More on Wood Props
April 1996
A correctly designed wood prop can operate successfully and safely for many years, provided it is carefully maintained and not abused. It is vey important to repair any damage to the finish ASAP. If bare wood is exposed to the elements it will rapidly absorb moisture and engine oil. This will significantly reduce the useful life of the prop.
Metal props are generally more efficient than wood props, mainly because they have thinner blade sections. You cannot simply cut your wood prop blades to thinner sections and expect them to give prolonged, safe use. They will not. If you are into performance tuning and feel you must thin down your prop blades, you will need to lay-up several plies of glass, or better yet, carbon and epoxy, from the hub to the tip on each blade. This is not generally recommended because it takes some design capability as well as skill to produce a balanced, symmetrical prop. This change also makes it difficult to have an urethane "rain proof" leading edge.
Props on pusher-type aircraft really have to work hard. They are running in the wake of the wing, fuselage and landing gear, which makes for a turbulent flow field (especially when compared to a tractor or "puller" installation). The exhaust plumes also create problems for these props, due to high speed jets of hot air flowing through the prop disc, which can actually scorch or burn the wood if the prop is not clocked" correctly.
Since the last newsletter, we have actually seen three different installations where one or more of the blades were damaged by hot exhaust gasses. The varnish was burned away, and in two of these installations the wood was blackened and scorched. It is critically important that you clock a two-blade pusher prop at the one o'clock/seven o'clock position when cylinder #I is at top dead center on a Lycoming engine.
If you have a three-blade propeller, inspect it carefully for any sign of scorching and rotate it one bolt hole either way if it does show evidence of heat distress, and try it again. The best way to ensure that your prop is not being damaged by hot exhaust gasses (particularly if you have an "in cowl" exhaust system that ends close to the prop disc) is to install a pair of sfick-on irreversible temperature indicators (about a buck apiece). These should be placed on the prop blades, centered in the area where where the exhaust plumes pass though the prop.
I currently have a pair of temperature indicators on my prop. They have a 150 degree - 175 degree - 200 degree - 225 degree temperature range, and with more than 50 hours of flying, the lowest temperature (150 degrees F) has yet to trigger. I have ordered more of these with a lower temperature range and will comment on the outcome in the next CP.
The bottom line is DO NOT FLY with evidence of heat distress on your prop!