Aircraft upholstery. Controls and signaling

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The aircraft fuselage consists of a frame and skin. There are three types of fuselages: truss, the load-bearing frame of which is a spatial truss; beam - their power frame is formed by longitudinal and transverse elements and working skin; mixed, in which the front part is a truss, and the tail is a beam, or vice versa.

Truss fuselages. As mentioned above, the power part of the truss fuselage is a frame, which is a spatial truss. The truss rods work in tension or compression, and the skin serves only to give the fuselage a streamlined shape. The truss is formed (Fig. 50) by spars located along the entire length or part of the length of the fuselage, struts and braces in the vertical plane, braces and braces in the horizontal plane and diagonals.

Instead of rigid braces and diagonals, the installation of wire or tape braces is widely practiced.

Knots are attached to the truss frame, which serve to attach the wing, plumage, landing gear and other parts of the aircraft to the fuselage. Fuselage trusses, as a rule, are made of welded pipes and less often riveted from duralumin profiles. Sheathing is made of canvas, plywood or sheets of duralumin. The streamlined shape of the truss fuselage is given by special non-power superstructures - fairings, called fairings.

The main advantages of truss fuselages over beam fuselages are ease of manufacture and repair, ease of installation, inspection and repair of equipment located in the fuselage.

The disadvantages include the imperfection of aerodynamic forms, low rigidity, short service life, the inability to fully use the internal volume to accommodate cargo. At present, truss structures are rarely used and mainly for light aircraft.

Beam fuselages are a beam, usually of oval or round section, in which reinforced skin and frame elements work on bending and torsion. There are three types of beam fuselages: spar-beam, stringer-beam (semi-monocoque), shell-beam (monocoque). Beam structures of the fuselages are more advantageous than truss ones, since their power section forms a streamlined surface, and the power elements are placed along the periphery, leaving the internal cavity free. This makes it possible to get a smaller midsection; a rigid working skin provides a smooth, undistorted surface, which leads to a decrease in drag. Beam fuselages are also more advantageous in terms of weight, since the material of construction is more distant from the neutral axis and, therefore, is better used than truss fuselages.

The frame of the spar-beam fuselage is formed by spars, stringers and frames. The frame is sheathed with duralumin sheets (plating).

The frame of the stringer-beam fuselage (Fig. 51) consists of often supplied stringers and frames, to which

the metal skin is attached to a greater thickness than that of the spar-beam fuselages.

The shell-and-beam fuselage (Fig. 52) does not have elements of a longitudinal set and consists of a thick skin reinforced with frames.

Currently, the prevailing type of fuselage is stringer-beam.

Stringers are elements of the longitudinal set of the fuselage frame, which interconnect the elements of the transverse set - frames. Stringers perceive mainly longitudinal forces and reinforce the rigid skin. Fuselage stringers are similar in design to wing stringers. The distance between the stringers depends on the thickness of the skin and ranges from 80-250 mm. The cross-sectional dimensions of the stringers vary both along the perimeter of the contour and along the length of the fuselage, depending on the nature and magnitude of the load on the fuselage frame.

The spars are also elements of the longitudinal set of the fuselage frame, which, working in compression-tension, perceive (partially) the moments bending the fuselage. As can be seen from the tasks and working conditions, the fuselage spars are similar to stringers.

The design of the spars is extremely diverse.

different. They are bent or extruded profiles of various sections; on high-capacity aircraft, they are riveted from several profiles and sheet elements.

Frames are elements of the transverse framing of the fuselage, they give the fuselage a given cross-sectional shape, provide transverse rigidity, and also perceive local loads.

In some cases, partitions are attached to the frames, dividing the fuselage into a number of compartments and cabins.

Frames are divided into normal and power. Power frames are installed in places where concentrated loads are applied, for example, in places where the wing is attached to the fuselage, landing gear, parts of the tail, etc.

Normal frames (Fig. 53) are assembled from arcs stamped from a metal sheet. The section of normal frames is most often channel-shaped, sometimes Z-shaped and less often T-shaped. Power frames are riveted from separate profiles and sheet elements. Sometimes such frames are pressed out on powerful aluminum alloy presses.

The distance between the frames usually ranges from 200-650 mm.

Sheathing is made of sheets of duralumin or titanium of various thicknesses (from 0.8 to 3.5 mm) and is attached to the frame elements with rivets or glued. Sheathing sheets are connected to each other along stringers and frames either end-to-end or overlapping, without undercutting. In the latter case, each front sheet overlaps the bottom one. A typical connection of the skin with stringers and frames is shown in fig. 53.

Cutouts in the beam-type fuselage skin dramatically reduce the strength of the structure. Therefore, in order to maintain the necessary strength, the skin at the cutouts is reinforced with reinforced stringers and reinforced frames. Small cutouts are edged with rings made of a material thicker than the skin, sometimes the necessary rigidity is provided by flanging the hole.

The fuselages of small aircraft are made, as a rule, one-piece. For larger aircraft, to simplify production, repair and operation, the fuselage is divided into several parts. The connection of the fuselage parts depends on its design scheme. The connection of the truss fuselages is made by butt joints mounted on the spars,

for beam fuselages, fastening is carried out along the entire contour of the connector.

On fig. 54 shows typical technological connectors of the fuselage of a transport aircraft. The fuselage consists of three parts, and each of the parts in turn is formed by panels representing skin sections with elements of a longitudinal set. The panels, connecting with the frames, are finally assembled in the assembly slipway. The connection of the panels is one-piece and is made with a riveted seam, separate parts of the fuselage are connected by bolts around the entire perimeter of the connector. Docking is carried out through fittings attached to the fuselage stringers (Fig. 55).

The floor in aircraft cabins is usually calculated on the maximum distributed static load. On passenger aircraft this load does not exceed 500 kg/m 2 , on cargo aircraft it reaches 750 kg/m 2 or more. The floor frame consists of a set of longitudinal and transverse beams, stringers and connecting nodes.

The transverse set of the floor consists of the lower beams of the frames. The chords of these beams are made from milled or stamped profiles. The panels covering the frame are made from sheets of pressed plywood 10-12 mm thick, from duralumin sheets reinforced with profiles attached from below

corner and channel sections or corrugation from pressed sheets of aluminum or magnesium alloy with subsequent mechanical or chemical processing. To prevent slipping, floor panels have a corrugated or rough surface, and in some cases are covered with cork chips. There are sockets on the floor for attaching passenger seats, and on cargo aircraft, rings for securing transported goods.

The windows of the passenger cabin are made rectangular or round. All cabin windows, as a rule, have double organic glass. Very often in pressurized cabins, the inner glass is the main working glass and takes the load from the excess pressure in the cabin. Only in the event of the destruction of the inner glass, the outer glass begins to receive excess pressure. The space between the panes is connected to the cavity of the pressurized cabin through a drying system that prevents the panes from fogging and freezing. Glazing is sealed with soft frost-resistant rubber, sometimes with non-drying putty.

