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Wood Prop Blade Failures
From CP83, Page 8 (January, 1996)
Over the past several years there have been
a number of blade failures, two of which have now been closely examined by the Forest
Products Laboratories (FPL). This group are experts in the field of properties of wood and
adhesives for wood.
Both of these props were two-blade Performance Props, and were flying on Long-EZ's. John Sheffles prop failed catastrophically in flight, loosing about 1/3 of one blade. Fortunately John made a safe landing on a road. The FPL engineers concluded that this prop had fluttered in flight, and the heat generated by the high frequency flexing at the trailing edge played a large part in the structural failure. Terry Schubert's prop fractured in a simple bending break. Again there was evidence of high temperatures. This failure occurred right in the area where the exhaust plume impinges on the prop blades.
These were contributory to the failures, but not the only reason. Both props had higher than normal moisture content in the area of the blade failures, and lower than normal moisture content in the hub and blade shank. The FPL believes that heating in the hub and shank area from the hot prop extension, tends to drive the moisture from the hub out into the blades (helped perhaps by centrifugal force? ed. comment). The combination of high moisture content and heat makes the wood glassy, crystalline and brittle. This condition in wood is known as depolymerization, and creates a small amount of acetic acid (vinegar). which has a strong and distinct smell.
In view of the number of wood props functioning normally on many hundreds of homebuilts (and factory airplanes, J3's, clamps etc.) We must conclude that this cannot be a wide spread problem.
What can you do to protect yourself?
1). If there is any evidence of overheating (charring) of either or both blades from the hot exhaust, rotate your prop one bolt hole. Our experience has shown that if a two-bladed prop is installed at approximately the 1 o'clock 7 o'clock position, with the number one cylinder at top dead center on the compression stroke, this overheating due to the exhaust, will be completely eliminated. You will still see gray exhaust deposits on the blades, but they will remain below 150 degrees F at all times.
2). Seal the inside of the center hole in the prop with an epoxy such as the West System. We seal all bolt hole and counter-bored holes as well.
3). Always leave the prop in the horizontal position when parked. This prevents the moisture that is in the wood from migrating into the lowest blade due to gravity.
4). Repair any damage to the finish on the wood prop immediately, and refinish and check the balance of the prop once a year or as required.
5). Inspect your prop carefully and often for signs of blistering and or checking or cracking on the surface, especially on the forward face (top of airfoil).
6). Editor Opinion - I would conduct this inspection even more often and carefully if you fly a prop with very thin and/or small blade sections, such as the Performance Prop and the Warnke "almost constant speed" type props. Be especially vigilant about flutter, If you feel a sudden unexplained vibration or roughness as you bring the power up to do your mag check, or in flight, reduce power, land and inspect for evidence of flutter. This may be in the form of a darker color in the wood especially near the trailing edges, at or near the 2/3 blade station (typically the highest activity area of the prop). There may also be checking or cracking of the finish along the trailing edges. Once you have experienced flutter of a prop, you are not likely to forget it. It is a harsh roughness or vibration that comes on rather suddenly. Any prop that exhibits this tendency to flutter, should be discarded and not flown.
NOTE - Terry Schubert, editor of Central States Newsletter, has been conducting a survey of a number of different pusher aircraft, with different exhaust configurations, looking for signs of high temperatures in the hub area (under the spinner) and in the area of the exhaust plume. We will publish the findings when we receive them from Terry. (By the way, if you do not already subscribe to the Central States Newsletter, you are missing a sure bet. This is an excellent source of neat ideas and information on all the RAF designs as well as other composite aircraft. Send $20 to Terry Schubert, 9283 Lindbergh Blvd, Olmsted Falls, Ohio, 44138).
Non recommended props
Be very cautious of narrow chord, two-blade un-reinforced wood props, particularly if the blades have sweep. If you are flying one of these be extremely vigilant for any unusual vibration. Read the following examples of wood prop failure, reprinted from CP46, page 7.
We recently heard from an EZ builder pilot
who was using a non-RAF recommended prop. After only 22 hours of operation, upon noticing a
new feeling or vibration, closely examined the prop and found compression failures in
the wood about 8 to 10 inches out from the spinner on the forward face of both
blades. Remember, most times you will get some type of warning before the prop really lets
go. Pay attention. Any new noise or vibration should be investigated.
We are becoming more and more advocates of the so called "multi-laminate" Canadian maple wood props. In our experience these props are stronger and allow more torque to be applied to the prop bolts without crushing the prop hub. We have routinely used 300 inch/lb of torque on the 3/8 inch prop bolts found on Lycoming 0-235 and Continental 0-200 with these props with no problems at all. Caution: Do not use more than 220 inch/lb of torque on the older style four or five laminations of birch-type props. Also, remember to check the prop bolts frequently, particularly when the prop is new.
The following is an incident report from VariEze builder/pilot and Defiant builder, Emerson Grooters of Norway. It concerns the failure of a propeller and points up the importance of selecting a good reliable prop. If you want to experiment with untested or unusual props, do yourself a favor and follow the Formula One racing guys lead, install a safety cable on your engine. This is at least a 1/8 inch aircraft cable that ties the engine to the airframe. If you lose a prop blade, and don't get the engine shut down in time, the engine could come loose from the firewall.
"During testing of a new wood prop which I intended to use for some altitude and speed records, the prop failed with multiple fractures in the root area of both blades - forward face. The prop was not one recommended by RAF, however, I think that there my be a good point here for everyone - that is, just because you have a wooden prop don't think that it will automatically work with your aircraft/engine combination. I had 2.15 hours on the prop when I retorqued the bolts prior to an altitude test of the aircraft, my RR 0-240 powered VariEze. I took off, climbed to 10,000 feet and checked various power/cruise settings for about 25 minutes. I then climbed direct to 20,000 feet and started full throttle cruise test prior to further climb. At about 107 KIAS and 2700 rpm I noticed an increase in vibration from the engine. The vibration was not severe; however as it was a change from the norm, I canceled my next planned step to 25,000 feet, reduced power to about 1/4 throttle and descended for landing. Total flight time 1.25 hours and total on the prop, 3.40 hours. On landing I saw the cracks in the prop. I was also glad that I had just had my chute inspected and repacked, even though I hadn't had to use it.
Last summer, my wife and I stopped to talk to another couple about their new beautifully executed homebuilt. They were both dead about 15 minutes later in a crash resulting from losing most of a prop blade. It was a one piece wooden prop recommended for their type mr--raft - not a RAF. I mention this because, just because you have a nice looking wood prop does not mean you are home free. Also any change from the normal operating conditions of your aircraft should be fully investigated as soon as possible. A precautionary landing may be inconvenient and take a little time but it could save your aircraft and yourself." Emerson Grooters