- The structure will not deflect such that something does not work anymore!
- Structure will meet specified durability/ damage tolerance/ fail safety requirements.
- Flight Loads : maneuver, gust, control deflection, buffet, inertia, and vibration.
- Ground Loads : vertical load factor, braking, bumps, truns, catapult, arested landingaborted takeoff, spin-up, spring back, one wheel/two wheel, towing. ground winds, and break away.
- Other Loads & Condition : jacking, pressurization, crash, actuation, bird strike, hair, power plant, thermal, fatigue, damage tolarance, fail safety, acoustics, ground handling.
Different Load Conditions are Critical for Different Areas
Typical Commercial Transport Critical Load Conditions
STRESSES AND STRAINS THAT AFFECT AIRCRAFT STRUCTURES
- has become fairly standard practice to interchange the words stress and strain.
- However, strain is the term used when an external force is applied to a structure that acts to deform it, while stress is the internal force within the structure that opposes the external force being applied
- the diagram, the tensile load strain applied is trying to pull the rod and elongate it.
- The tensile stress in the rod is resisting this strain.
- In the diagram, the compressive load strain applied is trying to squeeze the rod and shorten its length.
- The compression stress within the rod is resisting this strain.
- In the diagram, the torque load strain applied is trying to twist the rod along its length.
- The torsional stress within the rod is resisting this strain.
- In the diagram, the fastening holding the two plates together is subject to cutting action as the two plates are subject to a tensile load.
- The resistance to the two opposite tensile loads is shear stress.
- If a structure is bent, as per the diagram, it is subject to several different loads.
- The external radius of the bend is placed under tension, while the internal radius is placed under compression.
- A shear line is produced, shown by the red line, where the tension and compression act against each other.
- This is also referred to as radial stress, and is the stress raised in a container when it is filled, where the contents act to expand the container.
- For example, a balloon has hoop stress when inflated.
- Like a balloon, when an aircraft is pressurised, the pressure hull expands.
- Axial stress is also referred to as longitudinal stress. It occurs when the aircraft is pressurised and the pressure hull lengthens.
ELASTICITY OF MATERIAL
- The materials that modern air transport aircraft are made from have a degree of elasticity.
- As with a rubber band when it is stretched and then released, it returns to its original size.
- If the rubber band is over stretched, it is beyond its elastic limits.
- When released, the rubber band does notreturn to its original size.
- This is deformation.
- In aircraft structures, deformation can take the form of bending, buckling, elongation, twisting, shearing, or cracking, which ultimately leads to fracture and creep in materials.
- Where a material is subject to a load that is within its elastic limit but for a protracted period, the material can deform (elongation).
- This is termed creep.
- In Diagram 1.4, the bar is subject t o a tension load and has increased in length.
The factors that affect creep are:
- Material type
- Load applied
- Duration of the load
- When sufficient external force is applied to a structure so that it overcomes the structure’s ability to withstand the force (e.g. external force greater than the structure’s stress limitation), then the structure will deform due to strain.
- The amount by which the structure has deformed is found by comparing the original length with the deformed length.
- This is given as a ratio, which shows the magnitude of the strain.
- When a structure has a sudden increase in the load that is being applied to it (e.g. by a bird strike directly on to the compressor blades of a jet engine), this is termed a shock load.
- A heavy landing also creates a shock load. When the resultant load applied to a structure is greater than the elastic property of the material it is made from, the affected structure(s) becomes permanently deformed.
- Fatigue is inevitable in materials that are subject to alternating loads (i.e. an old rubber band stretches and then fails before the normal load is applied).
- This is called fatigue failure.
- A structure subject to load reversals (tensile loads in the main), suffers fatigue failure more quickly than the same structure that has been subject to a continuous load.
- The effects of cyclical loads on structures are cumulative and reach a point where the structure fails even under normal loads.
- Diagram 1.5 above is a graphical representation of Miner’s rule.
- In this diagram, a log scale graph shows three levels of stress that a structure could be subject to. These are S1, S2, and S3.
- Along the lower axis are the number of cycles that the material can withstand before failure, shown by N1, N2, and N3.
To make an aircraft viable, the structure and the material that it is made from must be as light as possible.
This results in the stress within the structure being high, especially as the Maximum Take-Off Mass (MTOM) may be up to two thirds of the structure failure load.
This combined with the varying S loads, which the aircraft is subject to throughout its operations cycles, have a cumulative effect on the fatigue within the structure.
Consequently, all structures are given a fatigue life.
- When a structure is subject to an average stress, scores, scratches, fastener holes, sharp edges, or sharp radial bends can build up local stress levels 2 or 3 times greater than the average.
- This leads to stress cracking as the stress level within the material tries to relieve itself.
- For example, if a sheet of paper is folded and a tear is started, further tension applied to the sheet across the tear line causes the tear to propagate. Each material has a critical crack length, CCL.