The glazed part of the fuselage, which provides a view to the crew, is called a lantern. The shape of the lanterns, their placement and dimensions are chosen to provide the best view and the least resistance. On fig. 56 shows the appearance of the lantern of the navigator and the appearance of the lantern of the cockpit. The angle of inclination of the visor of the lantern is taken equal to 50-65 ° (depending on the value of V max). Canopy windshields are usually electrically heated to prevent icing in flight. The lantern consists of a frame, cast or stamped from aluminum or magnesium alloys, and glasses. Glasses are fastened to the frame with bolts and pressed with duralumin tape. The glass is sealed with a rubber gasket, sealing tape and putty (Fig. 56, c).

Cutouts for the entrance doors of transport aircraft are most often located on the side surface of the fuselages, but in some cases they are also installed in the lower part. The width of the door usually does not exceed 800 mm, and the height - 1500 mm. The choice of dimensions of cargo doors (hatches) and their placement are made taking into account the dimensions of the cargo and the minimum time spent on loading (unloading) the aircraft. Doors open inside the cabin or move up or to the side. Doors are usually made in the form of a wedge, the base of which is the inner surface of the door leaf. Excessive pressure in the pressurized fuselage presses the door leaf against its base. In the closed position, the door is locked with a lock. When the door is open in the cockpit, a warning light comes on.

Cutouts under the door are reinforced by installing more powerful frames and stringers in the place of the cutout, installing additional skin. The door edging is included in the power frame of the fuselage. The door is metal, it usually consists of a bowl stamped from sheet duralumin, reinforced with a frame. Doors are sealed with rubber profiles.

Many modern aircraft fly at high altitudes, and to ensure the normal life of people on board such an aircraft, it was necessary to create the necessary pressure in the cabins. An aircraft cabin, inside which an increased (compared to atmospheric) air pressure is maintained during flight, is called hermetic. A pressurized cabin, made in the form of a separate power unit and installed in the fuselage without including it in the power circuit, is called suspended. The dimensions of such a cabin do not depend on the dimensions and contours of the fuselage, and therefore it can be made with the most advantageous shapes and minimum dimensions in terms of strength. Cabins of passenger aircraft, as a rule, are a pressurized compartment of the fuselage and are fully included in its power circuit. Such a cabin works like a vessel under the influence of internal pressure, and is also subjected to bending and torsion, like a conventional fuselage. For reasons of strength, the best form of a structure loaded from the inside with excess pressure is a ball, but due to the mismatch in the shape of the fuselage and the inconvenience of placing the crew and passengers in such a cabin, they tend to give the cabin the shape of a cylindrical shell, closed at the ends with spherical bottoms. The transition from the cylindrical walls to the bottom, if possible, should be smooth without fractures. In the presence of fractures, the bottom, loaded with excess pressure, compresses the walls of the cylinder in the direction of the radii, and then a reinforced frame must be installed in this place. Flat bottoms need to be reinforced especially strongly.

To maintain excess pressure in the cabin, it is necessary to ensure its tightness. Of course, it is very difficult to ensure complete tightness of the cabin, so some air leakage from the cabin is allowed, which does not reduce flight safety. The criterion of tightness can be the time of pressure drop from the value of the working excess to the value of 0.1 kg/cm 2 . This time should be at least 25-30 minutes.

The sealing of the cabins is achieved by: sealing the lining and glazing of hatches and doors, the conclusions from the cabins of rods, cables, aircraft and engine control rollers, electrical wiring, hydraulic pipelines, etc.

Sealing of skin sheets at the point of their connection and fastening to the elements of the fuselage frame is achieved by using multi-row seams, installing special sealing tapes laid between the skin and frame sheets. On the inside of the cabin, the rivet seams are covered with sealing putties. The sealing of entrance doors, loading hatches, emergency exits, moving parts of the lantern, windows (glazing), etc. is carried out with rubber profiles and gaskets. The following sealing methods are used: “knife-on-rubber” type seal; sealing with a rubber gasket having a pipe cross section; sealing with reed valve; sealed with a rubber tube inflated with air.

Hatches and doors opening inside the cabin are sealed according to the first three methods indicated. When sealing with a reed valve, a strip of lamellar rubber is reinforced on the inside along the contour of the cutout, then excess pressure presses the edges of the valve against the hatch and thereby seals the gaps.

It is more difficult to seal hatches that open outward and have relatively large dimensions, since internal overpressure will squeeze the hatch. Such hatches are most often sealed with a rubber tube inflated with air.

There are three types of pressure seals for control rods and cables, electrical wires and other elements: some of them are designed to provide reciprocating motion, others provide sealing of rotational movement, and still others seal stationary parts.

To ensure the tightness of reciprocating rods, a corrugated rubber hose of a cylindrical or conical shape is often used, or a device is made consisting of a body cast from a magnesium alloy with pressed bronze bushings in which steel rods move. There are felt and rubber seals between the rods and bushings. The internal cavity of the housing is clogged with grease through a special hole.

The cables are sealed with rubber plugs having through holes with a diameter smaller than the diameter of the cable, and a longitudinal section that allows you to put the plug on the cable. To reduce the friction force, the cable is coated with an antifreeze lubricant containing graphite over its entire length of travel. The sealing of parts that transmit rotational motion is carried out by rubber sealing rings. Sealing of pipelines is carried out with the help of special adapters fixed on the containment wall. Pipelines are attached to the adapter on one side and the other with union nuts. The wiring is sealed with special electrical inputs.

Used literature: "Fundamentals of Aviation" authors: G.A. Nikitin, E.A. Bakanov

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Applicable materials

Aircraft fabrics are designed to wrap wings, fuselages and control surfaces and, after coating, serve as their skin. The most durable are AST-100 cotton fabric or ALVK linen fabric.

Fabric AM-93, cotton from mercerized yarn has medium strength.

Fabric AM-100 and linen ALL have the lightest weight.

In each individual case, it is necessary to ensure that the brand of fabric used corresponds to that indicated in the drawing approved for the manufactured product.

Cotton fabrics - calico and calico - are used to cover the metal surfaces of windshields, trailing edges, wing ends and other surfaces in contact with the fabric sheathing.

To fasten the skin, various cotton tapes are used, which have the following purpose:

• calico tape, fabric or cut from calico, for wrapping ribs (winding tape) and others, in order to protect the linen sheathing or threads fastening the sheathing from contact with metal parts that have sharp corners.
• twill tape for wrapping strips of ribs (winding tape) for attaching linen sheathing and for reinforcing the sheathing (as a reinforcing tape) at the place of its attachment, with through stitching, semi-linen tape is used.
• double-breasted tape for fastening the skin to the shelves of the ribs, the tape is sewn with the fabric of the skin with a machine seam and subsequently sewn manually over the sides to the shelf of the rib.
• surface tapes: serrated grade LAP3 (SMTU-298) and with straight edges grade LAP (SMTU-293) for gluing over machine seams, skin attachment points

You can also use a toothed surface tape made from AM-100 fabric waste.

