Half-shell construction
.PNG)
With half-shell construction ( English stressed skin or semi-monocoque ) a construction method for the construction of aircraft and rocket fuselages , and more rarely also of wings , is referred to in aircraft construction , in which the "strength" of the outer skin (planking) of these components is dimensioned so that they one Can absorb most of the forces occurring. Compared to the pure shell construction ( monocoque ), which manages without internal reinforcements or with little stiffening through transverse ribs , the half-shell construction also uses longitudinal stiffeners . These are typically longerons (torso spars) and stringers (weaker longerons), which help to relieve the outer skin. This is to remedy the main weakness of the pure monocoque construction, namely to achieve sufficient strength with the lowest possible weight. However, a consistent distinction between the terms shell and half shell construction cannot be recognized in the literature. Often both terms are used synonymously. Occasionally, the shell construction, based on the self-supporting body of vehicles, is also referred to as a self-supporting structure.
history
The first proven use of monocoque construction was presented at the 1911 Olympia Aero Show with a machine designed by Handley-Page. The fuselage shell of type D (retroactively referred to as HP4 in 1924) consisted primarily of mahogany wood . Other early applications of the shell construction in aircraft construction are the Deperdussin Monocoque Racer from 1912, as a further example of the wood construction initially used. The hull of the 1919 Loughead S-1 designed by Jack Northrop was made in two concrete molds. Here the fuselage was made of several layers of plywood. In Germany, typical examples were the Albatros DV from the time of the First World War and the later Heinkel He 70 .
Dornier made the first attempts with an all-metal shell construction in 1917 with the Dornier Cl.I , in which the wooden shell construction of the Albatros two-seater was to be transferred to metal construction. To what extent the fuselage hull was really capable of absorbing forces, however, is unclear. The first aircraft actually designed in an all-metal shell construction were the Dornier Zeppelin DI , which was flown for the first time in June 1918, and the Short Silver Streak, which was shown at the Olympic Airshow in July 1920 .
At the end of the 1960s, the shell construction with fiberglass composite materials prevailed in glider construction . So the weight could be reduced. For the same reason, carbon fiber reinforced plastics (“carbon”) are increasingly being used in commercial aircraft construction.
Constructive execution
General
In the case of half-shell construction, a substructure is usually used to reinforce the outer skin and to preserve the cross-sectional shape, which can be composed of longerons, stringers , frames / bulkheads and formers. Bending forces acting on the trunk are absorbed by Longerons that run parallel to the longitudinal direction of the trunk. Stringers have a smaller cross-section than longerons and help them absorb compressive forces and increase the rigidity of the hull. Form frames and bulkheads are primarily used to maintain the cross-sectional shape and increase the stability of the stringers against kinking. Bulkheads are preferred where forces are concentrated in the fuselage or the wings. These are e.g. B. engine mountings, wing connections, the landing gear stop and the connections of the tail fins. The division into frames and stringers is also called "straken" in the technical term.
In addition to utilizing the high section modulus of closed circle-like fuselage cross-sections, the main advantage of the shell or half-shell construction over the truss fuselage is the possibility of achieving an aerodynamically designed fuselage without having to use additional shaping components. In addition, the interior can be fully used as there are no struts or bracing wires.
The practical production usually takes place in several individual shells that are riveted or glued together to form an almost round or oval cross-section. Cutouts in the shell structure represent a challenge, whereby the central middle part of the shell is made up of doors, hatches and the like. should be kept free. For example, in the case of the Vought F-8 , which had no longitudinal stiffeners, it was not possible to open all maintenance hatches at the same time during maintenance, as otherwise the fuselage would have warped when the aircraft was moved.
Wooden construction
_(2).jpg)
In timber construction, several layers of plywood, mostly in different thicknesses, are usually glued on top of each other and next to each other with the help of a positive mold. With the Loughead S-1 , on the other hand, an airbag was used in a negative mold, which was inflated after the mold was closed and kept under pressure for 24 hours. In the forms for the Lockheed Vega , the hull could be made for three different models. Depending on the position and shape of the openings cut into the shell, the Air Express or the low- wing aircraft Sirius could also be manufactured in addition to the Vega .
As types of wood z. B. Sitka spruce , birch , mahogany and the wood of the tulip tree are used. Balsa wood is also used in intermediate layers . Typical adhesives are the historically interesting casein glue and the current synthetic Aerodux.
Example mosquito
The hull of the De Havilland DH.98 Mosquito was made as a monocoque in two half-shells that managed with just a few longitudinal stiffeners. Neither pressure nor heat was required to manufacture the bowls. The two positive forms, about 12 m long, were either made of mahogany wood. B. in the Canadian production, molds made of concrete are also used. In the first construction stage, the hull bulkheads and other internal elements were built into slots in the mold. In the second stage, the inner hull skin was laid and the structural components were installed between the inner and outer plywood layers. At the rear of the fuselage, the plywood strips were glued at an angle in order to be able to better absorb the forces arising from the torsional load caused by the tail unit. The grain of the inner and outer stripes ran in opposite directions.
