The use of engineering plastics in the Zodiac 601 HD

By Alan Cozens

Modern engineering plastic materials have very valuable properties when properly applied in light aircraft construction. This note deals only with materials such as nylon which are readily available from plastics distributors in most major cities.

Some of these materials are particularly appropriate to the Zodiac, where for reasons of cost and weight many bearings are formed by providing a hole in the aluminium sheet structure so that a rotating or sliding steel tube may operate. This combination of materials is not normally used for engineering applications due to the tendency for aluminium alloys to gall or weld to steel, with attendant rapid wear. Aluminium also tends to 'load' with grit or dust where this is present, forming an abrasive matrix which will rapidly wear the steel member. Examples of aluminium/steel bearings on the Zodiac are the nose and main gear leg bearings and the control system torque tube bearings.

These bearings are in flat sheet aluminium, usually 1/8in. It is a relatively simple mater to 'back up' these bearings with say 6mm nylon sheet with minimum weight and without structural risk, by bolting the additional bearing material to the existing sheet metal surface. Bearing life, quietness and smoothness of operation are dramatically improved. To study the point, consideration of the main control stick torque tube bearings is suggested. A more dramatic example is the main and nose gear where runway dust is an additional hazard, and my aircraft after only 200 hours of operation is showing unacceptable wear on the main gear leg bearings which were left 'plain'. By contrast the nose leg nylon bearings have little perceptible wear, despite very active operation during taxiing. The difficulty of repairing the worn out aluminium bearings once the aircraft is in service should be borne in mind.

I would like to stress the importance of choosing the correct material from a bewildering range. In fact the number of possibilities drop dramatically when one considers the requirements for our application:-

good structural strength, good impact resistance, good bearing qualities, particularly resistance to abrasion, resilience to 'loading' with grit and dust, total resistance to hydrocarbons e.g. oil, Avgas, etc, reasonably easy to machine and cut.

There are possibly only 3 contenders that are widely available, and reasonably cheap. These are in my order of preference, nylon, fabric based phenolic (example-'Tufnol') and acetal (example-'Delrin'). There are various grades and types of these 3 but they are all suitable. Nylon 6.6 stands head and shoulders above the others, although its very abrasion resistance means cutting is more difficult. It is also available in a molybdenum disulphide filled grade, making lubrication unnecessary in most applications. Note none can operate above about 80C, nor are they suitable for high speed heavy load applications, as meltdown can occur! Do not forget PFA Engineering modification approval will be required in the UK, but approved drawings are available from the writer.

Suppliers usually have offcuts available, and for £20 I bought enough nylon for 2 aircraft. However a word of caution on offcuts - you must know exactly the material and grade being offered - a good supplier maintains this as a routine.