For stitching and attaching aircraft fabrics, threads are used that have the following purpose:

• threads No. 30, 20, 10 for machine stitching of panels of aircraft fabrics
• thread No. 1 for sewing fabric with a reinforcing double-breasted tape when attaching the sheathing with a machine seam.
• threads of trade numbers 0 and 00 for manual stitching along the leading and trailing edges of the wing panels, as well as when fitting the surfaces of the fairings with calico or coarse calico.
• Waxed linen threads NAR for fastening the fabric covering of the wings, fuselages and rudders (in 8 threads) and for fastening the ailerons and hatch skins (in 6 threads)

Natural beeswax is used to wax sewing threads by hand.

Cutting fabrics and sewing panels

Before unfolding the fabric on the cutting table, check the cleanliness of the table, then, for fabrics of the AM-100, AM-93 and AST-100 brands, determine the front side, i.e. the side with a smoother surface (less pile), and lay the fabric on the cutting table as follows so that when tightened, the front side is on the outside. Apply the fabric for cutting in several layers.

On the top layer of the fabric, mark according to the established templates, which should be placed on the surface of the fabric being cut in accordance with the technological cutting charts, minimizing fabric waste. Then cut the fabric exactly along the outlined contours.

Before sewing the cut panels on the sewing machine, it is necessary to check the correct thread tension, the number of stitches per centimeter and the correspondence of the number of threaded threads to the brand of the fabric being sewn.

When sewing airplane fabrics with a three-line machine seam, use the following threads: for cotton fabric AM-100 and linen ALL - No. 30, for cotton fabric AM-93 - No. 20, for cotton fabric AST-100 and linen ALVC - No. 10.

For all aircraft fabrics listed, there should be 40 - 42 stitches for every 10 cm of stitching.

Sew on a sewing machine individual panels from aircraft fabrics of all brands with an overlap seam with an inside stitch (Fig. 1)

Fig 1 Lap seam pattern with inside stitching and hem
a - stitching panels with the first line, b - view of the finished seam, 1 - second panel, 2 - seam lines

The stitching order with this seam is as follows:

• impose one panel on the other, so that the edges coincide (Fig. 1 a), stitched panels from fabrics with the front side to impose one on the other with the front sides.
• sew panels with one line at a distance of 20 mm from the edge, the line should be even, without zigzags
• Bend the top panel so that it covers the top line, and slightly pulling, smooth the folded fabric with your hand at the place of the line
• finally sew the panels with two lines, at a distance of 6 mm from each other (Fig. 2). Make sure that the top panel is folded correctly and that the seam lines are parallel to each other.

When sewing the first outer stitch, stretch the folded panel slightly so that the folded fabric is pressed against the threads of the inner stitch (this avoids the formation of a rib from a free fold of fabric). The correct and incorrect folding of the fabric after sewing with the first line is shown in Fig. 2.

Fig 2 Lap seam with inner stitching and folded edge
a - correct, b - incorrect

If the fabric has a perfectly straight, evenly woven and sufficiently strong edge, you can sew the panels with an overlap seam (Fig. 3) The stitching procedure is as follows:

Figure 3 Overlap seam

• overlap the edges to be sewn (Figure 3), so that they overlap each other by 20 mm. Cloths of fabrics having a front side should be laid one on top of the other with the front surface to the outside
• first sew one (first) line, which should be spaced from the edge of the top panel along the entire length to be sewn to a distance of no more than 1 mm, sew the first line very carefully, without zigzags
• finally sew the panels with two more lines, placing them at a distance of 6 mm from each other. Make sure that the lines are parallel to the first, and the fabric between them does not have wrinkles.

Notes:

1. When stitching panels, as well as when a thread breaks during sewing, tie the ends of the threads with a knot.
2. After stretching the fabric on the frame, seal all machine seams with a surface tape on A1H dope at the same time as gluing the surface tapes when varnishing the fabric sheathing

If the fabric is stretched over the part with a cover, sew the panels with a single-line seam along the edge to give them the shape of a cover (Fig. 4). The stitching order is as follows:

Figure 4 Fabric cover for steering wheel trim:
1 - a three-line seam with an inside line and a folded edge (at the place where the panels are sewn), 2 - a single-line seam, 3 - fabric

• fold the cut-out panels so that the stitched edges match exactly;
• mark the line of stitching the fabric panels at a distance of 10 mm from the edge; the seam allowance is taken into account by the template;
• sew on a sewing machine exactly along the intended line, otherwise the cover will not fit snugly around the frame, and the fabric will stretch evenly.

Notes.

1. The number of stitches for all brands of fabrics should be 40-42 by 10 cm.
2. Use for stitching thread No. 10 (GOST 6309-59).
3. At the beginning of stitching and at the end of it, as well as when the thread breaks during sewing, tie the ends of the threads with a knot.

Framework preparation

For frames of parts to be covered with fabric, all sharp corners and metal parts that come into contact with the skin can damage it during operation, cover it with fabric or tape. This will also protect the protective primer coating applied to the surface of the fabric-covered frame elements from dope when varnishing the stretched fabric.

Wrap the shelves of the ribs with tape: calico, if the linen sheathing is not attached to it, and keeper, if the sheathing is attached to the wound tape. Conduct work as follows:

Fasten the tape with NAR thread (in 8 threads) double-breasted or (in 6 threads) calico as follows:

Fig 7 Scheme of sewing the tape to the rib shelf

Large surfaces of elements should be sheathed with sheets of calico or calico instead of tape. You can use waste fabrics used as sheathing. The order in which these elements are covered is as follows:

Fig 9 Seam pattern:
a - a seam over the edge, b - a herringbone seam

in the process of stitching, every 200 - 250 mm, fasten the thread by tying a knot.

Aircraft fabric wrapping

Cover the wings, control surfaces and other aircraft elements in three ways: with a cover sewn according to the shape of the fitted frame, with a free panel, in a combined way (with a cover and a free panel).

In all cases, the fabric should be positioned so that the weft direction is perpendicular to the ribs (on the wings of the flight line). In the room where the fitting is performed, the relative humidity of the air should be 40 - 70%, temperature 12 - 25 degrees.

Fitting with a cover sewn according to the shape of the frame

The panels cut out according to the corresponding patterns are sewn with a machine seam in the form of a cover having the shape of a fitted frame. This tightening method is used when there are no protruding parts on the frame, and the skin is attached from the outside. It is not recommended to fit the wings of high-speed aircraft in this way, since when fitting with a cover, it is difficult to create the fabric tension necessary in these cases.

When fitting with a cover, you must perform the following operations:

• check the correct stitching of the cover
• pull the sewn cover onto the frame manually, while making sure that the fabric over the entire surface of the frame is evenly and well stretched Note
  • If the cover is made in several large sizes and is pulled over the frame freely, it must be changed, and not pulled up the fabric along the end of the frame
• after the cover is stretched over the frame, pull the fabric strongly from the side of the ends and temporarily fasten it with pins to the tape wound around the frame (for a metal structure) or attach with studs (for a wooden structure)
• check the tension of the skin with a TP tensiometer.
• if the tension corresponds to the norms, then sew the free ends of the cover by hand.
• remove the pins and studs with which the casing was temporarily attached to the end part.

