With very few exceptions, all aircraft need an undercarriage. This performs two main functions:
o It supports the aircraft on the ground.
o It absorbs the shock of landings and provides smooth taxying.
There is more to an undercarriage than just carrying out these functions, however. It must support the aircraft in the desired attitude on the ground, so that the drag on the take-off run is minimised, and the aircraft taxies without any tendency to float at normal speeds. It must withstand the loads that will occur during all movements on the ground, including braking and side loads. The undercarriage serves no function at all during flight, so it must be as small and light as possible.
There are many different layouts of undercarriage in current use. The type chosen depends on the type of aircraft and its intended use. For almost all aircraft, except some light aircraft, the tricycle layout is preferred, because it supports the aircraft in a horizontal attitude, giving low drag during the ground run. However, there are several different kinds of main unit, for different installations.
The designers main concern when choosing the type of main unit is how many wheels the unit will have, and their arrangement. This will depend on the weight of the aircraft and the way in which the undercarriage is to be retracted.
Aircraft wheels.
Each main-wheel unit may contain a single wheel, a pair of wheels side by side or in tandem, or four or more wheels. As aircraft become heavier, the loading on each wheel increases, leading to a considerable increase in the damage done to runways. By having the weight spread over a greater number of wheels, the contact pressure of the undercarriage is reduced. This also increases safety if a tyre bursts on landing. The Boeing 747 has 18 wheels - four main units, each with four wheels, and a dual nose-wheel unit.
Apart from the single-wheel main unit, the simplest type is the twin-wheel side-by-side (or dual) arrangement, which is used on many fighters, as well as medium-sized transports such as the Boeing 727 and 737, the Fokker F28 and many turboprop aircraft.
By far the most common arrangement of main units for large aircraft is the dual-tandem layout, also known as a bogey or truck. This is widely used on commercial aircraft, since it gives a good combination of low ground pressure and relatively easy retraction arrangements. The Boeing 747, 757, 767 and the Airbus series are just a few examples of the many aircraft using this arrangement. It is easily capable of retracting forwards or sideways, and the bogey can be rotated to fit into awkward spaces. If necessary, the bogey can be held parallel to the ground during retraction, to allow a shallow well to be used.
Retracting undercarriages.
One of the main reasons for the particular choice of undercarriage arrangement is the problem of retraction. The main units of low-wing aircraft are usually retracted into the wing, which is quite straightforward in most cases. With high-wing aircraft, this would require a long undercarriage, which increases weight. Twin turboprop aircraft have engine nacelles on the wing, and it is quite common to retract the main legs into these nacelles. Otherwise, they must be stowed in the fuselage. However, the points of contact of the undercarriage with the ground must be far enough apart to make the aircraft stable during take-off, landing and taxying, so the shape of the main units can become quite complex.
The tandem undercarriage is rarely used. However, a variation of the tandem arrangement is the jockey unit, which comprises two or three levered legs in tandem on each side of the fuselage, sometimes sharing a common horizontal shock absorber. It is particularly useful for high-wing medium-sized transport aircraft, because the undercarriage is easily retracted into panniers - bulges on the side of the aircraft. This gives a constant width of cargo area in the fuselage, and of course the widest load that can be carried is often restricted by the narrowest point in the load space. Among the advantages of this design are excellent rough-field performance and the ability to `kneel the aircraft by partially retracting the undercarriage to reduce the slope of loading doors. This is particularly useful where the aircraft is used to transport vehicles. The units also retract into a small space, without pentrating into the load space.
There are a number of other wheel arrangements in use, including tri-twin tandem, dual twin, dual-twin tandem and twin tricycle, but the more complex the type the less commonly it is used. However, as increasingly large aircraft are developed to take maximum advantage of crowded airspace, the number of wheels in undercarriages must be increased to keep ground pressures reasonably low, and limit damage to runways and taxiways.
With combat aircraft, the main undercarriage has another limitation, which is the requirement to clear stores fitted under the fuselage. The undercarriage must not interfere with these stores either in its extended position or during retraction. Many combat aircraft carry under-fuselage stores, and this can result in some rather awkward-looking undercarriage arrangements.
The undercarriage design will normally allow for steering, and a reasonable turn radius is needed for ground maneuvering. At the same time it must have a safety mechanism that prevents the nose wheel from being turned after retraction, and ensures that the wheel is straight for landing.
EASA/FAA wheel repair.
If the undercarriage hits a large obstacle that the aircraft wheels cannot climb, there is a risk that considerable damage may be done to the structure that supports the undercarriage. Shear pins are fitted, which will fail and allow the collapse of the undercarriage before the load rises beyond a safe level. The aircraft will still be damaged, of course, but not to the same extent as it would without this feature. The position of the undercarriage units is very important, particularly the main units. If they are too far forward, the aircraft may tip during loading and taxying. If they are too far aft, the aircraft will pitch forward violently during landing, which could cause the nose leg to collapse. If the main units are not sufficiently wide apart, the aircraft may tend to roll sideways on the ground, especially in side winds and during taxying. If they are too far apart, the aircraft may be prone to ground loops - a sudden violent turn to left or right, perhaps even more than a full circle. The nose leg must also be positioned carefully because its distance from the main units affects the proportion of the total weight that it carries. If it is too lightly loaded, the steering may not be effective, but the load must not be so high as to require the nose leg and associated structure to be unnecessarily strong and heavy. The designer will often be limited by the available structure and, as always, the position may be a compromise.
Most EASA FAA wheel repair is mercifully routine and not as a result of trauma. Airbus Boeing Tucano Honeywell Goodrich and Dunlop aircraft wheels repaired reclaimed and modified can be done at a choice of approved repair facilities.
