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  • Subject area(s): Engineering
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  • Published on: 7th September 2019
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Abstract:

This report provides a detailed investigation of the McDonnell Douglas F/A-18 fighter jet, including sophisticated analysis of the design, materials used in components, and the technology integrated with improved models. It also outlines the various tasks it may perform with underlying physics principals and a conclusion of its impact on society.

Introduction:

This report thoroughly investigates the aeronautical engineering module, specifically the McDonnell Douglas F/A-18 and includes:

- sophisticated detail of the design phase

- suitable properties and materials for major components

- technology used in the operation of the aircraft

- conclusion of the importance of the aircraft in society

Design and Development

The McDonnell Douglas F/A-18 Hornet, originally known as the McDonnell Douglas Model 267, is a drastically modified version of the YF-17 utilised to meet the American Navy and Marine Corp’s specifications for operational service. McDonnell Douglas, which today we know as Boeing, was designated in the 1970’s as a main contractor due to its aeronautic experience, and shared the structural workload through the production phase with Northrop in a 40:60 ratio for each aircraft unit. The replacement and improvement of various components has enabled the F/A-18 to be used in numerous international militaries, including the Australian Defence Force. The title F/A is derived from its operational functions as both a fighter and attack aircraft.

The aircraft’s aerodynamic structure enables high manoeuvrability and roll control, due to the strengthening of the airframe, undercarriage, and wings. The addition of folding wings and catapult attachments specifically for the ability to allow quick take-offs on short runways seen on aircraft carrier ships, and the landing gear was widened to increase the surface area for frictional forces, reducing the brake time. The twin-turbofan engine provides thrust for the mid-wing plane to achieve an excellent high angle of attack, controlled by canted vertical stabilisers, oversized horizontal stabilisers, full length leading edge flaps and trailing edge flaperons. Rigorous testing of the angle of attack was undertaken in the NASA F-18 High Alpha Research Vehicle to prove that the performance of the plane was some of the best at that time, sustaining 65-70 degrees to the horizontal. The Hornet was the first Navy aircraft to incorporate digital multiplexing avionics bus which enabled upgrades and alterations to be added easily, and was among the first to install a full digital fly-by-wire system and multi-function displays for flexible fighter manoeuvres. Here is a detailed overview of the major components seen in the F/A 18.

FUSELAGE

The fuselage is the main body of the aircraft, and the Hornet saw significant changes in the body structure to meet specifications. It follows a semi-monocoque design, composed of a lightweight shell supported by integrated framing. One of the main Navy requirements was increasing the F/A-18’s fuel capacity by 2020kg, which was achieved through the enlargement of the fuselage. Compared to the YF-17, the tail of the aircraft was widened by 11cm, the turbofan engines were slightly canted outwards towards the front, and the entire spine was enlarged in all dimensions just to increase fuel housing space. The swollen section of the fuselage’s dorsal spine contains the four main tanks, lined from the cockpit down to the engines. They are all coated with explosion suppression foam and are designed to self-seal any punctures. The only retractable refuelling probe is located on the starboard side of the fuselage, which can be used during flight. Towards the aft of the fuselage is the twin-turbofan engines, held in an engine bay (firewall) between the central and rear fuselage sections.

UNDERCARRIAGE

The undercarriage of the YF-17 was designed to exhibit a track width of 2.1m inches, which was increased on the updated Hornet design to 3.1m, mainly for suitable landing stability required for carrier landings. As the Hornet could maintain such a high angle of attack and descent, the bogies required considerable strengthening to resist the additional landing impact forces from a 14 feet/second descent rate used on carrier ships. The brakes used on the single pneumatic wheels are multi-disc plates, using several callipers rather than one. The three landing gears are all positioned so that they can be retracted and rotated 90 to sit horizontally beneath the fuselage. The nosegear has additional steel reinforcing to support a shuttle arm for catapult carrier take-offs and a rearward extended drag strut as a shock absorbent on landing. The two undercarriage single wheels retract beneath the air engine ducts.