Fitting with a loose cloth

When fitting with a loose cloth (Fig. 10), observe the following order of operations:

Tightening in a combined way

The combined method of covering with a fabric (cover and cloth) is used in cases where the frame has protrusions and it is difficult to cover the entire frame with a cover. When tightening in a combined way, be guided by the general provisions outlined above.

Fastening the skin to the ribs

There are the following ways of fastening the skin, which must comply with the instructions in the drawing for this product.

• Thread fastening.
• Screw fastening.
• Rivet fastening.
• Fastening with metal tape

Attaching the lining with threads

Sheathing can be fastened with threads in the following ways: through stitching, behind the shelf, behind the reinforcing tape, machine seam and through the holes of the rib shelf.

When fastening with threads, use NAR linen threads (in 8 threads). To prevent the fabric from being cut with a thread, lay a reinforcing keeper tape between the fabric and the thread at the attachment point. The width of the tape depends on the width of the flange of the rib and the method of attaching the skin.

End-to-end firmware

Through stitching is used only for attaching fabric sheathing to the wings and control surfaces of low-speed machines. The mounting process is as follows:

Fastening for the rib shelf

The method of attaching the ribs to the shelf should be used on the upper surface of the wings, when the shelf is open from below. A diagram of such fastening of the skin is shown in Figure 13. The procedure is as follows:

Figure 13 Fastening the skin to the rib flange
1 - scheme of thread lashing

• apply a reinforcing keeper tape on the stretched skin in the places where the skin is attached to the ribs. First, fasten one end of the tape, then, pulling it slightly, pin the other end of the tape to the skin with pins.
  • Note: the width of the reinforcing keeper tape with this method of fastening should be 18 - 20 mm more than the width of the rib shelf, i.e. the tape should overlap the shelf by 8 - 10 mm on each side.

• fasten the end of the thread on the rib shelf and pass it from below through the skin and reinforcing tape.
• take a step along the rib, pass the needle through the reinforcing tape and sheathing, as close as possible to the shelf and release the needle out on the other side of it.
  • Note: the thread should exit opposite the entry point, do not allow the exit point to deviate

• further flashing along the length of the rib should be carried out in a similar way (Fig. 13a) at certain distances, called the flashing step.

  Note:

  1. take the step of this fastening equal to 25 mm, observe the same step along the entire length of the rib
  2. pull the thread slightly when sewing

• after finishing the stitching in one direction, sew in the same way in the opposite direction (Fig. 13b), overlapping one thread after another.

  Note: when the thread is overwhelmed at each link of the firmware, it is good to tighten the thread, as with the first stitching, to prevent any possibility of its weakening

• at the end of the firmware, knot the threads of the last stitch

Reinforcing tape attachment

In most cases, the fabric covering on the lower surface of the wing and on the tail unit is fastened by sewing to a keeper tape wrapped around the rib shelf or sewn to it. The mounting procedure is as follows:

Machine seam fastening

When fastening with a machine seam, the fabric must be pulled over the wing frame twice:

This method of fastening ensures the smoothness of the surface of the skin and its sufficient strength at the point of fastening. It is used, as a rule, for fastening the skin to the upper surface of the wing when the fabric is tensioned with a free panel.

Fig. 17 Fastening of the sheathing with a machine seam for shelves with a solid wall
1 - fabric, 2 - double-breasted tape, 3 - stitching on a sewing machine with thread No. 1, stitching with NAR linen threads in 8 threads

Fig. 18 Fastening of the skin with a machine seam with box-section metal ribs
1 - double-breasted tape, 2 - fabric, 3 - stitching with NAR threads, 4 - rib shelf

Fastening through holes in the rib shelf

With this method of fastening, the rib shelf for drowning the thread that fastens the skin has a concave surface with holes in the middle (Fig. 19). Insert caps into these holes to protect the thread that fastens the skin from damage. Attach the skin to the shelves of the ribs with a waxed NAR thread (8 threads) with a semicircular needle with a radius corresponding to the step of the firmware (the distance between the holes). Step size 15 - 20 mm. Apply this method of sheathing on the rudders and ailerons.

The order of firmware in the specified way is as follows: (Fig. 19a)

Figure 19 Fastening the skin through the holes of the rib flange

• put a reinforcing keeper tape on the stretched skin at the place of the ribs, first strengthen one end. Then, slightly pulling the tape, fasten the other end of the tape with pins.
• stitch with two needles located at different ends of one thread, piercing the fabric with one needle, insert the thread under the rib shelf and bring it out into the next hole.
• With the other end of the thread, located at this moment above the skin, make a simple knot around the end emerging from under the shelf, pass the thread under the rib shelf and bring it out into the next hole (Fig. 19 b).
• pulling both ends of the thread, tighten the knot and sink it into the hole, thereby pressing the skin and reinforcing tape to the rib flange.
• further fastening of the skin along the length of the rib should be carried out in the same way.
• if the length of the rib to which the skin is attached is insignificant, it is possible to fasten with one needle, in this case, first flash in one direction, as shown by the arrows in the diagram (Fig. 19a), and then in the opposite direction, be sure to tie the stitches at the meeting point of the threads.
• when the last stitch is formed, tie the ends of the threads.

Fixing the cladding with screws

Fastening with screws should be used only for metal ribs, the shelves of which have a curved shape (Fig. 20) for drowning the fastening elements. The mounting procedure is as follows:

Fastening the cladding with rivets

Fasten the skin with rivets along the trailing edge of the fitted unit (for example, a steering wheel, etc.). First you need to prepare the frame, i.e. sheathe the rim 1 (Fig. 21) with fabric 2 and cover it with fabric 3. after that, fasten the skin in the following order:

Figure 21 Fastening the skin with rivets
1 - rim, 2 and 3 - fabric, 4 - fabric tape, 5 - round washer, 6 - rivet.

• stick a fabric tape 4 at the place of fastening of the skin using the dope of the first coating.

  Note: cut the tape from the fabric used for sheathing.

• in the places of the holes in the frame, pierce the holes in the casing and the fabric tape glued to it
• put round washers 5 in place of the holes
• fasten with special rivets 6

Fastening the cladding with shaped metal tape

To fasten the skin in this way, the metal shelves of the ribs must have a special shape (Fig. 22a). The mounting procedure is as follows:

Fig. 22 Fastening the cladding with metal tape
1 - rib, 2 - sheathing, 3 - metal tape, 4 - surface tape

• fit the frame
• put a shaped metal tape on the skin along the rib
• fasten the skin in the rib shelf by pressing the tape into the mesh with the skin by straightening it in the rib nest along its entire length with a special device, as a result of which the fabric is tightly clamped between the sides of the rib shelf and the fastening metal tape (Fig. 22 b)

  Note: cover the places of fastening of the skin with surface tapes when applying the dope of the first coating. Cut the tape from the fabric used for sheathing

Sewing panels along the edges of the frame

After attaching the fabric to the ribs on the upper and lower surfaces of the structure, sew the upper and lower panels at the place of their temporary attachment with a seam over the edge (roller).

