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.

7) Carving the final shape to match the templates:

Before you start carving the final shape, make sure you have both the leading and trailing edges finished. Don't leave extra material, it will just cause you grief. Likewise make sure you didn't leave extra material on front and rear of the prop. Why? When you are carving to match the templates it is extremely valuable to know that the four surfaces you have already cut are finished. This will help deter you from removing material from areas where you shouldn't. 

Use the flat half of the template to get your blade angle correct at each of the ten locations. Only after completing the back (flat) side of the prop do you proceed with the front (curved) side. I find it takes around 40 minutes to carve either side of prop to final shape.

                                                                                                      Materials required:

Fiberglass, both unidirectional (RA 7715) and bi-directional (RA 7725).

Fiberglass spar unidirectional (RA5177)

#1 grade pine wood 1x6” (I used 36 linear feet total). Available at certain lumber yards.

Epoxy (probably any epoxy that is not a one to one ratio is fine). 

Flox (Flox is milled cotton fibers. It's a filler that also has some strength.)

.75 od x .065 wall 2024t6 aluminum tube, less than 6 inches needed.

 

Equipment required:

Draw knife

Spoke shave

Drill press

Band saw

Low friction bearings (2)

Clamp fixture for laminating the pine boards

 

Method:

Designing the propeller:

I recommend the Bates Prop program. It’s not a real friendly program and you can make mistakes, but it’s great for providing blade shape info. He advertises in Kitplanes. See http://www.geocities.com/aeroopt/index.html   .  You need to input your prop diameter, and a table of rpm vs. hp. This allows the program to optimize the pitch for your aircraft. All of the rest of the program inputs are readily available from other program users.

The program will output the coordinates for your 9 propeller templates as well as performance predictions and lots of other goodies.

-Wood selection:

Each blade requires you to laminate 5 pieces of pine. You should use #1 grade pine boards (no knots) 1” x 6”. You will probably have to try a number of lumberyards. They will normally allow you to purchase a portion of a 20 ft board to obtain the highest quality pieces you can find. The strength of the wood is increased if the grain is tight and no knots. So why not strive for that? Even though the wood represents a very small portion of the prop strength.

 One of the outputs from the Bates program is “Blade section block summary”. This provides the dimensions that help you determine how many planks of wood you need for each blade location. A 68” diameter prop (84 pitch) used 2pcs at 34”, 1 pc at 28”, 1pc at 20” and 1 pc at 10”. That’s for each of the three blades. If your engine requires more pitch or larger diameter, then those board lengths would be changed. Optionally you can just cut 5 boards to 34” and not worry about specific thickness.

 

1) Laminating wood:

You need some simple way to tightly clamp the pieces while the glue cures. It’s nice if this fixture is flat. A table top can work great if you install about 14 pcs of 8” long threaded rod. These equally spaced fasters are adequate for clamping.

Use epoxy with a little flox added to laminate the boards. Don’t skimp on the glue. Lay some wax paper under the prop boards to prevent adhesion to the clamp fixture. See pic.

Carving tips: 

Start from the center of the prop and work from station to station until you get to the tip. Do all carving with draw knife until you are within appx. 1/16 inch of the final surface. The spoke shave (in pic above) is then your best tool to complete the shape. Finally you can use a sanding block with 60 grit (very coarse) sandpaper to blend and smooth. A belt sander drum is the perfect tool for blending the prop hub to your first station. In fact, I have found that the belt sander can eliminate the need for the spoke shave tool altogether. 

I lay my template on top of the station I'm carving and mark the high spots with a pencil. Then use spoke shave to cut away my marks, then once again lay template on prop and remark. Once I have completed a station I mark it with an "X" so I don't accidentally re-cut that location. It would be unfortunate to shape the prop using the wrong template. At times it's difficult to understand where to remove material. Just keep in mind that you should never remove material from the leading edge (LE) nor trailing edge (TE). Also keep in mind that you have already established the high point at each station in the step above "Remove excess thickness".

 

8) Gluing blades together:

1) Measure each blade length. Trim the tip as necessary to achieve identical lengths.

2) The blades will be glued with flat side (rear of prop) down. Find a perfectly flat surface. Verify by using a 3 ft level. You can see in the photo that I had to shim one blade to get all three at the same level as the center of the hub. 

3) Verify that blades are equally spaced by using tape measure from tip to tip. See photo. Tip to tip measure  must be identical.

