Elevator (aeronautics)
Elevators are flight control surfaces at the rear of an aircraft, which control the aircraft's pitch, therefore the angle of attack and the lift of the wing. The elevators are hinged to the tailplane or horizontal stabilizer, they may be the only pitch control surface present, sometimes located at the front of the aircraft or integrated into a rear "all-moving tailplane" called a slab elevator or stabilator. The horizontal stabilizer creates a downward force which balances the nose down moment created by the wing lift force, which applies at a point situated aft of the airplane's center of gravity; the effects of drag and changing the engine thrust may result in pitch moments that need to be compensated with the horizontal stabilizer. Both the horizontal stabilizer and the elevator contribute to pitch stability, but only the elevators provide pitch control, they do so by decreasing or increasing the downward force created by the stabilizer: an increased downward force, produced by up elevator, forces the tail down and the nose up.
At constant speed, the wing's increased angle of attack causes a greater lift to be produced by the wing, accelerating the aircraft upwards. The drag and power demand increase. At constant speed, the decrease in angle of attack reduces the lift, accelerating the aircraft downwards. On many low-speed aircraft, a trim tab is present at the rear of the elevator, which the pilot can adjust to eliminate forces on the control column at the desired attitude and airspeed. Supersonic aircraft have all-moving tailplanes, because shock waves generated on the horizontal stabilizer reduce the effectiveness of hinged elevators during supersonic flight. Delta winged aircraft combine ailerons and elevators –and their respective control inputs– into one control surface called an elevon. Elevators are part of the tail, at the rear of an aircraft. In some aircraft, pitch-control surfaces are in the front, ahead of the wing. In a two-surface aircraft this type of configuration is called a tandem wing; the Wright Brothers' early aircraft were of the canard type.
Some early three surface aircraft had front elevators. Several technology research and development efforts exist to integrate the functions of aircraft flight control systems such as ailerons, elevons and flaperons into wings to perform the aerodynamic purpose with the advantages of less: mass, drag, inertia and radar cross section for stealth; these may be used in 6th generation fighter aircraft. Two promising approaches are flexible wings, fluidics. In flexible wings, much or all of a wing surface can change shape in flight to deflect air flow; the X-53 Active Aeroelastic Wing is a NASA effort. The Adaptive Compliant Wing is a commercial effort. In fluidics, forces in vehicles occur via circulation control, in which larger more complex mechanical parts are replaced by smaller simpler fluidic systems where larger forces in fluids are diverted by smaller jets or flows of fluid intermittently, to change the direction of vehicles. In this use, fluidics promises lower mass and low inertia and response times, simplicity.
Rudder Aileron Aircraft Pitch Motion
Boeing 757
The Boeing 757 is a mid-size, narrow-body twin-engine airliner, designed and built by Boeing Commercial Airplanes. It is the manufacturer's largest single-aisle passenger aircraft and was produced from 1981 to 2004; the twinjet has a two-crew member glass cockpit, turbofan engines of sufficient power to allow takeoffs from short runways and higher altitudes, a conventional tail and, for reduced aerodynamic drag, a supercritical wing design. Intended to replace the smaller three-engine 727 on short and medium routes, the 757 can carry 200 to 295 passengers for a maximum of 3,150 to 4,100 nautical miles, depending on variant; the 757 was designed concurrently with a wide-body twinjet, the 767, owing to shared features, pilots can obtain a common type rating that allows them to operate both aircraft. The 757 was produced in two fuselage lengths; the original 757-200 entered service in 1983. The stretched 757-300, the longest narrow-body twinjet produced, began service in 1999. Passenger 757-200s have been modified to special freighter specification for cargo use, while military derivatives include the C-32 transport, VIP carriers, other multi-purpose aircraft.
Private and government operators have customized the 757 for research and transport roles. All 757s are powered by Rolls-Royce Pratt & Whitney PW2000 series turbofans. Eastern Air Lines and British Airways placed the 757 in commercial service in 1983; the narrow-body twinjet succeeded earlier single-aisle airliners, became used for short and mid-range domestic routes, shuttle services, transcontinental U. S. flights. After regulators granted approval for extended flights over water in 1986, airlines began using the aircraft for intercontinental routes. Major customers for the 757 included U. S. mainline carriers, European charter airlines, cargo companies. The airliner has recorded eight hull-loss accidents, including seven fatal crashes, as of September 2015. Production of the 757 ended in October 2004; the 757-200 was by far the most popular model, with 913 built. Diminished sales amid an airline industry trend toward smaller jetliners led Boeing to end production without a direct replacement, in favor of the 737 family.
