencil mark on the leading edge at that elevation. I repeat for all 9 stations. Now I just take my hand saw and cut down to the pencil mark at the nine stations. 

Use a "C" clamp to hold the prop blank firm to a bench. Then take the draw knife and remove all the excess wood from between the saw cuts. This only takes 35 minutes per blade.

Draw knife skills:

Of course, keep it sharp. I just hand stone the blade periodically. Some areas of the wood will tend to chip out when you use the draw knife. This means you need to rotate the prop blank 180 degrees and attack the wood from the other direction.