Aluminum or metal construction
planking
The outer skin is usually made of rolled aluminum sheet of special quality. It is stretched on a stretching machine with straps on a device corresponding to the aircraft fuselage shape and thus brought into shape. By stretching, the rolled structure is improved at the same time . Before riveting, the outer sheet is usually brought to the desired thickness in an acid bath. A protective varnish is applied to places where the sheet metal should retain its full thickness (e.g. in the area of windows, doors, frames and stringers). In places without protective varnish, the aluminum sheet is chemically removed by the acid. The time to etch exactly controls the thickness.
Assembly of the half-shells
The finished half-shells are riveted to their frames and the outer sheets with the other matching half-shell. The resulting fuselage sections are riveted lengthways to the stringers and the outer panels. Due to the slight overlap of the stringers, frames and outer sheets when riveting, these connections have almost exactly the same strength as a circular segment, are only slightly heavier, but are significantly cheaper to manufacture.
Large assemblies
Large commercial aircraft with one or two central aisles in the cabin usually have a circular cross-section and are often manufactured in three shells. This enables inexpensive ship, air or road transport from suppliers to the final assembly hall and space-saving and thus inexpensive intermediate storage.
Corrosion protection
Aircraft are subject to significant temperature fluctuations during operation. Humidity levels of up to 100% and temperatures of more than 40 ° C can often be found on a tropical airfield . At cruising altitude (often +12,000 m) the outside temperatures are in the range of about −55 ° C. That and the ongoing evaporation in the passenger cabin lead to a strong formation of condensation and thus to severe corrosion . Therefore, all aluminum parts must be degreased and painted before assembly. A rubber-like intermediate layer must be applied between the frames, stringers and sheets before assembly. After riveting, all connections are coated again with anti-corrosion paint. These measures are standard in today's international metal aircraft construction and guarantee a normal service life and operating life of approx. 15–40 years.
Example Bf 109
From the cockpit section onwards, the fuselage of the Messerschmitt Bf 109 is a half-shell construction that has been optimized especially with a view to industrial mass production. For example, the frames were integrated into the individual cladding elements by bending the edges ( flanging ) during production. This machining only had to be done on half of the segments (even numbering) on both sides. The other half of the eight cladding sheets were designed as simple flat sheets. It was no longer necessary to rivet additional ribs for stiffening. During the construction of the fuselage, all sheet metal segments of one half of the fuselage were stretched on a falsework and riveted together. The stringers were then pushed through prepared recesses in the “frame bends” and riveted to the 0.8 mm thick duralumin outer skin. The two halves of the fuselage were then joined together by riveting a further extra-wide stringer from above and below.
Fiber composite materials: GRP and CFRP
Principle and material
Plastics reinforced with glass (GRP) or carbon fibers (CFRP) have long been tried and tested materials in sailplanes and boat construction , which are now increasingly being used in commercial aircraft via military aviation (here: carbon) . A carrier material, usually a two-component - resin consisting of resin plus an accurately metered quantity of hardener, the parts take up thrust forces. A fiber material (such as glass or carbon fiber ) absorbs the tensile stresses. Since glass fibers and especially carbon fibers can absorb very high tensile stresses with little deformation, parts made of this material have a higher rigidity and strength than those made of aluminum with a lower mass . So a combination of two different materials becomes a new one with the advantages of both materials combined. However, since the material is brittle, new processes and methods are necessary to ensure the toughness, which is immensely important for aircraft construction.
Section size
In the case of composite materials, the aim is to build the components as large as possible in one piece, since each connection point has to be connected in a complex and overlapping manner and is therefore a cost and weak point. When building gliders, the whole fuselage from the cockpit to the vertical tail is usually laminated in one piece. For large components, such as B. With the modern Boeing B787 , for economic reasons (see above), the entire aircraft cannot be laminated "in one piece". With such dimensions, useful assemblies (sections) are formed again.
Positive / negative form
The positive shape of the aircraft fuselage is either reversed and produced directly as a negative shape or molded from a so-called master model . This form is treated with a release wax to prevent the material from sticking to the form. The first two-component resin layer or a special first filling layer is applied and later forms the outer skin. A layer of glass or carbon fiber is placed in this layer. On top of this is another layer of fiber material. The following layer is now inserted at a different angle (often crossing to the lower one). For prototypes or small series by hand, for medium-sized series with the support of a laser marking device (" scanner ") and for large series with a lamination robot. Layer by layer, between 5 and 20 layers are built up. This has to be done relatively quickly, as the "open glue" time runs out very soon after the resin and hardener are mixed together. The outer skin is very smooth after demolding and only needs to be processed minimally. The inside is often rough and uneven. Larger openings for doors, windows or flaps are already formed in the mold and the high rigidity and tangential tensile strength required there is created by laying the laminate fibers according to the outline.
