When The Rubber Meets The Runway
The landing gear is one of the most critical elements in any aircraft, providing safe takeoffs and landings. Aircraft tires and wheels are subject to conditions of extreme heat many times during the course of a day. The wheels and tires of today require a precision only dreamed of during the early days of commercial aviation. During this blog, we'll examine the current situation of aircraft tires and see where they're headed both from the standpoint of safety and market share.
Dedicated aircraft tires did not come into wide use until the 1930's, since many early planes were constructed of either fabric or wood materials and were not capable of lifting heavy weights by today's standards with many aircraft using bicycle or automobile tires. One of the earliest aircraft tire companies was the Goodyear Tire and Rubber Company in Akron, Ohio. Named for Charles Goodyear, the man who developed vulcanized rubber, the company was established in 1898 by Frank Seiberling. Seiberling had borrowed $3,500 from a brother-in-law to purchase the first factory. Though it began with only thirteen workers, Goodyear quickly emerged as a leader in the production of rubber items, to include hoses, horseshoes, pneumatic carriage, bicycle and automobile tires. By 1926 Goodyear was the largest rubber company in the world. Producing the first airplane tires in 1909, Goodyear built the first of a series of blimps ten years later. In the mid 1930's, Goodyear attempted to enter the aviation market, but was thwarted by a series of labor disputes. Goodyear management made the decision to both decrease wages and increase production, which proved to be a recipe for disaster. The newly formed United Rubber Workers struck the Akron plant, pursuing a two-pronged approach, utilizing picket lines as well as a sit-down strike. A sit-down strike meant workers could report to their work stations, but could not perform job duties. The Wagner Act, passed by Congress in 1935, made unions legal for the first time. Finally, the rubber workers became part of a larger labor organization, the Congress of Industrial Organizations. The CIO acted as an umbrella for a number of unions, giving them more bargaining power. After a year and a half, the strike was finally settled, but it delayed Goodyear's entry into the aviation market.
By World War II the production of aircraft tires were on the rise for both civil and military aircraft.
As the war began, a rival concept was considered by the USAAF leadership, which negated the use of tires on landing gear. The concept of tracked landing gear was first presented to General H.H. Arnold in November 1939 by J.W. Christie, inventor of the Christie tank. Following his meeting with Arnold, Christie was directed to construct a tracked landing gear on a Douglas A-20 at Wright Field from a series of drawings. For the track, Christie planned to use a belt made by Goodrich Corporation designed for heavy construction work. Dowty Equipment Corporation was awarded a contract in June 1941for the engineering design for the tracked gear on the A-20 in the amount of $20,000. The Dowty concept proposed the use of an air-inflated belt, two main rollers with brakes, two smaller auxiliary rollers sprung over the part of the belt touching the ground, and a smaller roller or idler mounted under the upper span of the belt to provide constant tension. In 1942 a new contract was issued to Goodyear for both design and fabrication of the A-20 gear in the amount of $100,000. Goodyear then subcontracted the developmental work on the track system to Firestone. Later that year Firestone built a rubber-belted track gear for a Stearman PT-17 aircraft and later installed the same gear on a Fairchild PT-19 utilizing a different shock absorber. While the tracked system worked well on both aircraft, the A-20 project was another story. The A-20 gear used a conventional nose wheel with the tracked gear. However, the volume and weight of the track made it a fixed gear rather than a retractable one. Though a number of tests conducted in early 1943 were successful under varying ground conditions, the tracked gear concept was abandoned shortly after the completion of testing. While the tracked gear offered superior traction and support over a wheeled system, it's two primary flaws were its weight (nearly twice that of a wheeled system) in addition to a fifteen per cent longer takeoff distance.
