|Propeller carving tips|
These notes assume that you're carving what I call an X-block propeller. This is the type of propeller that seems to be most common on older plans from the propeller carving days. Dannysoar's original instructions describe a different style that conserves some wood but requires some preliminary assembly.
Though I call this an X-block propeller, I'm including the second variation illustrated here, in which the X portion extends over only part of the propeller diameter. I call this location the 'shoulder.' The tips beyond the shoulder must be tapered in height to preserve the helical blade shape.
These instructions assume that you want a propeller with helical pitch. However, many experts advocate reducing the pitch slightly toward the tips for best performance. If that's what you want, use a 'shouldered' block and reduce the tip thickness slightly.
Most propeller plans I've seen show a block that's an odd size: 1" x 9/16" for example. Unless you have a more extensive selection of balsa blocks than I do, you'll have to saw or plane down a block from the next larger sized stock; say 1" x 3/4" or even 1" x 1"
But there's an easier way! Carve a propeller with the same pitch directly from the larger stock. Then cut the blades down to the smaller plan form.
To be sure you get a propeller that's the same pitch as your original plans, follow these steps:
* If the plans call for a shouldered block, try entering the overall radius and tip dimensions into the third calculator. This will give you the pitch at the propeller tips. If this doesn't match the 'shoulder' pitch, the specified block will not yield a helical prop.
If you'd like to take the more traditional approach, use the second calculator. This allows you to set the shoulder radius wherever you want it. I've heard that the widest portion of the blade should be at about 70% of the radius. Choose a height or width; the calculator computes the other dimension for you.
Most sources I've read suggest that you should carve the block to the X shape before beginning to shape the back of the propeller blades, as illustrated above. I did a few this way until I realized that this was just a lot of extra work. There's no reason you can't simply mark the block and begin carving the back immediately.
Half the lines shown on a traditional block are a waste of ink. The bold lines in this illustration are the only ones you need. This may also reduce the likely hood that you'll accidentally carve a pusher prop instead of a tractor.
Most props are tapered to a thin chord near the hub with the widest chord closer to the tips. If you want a helical prop, don't cut the blade to this shape until after you've carved the back of the blades. You can cut out the blade plan form immediately after shaping the back or you can shape the front of the blades first. In either case use a paper template to make sure they match.
On a related note, many diagrams call for a 'clearance' portion to be removed from the block, as illustrated above. Again, don't remove this until you carve the back of the blades or you won't have a helical prop.
A lot of plans show the tip tapered back from the leading edge, instead of evenly tapered as in my other diagrams. As long as you use the same tip thickness, you'll still get a helical propeller, but the center of the helix will flare backward slightly. I have no idea whether this affects performance, but I think it makes a slightly more attractive propeller.
Copyright 2000, Matt Keveney. All rights reserved.