The pavement-loading (i. e., flotation) limit is one of the drivers for tire design. This section presents relevant information for preliminary tire sizing to establish the section width (Wg), height (H), and diameter (D), as shown in Figure 7.14. The rim diameter of the hub is designated d. Under load, the lower half deflects with the radius, Rload. The number of wheels and tire size is related to its load-bearing capacity for inflation pressure and the airfield LCN for an unrestricted operation. For heavy aircraft, the load is distributed over the number of wheel and tires. The FAA regulates tire standards.
Table 7.5. Tire types (tire aspect ratio H/Wg and tire lift ratio, D/d)
Tires are rated based on (1) unloaded inflation pressure, (2) ply ratings for holding shape under pressure, (3) maximum static load for the MTOW (i. e., flotation consideration), and (4) maximum aircraft speed on the ground. Basically, there are mainly three types of tires from nine categories, as described herein. (See the Michelin and Goodyear data sourcebooks listed in the references.)
Types I and II: These types are becoming obsolete and are no longer produced. Type I is intended for a fixed undercarriage.
Type III: This type includes low-pressure tires that provide a larger footprint or flotation effect. They have a relatively small rim diameter (d) compared to overall tire diameter. Speed is limited to less than 160 mph. The tire designation is expressed by its section width, WG, and rim diameter, d (Figure 7.14). All dimensions are in inches. For example, a typical small-aircraft tire designation of 6.00-6 means that it has a width of 6.00 inches (in hundredths) and a rim diameter hub of 6 inches.
Types IV, V, and VI: These types no longer exist.
Type VII: These are high-pressure tires that are relatively narrower than other types. They are widely used in aircraft with pressure levels from 100 to more than 250 psi that operate on Type 2 and Type 3 airfields. Military- aircraft tire pressure can reach as high as 400 psi. Tire designation is expressed by the overall section diameter (D) and the nominal section width, WG, with the multiplication sign (x) in between. All dimensions are in inches. For example, 22 x 5.5 has an overall section diameter of 22 inches and a section width of 5.5 inches.
New Design Tire (three-part nomenclature): Except for Type III tires, all newly designed tires are in this classification. A Type VIII tire also has this designation. This type uses a three-part designation shown as (outside diameter, D) x (section width, WG) – (rim diameter, d). These are also known as biased tires, which are intended for high-speed aircraft with high tire-inflation pressures. Dimensions in FPS are in inches and dimensions in SI are in millimeters but the rim diameter is always in inches. For example, a B747 tire has the designation, 49 x 19.0-20, meaning that it has an outside diameter of 49 inches, a section width of 19 inches, and a rim diameter of 20 inches. New
Table 7.6. Tire pressure
* Depends on number of wheels.
** See Appendix E for more options. Also consult Jane’s manual.
tires also have radial types; the three-part designation has an “R” instead of a hyphen. An example of a radial tire in SI is 1400 x 530 R 23. There is a special designation that precedes the three-part nomenclature tires with a B, C, or H. The description of these construction details is beyond the scope of this book.
There are small tires not approved by the FAA that are used in the homebuilt aircraft category. This book addresses only Types III and VII and the New Design Tire.
Several tire manufacturers are available from which to choose, as in the case of the automobile industry. Tire manufacturers (e. g., Goodyear, Goodrich, Dunlop, and Michelin) publish tire catalogs, which provide important tire data (e. g., dimensions and characteristics) in extensive detail. Appendix E lists data from the manufacturers’ catalogs needed for the coursework in this book. Aircraft designers have the full range of tire catalogs and contact tire manufacturers to stay informed and benefit mutually from new tire designs.
Under load, a tire deflects and creates a footprint on the ground. Therefore:
load on tire = (footprint x tire pressure) (7.17)
For tire static deflection:
5tire = (maximum radius at no load) – (minimum radius under static load)
= D/2 – Rload, (7.18)
where Rload equals the radius of the depressed tire under load. It can be expressed as a percentage of the maximum radius.
Table 7.6 lists the typical tire pressures for the range of aircraft weights.
Under a typical static load, tire deflection is kept at a maximum of a third of the maximum height (H). As aircraft speed increases, the load also increases on tires as dynamic loading. During landing impact, the deflection would be higher and would recover sooner, with the tire acting as a shock absorber. Bottom-out occurs at maximum deflection (i. e., three times the load); therefore, shock absorbers take the
Figure 7.15. Ground friction coefficient
impact deflection to prevent a tire from bottoming-out. Section 7.9 discusses tire – deflection calculations; corresponding typical tire pressures for the sizes are given in Table 7.6.
Tire sizing is a complex process and depends on the static and dynamic loads it must sustain. This book addresses tire sizing for Type 2 and Type 3 runways. One of the largest tires used by the B747-200F has a main and nose gear tire size of 49 x 19-20 with an unloaded inflation pressure of 195 psi. Sizes used by existing designs of a class are a good guideline for selecting tire size.
Use of an unprepared runway (i. e., Type 1) demands a low-pressure tire; higher pressure tires are for a metal runway (i. e., Types 2 and 3). The higher the pressure, the smaller is the tire size. Civil aircraft examples in this book use a Type 3 airfield; military aircraft examples use Types 2 and 3 airfields. Small aircraft use a Type 1 airfield for club usage.