WING DESIGN

Further improving from the YF-17, the Hornet has a wing surface area increase of 4.65 square metres, which is now equivalent to 37.16m2. The additional area, span and chord allows the Hornet to be more agile and higher performing at low speeds and altitudes. The wings take on a trapezoidal shape, with the leading edge having a 20-degree sweepback for reduced drag and the trailing edge being perpendicular to the fuselage. Sweepback increases the time for sonic waves of air to hit the wing, limiting the amount of drag. The wings also incorporate variable cambers, achieved by hydraulic actuators repositioning the full-span leading edge and single-slot inner trailing edge flaps. These hydraulic flaps are constantly repositioned by computer by extensions and retractions to create an optimal angle and laminar for the pilot.

Leading edge root extensions (LERX) allow the F/A-18 to stay more controllable and low speeds and the high angles of attack required by inducing airflow vortexes across the wings, generating lift and delaying stall. Furthermore, a greater lift force reduces the need for large wing span, in-turn reducing the aircraft weight and allowing more directional stability during manoeuvres. Additional attachment joints also increase the rigidity and overall strength of the fuselage’s structure. Along the trailing edge of the Hornet’s wings is the ailerons, their primary purpose being controlling the roll of the plane but they also function as flaps further improving low speed performances.

Due to the F/A-18 having LEX’s, the design required canted vertical stabilisers to offset the high velocity vortex ejected from the extensions which if not treated effects yaw. To create drag-free airflow along the surface of the fuselage, the vertical tails are positioned as far forward as possible to close the aerodynamic gap. Furthermore, situating the tails far forward counters the necessity of fuselage body attachments that reduce induced airflow from the engines.  Between the vertical tails is the hydraulic airbrake, mounted to the dorsal fuselage. It is configured in a manner that gives minimum pitch change once extended. Also on the dorsal fuselage is oversized horizontal stabilisers, exerting a vertical force to maintain longitudinal balance.

ENGINE

For fighter jets to achieve transonic and supersonic speeds, they require incredibly powerful engines that generate large amounts of thrust. New and improved General Electric F404-GE-400 turbofans were installed in the Hornet, and the twin engine also included two afterburners for additional thrust. Afterburners supply additional fuel injections to the rear of the engine, and when ignited creates a combustion reaction generating more gas. The jet engine net thrust is increased due to the principle of increased temperature and gas mass. General Electric manufactured the engine to compete with other products, especially the Pratt and Whitney F100 turbofan which won the competition for use in the YF-17. The F404 is a low-bypass turbofan, that is, untreated air runs exterior to the engine rather than through the combustion chamber. The air which is bypassed has a ratio of 0.34, meaning that there is too much spilt air to be considered a turbojet or “leaky” turbojet. It is comprised of an initial three stage fan for air intake, the first creating air intake, the next providing a fixed guide inlets and the last has variable guide vanes. The compression stage

of the engine has seven sets of blades, the first tree also being variable stators depending on air pressure and intake volume. The engine’s design is resilient to compressor stalls during flight and at high angles of attack, and can recover quickly due to the afterburners reigniting the burners automatically. Because the design has relatively few moving components compared to other turbofans in history the replacement costs are considerably low.

ENGINE INLETS

The engine air inlets of the Hornet follow a ‘fixed’ design similar to that of the F-16 rather than their predecessors’ variable geometry intake ramp air inlets seen on the F-4, F-14 and F-15. The simplicity of the two-dimensional D-shaped faces separated by fixed splitter plates allow sufficient air intake for the compression of air in the jet engine at transonic speeds. Fixed geometry inlets are designed to generate shock waves that assist with internal air compression and limit the air spillage and pre-entry drag. The two intake ramps are situated below the leading-edge root extensions (LERX) direct the slower air towards the engine inlets. They also allow lethargic boundary layer air to be dumped away from the engine via spills as the pressure is too low. Within the compression phase of the engine, bleed air is created and is utilised to remove ice formed on the wings due to the air’s high temperature and pressure. Two controllable ducts permit the bleed air to be ejected onto the LERX and further dispersed by generated airflow across the wing. By having the bleed air intakes remain stationary the maximum obtainable speed is limited. Although this is not as effective as using variable geometry inlets, the Hornet only needs to reach speeds of Mach 2, making the reduced amount of air intake from the bleed air technique benefit for the aircraft. Another benefit is their reduced weight and construction time, providing a smaller cross-sectional area which is more difficult to detect by a radar from a head-on position.