The last 757 was delivered to Shanghai Airlines in November 2005. In July 2017, 666 of the narrow-body twinjets were in airline service. In the early 1970s, following the launch of the wide-body 747, Boeing began considering further developments of its narrow-body 727 trijet. Designed for short and medium length routes, the three-engined 727 was the best-selling commercial jetliner of the 1960s and a mainstay of the U. S. domestic airline market. Studies focused on improving the most successful 727 variant. Two approaches were considered: a stretched 727-300, an all-new aircraft code-named 7N7; the former was a cheaper derivative using the 727's existing technology and tail-mounted engine configuration, while the latter was a twin-engine aircraft which made use of new materials and improvements to propulsion technology which had become available in the civil aerospace industry. United Airlines provided input for the proposed 727-300, which Boeing was poised to launch in late 1975, but lost interest after examining development studies for the 7N7.
Although the 727-300 was offered to Braniff International Airways and other carriers, customer interest remained insufficient for further development. Instead, airlines were drawn to the high-bypass-ratio turbofan engines, new flight deck technologies, lower weight, improved aerodynamics, reduced operating cost promised by the 7N7; these features were included in a parallel development effort for a new mid-size wide-body airliner, code-named 7X7, which became the 767. Work on both proposals accelerated as a result of the airline industry upturn in the late 1970s. By 1978, development studies focused on two variants: a 7N7-100 with seating for 160, a 7N7-200 with room for over 180 seats. New features included a redesigned wing, under-wing engines, lighter materials, while the forward fuselage, cockpit layout, T-tail configuration were retained from the 727. Boeing planned for the aircraft to offer the lowest fuel burn per passenger-kilometer of any narrow-body airliner. On August 31, 1978, Eastern Air Lines and British Airways became the first carriers to publicly commit to the 7N7 when they announced launch orders totaling 40 aircraft for the 7N7-200 version.
These orders were signed in March 1979, when Boeing designated the aircraft as the 757. The shorter 757-100 was dropped; the 757 was intended to be more capable and more efficient than the preceding 727. The focus on fuel efficiency reflected airline concerns over operating costs, which had grown amid rising oil prices during the Yom Kippur War of 1973. Design targets included a 20 percent reduction in fuel consumption from new engines, plus an additional 10 percent from aerodynamic improvements, versus preceding aircraft. Lighter materials and new wings were expected to improve efficiency; the maximum take-off weight was set at 220,000 pounds, 10,000 pounds more than the 727. The 757's higher thrust-to-weight ratio allowed it to take off from short runways and serve airports in hot and high climates, offering better takeoff performance than that offered by competing aircraft. Competitors needed longer takeoff runs at airports at higher elevations, with higher ambient temperatures and thinner air.
Boeing offer
Wing tip
A wing tip is the part of the wing, most distant from the fuselage of a fixed-wing aircraft. Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of shapes, including: Squared-off Aluminium tube bow Rounded Hoerner style Winglets Drooped tips Raked wingtips Tip tanks Sails Fences End platesWinglets have become popular additions to high speed aircraft to increase fuel efficiency by reducing drag from wingtip vortices. In lower speed aircraft, the effect of the wingtip shape is less apparent, with only a marginal performance difference between round and Hoerner style tips The slowest speed aircraft, STOL aircraft, may use wingtips to shape airflow for controlability at low airspeeds. Wing tips are an expression of aircraft design style, so their shape may be influenced by marketing considerations as well as by aerodynamic requirements. Wing tips are used by aircraft designers to mount navigation lights, anti-collision strobe lights, landing lights and identification markings.
Wing tip tanks can distribute weight more evenly across the wing spar. On fighter aircraft, they may be fitted with hardpoints, for mounting drop tanks and weapons systems, such as missiles and electronic countermeasures. Wingtip mounted. Aerobatic aircraft use wingtip mounted crosses for visual attitude reference. Wingtip mounted smoke fireworks highlight rolling aerobatic maneuvers; some airshow acts feature the pilot dragging the wingtip along the ground. Aircraft with a single main landing gear or high aspect ratio wings such as gliders, may place small landing gear in the wingtips; some uncommon designs,like the Rutan Quickie, Convair XFY placed the main landing gear in the wingtips. Some early World War I aircraft used wooded skids on the wingtips to minimize damage on ground looping incidents. Several amphibious aircraft such as the Consolidated PBY Catalina, use retractable wingtips as floats. Moveable wingtips can affect the controlability of a wing. Wing warping the ends of the wing, produced roll control on the earliest of aircraft such as the Wright Flyer.
