author, Sensenich Wooden Propellers
Several instances of loose propellers in the last year have prompted a review of wooden propeller installation and operating procedures, in particular the proper technique for applying and maintaining propeller bolt torque.
All loose propellers have shown signs of burning on the hub mounting face, sometimes accompanied by bolt hole and/or counter bore elongation, indicating relative motion between the propeller hub faces and contacting flanges.
Maintaining proper bolt torque is the most important maintenance item for a wooden propeller. Loss of proper bolt torque will result in the decrease or loss of hub compression and thus the loss of drive friction between the propeller mounting hub face and the engine or spool drive flange.
At this point the drive torque is transferred only by the propeller bolts and/or flange drive bushings, which will then begin to elongate the bolt holes and/or counterbores in the mounting face of the wooden propeller and can cause cracking in the hub.
The propeller bolt torque must be carefully applied and checked periodically in order to maintain adequate hub compression. Do not torque above the recommended levels as this will crush the wooden hub, breaking its moisture seal and slightly reducing the drive-torque capacity of the installation.
The main factor that leads to the loss of propeller bolt torque is the variation of wood hub thickness with long term environmental changes. Propeller bolt torque should be checked according to the following schedule:
1. Stabilization Period – When installing a propeller, check the propeller bolt torque after first flight, then recheck every 10 hours OR 10 days, whichever comes first, until the torque stabilizes. The torque should be completely removed and then re-torqued to the recommended values using the pertinent installation instructions.
2. After Stabilization Period – Once the propeller bolt torque has stabilized, a torque check should be performed every 50 hours or annually.
3. Environment Changes – Should the operating environment change significantly in temperature and/or humidity for a long period of time, the propeller should undergo another stabilization period.
ADDITIONAL INSTALLATION TIPS
Spinner or Spacer Installations: For propeller installations that use spinner bulkheads or spacers mounted in-between the propeller and mounting flange, ensure that both faces bulkheads/spacer are smooth and clean.
Testing has found that painted surfaces provide the greatest drive friction and larger safety margin if hub compression is not maintained.
Installation Hardware: Ensure that all threads on propeller attaching bolts, lock nuts or drive bushings (where applicable) are clean and dry. Any lubricants on the threads can result in over-tightening and possible crushing of the wood hub.
Wood propellers have been installed on hundreds of thousands of aircraft over the years with excellent service histories. Following the maintenance procedures above will help ensure safe operation of your wooden propeller.