Thursday, December 4, 2008
Propeller 01 The Blank & Other Preparations
A good propeller begins with a good blank. The one shown here is not especially good. It lacks sufficient squeeze-out at the glue line. But it's good enough to serve as a training aid.
The blank shown here is made of pine weighing about 32 pounds per cubic foot. Which is pretty heavy for pine. The blank is a full six feet long, four inches more than required. Reducing the blank to sixty-eight inches will of course reduce its weight, as will carving the blank to the usual propeller shape. As shown the blank weighs twenty-two and a half pounds.
I've taken the trouble to mention the weight -- and the fact it will weigh less as the work progresses -- because I can't lift it, thanks to the cancer (mentioned in earlier posts). And since I can't lift it you may see me doing some rather silly things with the blank. That doesn't mean you should do the same :-)
The blank began as four pieces of shelving. To make-up the required three-inch thickness I had to stack the four pieces atop one another. Since this blank is specifically for training you may elect to use a different number of laminations, so long as you adhere to the basic rule: More laminations is better than fewer laminations.
In the same vein, I've used pine. You may elect to use a different specie of wood such as hemlock, cedar or what-have-you. But I think it would be wise to stick with softwoods, at least for this particular prop. 'Real' props are often carved from hardwoods but as you are about to see, a good deal of what you must learn has nothing to do with the type of wood you are using; in theory you could learn with a styrofoam blank.
I've used Weldwood 'Plastic Resin' glue on this practice blank and intend to use it on the real blank as well. 'Real' propellers generally use Resorcinol but it has become difficult to find. Some prop-makers are using epoxy and I've even heard of urethane being used, but the choice of adhesive is really up to you, since all commonly-available adhesives are stronger than the wood you'll be using.
In any case, given the amount of adhesive you'll be using, it's cost is insignificant compared to the cost of the airplane as a whole. Which is one of the reasons you want a good squeeze-out. A good, sloppy squeeze-out is the best insurance against a glue-starved lamination. If you'll examine the photos you will see that I did not get a good squeeze-out with this blank, which is why it would normally be rejected.