Ben Owen

DRIVING THE PROP In our experimental aircraft, we have several experiments going at once, and frequently one of these is the experimental propellers that we use to push or pull our airplanes. Ed Sterba, EAA Technical Counselor 1356, has some suggestions from his

experiences as a prop builder. Slip-In Drive Bushings: The usual propeller hub has a series of bolts that hold the propeller to the hub. On the hub itself, what are known as driving bushings align the propeller to the hub, and, of course, the attach-

ment bolts pass through the propeller, through these bushings, through the spinner bulkhead extension (if any) and the drive hub itself. The cutaway shows drive bushings. Lycoming and Continental drive bushings are 3/4" deep and are pressed in place. There are some propeller extensions that have a slip fit into the aluminum prop extension which requires the propeller itself to be attached to ensure that they do stay in place. The propeller is primarily driven by the friction between the rear propeller disk and the face of the prop extension or face of the propeller hub if no extension is used. This is why it is very necessary to keep these propeller bolts tight. The drive bushings assist this attachment, and these propeller forces are primarily taken in shear through the drive bushing to the prop extension or hub. As shown in the diagram, if you have a slip fit drive bushing set and if the counter bores in the propeller are too deep for the drive bushings, they can eventually work themselves into the propeller and out of the extension or hub. The bolt on the prop extension itself then is going at least partially through a 5/8" hole near the propeller when it is a 3/8" bolt. This makes it possible for the propeller bolts themselves to fail. This has happened! The addition of a thick spinner bulkhead between

the propeller and hub extension can aggravate this condition. The shorter the bushings, the greater the potential for a problem. On one particular extension, these bushings only extend 112" from the face of the extension. If the propeller bolt holes are drilled deeply for this bushing, you can see where the problem may develop. The opposite problem can occur if the counter bore holes in the propeller are bored to the exact depth of the drive bushings and the propeller is then installed without the spinner rear bulkhead. In this case, the drive

bushings will bottom out in the counter bore holes before the propeller is tight against the extension face. This problem has occurred when the pilot was forced to remove the spinner on a cross country flight because it was cracked or damaged. The bolts can be correctly torqued, but there is no contact between the prop face and the extension face. The failure in this case will probably occur quite quickly. The solution to this problem is

to make sure that the combination of drive bushing, bulkhead and propeller add up correctly to hold the bushings in place. A change to press-in fit bushings can also be made, or in this case, 3/8" I.D. by 5/8" O.D. spacers can be cut that will minimize the amount of movement possible for the bushings. This condition can exist on either wood or metal propellers with slip fit drive bushings. It can also occur on press-in-place drive bushings when the bushing hole in the propeller is too shallow.

Castle Nuts vs. Plastic Stop Nuts On Your Propeller: When the propeller is held to the hub by nuts and bolts, the question often arises whether to use castle nuts and cotter pins or the self locking, plastic stop nuts. On VW engines, we normally use the

plastic stop nuts, since it is easier to install and allows a quick check on the bolt torque.

-.090" Spinner Bulkhead

For instance, if the nuts are on the back face of the propeller flange, these can often be reached on a preflight. If they can be turned by the nut, it is obvious that the bolt is too loose. People being what they are, a person is more likely to check the torque on these important nuts and bolts when they don't have to go through the safety procedure of cotter pins or safety wires. In other words, it is hoped that they will check the torque more often. A wood propeller will change thickness with moisture and temperature changes, so four to six times per year is not too often to check. A number of the pusher type aircraft have had propeller bolt failures, which may be attributed to the engine and exhaust heat on long, cross country flights, drying out the wood in the hub area and causing it to shrink enough to reduce the prop to hub friction. Loose bolts allow movement, creating more friction and heat to the point that the wood can be charred black and the bolts can fail. In this area, the pusher propeller of wood may be more susceptible to looseness than a tractor type installation.

3/8" Did.

Bolt hole

Preflight On Wood Props: Most people agree that a wood prop is less susceptible to fatigue crack failures than a metal prop.

5/8" Thick Prop Hub on Engine 5/8" Did. 3 / 4 "

There is still a need to give the wood propel-

ler a careful preflight to make sure damage long

Drive Bushing Slip-fit on prop

extension or hub

didn't occur the last time it was flown. Since the trailing edge of the prop is thinner than

the leading edge, it needs special attention. The wood prop will split along the grain,

rather than across the blade, as with the metal prop. So any damage to this delicate trailing edge is cause for concern. The normal thrust loads will put this section of wood in tension. SPORT AVIATION 37