4) Mix a generous amount of epoxy, add flox and apply to the hub joints. DON'T add too much flox. We want a runny glue mix. It's difficult to compress these joints adequately if the mix is too thick. Double check that the point of the "V" of all three blades align with each other. Ignore how the outer diameter of the hub aligns. You may be able to see that my hub outer diameter has 3/8 inch mismatch blade to blade. That's because I offset my largest template in step 3b. I used a very long piece of solid core wire strung from tip to tip. This was twisted at the ends to force the blades together at the center. Without this wire, you will end up with glue voids Allow to cure overnight. 

9) Drilling:

All drilling operations should be done using a drill press or milling machine that is perfectly square. Sequence is important in most operations below. 

A) Use a hole saw to create a counter bore 1 3/4" diameter by 3/8 inch deep. Make sure you drill dead center of the prop on both sides of the hub. This counter bore will later be filled with your glass spar material.

B) Drill 3/4" diameter thru hole, also precisely centered and square. Drill it from the curved (forward) side of the prop to maximize drill accuracy at that end of the prop. Tractor installations should be drilled from the flat (rear) side of prop.

C) Cut a piece of 3/4" x .058 wall aluminum tubing (6061 or 2024 T6 alloy) to fit the full thickness of the prop at hub. Mix some runny flox and epoxy and glue this tube into the center of the hub. Allow to cure over night. This tube will pilot the propeller. Also, all of the spar material will be pulling on it when the prop flexes.

11) Drill counter bore at prop bolt locations:

These counter bores just serve to reduce compression of wood when the prop is torqued down. Use 1" diameter wood bit to drill each bolt hole location 3/8" deep. Both sides of prop. I just pinned my paper template (cad generated) of the prop hub onto the prop. Then center punched each of the six holes. Fill these 12 counter bores with flox after step 12 below and allow to cure. I place wax paper on top of hub and a weighted flat board atop. This counter bore location is not overly significant. You will drill the holes for the bolts later on.

12) Install spars:

40% of each blade has 2 fiberglass spars installed. Since my blades are 34 inches long, my spars will be 13.6 inches. The above picture shows two pencil lines from the center of prop to 13.6 inches. I just laid a long piece of sheet metal along blade to allow tracing of a straight line. Along each of these two pencil lines I will route a notch 1/4 inch wide by 1/4 inch deep. Well actually it starts out at 1/4 inch, but gradually tapers to darn near nothing at the 13.6" dimension. There must be a better way to cut this groove, but so far my best method is to use my Dremel with a cylinder shaped stone bit. I then follow up with another stone bit which is cone shaped. The groove looks like hell, but is functional. 

Use felt pen to number the blades 1,2, and 3. Wet out fiberglass spar material and apply a couple strands at a time. Strand starts at 13.6" dimension, wraps around the center tube, then back to the 13.6" dimension. So you do two strands on blade 1, two on blade 2, two on blade 3, then repeat. We want a tapered spar to reduce stress concentration. So the first time you do the three blades, your spar reaches the 13.6" dimension... but the next pass you only go to perhaps 12". Then perhaps 10", etc. 

Don't forget to repeat this process for the other side of the prop. We end up with 4 spars per blade, two on each side. Allow to cure and then sand the excess spar material to the proper blade shape.

13) Apply Fiberglass to flat side of prop:

Only apply fiberglass to the back side (flat side) of the prop. We won't do the front side until we strengthen the TE later. Each layer of glass below is applied to all three blades. Only then do you proceed with the next layer.

How do I orientate the glass weave? 

Uni-directional glass: All uni has the fibers running parallel to the TE. 

Bi-directional fiberglass: All bid has the fibers at 45 degrees to the TE. This is not natural. Your scissors must cut into the roll of material at a 45 degree angle. Then that cut edge must be laid along the TE. 

Layer #1 is bi-directional weave (RA 7725) full length of prop. So for my 34" blade length,  this piece is 38 inches x 6". Note: If your engine is rated over 120 hp, then add a second layer of this same material to each blade. I found it only necessary to add this to the curved (front) side of prop for my 165 hp engine.

Layer # 2 is Uni-directional weave (RA 7715) full length.

Layer # 3 is Uni 50% of full length. 

Layer # 4 is Uni 25% of full length.

Layer # 5 is Bi-directional weave at 15% of prop length.

Layer #6 is Bi-directional at 15%. 

Before the glue hardens, make sure the fiberglass overlaps onto the curved side of prop appx 1/2". You will be tempted to flip the prop over to do this overlap. Don't! This overlap is a pain. You must use great care to prevent large trapped air pockets due to the sharp bend the fiberglass must take. Why do this overlap? It connects the glass from the back side to the front and seems to cause the prop to resist twisting.