Pre-cut the excess fabric in these places with scissors in such a way that the ends of the panels to be sewn can be bent by 8-10 mm, the edges of the panels to be sewn should come close to each other. When stitching, do not allow the weakening of the tissue tension (Fig. 23)

Figure 23 Sewing panels along the edges of the frame

At the end of the stitching of the panels along the edges of the frame and the fastening of the sheathing at the hatches and holes, remove all pins and cloves that temporarily secure the fabric.

Quality control of cladding fastening

The fabric sheathing should be well pressed against the shelves of the ribs. If this condition is not met, then in flight, when the skin vibrates, the threads securing it will fray and the skin will come off. Therefore, when accepting wings, empennage and fuselages with finished fabric fastening, it is necessary to carefully check the quality of fabric fastening to the ribs. For this you should:

• When fastening according to the schemes shown in Figures 12, 13, 14 and 19, in a number of places, slip a rounded (without sharp corners) rod with a diameter of 2 - 3 mm under the thread and lift the thread passing over the skin. If the thread moves away from the sheathing freely and remains in this position, the fastening is done poorly and the firmware must be redone, and excessive thread tension should also not be allowed, as this may lead to cutting through the fabric.
• when attaching the sheathing to the tape (Fig. 14), make sure that the thread captures the reinforcing tape without gaps and passes under it, as indicated in Fig. 15.
• when fastening with a machine seam, make sure that the seam lines are spaced from each other at the same distance along the entire length of the fastening, without zigzags and do not find each other; rib flanges, as this will create local overstresses in the machine seam and may lead to sheathing failure.
• when fastening with screws and metal tape, pay attention to the fact that the metal tape tightly presses the skin along the entire length of the rib flange and the screws are screwed up to failure.

Aircraft skin - a shell that forms the empennage and the outer surface of the aircraft body. It is necessary to give the aircraft a streamlined shape. The aerodynamic performance of the aircraft largely depends on how high-quality the skin will be.

Sheathing material

Modern aircraft skins are made from panels or individual sheets of aluminum alloys (or titanium and stainless steel) molded to the surface of the wings or fuselage. Fixed panels or sheets are most often attached to the frame with countersunk riveting, while removable ones are connected with flush-head screws. Sheathing sheets are joined end-to-end. Quite often, large-monolithic finned panels and a skin layer with honeycomb core are used for sheathing fuselages. Antenna fairings (radio-transparent skin elements) are made of honeycomb or monolithic composite material. Recently, composites have also been used as sheathing panels and power units.

Depending on the material used for the construction of the aircraft, the aircraft skin can be:

• metal: steel, aluminum alloys, titanium;
• wood (veneer or plywood);
• percale (linen);
• composite materials;
• laminated film.

Aircraft skin history

The first aircraft had a skin made of linen, which was impregnated with varnish (hence, in fact, the name itself appeared), the fuselages quite often had no skin at all. Later, sheathing began to be made of wood - plywood and veneer, which were also impregnated with varnish.

With the development of technology, the skin was made of aluminum, smooth and corrugated. To date, exclusively smooth metal skin is used. True, on light aircraft you can still find linen sheathing. This is an extremely rare phenomenon, since it is effectively replaced by polymer films.

Types of skins

In aviation, there are two types of skin - soft "non-working" and hard "working". Nowadays, rigid metal skin has an advantage, as it fully meets the requirements of strength, aerodynamics, weight and rigidity. It perceives loads in the form of twisting and bending moments, external aerodynamic loads and loads of shear forces acting on the aircraft frame. Materials for the production of working skin: titanium, aluminum and steel alloys, aircraft plywood, composite materials. Titanium and steel are most commonly found in supersonic aircraft designs.

The non-powered sheathing is not included in the power circuit, since the load from the sheathing is immediately transferred to the frame. The material for its manufacture can be percale (canvas).

Wing skin

Depending on the type of construction, the empennage and wing skin can be thick, consisting of a monolithic milled or pressed panel, three-layer or thin, reinforced with a special stringer set. At the same time, a special filler is located in the inter-sheathing space (honeycombs made of foam, foil or special corrugation). It is important that the wing skin retains its predetermined shape and is rigid. The formation of folds on it provokes aerodynamic resistance.

The upper skin of the wing under the action of a bending moment is loaded with cyclic compressive forces, and the lower skin, respectively, with tensile forces. For this reason, high-strength materials that perform well in compression are typically used for top compression panels. In turn, materials with high fatigue characteristics are used for the lower tensioned skin. The skin material for supersonic aircraft is selected taking into account the heating in flight - conventional aluminum alloys, heat-resistant aluminum alloys, steel or titanium.

To increase the strength and survivability of the skin along the length of the wing of the aircraft, the number of joints that have a smaller resource compared to the main skin is sought to be minimized. The weight of the wing skin is 25-50% of the total mass.

fuselage skin

It should be noted right away that it is selected taking into account the current load. The lower area of ​​the skin perceives compressive loads by the part that is attached to the stringers, and the upper area perceives tensile forces with absolutely the entire area of ​​the skin. The thickness of the skin in the pressurized fuselage is selected depending on the internal overpressure. To improve the survivability of the fuselage on the skin, stopper tapes are often used to prevent the spread of cracks.

The connection of the skin and frame elements

There are three ways to connect the frame with the skin:

• the skin is attached to the frames;
• the skin is attached to the stringers;
• the skin is attached to both frames and stringers.

In the second case, only longitudinal rivet seams are formed, while there are no transverse ones, which has a positive effect on the aerodynamics of the fuselage. Loose skin on the frames loses stability at lower loads, which increases the mass of the structure. In order to avoid this, the skin is connected with an additional pad (compensator) to the frame. The first method of fastening is used exclusively in stringerless (skinned) fuselages.

A honeycomb skin is attached to the frames. It includes a core and two metal panels. Honeycomb construction is a hexagonal type material consisting of metal. There is glue in the core, which allows you to not use rivets at all. This design is capable of transmitting stress over the entire surface and is characterized by high resistance to deformation.

An airplane is an aircraft, without which it is impossible to imagine the movement of people and goods over long distances today. The development of the design of a modern aircraft, as well as the creation of its individual elements, is an important and responsible task. Only highly qualified engineers, specialized specialists are allowed to this work, since a small error in calculations or a manufacturing defect will lead to fatal consequences for pilots and passengers. It is no secret that any aircraft has a fuselage, carrying wings, a power unit, a multidirectional control system and take-off and landing devices.

The following information about the features of the design of aircraft components will be of interest to adults and children involved in the design development of aircraft models, as well as individual elements.

aircraft fuselage

The main part of the aircraft is the fuselage. The remaining structural elements are fixed on it: wings, tail with plumage, landing gear, and inside the control cabin, technical communications, passengers, cargo and aircraft crew are located. The body of the aircraft is assembled from longitudinal and transverse power elements, followed by metal sheathing (in light versions - plywood or plastic).