The North American XB-70 Valkyrie raised and lowered its wingtips in flight to adjust its stability in supersonic and subsonic flight. Wingtips can house the power plant or thrust of an aircraft; the EWR VJ 101 used tip mounted jets, the V-22 uses tilting wingtip mounted engines, the Harrier uses wingtip thrust for stability while hovering. Rotary wing aircraft wingtips may be curved to reduce noise and vibration; some rotary wing aircraft place their propulsion in wingtip tip jets. The Boeing 777X will feature 3.5 m folding wingtips supplied by Liebherr Aerospace from Lindenberg. The mechanism was demonstrated for Aviation Week at the Boeing Everett Factory in October 2016; the folding takes 20 seconds to complete. Wingtip device
Flow separation
All solid objects traveling through a fluid acquire a boundary layer of fluid around them where viscous forces occur in the layer of fluid close to the solid surface. Boundary layers can be either turbulent. A reasonable assessment of whether the boundary layer will be laminar or turbulent can be made by calculating the Reynolds number of the local flow conditions. Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer relative to the object falls to zero; the fluid flow becomes detached from the surface of the object, instead takes the forms of eddies and vortices. In aerodynamics, flow separation can result in increased drag pressure drag, caused by the pressure differential between the front and rear surfaces of the object as it travels through the air. For this reason much effort and research has gone into the design of aerodynamic and hydrodynamic surfaces which delay flow separation and keep the local flow attached for as long as possible.
Examples of this include the fur on a tennis ball, dimples on a golf ball, turbulators on a glider, which induce an early transition to turbulent flow regime. Boundary layer separation is the detachment of a boundary layer from the surface into a broader wake. Boundary layer separation occurs when the portion of the boundary layer closest to the wall or leading edge reverses in flow direction; the separation point is defined as the point between the forward and backward flow, where the shear stress is zero. The overall boundary layer thickens at the separation point and is forced off the surface by the reversed flow at its bottom; the flow reversal is caused by adverse pressure gradient imposed on the boundary layer by the outer potential flow. The streamwise momentum equation inside the boundary layer is stated as u ∂ u ∂ s = − 1 ρ d p d s + ν ∂ 2 u ∂ y 2 where s, y are streamwise and normal coordinates. An adverse pressure gradient is when d p / d s > 0, which can be seen to cause the velocity u to decrease along s and go to zero if the adverse pressure gradient is strong enough.
The tendency of a boundary layer to separate depends on the distribution of the adverse or negative edge velocity gradient d u o / d s < 0 along the surface, which in turn is directly related to the pressure and its gradient by the differential form of the Bernoulli relation, the same as the momentum equation for the outer inviscid flow. Ρ u o d u o d s = − d p d s But the general magnitudes of d u o / d s required for separation are much greater for turbulent than for laminar flow, the former being able to tolerate nearly an order of magnitude stronger flow deceleration. A secondary influence is the Reynolds number. For a given adverse d u o / d s distribution, the separation resistance of a turbulent boundary layer increases with increasing Reynolds number. In contrast, the separation resistance of a laminar boundary layer is independent of Reynolds number — a somewhat counterintuitive fact. Boundary layer separation can occur for internal flows, it can result from such causes such as a expanding duct of pipe.
Separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculating flow and the flow through the central region of the duct is called the dividing streamline; the point where the dividing streamline attaches to the wall again is called the reattachment point. As the flow goes farther downstream it achieves an equilibrium state and has no reverse flow; when the boundary layer separates, its displacement thickness increases which modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag, if severe enough will result in loss of lift and stall, all of which are undesirable. For internal flows, flow separation produces an increase in the flow losses, stall-type phenomena such as compressor surge, both undesirable phenomena. Another effect of boundary layer separation is shedding vortices, known as Kármán vortex street.
