Basic Propeller Construction - Materials
Just as there were many variations in propeller design, various manufacturers held firm beliefs that certain materials and construction methods produced a superior product. Much of the information provided below is taken from a 1920 publication of the American Propeller Company, which built thousands of wooden propellers under the trade name of Paragon. It is focused primarily on WW1 era propellers. At that time, wood was the ideal material for use in making propellers. It was strong, durable, readily available and easily milled to a variety of shapes and styles.
Types of Wood Used
Virtually every conceivable type of wood, at one time or another, has been used in the construction of propellers, and many examples of this can still be found today. Overall, mahogany was the wood of choice in early wooden propeller construction. Whether this made sense purely from its mechanical properties or whether it simply evolved from its widespread use in the furniture industry is a matter of speculation, and there is evidence that political pressures from suppliers may also have influenced its choice. It is light, stable, easily workable, has a uniform grain pattern and is reasonably strong. Walnut is similar, although perhaps stronger, and was used commonly along with mahogany by European manufacturers, and the in U.S. was used almost exclusively by Hartzell, which was a walnut lumber company when it began producing propellers in 1917. A variety of other hardwoods, as well as spruce, were used in propeller manufacturing prior to 1920 and examples of almost every kind of wood may still be found.
Around 1915 oak became the material of choice for Paragon propellers built by the American Propeller Company , after it had produced a number of beautifully constructed propellers of oak exteriors combined with spruce interiors prior to that time. The decision to use all oak was driven by demands of cost and speed of production. The company had experience with every kind of wood ever used for propellers, but firmly believed that quarter-sawn oak was "incomparably the best material we have ever used." They felt that mahogany was too soft and crumbly and had poor resistance to wear, and that walnut was too brittle to justify its increase in strength. As a testimonial to oak, they pointed out that of 8,000 oak propellers delivered in 1916 to the Canadian RFC over an 18 month period, only 4 failed to pass final inspection in Canada and not one was ever returned from the fields with any complaint
Reinforcing the Wood
All wooden propellers, with rare exceptions, are laminated. This has more to do with the efficiencies of cutting, drying, and milling of the materials than anything else. Additional strength could be obtained by using additional materials, like fabric and metal.
Many wooden propellers have fabric wrapped around the tips, or sometimes extending the length of much of the blade. Some early propellers even used animal hide, such as pigskin for this purpose. The wrapping improved the strength of the ends of the propeller, particularly to splitting. As testimony to the strength of oak mentioned above, it should be noted that many oak propellers do not have fabric tips, whereas almost all mahogany and walnut propellers do (at least as originally manufactured.) Most propellers that appear to have painted tips, on closer inspection will be found to have fabric beneath the paint.
Metal sheathing was added to some early propellers, but by 1913 there were so many fatal accidents in Europe from throwing off sheet metal that the whole practice of sheathing was nearly abandoned for some time. The need for sheathing, however, became apparent with propellers used by flying boats, so after carefully calculating and testing the methods for attaching the sheathing, the practice was re-instituted several years later. Copper, tin, and monel were all used along the tips and leading edges of many propellers. Typically, the sheathing was applied to the tips and held in place with either screws or through rivets, the exposed heads of which were then soldered and smoothed down. Sheathing has continued to be used in manufacturing of propellers to this day, although plastic and epoxy have replaced the traditional metal in most of them.
Propeller Construction Techniques
Example showing both fabric and metal covering.