-Ben Owen-

Submitted by Ed Sterba Sterba Aircraft Propellers 412 S. 5th St. Delavan, Wl 53115

With the advent of higher performance homebuilts, many builders are faced with the choice of a fixed pitch propeller or a constant speed prop. The weight and cost (both initial and maintenance) of the constant speed prop make the fixed pitch propeller look more attractive to many. As seen in the recent CAFE propeller contest the performance edge of the constant speed prop is not as great as a lot of people probably thought. The fixed-pitch propellers available are usually constructed of metal or wood. As people have found out, taking an existing fixed pitch metal prop, sawing off the ends and then repitching to the new, higher blade angles does not always result in a safe product. Unless a vibration analysis is carefully conducted the possibility of blade failure is higher than many pilots are willing to accept. The performance of the new metal propeller is not always as good as expected since the blade angles may not have been changed at all stations to meet the new pitch. Therefore, you don't have a true pitch except for the one spot twisted. Which brings us finally to the wood fixed pitch propeller and its major drawback - rain erosion. The method applied in the past to protect the wood from rain has normally been to wrap the leading edge of the blade with metal riveted in place. This would certainly do the job but is not without its disadvantages - cost, difficulty in getting a smooth fit and the possibility of its coming lose. One replacement for this type edge has been the use of the urethane leading edge. The

urethane family of materials have proven to be one of the toughest around. If it is good enough for outdoor roller skate wheels, it should stand up in rain. One of the KRs coming to Oshkosh this year spent 3 hours in rain and had very little damage to show. Probably light sanding of the urethane surface would restore the radius. It is not a perfect material but it is a good compromise. The urethane I purchase is made for the foundry industry for the protection of core boxes to prevent sand erosion and has a Shore hardness of 95A, if that means anything to you. It is tough stuff but luckily requires a long enough cure time that allows it to be easily worked with hand tools 8 to 24 hours after pouring. It is a two part resin with a 40 to 100 ratio by weight that I enhance by zapping in the microwave to thin for mixing. (You will forget to select a low heat range only one time.) The mixed product looks and pours like maple syrup which means your form must be set on the leading edge of the prop carefully. The plastic form is waxed and then clamped in place with a batten underneath. Both ends are then dammed up with tape. The blade must be leveled and then baby-sat until

the resin gells and quits trying to run on the floor. Practice, practice. The liquid material is not difficult to use but should be protected from moisture in the air as best as possible. After an evening cure, the urethane may be shaped with normal hand tools such as planes, files and abrasives. With sufficient cure it can be machined. Which brings up one of its unusual characteristics - it continues to release a vapor for several weeks that keeps many finishes from sticking or curing on top of it. Most tapes, including electrical tape, will not adhere to it. This makes painting difficult in the first few weeks. After the propeller has spent time in the field, the tips can then be painted. There have been a few cases where the very tip of the urethane has come loose from the wood. It was even noticeable to people on the ground as a noise change as it fluttered. In most instances the leading edge can be glued back in place with epoxy and will cause no further problem. I normally drill small holes in the wood before pouring the urethane to allow for a mechanical lock. There is probably a better material for prop leading edges but it will have to be very good to replace this extremely tough rubber. SPORT AVIATION 67