When the vortices begin to shed off the bounded surface they do so at a certain frequency. The shedding of the vortices could cause vibrations in the structure that they are shed
Ultralight trike
An ultralight trike is a type of powered hang glider where flight control is by weight-shift. These aircraft have a fabric flex-wing from, suspended a tricycle fuselage pod driven by a pusher propeller; the pod accommodates a pilot and a single passenger. Trikes grant affordable and exciting flying, have been popular since the 1980s. Trikes are referred to as "microlights" in Europe; such aircraft are known as 2-axis microlights, flex-wing trikes, weight-shift-control aircraft, microlight trikes, deltatrikes or motorized deltaplanes, The history of the trike is traced back to the invention of Francis Rogallo's flexible wing and subsequent development by the Paresev engineering team's innovations and others. On 1948, engineer Francis Rogallo invented a self-inflating wing which he patented on March 20, 1951, as the Flexible wing, it was on October 4, 1957, when the Russian satellite Sputnik shocked the United States and the space race caught the imagination of its government, causing major increases in U.
S. government spending on scientific research, education and on the immediate creation of NASA. Rogallo was in position to seize the opportunity and released his patent to the government and with his help at the wind tunnels, NASA began a series of experiments testing Rogallo's wing –, renamed Para Wing – in order to evaluate it as a recovery system for the Gemini space capsules and recovery of used Saturn rocket stages. F. Rogallo's team adapted and extended the flexible principle into semi-rigid variants; this involved stabilizing the leading edges with compressed air beams or rigid structures like aluminum tubes. By 1960, NASA had made test flights of a powered framed cargo aircraft called the Ryan XV-8 or Fleep and by March 1962, of a weight-shift experimental glider called Paresev. By 1967, all Para Wing projects were dropped by NASA in favor of using round parachutes without considering development of personal ultralight gliders, but the airfoil's simplicity of design and ease of construction, along with its capability of slow flight and its gentle landing characteristics, did not go unnoticed by hang glider enthusiasts.
The challenge was to modify and fit a Rogallo flexible wing with an appropriate frame to allow it to be used as a hang glider. A crucial development toward the trike was introduction of several mechanical innovations developed by the Paresev and the Fleep engineers. Publicity from the Fleep and the Paresev tests sparked interest in the design among several tinkerers, including Barry Palmer, Sport Aviation of 1962 shown Jim Hobson of Experimental Aircraft Association. An Australian engineer Mike Burns developed and used the boat-towed Rogallo-airfoiled SkiPlane starting in 1962 through the 1960s. A fellow countryman of Mike Burns, John W. Dickenson, made ski-kites and partnered with Mike Burns to improve the ski-kite. An influence through John Dickenson's duplication of his device, who named his flexible-wing ski-kite the Ski Wing. Dickenson fashioned a water ski kite airframe to fit on a Rogallo airfoil where the pilot sat on a swinging seat while the control frame and wire bracing distributed the load to the wing as well as gave a frame freedom to be pushed/pulled for weight-shift control.
Dickenson's Ski Wing turned out to be stable and controllable under tow, unlike the flat manned kites used at water ski shows. The Ski Wing kite was first kited in public at the Grafton Jacaranda Festival in September 1963 by Rod Fuller while towed behind a motorboat. Australian manufacturers like Bill Bennett and Bill Moyes developed and marketed Dickenson's innovations to the world, which fueled the hang glider revolution. Although by the early 1970s many rigid wings were developed, none sold well, while dozens of flexible-wing hang glider companies were springing up worldwide, building variants of Dickenson's Ski Wing. In 1972, Popular Mechanics and Popular Science magazines published articles on hang gliding which further increased its popularity, as the Sky Raiders hang gliding movie released in 1975. Francis Rogallo, Barry Palmer, John Dickenson, others never made any money out of their innovations. Profit to manufacturers of hang gliders and Rogallo-winged hang gliders came once organized and insured sporting events grew in popularity.
