This document is presented to compare the two commercially successful and super-efficient airplanes, the Boeing 707-320B and Boeing 787-9. This document will identify the key innovations in airframe and propulsion technology, and also further discuss on why the basic design and appearance of aircraft remain unchanged over 50years.
Innovations in Airframe
Throughout the years since aircraft was created, engineers are constantly improving the efficiencies, durability and speed of its Airframe. From the beginning of 1920s, the all aluminium structures to the high-strength alloys and high-speed airfoils in the beginning of the 1940s. However as flying becomes more commercialised, people were not satisfied with just travelling at higher speed; they want to travel a longer distance with lesser fuel burnt! Hence, by the beginning of 1960s and 1980s, long-range design air frames and light weight composite researches were developed respectively.
The materials used to construct airframe ideally require light, durable characteristics and at the possible lowest cost. The Boeing 707-320b airframe is constructed mainly using aluminium. The properties of having high tensile strength, light in weight, easily alloyed with other various metals, make aluminium very favourable in meeting the requirements of the aircraft construction.
Many suggested that they would much rather fly a metal plane then a plastic one. However, as for Boeing 787-9, it is made up as much as 50% of composite material, approximately 32000 kg of carbon fiber reinforced plastic made from 23 tons for carbon fibre. These composites used to construct the B787 is not like any common plastic, it is stronger, lighter and offers greater strength to weight ratio than anything else. The boldly introduced airframe construction weighs 20% lighter than the conventional aluminium designs. This approach allows the airplane to carry more payloads and fly a further distance. In addition to the overall weight saving, moving to a composite primary structure also promises to increase resistant to fatigue and corrosion, reducing both the scheduled and non-routine maintenance burden on airlines.
With rising fuel prices, all airline operators hope for an engine with low fuel consumption.
The B707-320B uses 4 Pratt and Whitney JT3D engines. Each of these low-by pass engines could only produce 80kN of thrust. In the making of aircraft engines in the early generation, there were many constraints. Materials and technology were not developed and advance enough to overcome those limitations.
On the other hand with mature technology now, the B787-9 uses a standard electrical interface that allows the aircraft to be fitted with either Rolls Royce Trent 1000 engines or General Electric engines. Each of these high-by pass engines produces 240 to 330kN of thrust. The aim of being compatible to these 2 models of engines is to save time and cost when changing engine types.
Departing from the traditional aircraft design, the B787 also operates without the use of bleed air. The approach improves engine efficiency, as there is no loss of mass airflow and therefore energy from the engine, leading to lower fuel consumption.
The B787 claimed to be 70% more fuel efficient than the company’s first 1950s-era four-engine Pratt & Whitney JT3D-powered B707 and 20% more fuel efficient than the modern aircraft of the similar size.
The basic appearance and design of B787 appears unchanged from its predecessor B707. The basic swept wing, under-wing engine configuration has served as the basis for all of almost all of the new aircraft’s airframe. The reason is because the way how aircraft is going to fly and how lift is being created is not going to change considerably.
Changes and improvements are often instead made on aircraft weight, performance, noise and passenger comfort.
Breguet Range Equation
• V-Speed of aircraft
• G-Gravitional pull
• SFC-Specific Fuel consumption
(Reference to the equation above) With a given specific plan or profile, the Breguet Range Equation is used to calculate the aircraft’s range. We use this equation to predict and estimate the distance an airplane is capable to fly, accounting for its flight performance and the changes in weight as fuel is burned. The Specific fuel consumption is the measure on how efficiently an engine uses the fuel supplied to produce work. It allows engines of all different sizes to be compared to see which is the most fuel efficient.Using high by pass design and advanced materials, modern aircraft engine is able burn fuel more efficiently and overcome limitations in early generation such as high turbine temperatures. A decrease in SFC would mean an increase in range. Reducing the aircraft weight is always the goal for all aircraft designer.
In case of B787, composite CFRP was boldly used up to 50% in the construction of the airplane. With reduced weight would means lesser thrust required. With lesser thrust would means decrease in fuel consumption rate. With a decreased fuel consumption rate, airplane will be able to fly a longer range. The lift to drag ratio refers to the amount of lift created by the aircraft, divided by the drag it produces when moving through air. Aircraft companies have been going towards the direction of achieving a higher L/D design; since a particular aircraft’s required lift is determined by its weight, delivering that lift with drag reduced, results directly to better fuel economy, longer range and at the same time a better climb performance and glide ratio .
With improved technology, aircraft engines will get increasingly fuel efficient; aircraft will get lighter and stronger. Aircraft will be able to fly cheaper, faster and better.
Courtney from Study Moose
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