When designing an aircraft fuselage, requirements are imposed on the weight of the structure and maximum strength characteristics. This can be achieved using the following principles:

1. The body of the aircraft fuselage is made in a form that reduces drag on air masses and contributes to the emergence of lift. The volume, dimensions of the aircraft must be proportionally weighed;
2. When designing, they provide for the most dense layout of the skin and power elements of the hull to increase the usable volume of the fuselage;
3. They focus on the simplicity and reliability of fastening wing segments, takeoff and landing equipment, power plant;
4. Places for securing cargo, accommodating passengers, consumables must ensure reliable fastening and balance of the aircraft under various operating conditions;

1. The location of the crew should provide conditions for comfortable control of the aircraft, access to the main navigation and control devices in emergency situations;
2. During the maintenance of the aircraft, it is possible to freely carry out diagnostics and repair of failed components and assemblies.

The strength of the aircraft body must provide resistance to loads under various flight conditions, including:

• loads at the attachment points of the main elements (wings, tail, landing gear) during takeoff and landing;
• during the flight period, withstand the aerodynamic load, taking into account the inertial forces of the weight of the aircraft, the operation of the units, the functioning of the equipment;
• pressure drops in hermetically limited sections of the aircraft, which constantly occur during flight overloads.

The main types of aircraft body construction include flat, one- and two-story, wide and narrow fuselages. Beam-type fuselages have proven themselves and are used, including layout options that are called:

1. Sheathing - the design excludes longitudinally located segments, reinforcement occurs due to frames;
2. Spar - the element has significant dimensions, and the direct load falls on it;
3. Stringer - have an original shape, area and cross section is less than in the spar version.

Important! The uniform distribution of the load on all parts of the aircraft is carried out due to the internal frame of the fuselage, which is represented by the connection of various power elements along the entire length of the structure.

Wing structure

The wing is one of the main structural elements of the aircraft, which provides the creation of lift for flight and maneuvering in air masses. Wings are used to accommodate take-off and landing devices, power unit, fuel and attachments. The operational and flight characteristics of the aircraft depend on the correct combination of weight, strength, structural rigidity, aerodynamics, and workmanship.

The main parts of the wing is called the following list of elements:

1. Hull formed from spars, stringers, ribs, skin;
2. Slats and flaps for smooth takeoff and landing;
3. Spoilers and ailerons - through them, the aircraft is controlled in the airspace;
4. Brake flaps designed to reduce the speed of movement during landing;
5. Pylons necessary for mounting power units.

The structural power scheme of the wing (the presence and location of parts under load) must provide a stable resistance to the forces of torsion, shear and bending of the product. It includes longitudinal, transverse elements, as well as external skin.

1. The transverse elements include ribs;
2. The longitudinal element is represented by spars, which can be in the form of a monolithic beam and represent a truss. They are located throughout the volume of the inner part of the wing. Participate in stiffening the structure, when exposed to bending and transverse forces at all stages of flight;
3. Stringer is also referred to as longitudinal elements. Its placement is along the wing along the entire span. Works as an axial stress compensator for wing bending loads;
4. Ribs - an element of transverse placement. The design is represented by trusses and thin beams. Gives a profile to the wing. Provides surface rigidity when distributing a uniform load during the creation of a flight air cushion, as well as fastening the power unit;
5. The skin gives shape to the wing, providing maximum aerodynamic lift. Together with other structural elements, it increases the rigidity of the wing and compensates for the effect of external loads.

The classification of aircraft wings is carried out depending on the design features and the degree of work of the outer skin, including:

1. Spar type. They are characterized by a slight thickness of the skin, forming a closed contour with the surface of the spars.
2. Monoblock type. The main external load is distributed over the surface of the thick skin, fixed by a massive set of stringers. Sheathing can be monolithic or consist of several layers.

Important! Docking parts of the wings, their subsequent fastening must ensure the transmission, distribution of bending and torque that occur during various operating modes.

Aircraft engines

Thanks to the constant improvement of aircraft power units, the development of modern aircraft construction continues. The first flights could not be long and were carried out exclusively with one pilot, precisely because there were no powerful engines capable of developing the necessary traction force. Over the entire past period, aviation has used the following types of aircraft engines:

1. Steam. The principle of operation was to convert the energy of steam into translational motion transmitted to the propeller of the aircraft. Due to the low efficiency, it was used for a short time on the first aircraft models;
2. Piston - standard engines with internal combustion of fuel and torque transmission to the propellers. The availability of manufacturing from modern materials allows their use to date on individual aircraft models. Efficiency is presented no more than 55.0%, but high reliability and unpretentiousness in maintenance make the engine attractive;

1. Reactive. The principle of operation is based on the conversion of the energy of intensive combustion of aviation fuel into thrust necessary for flight. Today, this type of engine is most in demand in the aircraft industry;
2. Gas turbine. They work on the principle of boundary heating and compression of the fuel combustion gas, directed to the rotation of the turbine unit. They are widely used in military aviation. Used in aircraft such as Su-27, MiG-29, F-22, F-35;
3. Turboprop. One of the variants of gas turbine engines. But the energy received during operation is converted into drive for the propeller of the aircraft. A small part of it is used to form a jet pusher jet. They are mainly used in civil aviation;
4. Turbofan. Characterized by high efficiency. The applied technology of injection of additional air for complete combustion of fuel ensures maximum efficiency and high environmental safety. Such engines have found their application in the creation of large airliners.

Important! The list of engines developed by aircraft designers is not limited to the above list. At different times, attempts were repeatedly made to create various variations of power units. In the last century, work was even carried out on the design of atomic engines in the interests of aviation. Prototypes were tested in the USSR (TU-95, AN-22) and the USA (Convair NB-36H), but were withdrawn from testing due to the high environmental hazard during aviation accidents.

Controls and signaling

The complex of on-board equipment, command and executive devices of the aircraft are called controls. Commands are given from the pilot cabin, and are carried out by elements of the wing plane, tail plumage. Different types of aircraft use different types of control systems: manual, semi-automatic and fully automated.

Controls, regardless of the type of control system, are divided as follows:

1. The main control, which includes actions responsible for adjusting flight modes, restoring the longitudinal balance of the aircraft in predetermined parameters, these include:

• levers directly controlled by the pilot (steering wheel, elevators, horizon, command panels);
• communications for connecting control levers with elements of actuators;
• direct executing devices (ailerons, stabilizers, spoler systems, flaps, slats).

1. Additional control used during takeoff or landing.

When using manual or semi-automatic control of the aircraft, the pilot can be considered an integral part of the system. Only he can collect and analyze information about the position of the aircraft, load indicators, compliance of the flight direction with planned data, and make a decision appropriate to the situation.

To obtain objective information about the flight situation, the state of the aircraft components, the pilot uses groups of instruments, let's name the main ones:

1. Aerobatic and used for navigational purposes. Determine the coordinates, horizontal and vertical position, speed, linear deviations. They control the angle of attack in relation to the oncoming air flow, the operation of gyroscopic devices and many equally important flight parameters. On modern aircraft models, they are combined into a single flight and navigation complex;
2. To control the operation of the power unit. Provide the pilot with information about the temperature and pressure of oil and aviation fuel, the flow rate of the working mixture, the number of revolutions of the crankshafts, the vibration indicator (tachometers, sensors, thermometers, etc.);
3. To monitor the operation of additional equipment and aircraft systems. They include a complex of measuring instruments, the elements of which are located in almost all structural parts of the aircraft (pressure gauges, air consumption indicator, pressure drop in hermetically closed cabins, flap positions, stabilizing devices, etc.);
4. To assess the state of the surrounding atmosphere. The main measured parameters are the outdoor air temperature, the state of atmospheric pressure, humidity, and the speed indicators of the movement of air masses. Special barometers and other adapted measuring instruments are used.