Dickenson's adaptation and innovations produced a foldable hang glider that reduced difficulty in control, transport and repair. In addition, the flexible wing lends itself to design changes for possible improvements; the crucial developments put together by the Paresev engineers, Barry Palmer, John Dickenson, Bill Bennett, Bill Moyes, Richard Miller, hundreds of other innovators gave success to the flexible-wing hang glider. In 1961, Engineer Thomas Purcell built a towable Rogallo-wing glider with an aluminum frame, wheels, a seat and basic control rods. In 1964, Swiss inventor Pierre Aubert saw a photo of NASA's Fleep and completed construction of a similar trike; as with the Fleep, his Rogallo wing did not allow for pendulum weight-shift control. In March 1967, aeronautical engineer Barry Palmer completed the earliest example of a true weight-shift powered trike: the Paraplane; the Paraplane u
Landing gear
Landing gear is the undercarriage of an aircraft or spacecraft and may be used for either takeoff or landing. For aircraft it is both, it was formerly called alighting gear by some manufacturers, such as the Glenn L. Martin Company. For aircraft, the landing gear supports the craft when it is not flying, allowing it to take off and taxi without damage. Wheels are used but skids, floats or a combination of these and other elements can be deployed depending both on the surface and on whether the craft only operates vertically or is able to taxi along the surface. Faster aircraft have retractable undercarriages, which fold away during flight to reduce air resistance or drag. For launch vehicles and spacecraft landers, the landing gear is designed to support the vehicle only post-flight, are not used for takeoff or surface movement. Aircraft landing gear includes wheels equipped with simple shock absorbers, or more advanced air/oil oleo struts, for runway and rough terrain landing; some aircraft floats for water, and/or skids or pontoons.
It represents 2.5 to 5% of the MTOW and 1.5 to 1.75% of the aircraft cost but 20% of the airframe direct maintenance cost. The undercarriage is 4–5% of the takeoff mass and can reach 7%. Wheeled undercarriages come in two types: conventional or "taildragger" undercarriage, where there are two main wheels towards the front of the aircraft and a single, much smaller, wheel or skid at the rear; the taildragger arrangement was common during the early propeller era, as it allows more room for propeller clearance. Most modern aircraft have tricycle undercarriages. Taildraggers are considered harder to land and take off, require special pilot training. Sometimes a small tail wheel or skid is added to aircraft with tricycle undercarriage, in case of tail strikes during take-off; the Concorde, for instance, had a retractable tail "bumper" wheel, as delta winged aircraft need a high angle when taking off. Both Boeing's largest WWII bomber, the B-29 Superfortress, the 1960s-introduced Boeing 727 trijet airliner each have a retractable tail bumper.
Some aircraft with retractable conventional landing gear have a fixed tailwheel, which generates minimal drag and improves yaw stability in some cases. Another arrangement sometimes used is central nose gear with outriggers on the wings; this may be done where there is no convenient location on either side to attach the main undercarriage or to store it when retracted. Examples include the Harrier Jump Jet; the B-52 bomber uses a similar arrangement, except that each end of the fuselage has two sets of wheels side by side. To decrease drag in flight some undercarriages retract into the wings and/or fuselage with wheels flush against the surface or concealed behind doors. If the wheels rest protruding and exposed to the airstream after being retracted, the system is called semi-retractable. Most retraction systems are hydraulically operated, though some are electrically operated or manually operated; this adds complexity to the design. In retractable gear systems, the compartment where the wheels are stowed are called wheel wells, which may diminish valuable cargo or fuel space.
] Pilots confirming that their landing gear is down and locked refer to "three greens" or "three in the green.", a reference to the electrical indicator lights from the nosewheel/tailwheel and the two main gears. Blinking green lights or red lights indicate the gear is in transit and neither up and locked or down and locked; when the gear is stowed up with the up-locks secure, the lights extinguish to follow the dark cockpit philosophy. Multiple redundancies are provided to prevent a single failure from failing the entire landing gear extension process. Whether electrically or hydraulically operated, the landing gear can be powered from multiple sources. In case the power system fails, an emergency extension system is always available; this may take the form of a manually operated crank or pump, or a mechanical free-fall mechanism which disengages the uplocks and allows the landing gear to fall due to gravity. Some high-performance aircraft may feature a pressurized-nitrogen back-up system; as aircraft grow larger, they employ more wheels to cope with the increasing weights.