NOSE

The American Navy wanted to have nose technology housing that could be utilised in all weathers, so the previous hub had to be replaced by a more powerful radar. Engineers decided to opt for the Hughes AN/APG-65 digital multi-mode radar system which was significantly larger. To install it, the nose was enlarged to 28 inches and allowed for the incorporation of more sophisticated technology, including a velocity search up over 30 nautical miles providing a detection range, range-while-search on all targets including missiles, track-while-scan giving the aircraft lock on and fire capability when combined with the AIM-7 Sparrow homing missiles, single track target, gun director and raid assessment operational modes.

COCKPIT AND PILOT SEAT

The F/A-18 Hornet was, like many fighter jets, intended to be for single crew members. However, monitoring and operating the many electrical systems is distracting and time consuming, so engineers incorporated a second tandem seat for Air Combat Officers as well as the option for just a single pilot. Previous fighter aircrafts that used control systems where the controls were physically attached to the components surface were much heavier and required a greater workload, so the Hornet became the first plane to use a quadruple digital fly-by wire system to provide pilots with greater manoeuvrability. Engineers introduced a pressurised “glass” cockpit, where traditional gauges were removed and replaced with cathode-ray tube (CRT) displays. The heads-up display is controlled by two multi-function CRT’s and a horizontal situation CRT to separate the dials. Hands-on throttle and stick mechanisms are used in the design to simplify the degree of thought on controls during combat situations. All controls are mounted directly on the joystick or on the main control column.

The seats are specifically Martin Baker US10S (SJU-5/6) designs, equipped with a safety kit within the seat bucket, emergency oxygen on the right-hand side of the pilot, and a rocket assisted ejector seat. The ejection can be operated electronically or manually if there are any malfunctions, and the parachute is stored in the box headset.

The hydraulically operated airbrake is located between the vertical fins and is made of graphite epoxy material.

WEAPON HARDPOINTS

The Hornet boasts nine external hardpoints for weapon attachment. The wings support 6 weapons, the fuselage holds two behind each air intake, and the undercarriage has a centre-line ventral. It is also noted that the nose can also hold a machine gun but this affects the transonic conical airflow that enters the engine inlets. The wingtips house Sidewinder infrared-homing air-to-air missiles and a 20-mm M61 cannon. When in intercept mode the fuselage hardpoints carry Sparrow missiles but in attack they carry FLIR (infrared camera) and laser designation pods for precision guided munitions.

Materials

The F/A-18 incorporates a variety of materials including metals and composites to maintain a good strength-to-weight ratio. The structural weight is comprised of these materials:

• 49.6% avionic-grade aluminium alloys

16.7% steel

• 12.9% titanium

• 9.9% graphite epoxy composite coatings

• 10.9% other materials (rubbers, tyres, foams, fluids etc.)

The majority of the surfaces covering the flight panels are configured in a honeycomb sandwich pattern supported by the rigidity, strength and integrity of composite materials. This method of production is specifically utilised in the leading-edge flaps and the ailerons/stabilisers where additional forces generated for flight control impact the materials. Aluminium is a suitable aeronautic material due to its lightweight, high strength and great corrosion resistance, increasing the surface life of manufactured components. Pure aluminium is used in a honeycomb pattern in the two rudders behind the canted vertical stabilisers, coated with two graphite epoxy (AS4/3501-6) sheets for increased specific stiffness. However, any ingresses in the sheet results in water entering the plates and separating the bond between the surfaces, damaging the entire component and requires a $75000 replacement. The fuselage is manufactured from the aluminium 7050 alloy, 87.3% aluminium, 6.2% zinc, 2.3% copper, 2.25% magnesium, and 0.12% zirconium. This alloy from the 7000 series has exemplary corrosion resistance while maintaining strength, rigidity and a good strength-to-weight ratio.  

The majority of the steel used in the Hornet is located in the engine, where high heat thresholds are required around the afterburners. Stainless steel alloys can be utilised in the fan due to temperatures only reaching maximum 150C, but not in the compression and combustions stages where temperatures of 1800C exceed steel alloy melting points, making them ineffective. The exhaust must maintain high strength and corrosion resistance with the large volumes of hot gasses constantly flowing past, so engineers use a nickel-chromium-iron alloy known as Inconel combined with stainless steel as together they are resilient to failure and corrosion at extremely high temperature conditions.

Titanium exists in the wing fold joints and the horizontal and vertical stabiliser flaps and rudders, making up 12.9% of the total weight. Titanium has the properties of high tensile strength to density ratio, high corrosion resistance, and the ability to withstand moderately high and low temperatures without failing. It acts as an additional reinforcement against fatigue whilst avoiding and increase in structural weight.

