Aircraft Tyres — Page 122, Lesson 160

Let’s start with the big picture. The aircraft wheel is fitted with a pneumatic tyre, and that tyre can be either tubeless or it can have an inner tube. Tubes tend to be fitted to light and older aircraft. The tyre is usually inflated with nitrogen. Nitrogen does two jobs: it absorbs shock, and it supports the weight of the aircraft. The cover — that’s the outer casing of the tyre — restrains and protects the tube from damage, maintains the shape of the tyre, transmits braking, and provides a wearing surface. Now let’s look at the construction of that cover. The tyre cover is a casing made of rubber, reinforced with plies of cotton, rayon, or nylon cords. Here’s a key point: those cords are not woven. They are arranged parallel in single layers, and they’re held together by a thin film of rubber. That film prevents cords of adjacent plies from cutting one another as the tyre flexes in use. During construction, the plies are fitted in pairs, and they’re set so that the cords of adjacent plies are at 90 degrees to one another — that’s for a bias, or cross-ply, tyre. In a radial tyre, the cords run from bead to bead at approximately 90 degrees to the centre line of the tyre. So you have two different cord orientations depending on the tyre type. To absorb and distribute load shocks, and to protect the casing from concussion damage, there are two narrow plies embedded in thick layers of rubber, situated between the casing and the tread. Those special plies are called breaker strips. Now, how does the casing stay on the wheel? The casing is retained on the rim by interlocking the plies around inextensible steel wire coils, forming what are called ply overlaps. That portion of the cover is known as the bead. Each tyre is given a ply rating by the manufacturer. This is important: the ply rating does not relate directly to the number of plies in the tyre. It is the index of the strength of the tyre. For example, a 49 × 17 size tyre with a ply rating of 32 only has 18 plies. So the rating is a strength index, not a ply count. The wire coils are made rigid by bonding all the wires together with rubber. To ensure a strong bond, each wire is copper plated. The bead coil is also reinforced by winding it with strips of fabric before the apex and filler strips are applied. The apex strips are made of rubber and are located by rubberized fabric filler strips. They provide greater rigidity and less acute changes of section at the bead, and they also provide a greater bonding area. Finally, the bead portion is protected on the outside by chafer strips of rubberized fabric. Let me show you the make-up of the tyre so you can see how all these layers fit together. Now, to help describe the cover, it’s divided into regions or sections. The tread of the tyre is situated in the crown and shoulder section. And note this: the term ‘tread’ is applied whether the rubber is plain and smooth, or moulded on a block pattern. The most popular tread pattern is called Ribbed. It has circumferential grooves around the tyre. Those grooves assist in water dispersion and help prevent aquaplaning — also called hydroplaning. The grooves also help improve traction and contact grip between the tread and the runway surface. There’s another pattern, not seen so frequently now, but still termed the all weather pattern — that’s the Diamond tread pattern. So to summarise the key points: nitrogen inflation absorbs shock and supports weight; the cover protects the tube, maintains shape, transmits braking, and provides the wearing surface; plies are parallel cords, not woven; bias tyres have cords at 90 degrees to each other, radial tyres run bead to bead at 90 degrees to the centre line; breaker strips absorb load shocks; the bead interlocks plies around steel wire coils; ply rating is a strength index, not a ply count; and the tread sits in the crown and shoulder, with Ribbed being the most popular pattern for water dispersion and grip.