 

After cure, trim the excess glass at edges. Then flip the prop over to the curved (front) side. The wood at the trailing edge won't stand up to the environment, so we remove all wood from trailing edge. I used a Dremel stone bit to remove most of this .2 inches of wood, followed by a wood chisel to remove the last shreds of pine. I found it useful to support the section I'm working on with a scrap piece of flat wood. Now apply a thick mix of flox. Let it cure and then sand the flox to proper profile.  

At this point, I find it helpful to drill thru the center of prop. This because it's tough to locate the center after you apply fiberglass to the other side of the prop. Likewise, I also mark with pencil the seam locations where the 3 blades connect. I use these seams later to help get the bolt holes properly timed to the spars.  

We are now ready to apply the layers of glass to the front (curved ) side of prop. Same number and type of layers described above. 

14) Drill mounting holes:

Use the prop crush plate as a drill guide. Also use your cad template to make sure you are not drilling into your spars. Using drill press, drill all 6 holes thru. Drill from the front side of prop as that is the primary mounting surface. Use good machining practices when drilling... surfaces clean, square. Crush plate held firm. 

There are two critical dimensions on props. The flat surface of the hub on the front side is a primary locating surface. The center tube at that same surface is the control for centering and balance. 

Prop balance is critical. Blade tracking is significant (and easily adjusted). Blade angle on flat side of prop is significant, as is the shape of the leading edge. All other characteristics are of reduced importance.

15) Balance prop

You've sanded the prop, it's all drilled, almost ready to start the painting process. Time for another preliminary balance as shown in step 10 picture. You will likely find one blade much lighter than the rest. A good way to resolve this is to find the lightest blade, then add a small piece of Uni- directional glass to the prop tip on the back side of the blade. After that glass cures, you just sand it away until your prop is perfectly balanced. Also note that the prop tips already have 1 layer more than needed. So there are no concerns if you sand into the glass near the tip. The last six inches of the prop tips only need 1 layer of glass per side. 

16) Adjust blade track

Mount the propeller to your engine. Torque it fully. I don't know the ideal torque value. I use 25 ft-lbs. Re-torque after a couple hours. Allow it to sit for 24 hours, then re-torque and measure blade tracking. Tracking changes as the wood compresses over time. That's why you wait. I place a bucket on the floor near the blade and then pull each blade past the bucket. The variation in how close the blade gets to bucket is the blade track runout. We are talking about fore/aft runout, not radial runout (which should be 0). You should have less than 1/16" runout in the fore/aft axis. My prop started with .375 inch runout. So I added 3 layers of glass to my prop mounting surface. I added it to most of the prop mounting surface so as to cause the blades to tip relative to the mounting surface. The math?  Prop diameter of 68 inches divided by 6 is  ratio of mounting surface to blade diameter = 11.33. The .375" runout divided by 11.33 ratio = amount of material to add to mounting surface = .033 inches. Each layer of glass is .010 inches thick, so I need 3 layers. Clear as mud eh?

Don't EVEN consider flying with this prop until you wait 24 hours after mounting. If bolt torque is not stable, then don't fly. Give it time for the wood fibers to compress. I will torque to 20 ft-lbs., wait 24hrs , re-torque, ground test, re-torque, wait 24 re-torque, fly with prop, re-torque, fly, re-torque, then just check the torque before each flight until stable. Can you tell I am very conservative with prop torque? What do I mean by re-torque? I just tighten further each bolt to verify that torque is at least 20 ft-lbs. You have to get the bolt to move to know it's torque value. Don't be tempted to exceed 20 ft-lbs, you would just be crushing wood fibers endlessly.

 

17) Painting

I use Superfill to fill the glass weave. Epoxy mixed with glass beads would work too. Let it cure and then sand smooth. I also use auto body glazing putty to fill the weave. Sand with 120 grit, then re-balance. I've been happy using a sandable primer and enamel spray cans. I mount the prop on my balancing system (see step 10), then paint the lightest blade. I place a paper shield over the balance bearing as the paint quickly affects the bearings resistance. It's reassuring to see the prop rotate as the paint throws it out of balance. When I'm done I verify that prop is balanced by attaching a postit note to any blade and observing it rotating to the bottom position. 

I haven't found a need for leading edge tape. So I don't apply it. My prop tip speed is well below Mach, so I don't bother replacing the wood at the prop tips with graphite. 

10) Preliminary balance operation:

Insert two ball bearings into the center tube of the prop and suspend as shown in the photo below. Use spoke shave and sanding block to remove material from the heaviest blade until balanced. If you place a post-it note on any one blade you will be able to prove that your balance device is sufficiently sensitive and also that all the blades are equal weight. A post-it note attached to any blade always causes that blade to rotate to the bottom. My source for ball bearings was the head of a broken VCR (video recorder). They are very low friction.