The other day, Andrew Sego asked: “Can you share how you let-in the plywood on your prop? Was it before or after carving?” Well, you can’t start in the middle of something like that, so I went into the workshop and got the nose off of my Elmendorf Special Greve Racer. The existing prop had some issues over the past season, so I decided to replace it and record all of the steps.
STEP 1 – I weighed the nose assembly. Why? Because this prop flies the plane well and if I swap out the prop, I need to replicate the overall nose weight so that I don’t disrupt the general stability of the model. So, 6.9 grams will be my target weight for the finished new prop. Note, I am only replacing the prop, not the entire assembly (although the prop shaft had to be replaced, too).
Step 2 – The old prop was a 6″ Superior Prop; I can’t remember exactly, but I am sure it was a 7″ cut down to 6″. I am replacing this with a 7″ cut down to 6.5″, so I start with a Superior Props 7″ blank. This has a 1.3:1 Pitch, so it was cut to 9″ Pitch. This is how you receive the product from us – most of the wood cut away, true pitch cut into the back side, and blades slightly tapering from a thick hub to a thin tip. IMPORTANT NOTE: we always mark the TOP or LEADING EDGE of the prop. It is important that you remember that and know which side of the prop you are working on. The blade faces you can see when looking down on these markings are the back side of the prop blade – you will sand and shape these faces, but NOT the other faces (the ones you can’t see here). Those faces are the working faces – where the pitch has been cut. you must only lightly finish-sand those faces or you will change the pitch. THIS IS CRITICAL.
Step 3 – Lay out your favorite prop blade shape. This is a 7″ Chinese prop (which weighs 4 grams by itself). I make sure the blade is aligned properly then trace around it with a pencil.
Step 4 – This is the part Andrew asked about. After locating where the hub would be (front to back), I located and drew a line about 2″ long and 1/4″ up from the prop base. Do this on both sides. If you use an Xacto, slowly cut over the line repeatedly, switching sides after each pass – and being careful to keep parallel to the base (or perpendicular to the prop shaft). This time, I tried doing that with the balsa stripper seen here. It worked fine – probably better than guessing at parallel. You do have to be careful with the stripper, too, as you can gouge the blade surface (see the gouge to the left of the cut line). Cut this slot all the way through, but only as long as the line. The idea here is to strengthen the prop hub so it doesn’t break as easily.
Step 5 – cut a piece of 1/64″ plywood the same length as the slot (2″ in this case) and wider than the blade (1/2″ in this case). You want to center this ply in the slot, leaving excess on either side.
Step 6 – Test fit the ply into the slot. Be careful and take your time. You will probably need to do this two or three times as the slot is not the same thickness as the wood.
Step 7 – Soon you will be able to insert it completely. Test fit to insure you know how you want it to end up. That is, centered end-to-end and fore-and aft, so that equal amounts of the ply are visible on either side of the prop blank. Make sure you can slide this in and out relatively easily – like a drawer.
Step 8 – Pull the ply out so that just the edge is in the slot. Lay a bead of medium CA on the top and bottom of the plywood. Then carefully press the spine back into the slot, aligning so that it matches how you had it set up in Step 7. Use a stick of balsa to wipe off the excess CA after you slid it in. On the other side, run some thin CA along the top and bottom intersection of the ply and the balsa. Do this because it is likely that the medium CA did not go all the way into the wood. Thin CA from the other side should seep in and complete the gluing process.
Step 9 – Using a SHARP knife, carefully trim off the plywood that sticks out of the prop blank. Take your time and be careful – avoid a slipping knife that gouges the balsa.
Step 10 – Once you have the plywood trimmed close, use a sanding stick to sand the ply down to the surface of the prop blank. You will probably need to sand the plywood more later when you shape the blade, but this is a good start.
Step 11 – Using your sharp knife, cut NEAR the pencil line. Try to keep the blade perpendicular to the surface of the balsa; it is not super critical, but will help with the blade shape. Carve down to about 1/16″or 1/32″ close to the line. You will sand down to the line in the next step, so removing most of the required wood with the knife will make sanding a little easier later.
Step 12 – Using a sanding stick or board, sand away the remaining excess balsa. Again, try to keep the sander perpendicular to the surface of the blade. In this photo, you can see the edge in the upper right has been sanded and I am now working on the other side. I use up-and-down sanding here to rough in and then side-to-side to smooth the line.
Step 13 – This photo is just to show the blank after the blade shape sanding is completed. Note that this is a strange shape of prop since I could not have any blade behind the nose bearing – downthrust was already hitting the long vertical nose. So all of the prop blade in front of the hub and base makes these blades look strange. The cutout will be covered by the spinner and this is where the Nason clutch will be installed. Nason clutches are great for use inside a spinner since they automatically catch the prop shaft and require no manual setting (like the Garami clutch sometimes does). This prop could work as it is, but there is much to be done to finish it.
