Hub assembly Laminated wood blade Fabric sheathing Metal tipping Notch to prevent buckling when forming Notch to prevent buckling when forming Solder Solder Drainholes Drainholes Rivets or screws Rivets or screws Figure 7-20.  Fix-pitch wooden propeller assembly.  Figure 7-21.  Installation of metal sheath and tipping.  propeller is well suited for such small aircraft because of its  light weight, rigidity, economy of production, simplicity of  construction, and ease of replacement. A wooden propeller is not constructed from a solid block  but is built up of a number of separate layers of carefully  selected and well-seasoned hardwoods. Many woods, such  as mahogany, cherry, black walnut, and oak, are used to  some extent, but birch is the most widely used. Five to nine  separate layers are used, each about  3 ⁄ 4 inch thick. The several  layers are glued together with a waterproof, resinous glue and  allowed to set. The blank is then roughed to the approximate  shape and size of the finished product. The roughed-out  propeller is then allowed to dry for approximately one week to  permit the moisture content of the layers to become equalized.  This additional period of seasoning prevents warping and  cracking that might occur if the blank were immediately  carved. Following this period, the propeller is carefully  constructed. Templates and bench protractors are used to  assure the proper contour and blade angle at all stations. After the propeller blades are finished, a fabric covering is  cemented to the outer 12 or 15 inches of each finished blade.  A metal tipping is fastened to most of the leading edge and tip  of each blade to protect the propeller from damage caused by  flying particles in the air during landing, taxiing, or takeoff.  [Figure 7-21]  Metal tipping may be of terneplate, Monel  metal, or brass. Stainless steel has been used to some extent.  It is secured to the leading edge of the blade by countersunk  wood screws and rivets. The heads of the screws are soldered  to the tipping to prevent loosening, and the solder is filed  to make a smooth surface. Since moisture condenses on  the tipping between the metal and the wood, the tipping is  provided with small holes near the blade tip to allow this  moisture to drain away or be thrown out by centrifugal force.  It is important that these drain holes be kept open at all times. Since wood is subject to swelling, shrinking, and warping  because of changes of moisture content, a protective coating  is applied to the finished propeller to prevent a rapid change  of moisture content. The finish most commonly used is  a number of coats of water-repellent, clear varnish. After  these processes are completed, the propeller is mounted on  a spindle and very carefully balanced. Several types of hubs are used to mount wooden propellers  on the engine crankshaft. The propeller may have a forged  steel hub that fits a splined crankshaft; it may be connected  to a tapered crankshaft by a tapered, forged steel hub; or it  may be bolted to a steel flange forged on the crankshaft. In  any case, several attaching parts are required to mount the  propeller on the shaft properly. Hubs fitting a tapered shaft are usually held in place by a  retaining nut that screws onto the end of the shaft. On one  model, a locknut is used to safety the retaining nut and to  provide a puller for removing the propeller from the shaft.  This nut screws into the hub and against the retaining nut.  The locknut and the retaining nut are safetied together with  lock-wire or a cotter pin. 7-13