9.9% of the Hornet’s weight comes from its graphite epoxy composite resin that covers the entirety of the fuselage for weight reduction. About 40 percent of the aircraft\'s surface area is covered by the resin, and is incredibly suitable because no riveting is involved with the construction process. The resin can be heated up and then cured into a hard, rigid substance with the constituents complementing each other’s’ compressive and tensile strength when configured in the honeycomb pattern.

Technology

Technology in aeronautical engineering has evolved drastically overtime, especially in aircrafts. The F/A-18 was the first plane to incorporate a quadruple-redundant digital fly-by-wire. Digital fly-by-wire (FBW) is a system that replaces previous mechanical systems, where the pilot’s manual controls operated flaps and other surfaces through cables and pulleys rather than through electric signals. If one of the electronic systems fail, the other three make an instantaneous judgement and decision to ignore the malfunction and shut it down, as the three can keep full functionality. Engineers incorporated four systems to eliminate the possibility of a single shot from an enemy wiping out all controls, similar to the events of the Vietnam War. If the remaining FBWs also shut down the pilot can still use manual control only on the horizontal tail plane for emergencies to land, through hydraulically operating the functioning surfaces of the tail. The digital system first has the joystick and rudder controls inputted into the main computer which interprets the movements and reciprocates the demands through hydraulics. The advantage of digital fly-by-wire systems over previous mechanical and hydraulic actuators is the weight reduction and replacement cost. Electric cable is easily produced and insulated, so when one is damaged a single area can be targeted and replaced with another wire. Also, the weight of cables, chains, rods and pulleys is considerably more than wires, and replacing the components would also be more tedious, impeding the production and performance of the F/A-18.

The Hornet was also the first plane to heavily include multi-function displays rather than analogue dials. A multi-function display is a cathode ray or liquid-crystal screen that displays numerous sets of information to the pilot at the same time. It displays weather radars, moving maps of local geography, navigation routes and air traffic. Configurable keys surrounding the screen allow the pilot to input certain information, such as altitude pre-sets, GPS coordinates or routes. More importantly, it provides the pilot with a range of fighter and attack manoeuvres at the touch of a button, making combat situations easier and safer to a degree. This level of simplicity provides additional flexibility in combat and can allow attention to be focused on stabilising the aircraft. The advantage of a multi-function display is that it does not consume much of the cockpit’s internal area and control surface, and all data can be presented in multiple pages rather than permanent gauges which may need individual replacement.

Conclusion of Effectiveness

The innovation of the F/A-18 Hornet has undoubtedly improved the standards of aeronautical engineering, effectiveness in combat and prospect for jobs. Prior to the Hornet existed a vast range of fighter jets which all displayed numerous flaws and areas that could be significantly improved. Aeronautical engineers focused on the design of the YF-17 to develop a world class aircraft that is still in operation today. During the production and design phase, many aeronautical engineers were employed from society for this excellent opportunity of innovating the aeronautic industry, introducing new ideas such as the quadruple digital fly-by-wire system and self-sealing fuel tanks. The invention of the turbofan engine significantly reduced the noise pollution of aircraft during flight without effecting stability and achievable velocities.

With consideration of past operations such as wars involving fighter aircraft, the military’s approach to combat situations has adapted, being more regarding for the safety of the pilot and the country. The success of the Hornet in defence forces has promoted protection, ensuring society has the best equipment and technology fighting for them. However, the culture of some people perceives the production of weapons as disturbing and actually promoting violence, but it is more commonly accepted that Governments are behind the political decisions and engineers follow the orders. Furthermore, the extensive research conducted on the design and materials selected for the plane has established the F/A-18 in the history of attack aircraft as one of the most manoeuvrable and efficient at the time. Each composite and alloy has been expertly picked to maintain high strength, rigidity and corrosion resistance. Technology developed for military purposes has also been introduced into civilian applications including radar, demonstrating the impact of aeronautics on society.

Thus, the F/A-18 Hornet is a sophisticated piece of aeronautical engineering work which will remain in operation in the Australian Defence Force and many others forces for numerous years, due to engineer’s consideration of social and cultural factors alongside their vast knowledge of flying principles. This is why we use this excellent low-speed performing aircraft.

Proudly written by Haydon Illingworth for one last time.

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