- Up to the CCL, the material that the structure is made from maintains its integrity.
- LIFT — The lift generated by the wings in flight acts to pull the wings up and is evident by the flexing upward of the wing tips. This creates compression on the upper surfaces and tension in the lower surfaces. Due to lifting forces produced at the trailing edge, the wing also has a torsional force trying to rotate the leading edge nose down.
- DRAG — The impedance of the airflow by components, such as fixed undercarriage legs, tries to bend the component backward out of the airflow.
MASS — The mass of the aircraft acts to pull the aircraft vertically downward.
WEIGHT –To calculate weight, multiply mass by acceleration due to gravity, which is assumed to be 9.81 m/s2. (Some questions in the JAR examination specify 10 m/s2 for acceleration due to gravity. If no value is given, use 9.81 m/s2.) Example:
A mass of 100 kg has a weight of …..? (gravitational constant of 10 m/s2) = 1000 Newtons
A mass of 100 kg has a weight of …..?
When an aircraft is in straight and level unaccelerated flight, it is subject to 1g (acceleration of 9.81 m/s2). This is the same as when the aircraft is on the ground.However, as the aircraft turns or dives and pulls up or is displaced by a gust of wind, the aircraft feels a change in the gravitational force. A steeply banked turn increases g, while an up gust can reduce it. This alters the effective weight of the aircraft and all items within it.
- ACCELERATION — Accelerating increases the drag on those components that are in the airflow. It also increases the momentum of the aircraft.
INERTIA — The structure and components of the aircraft want to continue in the original direction of travel when the aircraft turns, pulls up, dives, or levels out.
- AIRSPEED — At supersonic airspeeds, the friction of the air against the leading edges of the aircraft acts to heat them.
- TEMPERATURE — A decrease in temperature makes many materials more brittle. An increase in temperature makes most materials more ductile.
- ALTITUDE — An increase in altitude reduces the static ambient pressure acting against the aircraft. If the aircraft is pressurised, there is a differential acting across the skin that creates radial and axial load. Increasing the aircraft’s altitude or increasing the cabin pressure, increases the differential and, therefore, the hoop and axial loads on the pressure hull.
ON THE GROUND
- DURING LANDING — Momentum and inertia have a downward vertical component as well as a forward component as the aircraft touches down on the runway (e.g. the wings flexing downward on landing). As the runway is effectively in the way of the aircraft, an external force alters the aircraft’s direction. This subjects the landing gear and its mounting structure to a compression load.
- FRICTION — The friction created between the wheels and the ground (rolling friction) acts to hold the wheels back and, therefore, the lower leg of the undercarriage as the fuselage of the aircraft moves forward with its momentum. For a tricycle aircraft, the friction between the main gear wheels and the ground also acts to rotate the nose downward and if not controlled, the nose gear can be rotated into the ground at a greater rate than the structural limitation.
PRESSURISATION — If an aircraft lands with its cabin pressurised, the total load on the airframe is the sum of the landing load an
THRUST REVERSAL — When reverse thrust is selected, there is an angular change in direction of the thrust. This is 45° from normal thrust line. This retardation creates a load on the engine’s mounting structures that have inertia from forward velocity.d the pressure load combined.
BRAKING — Applying the main wheel brakes causes the nose to pitch down and the aircraft to slow. Items not fastened down continue to move forward due to momentum.
STATIONARY — When the aircraft is stationary, its undercarriage supports the total mass of the aircraft. Since it is stationary, the weight is its mass times 1g. When the aircraft starts to move, the increase in thrust is used to overcome inertia and accelerate the aircraft forward.
TAXIING — When the aircraft is taxiing over uneven surfaces, the wings flex up and down. The rolling friction between the ground and tyres also prevents the aircraft from turning and produces side loads on the gear legs during a turn.
ON TAKE-OFF — From a stationary start, the thrust of the engines has to overcome the inertia of the aircraft, the rolling friction until take-off, and the increase of drag as the speed increases.
The regulations classify the aircraft’s structure into three groups:
- Primary structure — This is structure that is stressed. In the event it fails, the structural integrity of the aircraft would be compromised to such a point that the aircraft could suffer a catastrophic failure.
- Secondary structure — This structure is stressed but to a lesser degree. In the event of a failure, the aircraft would not suffer a catastrophic failure, but could be limited in operation.
- Tertiary structure — This structure is not stressed or nominally stressed and would not cause a catastrophic failure in the event it fails.
- Critical parts must achieve and maintain a particularly high level of integrity if hazardous effects are not to occur at a rate in excess of “extremely remote”. (See table on next page.)
- The failing of a major part might adversely affect the operational integrity of the unit in which it is installed.
Table compiled by the JAA on the probability of failure and the likely consequences