Aircraft Loads and Types, Fuselage, Wing Tail, Landing Gear Loads

Aircraft loads refer to the forces and moments acting on an aircraft’s structure during various phases of flight, including takeoff, climb, cruise, descent, and landing. These loads are essential considerations in aircraft design, as they determine the structural integrity and performance of the aircraft under different operating conditions.

A commercial aircraft needs to handle two main types of loads: ground loads and air loads.

Ground loads refer to the forces the aircraft experiences while it’s on the ground, like when it’s taxiing, landing, or being towed.

Air loads are the forces that affect the aircraft during flight. These include the pressure from the air pushing on the aircraft’s surfaces, such as the wings and fuselage.

Both ground and air loads can be broken down further into two types of forces:

1. Surface forces: These forces act on the outer surfaces of the aircraft. For example, aerodynamic forces come from the air pressing against the aircraft’s wings and body. Hydrostatic pressure is another type of surface force.
2. Body forces: These forces act throughout the entire volume of the aircraft’s structure. Gravity is the main body force, pulling the aircraft downward.

In essence, air loads result from the pressure of the air against the aircraft’s surfaces during flight. These pressures cause various types of stress on the aircraft’s structure, including direct loads, bending, shear, and torsion. These stresses affect different parts of the aircraft’s structure and must be carefully considered during the design and construction process to ensure the aircraft’s safety and durability.

1. Fuselage of an Aircraft

The fuselage of an aircraft is subject to various types of loads from different sources. Here’s an explanation of the loads it typically experiences:

1. Weight Load: The weight of the fuselage structure itself, along with the payload it carries, causes the fuselage to bend downwards from its support at the wing. This puts the top of the fuselage in tension (being stretched) and the bottom in compression (being squeezed).
2. Maneuvering Flight: During maneuvers, such as turns or sudden changes in direction, the loads on the fuselage can be greater than during steady flight. These dynamic forces must be accounted for in the design.
3. Landing Loads: The impact forces experienced during landings can also be significant. The structure of the fuselage must be able to withstand these loads without deformation or failure.
4. Pressurization: Most commercial aircraft have pressurized cabins, which means the internal pressure is higher than the external atmospheric pressure, similar to being at an altitude of 2000-2500 meters during cruise. This pressurization creates bending loads in the fuselage frames, requiring reinforcement to handle the stress. Designers also need to consider the potential consequences of sudden depressurization, which could result in higher loads than normal operation.
5. Doors and Hatches: Designing doors and hatches presents a challenge, as they may need to support some of the fuselage’s structural load. The design of these openings must be carefully considered to ensure they do not compromise the integrity of the fuselage.
6. Windows and Floors: Windows, due to their small size, generally do not pose a significant structural challenge. However, the floor of the fuselage can experience high localized loads, especially from passengers wearing high-heeled shoes. Therefore, the floor must be reinforced to withstand these stresses and ensure passenger safety.

2. Wing and Tail Loads

The wing and tail of an aircraft experience specific types of loads:

1. Wing Loads:

• Lift Forces: The primary load on the wing is the lift generated by the airflow over its surface. This lift creates a shear force and a bending moment, with the highest values occurring at the root of the wing, making it one of the most structurally demanding areas of the aircraft.
• Engine and Fuel Load: In the case of wing-mounted engines, the weight of the power plant adds additional loads to the wing structure. Furthermore, the jet fuel stored inside the wing contributes to the overall weight distribution and affects the bending moment. Proper placement of the engines and fuel tanks helps balance the lift forces during flight.
• Ground Loads: When the aircraft is on the ground, the lift generated by the wings is negligible compared to the weight of the aircraft, engines, and fuel. Therefore, the wing must be designed to withstand these static loads as well.

1. Tail Loads:

• Lift from Tail Surfaces: The tailplane, rudder, and ailerons also generate lift, which creates a torsional force in the fuselage. This torsion is effectively resisted by the cylindrical shape of the fuselage, ensuring structural integrity.

In summary, the wing must be designed to handle the complex combination of lift, engine and fuel loads, and ground loads, with particular attention to the structurally demanding wing root. Similarly, the tail surfaces contribute to the overall aerodynamic forces and require consideration to ensure stability and structural integrity.

3. Landing Gear Loads

The landing gear of an aircraft exerts significant loads on the structure, particularly during landing. The primary force generated by the landing gear is an upward shock as the aircraft touches down on the runway. To mitigate the impact of this shock, shock absorbers are incorporated into the landing gear system. These shock-absorbers work to absorb the landing energy, thereby reducing the force transmitted to the aircraft structure.

During a typical landing, the shock absorbers effectively cushion the impact, ensuring a smoother touchdown and minimizing the stress on the aircraft structure. However, in the event of a hard landing or excessive force, such as during an emergency landing, the shock-absorbers may be subjected to extra work, resulting in a sudden increase in the force transmitted to the structure.

Overall, the design and performance of the landing gear, including the effectiveness of the shock-absorbers, are crucial in ensuring the structural integrity of the aircraft during landing maneuvers.

4. Other loads

In addition to the primary loads discussed earlier, aircraft structures are subjected to various other loads that can impact their integrity and performance. These include:

1. Engine Thrust: The thrust generated by aircraft engines, whether mounted on the wings or fuselage, exerts a force in the plane of symmetry. In the event of an engine failure, asymmetric thrust can induce severe bending moments on the fuselage, requiring the structure to withstand these unexpected loads.
2. Catapult Launch Loads: Fighter aircraft operating from aircraft carriers often undergo catapult launches, where they are rapidly propelled from the carrier deck to achieve takeoff speed. These launches subject the aircraft to concentrated shock loads, requiring the structure to withstand the sudden and intense forces exerted during the launch sequence.
3. Hydrodynamic Pressure: Seaplanes and amphibious aircraft equipped with floats or fuselage hulls encounter hydrodynamic pressure when operating on water surfaces. The pressure exerted by the water on the fuselage or floats must be considered in the design to ensure the structural integrity of these components while taxiing, taking off, and landing on water.

These additional loads highlight the diverse and dynamic operating conditions that aircraft structures must be designed to withstand, emphasizing the importance of robust engineering and structural analysis to ensure the safety and reliability of aircraft during various phases of operation.