The earliest "giant" aircraft placed in quantity production, the Zeppelin-Staaken R. VI German World War I long-range bomber of 1916, used a total of eighteen wheels for its undercarriage, split between two wheels on its nose gear struts, a total of sixteen wheels on its main gear units — split into four side-by-side quartets each, two quartets of wheels per side — under each tandem engine nacelle, to support its loaded weight of 12 metric tons. Multiple "tandem wheels" on an aircraft — for cargo aircraft, mounted to the fuselage lower sides as retractable main gear units on modern designs — were first seen during World War II, on the experimental German Arado Ar 232 cargo aircraft, which used a row of elev
Fuselage
The fuselage is an aircraft's main body section. It holds crew and cargo. In single-engine aircraft it will contain an engine, as well, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage, which in turn is used as a floating hull; the fuselage serves to position control and stabilization surfaces in specific relationships to lifting surfaces, required for aircraft stability and maneuverability. This type of structure is still in use in many lightweight aircraft using welded steel tube trusses. A box truss fuselage structure can be built out of wood—often covered with plywood. Simple box structures may be rounded by the addition of supported lightweight stringers, allowing the fabric covering to form a more aerodynamic shape, or one more pleasing to the eye. Geodesic structural elements were used by Barnes Wallis for British Vickers between the wars and into World War II to form the whole of the fuselage, including its aerodynamic shape. In this type of construction multiple flat strip stringers are wound about the formers in opposite spiral directions, forming a basket-like appearance.
This proved to be light and rigid and had the advantage of being made entirely of wood. A similar construction using aluminum alloy was used in the Vickers Warwick with less materials than would be required for other structural types; the geodesic structure is redundant and so can survive localized damage without catastrophic failure. A fabric covering over the structure completed the aerodynamic shell; the logical evolution of this is the creation of fuselages using molded plywood, in which multiple sheets are laid with the grain in differing directions to give the monocoque type below. In this method, the exterior surface of the fuselage is the primary structure. A typical early form of this was built using molded plywood, where the layers of plywood are formed over a "plug" or within a mold. A form of this structure uses fiberglass cloth impregnated with polyester or epoxy resin, instead of plywood, as the skin. A simple form of this used in some amateur-built aircraft uses rigid expanded foam plastic as the core, with a fiberglass covering, eliminating the necessity of fabricating molds, but requiring more effort in finishing.
An example of a larger molded plywood aircraft is the de Havilland Mosquito fighter/light bomber of World War II. No plywood-skin fuselage is monocoque, since stiffening elements are incorporated into the structure to carry concentrated loads that would otherwise buckle the thin skin; the use of molded fiberglass using negative molds is prevalent in the series production of many modern sailplanes. The use of molded composites for fuselage structures is being extended to large passenger aircraft such as the Boeing 787 Dreamliner; this is the preferred method of constructing an all-aluminum fuselage. First, a series of frames in the shape of the fuselage cross sections are held in position on a rigid fixture; these frames are joined with lightweight longitudinal elements called stringers. These are in turn covered with a skin of sheet aluminum, attached by riveting or by bonding with special adhesives; the fixture is disassembled and removed from the completed fuselage shell, fitted out with wiring and interior equipment such as seats and luggage bins.
Most modern large aircraft are built using this technique, but use several large sections constructed in this fashion which are joined with fasteners to form the complete fuselage. As the accuracy of the final product is determined by the costly fixture, this form is suitable for series production, where a large number of identical aircraft are to be produced. Early examples of this type include the Douglas Aircraft DC-2 and DC-3 civil aircraft and the Boeing B-17 Flying Fortress. Most metal light aircraft are constructed using this process. Both monocoque and semi-monocoque are referred to as "stressed skin" structures as all or a portion of the external load is taken by the surface covering. In addition, all the load from internal pressurization is carried by the external skin; the proportioning of loads between the components is a design choice dictated by the dimensions and elasticity of the components available for construction and whether or not a design is intended to be "self jigging", not requiring a complete fixture for alignment.
Early aircraft were constructed of wood frames covered in fabric. As monoplanes became popular, metal frames improved the strength, which led to all-metal-structure aircraft, with metal covering for all its exterior surfaces - this was first pioneered in the second half of 1915; some modern aircraft are constructed with composite materials for major control surfaces, wings, or the entire fuselage such as the Boeing 787. On the 787, it makes possible higher pressurization levels and larger windows for passenger comfort as well as lower weight to reduce operating costs; the Boeing 787 weighs 1500 lb less than. Cockpit windshields on the Airbus A320 must withstand bird strikes up to 350 kt and are made of chemically strengthened glass, they are composed of three layers or plies, of glass or plastic: the inner two are 8 mm thick each and are structural, while the outer ply, about 3 mm thick, is a barrier against foreign object damage and abrasion, with a hydrophobic coating. It m
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