The proliferation and variety of aircraft tires after World War II has paralleled that of the automobile industry. An aircraft tire is constructed for the purpose it serves. Unlike an automobile or truck tire, it does not have to carry a load for a continuous period of time, but must be able to absorb the high impact loads of landing, as well as being able to operate at high speeds during landings and takeoffs. To gain an understanding of how an aircraft tire operates, we must be able to identify the various components of tires and their purpose. The tire bead is an important part of the tire, since it anchors the tire carcass and provides a firm mounting surface for the tire on the wheel rim. Because of this, tire beads are strong and are usually made from high-strength carbon steel wire bundles encased in rubber. One, two, or three bead bundles may be found on each side of the tire depending on its size and the load it is designed to carry. Radial tires have a single bead bundle on each side of the tire. The bead transfers the impact loads and deflection forces to the wheel rim. Apex strips are additional rubber formed around the bead to give a contour for anchoring the ply turn-ups. Layers of fabric and rubber called flippers are placed around the beads to insulate the carcass from the beads and improve tire durability. Chafer strips made of fabric or rubber are used in this area. These strips are laid over the outer carcass plies after the plies are wrapped around the beads. The chafers protect the carcass from damage during mounting and demounting of the tire. They also help reduce the effects of wear and chafing between the wheel rim and the tire bead, especially while the tire is moving. Carcass piles, or casing piles, are used to actually form the tire. Each ply consists of fabric, often nylon, sandwiched between two layers of rubber. The piles are applied in layers to give the tire strength and form the carcass body of the tire. The ends of each ply are anchored by wrapping them around the bead on both sides of the tire to form the ply turn-ups. The angle of the fiber in the ply is adjusted to create a bias tire or radial tire as desired. Radial tires usually require fewer plies than bias tires. After the plies are placed in the tire, bias tires and radial tires each have their own type of protective layers on top of the plies but under the tread of the running surface of the tire. On bias tires, these single or multiple layers of nylon or rubber are called tread reinforcing piles. On radial tires, an undertread and a protector ply perform a similar function. These additional piles strengthen and stabilize the crown area of the tire while reducing tread distortion under load and increase stability of the tire at high speeds. The reinforcing piles and protector piles serve to resist puncture and cutting while protecting the carcass body of the tire. The tread is designed to stabilize the aircraft on all operating surfaces and is subject to wear. Many aircraft tires are constructed with protective undertread layers with extra tread reinforcement accomplished by breakers. These nylon cords strengthen both the tread and carcass piles. A number of tires designed with reinforced tread, may be retreaded as the need arises. The sidewall of an aircraft tire is designed to protect the carcass piles. It may contain compounds designed to resist the effects of ozone on the tire. The tire sidewall contributes little strength to the core body, with its main function being overall protection of the tire. The sidewall area also contains information about the tire. Tubeless tires are lined with a thicker, less permeable rubber. This replaces the tube and contains the nitrogen or inflation air within the tire and keeps it from seeping through the carcass piles.
Aircraft tire treads are designed for different uses. For example, a rib tread tire, which has several prominent grooves in the tread area, is designed for use on paved surfaces, while a diamond tread aircraft tire is designed for use on unpaved ones. A tire with an all weather tread combines a rib center tread with a diamond tread pattern on the edges. Smooth tire treads are found primarily on older and slower aircraft, which are relatively small and not as dependent on brake application, as with larger aircraft. Chine tires are small tires used on the nose wheels of jet aircraft. They have six prominent grooves, which serve to deflect runway water away from jet intakes. Aircraft tires are both bias ply and radial, like their automotive counterparts. A bias ply tire has the fabric bias oriented with and across the direction of rotation and the sidewall. Since fabric can stretch along the bias, the tire is flexible and can absorb loads. The addition of piles will serve to strengthen the tire. A radial tire has the fiber strands oriented with and at a ninety degree orientation to the direction of rotation and the tire sidewall. This restricts directional flexibility and the flexibility of the sidewall while it strengthens the tire to carry heavy loads. The aircraft tires market has made significant gains over the past few years, due to the expanding airline network, in addition to a surging global demand for both commercial and military aircraft. From initially converting radial tires to materials such as Kevlar, an aromatic polyamide, aircraft manufacturers are adopting hybrid composite materials for the manufacture of military aircraft tires. These efforts are bearing fruit, with the military segment alone registering a projected growth rate of six per cent per year through 2031, in spite of setbacks during 2020 as a result of the COVID pandemic. The rising production and usage of aircraft have led to increased maintenance activities with technological advances contributing to the growth of the tire market, as well. Though narrowbody aircraft are currently experiencing the most growth, the recovery of the widebody aircraft segment is not far behind with the resurgence of the travel industry as a whole.