Step 14 – Remember when I said it was critical to know the side of the blade to carve and the side that must be left alone? (If not, go back and read Step 1.) Now is when you need to know that. These blades are way too think and you must carve and sand them down to their final thickness. It only takes minutes once you know what to do. Remember – LEAVE THE WORKING SIDE ALONE – only work other surface – the one that you can see when you look down on the prop from the front.
Using your sharp knife, carefully cut off the corners, beveling that edge. Be careful the direction you cut – sometimes the knife will get into the grain and split the wood – that is bad. Always cut across the grain to avoid that. Once you have beveled those corners, use the knife to thin the tip. You only need to do a rough job here because sanding will take care of the rest. Also, don’t over cut with the knife or you will cut away wood that should remain. Use your sanding board to bring the thickness of the prop down to where you want it to be. It takes a couple of props to get better and better, so don’t be discouraged if your first prop isn’t as good as you would like.
Step 15 – Here is the prop sanded to (nearly) final shape. Remember, do all of your work on the back side – only lightly sand to clean up the surface of the working side. Superior Props as cut to true pitch at all stations and any aggressive sanding on that working face will change the blade.
At this point, the prop is read for your favorite sealing or finishing process. I use thin CA. I lay a couple of beads on one surface of one blade and rub it in evenly with my pinky finger. I make sure it is as smooth as possible. Then I use my sanding stick to sand down and smooth out that layer of CA. I do it again, this time with less CA. Then I sand that surface again with a finer grade of sander, removing all high spots. The finished prop has a satin finish that is harder than the balsa. That process on this prop added 0.4 grams of weight. This provides basic protection only. The blade can still nick or crack or break, but it is better than bare balsa.
Step 16 – Every Superior Prop comes with a 3/32″ hole drilled in the center for a prop shaft. I have had problems with this area in the past – after a few flights, my aluminum tube bushing wears the soft balsa and the prop gets a little wobbly. The new spine is just one of the things that I am hoping will fix the problem. Of course, the spine did not have a hole drilled in it.
Use a piece of 3/32″ OD aluminum tubing to act as a drill guide. And use a 1/16″ drill. I do this next step by hand.
Step 17 – Using the tubing as a drill bushing, gently drill through the plywood with the 1/16″ drill. Again, I just do this by hand, without even a pin vise to hold the drill. Just be careful – don’t stress the sides of the balsa hole. Remove the tubing and do the same process again with a 3/32″ drill (without the tubing this time). Again, be careful not to disturb the existing balsa hole.
Step 18 – Eventually, you will be able to fit the 3/32″ OD aluminum tube all the way through the prop.
Step 19 – The second part of strengthening the hub for the tube bushing is to glue small plywood washers on the front and rear of the prop hub. It is my hope that this will anchor the tubing in three strong places – the front and back and the center spine.
I insert extra tubing, put medium CA around the hole on the prop, and slip on a washer. I press it down into the glue and rotate the tubing so that it doesn’t get stuck. I don’t use too much glue – just enough to hold the ply washer in place – final gluing will come later. Do that front and back.
Step 20 – This is the final tube bushing installation. I insert the tubing so that about 1/32″ is sticking above the surface of the ply washer on the other side. Then I mark about 1/32″ excess on this side, scribing a mark with a knife. Remove the tube and cut the tube to the newly defined length. Similar to the plywood spine gluing in Step 8 above, I just start to insert the new tube into the hole and apply medium CA around the tube. I press it into the prop, making sure to leave some excess tubing on each side.
I let that CA set up and then I put just a drop of thin CA on the washer so the CA flows under the washer and at the tubing and washer junction. It should wick into the assembly and hold it firm permanently. Note – this is why the aluminum tubing was a little long – so the thin CA didn’t get inside the tubing. Be very careful with that part.
Once the CA is set, use a coarse sanding stick or a fine file to grind down the excess tubing. It should be flush with the ply washers. On my Nason and Garami clutches, I use a 1/16 OD thin-walled brass tube inside the 3/32″ tubing. This should be cut to nearly 1/16″ longer than the aluminum. This tubing will accommodate an 0.047″ prop shaft wire.
Step 21 – Here is the nose block with the new replacement prop. The new prop was slightly heavier than the original. I did have some clay in the nose, so I just removed that until I replicated the original nose weight. A spinner will be added once I have re-trimmed the plane for flight.
This process took a few hours; maybe 4 or 5. But much of that time was taken planning and taking the photos.