Longerons In Aircraft - The fuselage is one of the primary contributors to the total drag force produced by an aircraft in flight and so must be carefully shaped to be as aerodynamic as possible in an effort to minimize drag.
There is of course some balance that must be sought between low aerodynamic drag and payload and passenger comfort. The probe is also designed to be semiflexible, making it capable of conforming to skin curves. In addition, the rugged casing is much more adapted, compared to other available solutions, to the rough handling to which probes are often submitted.
Longerons In Aircraft
In a semi-monocoque structure both the outer skin and the internal substructure are load bearing, and both contribute to the overall stiffness of the structure. This design methodology was born out of the use of aluminum, rather than steel or wood, as the primary structural material used to manufacture airframe structures.
Fuselage Loading
Aluminum has many advantages over steel. The density of an aluminum alloy is approximately one-third that of steel which allows for thicker structural sections to be built without any weight penalty. Thicker skins are advantageous as these are less likely to buckle under load, resulting in a more efficient structure.
The fuselage will see a combination of loads from multiple sources during a typical flight. Large bending loads are introduced from the wing and tail sections, as well as a torsional load from the pitching moment of the wing.
The cracks were discovered in two US F-16D groups, with the first including 39 aircraft with an average age of 27 years old with 6,455 actual flight hours (AFH) and 7,016 equivalent flight hours (EFH); the second group comprised of 43 F-16s averaged 21 years old with 5,934 AFH and 4,867 EFH.
Faster execution Switching from the crack inspection setup to the corrosion inspection setup is very quick. Because it does not require a new hardware setup and that the array probe offers wider coverage, inspections have gone from hours to mere minutes.
Stringers And Longerons
The load-bearing skins are attached to the stringers and frames of an aluminum aircraft through rivets. The skins carry load through shear and transmit this shear into the stiffeners. In a pressurized aircraft the skin works with the frames to oppose the internal pressure load.
The skin's ability to carry and transmit shear is reduced if the skin is allowed to buckle; this forms a constraint that determines the spacing of the stringers and frames. These make up the longitudinal components of the structure.
Their primary aim is to transmit the axial loads (tension and compression) that arise from the tendency of the fuselage to bend under loading. The stringers also support the skin, and when combined with the frames, create bays over which the skin is attached.
The cracked longerons are original to the aircraft, and were not upgraded under the Falcon Star programme, according to Murphy, who also noted that a temporary fix would have enabled quick resumption of flights, but was considered risky and hence abandoned.
In August, the USAF grounded 82 of its F-16D aircraft after an immediate time action compliance technical order (TCTO) found structural cracks in the canopy sill longerons between the front and rear pilot seats, as reported by Flightglobal.
These situations present some interesting technical problems. First, potential cracks existing in the vicinity of fasteners are short, they spread in all directions, and they are often subsurface, making them difficult to detect. Second, hidden corrosion between the layers of multilayered aluminum structures is hard to locate.
The required corrosion detection threshold is typically 10% in material loss in such structures. Longerons usually carry larger aircraft loads and help to transfer skin loads to the internal structure. Longerons nearly always attach to frames, ribs, or the skin of the aircraft.
Fatigue and stress cracks eventually appear around the fasteners used to secure longerons to structures. Such cracks risk going undetected because they are small (typically 1.25mm or 0.05in.), they are near and under fastener heads, and often under surface coatings.
The faying surface between longerons and skin is also prone to corrosion and must be inspected. The fuselage is the name given to the main body of the aircraft and houses the pilots, crew, passengers, and cargo.
The wings and tail section are attached to the fuselage, and depending on the design of the aircraft, may include engine attachments too. Eddyfi Technologies developed a hybrid Eddy Current Array (ECA) probe that would detect both types of defects within their respective tolerances.
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The probe therefore incorporates two arrays of coils using proprietary coil topology. The first is a high-resolution array using small, high-frequency coils to detect the surface cracks around fasteners. The second array is designed to detect subsurface corrosion.
Welcome to part two in this five-part series on airframe structures. In this post we'll be focusing on the fuselage; specifically, we discuss the design of a typical semi-monocoque structure, and the various structural components and loadings that contribute to the final design.
Thanks to this powerful all-in-one design, the probe is able to detect corrosion build-ups ±5% of the layer thickness (e.g., down to 0.5 mm of a 10 mm layer), which is approximately twice as powerful as the initial requirement.
The fuselage does more than just house the occupants of the aircraft; it must be sized and designed to ensure that the wings and tail are positioned in such a way as to keep the aircraft statically stable through the designed center of gravity envelope.
A statically stable aircraft is one that will tend to return to straight and level flight if the controls are released, which is a requirement for all civil and general aviation aircraft. Commenting on return-to-flight timeframe, Murphy said the USAF is still trying to determine whether the repairs can be done in the field and the scope of testing required for return-to-flight will remain known until the repair process is complete.
Frames are transverse elements that define the cross-section of the fuselage. They are typically spaced approximately 20 inches apart and define the aerodynamic shape. The frames and stringers are spaced in such a way to ensure that the resulting bays that are created support the skins against buckling.
Frames also provide a means to introduce point loads into the fuselage. Large frames are required at the wing-fuselage and tail-fuselage interface to transmit the loads generated by these lifting surfaces into the fuselage. All these load cases, and the interaction between cases must be considered to arrive at a final design.
The structure must be strong enough to withstand these loads at the Ultimate Load Factor determined by the applicable airworthiness regulations in order to ensure the safety of the crew and passengers. The fuselage structure must be strong enough to ensure safe operation throughout the flight envelope.
A semi-monocoque structural design is usually favoured; where the sub-structure and the skins work together to absorb and transfer the loads generated during flight. The instrument and the software to drive the solution—Eddyfi® Ectane® and Magnifi®—already existed at the moment Eddyfi Technologies tackled this application.
What was therefore necessary was a new type of probe capable of detecting surface and subsurface defects.
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