Important! The measuring instruments used to monitor the state of the machine and the environment are specially designed and adapted for difficult operating conditions.

Takeoff and landing systems 2280

Takeoff and landing are considered critical periods in the operation of the aircraft. During this period, there are maximum loads on the entire structure. Only a well-designed landing gear can guarantee an acceptable take-off acceleration and a soft touch on the runway surface. In flight, they serve as an additional element to stiffen the wings.

The design of the most common chassis models is represented by the following elements:

• folding strut, compensating lot loads;
• shock absorber (group), ensures the smoothness of the aircraft when moving along the runway, compensates for shocks during contact with the ground, can be installed in a set with stabilizer dampers;
• braces that act as a structural stiffener, can be called rods, are located diagonally with respect to the rack;
• traverses attached to the fuselage structure and landing gear wings;
• orientation mechanism - to control the direction of movement on the lane;
• locking systems that secure the rack in the required position;
• cylinders designed to extend and retract the landing gear.

How many wheels are on an airplane? The number of wheels is determined depending on the model, weight and purpose of the aircraft. The most common is the placement of two main racks with two wheels. Heavier models - three rack (placed under the nose and wings), four rack - two main and two additional support.

Video

The described device of the aircraft gives only a general idea of ​​the main structural components, allows you to determine the degree of importance of each element in the operation of the aircraft. Further study requires deep engineering training, special knowledge of aerodynamics, strength of materials, hydraulics and electrical equipment. At aircraft manufacturing enterprises, these issues are dealt with by people who have undergone training and special training. You can independently study all the stages of creating an aircraft, only for this you should be patient and be ready to gain new knowledge.

Aircraft fuselage design has evolved from early wood truss structures through the monocoque shell to the modern semi-monocoque shell.

farm structure. The main disadvantage of the truss structure is the lack of a streamlined shape. The design is based on pipe segments called spars. Welded together, they form a well-reinforced frame. Vertical and horizontal brackets are welded to the spars, due to which such a structure acquires a square or rectangular section. Additional brackets are added to the structure to provide resistance to external pressure that can occur from either side of the structure. Stringers and frames (or accessory ribs) create the shape of the fuselage and support the skin.

As technology progressed, designers began to cover the trusses to give the fuselage a more streamlined shape and improve its aerodynamic performance. This was originally done with cloth. Subsequently, light metals (aluminum) began to be used. In some cases, the outer skin can carry all or a significant part of the flight load. Most modern aircraft use a load-bearing skin structure known as a monocoque or semi-monocoque (Figure 2-14).

Monocoque. The monocoque design uses a load-bearing skin, which, like the wall of an aluminum can, takes almost all the load. Being sufficiently rigid, such a design does not respond very well to the deformation of its surface. For example, an aluminum can can withstand a significant load if this load falls on the edges. But if the side surface of the can is even slightly deformed, even slight pressure can crush the can.

Since most of the bending load is on the skin and not on the exposed truss frame, there is no need to reinforce the structure internally. This reduces its weight and increases interior space. One of the original methods of using a monocoque was first proposed by American engineer Jack Northrop. In 1918, he developed a new method for manufacturing a monocoque fuselage, which was later applied to the creation of the Lockheed S-1 Racer aircraft. The design consisted of two plywood halves of the shell, which were glued to wooden hoops-stringers. In order to get the halves, the designer used three large pieces of spruce plywood, which were soaked in glue and placed in a semi-circular concrete mold resembling a bathtub. Then the form was covered with a tight-fitting lid, and a rubber ball was inflated inside it, which pressed the plywood against the surface of the form. A day later, a smooth and even half of the shell was ready. Both halves had a thickness of no more than 6 millimeters.

Due to the difficulties in industrial production, the monocoque did not become widespread until a few decades later. Today, monocoque construction is widely used in the automotive industry, where the monocoque body is the de facto industry standard.

Semi-monocoque. The semi-monocoque design (partial or half) uses an additional structure to which the aircraft skin is attached. Consisting of frames and/or ribs of various sizes, as well as stringers, this structure reinforces the load-bearing skin, partially removing the bending load from the fuselage. On the main section of the fuselage there are also places for fastening the wings and a heat-shielding casing.

On single-engine aircraft, the engine is usually mounted at the front of the fuselage. A fireproof partition is installed between the rear wall of the engine and the cockpit, which serves to protect the pilot and passengers in the event of a sudden fire in the engine. It is usually made of heat-resistant material (eg stainless steel). Recently, however, composite materials have been increasingly used in aircraft construction. Some aircraft are completely made from them.

Composite construction. Story. The use of composite materials in aircraft construction began during World War II. It was then that fiberglass began to be used in the production of the fuselages of the B-29 strategic bombers. In the late 50s, this material began to be widely used in the manufacture of gliders. In 1965, the first aircraft made entirely of fiberglass was certified. It was a Swiss-made Diamond HBV glider. Four years later, the all-fiberglass, four-seat, single-engine Windaker Eagle aircraft was certified in the United States. Currently, more than a third of all aircraft in the world are made from composite materials.

Composite material is a broad concept. These materials include fiberglass, carbon fiber, bulletproof fiber "Kevlar", as well as their combinations. Composite construction has two important advantages: an extremely smooth surface and the ability to fabricate complex curved or streamlined structures (Figure 2-15).

Airplanes made of composite materials. Composite material is an artificially created heterogeneous material consisting of a filler and reinforcing elements (fibers). The filler acts as a kind of “glue”, fastening the fibers and (during vulcanization) giving the product a shape, and the fibers take on the bulk of the load.

There are many different types of fibers and fillers. In the manufacture of aircraft, epoxy resin is most often used, which is a type of thermosetting plastic. Compared to other similar materials (such as polyester resin), epoxy resin is significantly stronger. In addition, it is better able to withstand high temperatures. There are many varieties of epoxy resins that vary in performance, curing time and temperature, and cost.

Fiberglass and carbon fiber are most often used as reinforcing fibers in the production of aircraft. Fiberglass has good tensile and compressive strength, high impact resistance. It is an easy-to-work, relatively inexpensive and widespread material. Its main disadvantage is its relatively large weight. Because of this, it is difficult to make a load-bearing body from fiberglass, which could compete with similar aluminum in lightness.

Carbon fiber is generally stronger in tension and compression than fiberglass, and much more rigid in bending. It is also significantly lighter than fiberglass. However, its resistance to impact loads is somewhat lower, the fibers are quite brittle and break with a sharp impact. These characteristics are greatly improved in the "reinforced" epoxy type of carbon fiber used in the horizontal and vertical stabilizers of the Boeing 787.