Formers and stringers
In places with particularly high loads, such as engine , landing gear or wing mountings, as well as in metal and wooden aircraft construction, bulkheads, frames and, if necessary, stringers are installed as reinforcement. These are glued to the outer skin in several layers in the corners.
Laminating direction and number of layers
Since (ideally) the stress curves in the fuselage are known in advance through tests with fracture cells or FEM methods, the number of layers and the direction of the fiber strips can be determined in advance. The higher the expected tension, the more layers. The angle of the layers to one another largely determines the strength and is determined in complex numerical simulations .
assembly
The half-shells modeled in this way have a protrusion after demoulding that must be removed. To disconnect , so removing the excess margin, usually 5-axis are HSC - milling - or water jet cutting machines used. In special assembly devices, the shell elements are aligned, fixed and glued together with additional layers of resin plus fiber material so that they overlap.
Environmental sustainability, repair, recycling
Functioning material cycles exist for aluminum . For the brittle fiber composite materials, these maintenance procedures still have to be developed or established in broad application. The cost-effective repair of dents, dents or even holes, such as those that quickly occur in everyday airfields due to collisions with ground vehicles, still needs to be optimized. However, the weight advantage outweighs this disadvantage by the fuel saved. Insurance companies do not yet have reliable empirical values everywhere.
Example Airbus A350
A mixed construction of aluminum alloys and CFRP is used for the Airbus A350 , with CFRP making up 52% of the total aircraft. A fuselage section of the Airbus A350 consists of four CFRP individual elements that are riveted onto aluminum frames. The stringers on the A350 are also made of CFRP and are glued to the outer skin in a previous work step.
Space travel
Large rockets in space travel are also mostly manufactured in half-shell construction. One of the first rockets of this type was the V2 rocket towards the end of World War II. It was divided into four larger assemblies :
- Drive (with motor, fairing, four control surfaces)
- Hull consisting of two large half-shells riveted together
- two tanks including piping for fuel and oxygen
- Tip with flight controls and warhead
The two half-shells and the two tanks were manufactured from 1943 to 1945 in Saulgau in Württemberg, far away from final assembly.
Other areas of application
Vehicle construction silencers
Silencers in the exhaust system are hollow bodies that usually consist of two deep-drawn sheet metal shells. These are fixed in welding devices and welded together.
literature
- The 160 HP Deperdussin Racing Monoplane . In: FLIGHT November 22, 1913, p. 1269 ( online )
- Maurice F. Allward: `` Mononcoques - A Brief Survey of the Development and some of the Problems Involved in the Design of stressed-skin Fuselages ''. In: FLIGHT January 18, 1945, pp. 65–97 ( online )
- Philipp Hassinger: Between Evolution and Revolution - The Material Change in Aircraft Construction, Technology Discourses, Volume 12, Karlsruhe Studies on the History of Technology, 2013
Individual evidence
- ↑ Definition of the shell construction on merriam-webster.com (accessed on July 28, 2015)
- ↑ Definition of the shell construction on thefreedictionary.com (accessed July 28, 2015)
- ↑ Definition of the half-shell construction at navyaviation.tpub.com (accessed on July 28, 2015)
- ^ Hugh Driver, The Birth of Military Aviation: Britain, 1903-1914, p. 90
- ↑ Frost, Kössler, Koos: Dornier - From the Beginning to 1945 , 2010, p. 23
- ↑ The Crusaders of the Marines . In: Airplane - Take-off in die Welt des Fliegens (The new collection) Issue 92, p. 2555.
- ↑ Richard Sanders Allen: `` The Northrop Story, '' p. 5
- ↑ Lockheed Vega as an example of a wood monocoque
- ↑ Tony Harmsworth: The Return of "Wooden Wonderment"! . In: Airplane Monthly December 2012, p. 51 f.
- ↑ Jean-Michel Goyat, Russ Snadden: Making the Messerschmitt Part 1 - Manufacturing Techniques . In: Airplane Monthly July 1999, p. 71
- ↑ Fuselage panels and sections on the A350
- ↑ Gluing the stringers to the outer skin of the A350 ( memento of the original from March 5, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (accessed on July 30)
- ↑ Georg Metzler: Secret command matter - rocket armor in Upper Swabia. The Saulgau satellite camp and V2 (1943–45) . ISBN 3-89089-053-9
- ↑ Patent specification EP 1 110 659 A2 from June 27, 2001 by the Eberspächer company
