Landing Gear Arrangement

Definition

The landing gear is that portion of the aircraft that supports the weight of the aircraft while it is on the ground. The landing gear contains components that are necessary for taking off and landing the aircraft safely. Some of these components are landing gear struts that absorb landing and taxiing shocks; brakes that are used to stop and, in some cases, steer the aircraft; nose wheel steering for steering the aircraft; and in some cases, nose catapult Components that provide the aircraft with carrier deck takeoff capabilities. The landing gear is the principle support of the airplane when parked, taxiing, taking off, or when landing. The most common type of landing gear consists of wheels, but airplane0s can also be equipped with floats for water operations, or skis for landing on snow. The landing gear consists of three wheels—two main wheels and a third wheel positioned either at the front or rear of the airplane. Landing gear employing a rear mounted wheel is called conventional landing gear. Airplanes with conventional landing gear are sometimes referred to as tail wheel airplanes.

When the third wheel is located on the nose, it is called a nose wheel, and the design is referred to as a tricycle gear. A steerable nose wheel or tail wheel permits the airplane to be controlled throughout all operations while on the ground. The landing gear is the structure that supports an aircraft on the ground and allows it to taxi, take-off, and land.

The primary functions of a landing gear are as follows:

1. To keep the aircraft stable on the ground and during loading, unloading, and taxi

2. To allow the aircraft to freely move and maneuver during taxing

3. To provide a safe distance between other aircraft components such as wing and fuselage while the aircraft is on the ground position to prevent any damage by the ground contact

4. To absorb the landing shocks during landing operation

5. To facilitate take-off by allowing aircraft acceleration and rotation with the lowest friction.

Introduction:

In order to allow for a landing gear to function effectively, the following design requirements are established:

1. Ground clearance requirement
2. Steering requirement
3. Take-off rotation requirement
4. Tip back prevention requirement
5. Overturn prevention requirement
6. Touch-down requirement
7. Landing requirement
8. Static and dynamic load requirement
9. Aircraft structural integrity
10. Ground lateral stability
11. Low cost
12. Low weight
13. Maintainability
14. Manufacturability

The first job of an aircraft designer in the landing gear design process is to select the landing gear configuration. Landing gear functions may be performed through the application of various landing gear types and configurations. Landing gear design requirements are parts of the aircraft general design requirements including cost, aircraft performance, aircraft stability, aircraft control, maintainability, producibility and operational considerations.

Single Main

The simplest configuration of landing gear is the single main (see Fig. 1). It includes one large main gear that carries a large portion of the aircraft weight and load; plus a very small gear under the nose. In terms of size, the main gear is much larger (both strut and wheel) than the secondary one. Both of these gears are in the aircraft symmetrical plane. The main gear is close to the aircraft cg, while the other gear is far from it. In majority of cases, the main gear is located in front of the aircraft cg and the other one is behind cg (under the tail section). In case, where the main gear is aft of aircraft cg, the secondary gear is usually converted to a skid under the fuselage nose. Majority of sailplanes are employing single main landing gear because of its simplicity. The single main landing gear is not usually retracted, so it is very short in height.

An aircraft with a single main landing gear is not stable on the ground, so the aircraft will tip over one side (usually on wing tips) while staying on the ground. In such landing gear configuration, an operator must hold the wing level when an aircraft is stationary and prior to take-off. To prevent a sideway tipping, some aircraft are equipped with two auxiliary small gears under two wing sections. In an aircraft without auxiliary wheels, the wing tips must be repaired in a regular basis, since the wing tips are damaged during each tipping. Two advantages of this arrangement are the simplicity and the low weight of the landing gear. On the other hand, beside the ground instability, a disadvantage of this configuration is the longer take-off run, since the take-off rotation is limited.

The single main landing gear is not usually retracted, so it is very short in height. An aircraft with a single main landing gear is not stable on the ground, so the aircraft will tip over one side (usually on wing tips) while staying on the ground. In such landing gear configuration, an operator must hold the wing level when an aircraft is stationary and prior to take-off. To prevent a sideway tipping, some aircraft are equipped with two auxiliary small gears under two wing sections. In an aircraft without auxiliary wheels, the wing tips must be repaired in a regular basis, since the wing tips are damaged during each tipping. Two advantages of this arrangement are the simplicity and the low weight of the landing gear. On the other hand, beside the ground instability, a disadvantage of this configuration is the longer take-off run, since the take-off rotation is limited.

Tail Gear

Tail-gear landing gear has two main wheels forward of the aircraft cg and a small wheel under the tail. Figure 5 illustrates the side and top views of the gear in a typical aircraft. The wheels in front of the aircraft cg is very close to it (compared with aft wheel) and carries much of the aircraft weight and load; thus is referred to as the main wheel. Two main gears are in the same distance from the cg in the x-axis and the same distance in y-axis (in fact left and right sides); thus both are carrying the same load. The aft wheel is far from cg (compared with main gear); hence it carries much smaller load and then is called an auxiliary gear.

The share of the main gear from the total load is about 80 to 90 percent of the total load, so the tail gear is carrying about 10 to 20 percent. This configuration of landing gear is referred to as a conventional landing gear, since it was the primary landing gear during the first 50 years of aviation history. But currently, only about 10 percent of the aircraft produced are employing tail-gear. In order to reduce drag, in some aircraft, a skid (vertical flat plate) is used instead of the tail wheel. Such landing gear is referred to as the tail-dragger. Most agricultural some GA aircraft are equipped with tail gear.

Conclusion

Different systems of landing gears are present out of which few are obsolete now. In the early age landing gear development went on from two wheels only which later on scattered to various wheels arrangement. For General Aviation industries now a day’s only following system are in use:

1. Tail-gear

2. Tricycle or nose-gear

3. Quadricycle

4. Multi-bogey 99% of the aircraft are using the same system which is shown above.

Out of the above shown list Nose gear arrangement is more popular due to its added advantage as compared to other systems. Off course here comes the weight parameter also so depending on the weight of the aircraft and the available space system the arrangement can shift to Multi bogey system as well.