Carbon fiber has a higher cost than fiberglass. Prices have fallen somewhat since the innovations of the B-2 bomber (in the 1980s) and the Boeing 777 (in the 1990s). Well-designed carbon fiber structures can be significantly lighter, than similar aluminum ones, sometimes by more than 30%.

Advantages of composite materials. Composite materials have several significant advantages over metals, wood or fabric. Most often, lighter weight is cited as the main advantage. However, it should be understood that an aircraft body made of a composite material will not necessarily be lighter than a metal one. It depends on the characteristics of the case, as well as on the material used.

A more important advantage is the ability to create a very smooth and complexly curved airfoil using composite materials, which can significantly reduce air resistance. It is for this reason that in the 60s of the last century, glider designers switched from metal and wood to composite materials.

Composite materials are widely used by aircraft manufacturers such as Cirrus and Columbia. Due to the reduction of air resistance, the aircraft of these companies are distinguished by high flight characteristics, despite the presence of a fixed landing gear. Composite materials also help mask radar signatures in stealth designs (in aircraft such as the B-2 strategic bomber and the F-22 multirole fighter). Today, composite materials are used in the manufacture of any aircraft - from gliders to helicopters.

The third advantage of composite materials is the absence of corrosion. Thus, the fuselage of the Boeing 787 is made entirely of composite materials, which allows this aircraft to withstand a greater pressure drop and greater humidity in the cabin than previous generations of airliners allowed. Engineers are no longer concerned about the problem of corrosion due to moisture condensation on hidden parts of the fuselage skin (for example, under an insulating coating). As a result, the long-term operating costs of airlines can be substantially reduced.

Another advantage of composite materials is good performance in a bending environment (for example, when used in helicopter rotor blades). Unlike most metals, composite materials do not suffer from metal fatigue and cracking. With proper design, rotor blades made of composite material have a significantly higher standard operating time than metal ones. Because of this, most modern large helicopters have fully composite blades, and sometimes a composite rotor hub.

Disadvantages of composite materials. Composite structures have their drawbacks, the most important of which is the absence of visual signs of damage. Composite materials react differently to impact than other materials, and damage is often invisible to the naked eye.

For example, if a car crashes into an aluminum fuselage, a dent will be left on the fuselage. If there is no dent, there is no damage. If a dent is present, the damage is determined visually and repaired. In composite structures, low-impact impact (for example, from a collision or a dropped tool) often leaves no visible damage on the surface. In this case, a wide delamination zone can occur in the impact zone, which spreads funnel-shaped from the impact point. Damage to the posterior surface of a structure can be significant—and yet completely invisible. As soon as there are reasons to believe that an impact (even of a minor force) has occurred, it becomes necessary to invite a specialist to inspect the structure and look for internal damage. A good sign of delamination of the fiber structure when using fiberglass is the appearance of "whitish" areas on the surface of the case.

A moderate impact (for example, in a collision with a car) leads to local damage to the surface, which is visible to the naked eye. The damage zone is larger than the damage on the surface and needs to be repaired. A high-impact impact (for example, a bird or hailstone hitting the aircraft body during flight) results in a hole and significant structural damage. In the case of impacts of medium and high strength, the damage is visible to the eye, but the impact of low strength is difficult to determine visually (Fig. 2-16).

If the impact caused delamination, surface destruction or a hole, it is imperative to carry out repairs. Pending repair, the damaged area should be covered and protected from rain. Parts made of composite material often consist of a thin shell with a porous inner layer underneath (the so-called “sandwich” construction). Excellent in terms of structural rigidity, this structure is susceptible to moisture penetration, which can later lead to serious problems. Putting a piece of "duct tape" over the hole is a good way to temporarily protect against water, but it's not a structural repair. Nor is it a repair to use paste to fill holes, although this method can be used for cosmetic purposes.

Another disadvantage of composite materials is their relatively low heat resistance. While the temperature limits of use vary with different resins, most resins begin to lose strength at temperatures above 65°C. To reduce temperature exposure, the composite body is often painted white. For example, the underside of a wing painted black and located above a hot asphalt pavement on a sunny day can heat up to more than 100°C. The same structure painted white rarely warms up to more than 60°C.

Composite aircraft manufacturers often give specific recommendations on acceptable hull colors. When repainting the aircraft, these guidelines must be followed exactly.

The cause of thermal damage can often be a fire on board. Even a quickly extinguished fire in the brake system can damage the lower wing skins, struts or landing gear wheels. Composite materials are also easily damaged by various solvents, so composite structures cannot be treated with such chemicals. To remove paint from composite parts, only mechanical methods are used, such as metal powder blasting or sandblasting. Solvent damage to high-value composite parts is relatively uncommon and such damage is usually beyond repair.

Fluid leakage on composite structures. Concerns are sometimes expressed about fuel, oil or hydraulic fluid getting on composite structures. It should be said that with modern epoxy resins this is usually not a problem. As a rule, if the leaking liquid does not corrode the paint, it cannot damage the composite material underneath. For example, some aircraft use fiberglass fuel tanks, in which the fuel is in direct contact with the composite surface without the use of a sealant. Some inexpensive polyester resins can be damaged if they come into contact with a mixture of motor gasoline and ethyl alcohol. More expensive resins, like epoxy, can safely come into contact with automotive gasoline, as well as aviation gasoline (100 octane) and jet fuel.

Lightning strike protection. An important factor in aircraft design is protection against lightning strikes. When lightning strikes an aircraft, its structure is exposed to enormous power. Whether you are flying a general purpose aircraft or a large airliner, the basic principles of lightning protection are the same. Regardless of the size of the aircraft, the energy from the impact must be distributed over a large surface area - this allows you to reduce the current per unit area of ​​\u200b\u200bthe skin to an acceptable level.

When lightning strikes an aircraft made of aluminum (due to its electrical conductivity), electrical energy is naturally distributed throughout the aluminum structure. In this case, the main task of designers is to protect electronic equipment, fuel system, etc. The outer skin of the aircraft must provide a path of least resistance for electrical discharge.

In the case of an aircraft made of composite materials, the situation is different. Fiberglass is an excellent electrical insulator. Carbon fiber conducts electricity, but not as well as aluminum. Therefore, the outer layer of the composite skin must have additional electrical conductivity. This is usually achieved with a metal mesh embedded in the skin. The most commonly used meshes are aluminum or copper - aluminum for fiberglass, copper for carbon fiber. Any structural repair of lightning protected surfaces must include the restoration of the metal mesh.

In the event that the design of a composite aircraft requires the presence of an internal radio antenna, special “windows” must be left in the lightning protection grid. Internal radio antennas are sometimes used in composite aircraft because fiberglass is transparent to radio waves (whereas carbon fiber is not).

The future of composite materials. In the decades since the end of World War II, composite materials have taken an important place in the aviation industry. Thanks to their versatility and corrosion resistance, as well as their good strength-to-weight ratio, composite materials allow the most daring and innovative design ideas to be realized. Used in aircraft ranging from the Cirrus SR-20 light monoplane to the Boeing 787 airliner, composite materials play a significant role in the aviation industry and their use will only expand (Figure 2-17).