Manufacture
The HOFFMANN variable pitch propeller consists of 3 sub-assemblies:
Hub assembly
The hub is made of forged aluminum alloy which guarantee high strength.Axial movement of the piston is transferred to the pitch change knob by a fork and pitch change blocks, whereby the blade angle is changed. The pitch change is limited by mechanical stops. The corresponding blade angle can be adjusted when the propeller is installed on the craft.
Blade assembly
The HOFFMANN composite blade is a joint construction. The blade root is made of highly compressed hardwood and the blade part is made of lightwood (spruce). Special lag screws connect the compressed wood of the blade with a metal ferrule. An aluminum or PU strip protects the leading edge against erosion.To increase torsional stiffness the blade is covered with fibre reinforced epoxy (FRP). Fatigue failures due to vibrations are unlikely because the internal damping of wood is considerably higher than that of duraluminum.
Several layers of polyurethane lacquer are sprayed onto the fibre reinforced epoxy covering which assure high resistance against moisture, erosion and other mechanical effects without loss of necessary elasticity. Additionally, the lacquer protects against UV-rays. The thrust side is painted black to avoid reflections of sunlight.
For safety reasons the tips are painted with a different colour to make the transparent disc visible when the propeller is running.
Spinner assembly
The spinner is one piece of aluminum alloy and is screwed to the spinner bulkhead which is also made of an aluminum alloy sheet.To increase torsional stiffness the blade is covered with fibre reinforced epoxy (FRP). Fatigue failures due to vibrations are unlikely because the internal damping of wood is considerably higher than that of duraluminum.
Several layers of polyurethane lacquer are sprayed onto the fibre reinforced epoxy covering which assure high resistance against moisture, erosion and other mechanical effects without loss of necessary elasticity. Additionally, the lacquer protects against UV-rays. The thrust side is painted black to avoid reflections of sunlight.
For safety reasons the tips are painted with a different colour to make the transparent disc visible when the propeller is running.
