Every commercial aircraft is fundamentally designed for safety, passenger acceptance, fuel efficiency and airport compatibility among other important features. Numerous challenges face all current passenger aircrafts, for instance the conventional wing consists of a number of moving parts which increase the weight of the plane and are difficult to adjust and maintain. The current control surfaces also increase turbulence and drag since the smooth flow of air is disrupted, and the aircraft cannot adapt to different flight stages and conditions. Design architectures such as the vertical and horizontal stabilizer surfaces increase the overall drag of the plane, for example, the Boeing 747 and the Airbus A380. In the recent past, the GE Aviation team designed a 20-passenger aircraft that includes a shape that smoothes air flow over all surfaces, and electricity-generating fuel cells to power advanced electrical systems. “The aircraft's advanced turboprop engines sport low-noise propellers and further mitigate noise by providing thrust sufficient for short take-offs and quick climbs” (Deidrich 104). Although this is a major step, the limitation to this however is the low carrying capacity of the aircraft that is only profitable for military activities as opposed to commercial transport.
There have been three considered concepts for aircraft design; the Blended Wing Body configuration, the first conventional design and the second conventional configuration.
1. The first conventional design features a T-tail with rear mounted low wings and engines fitted at the rear of the fuselage. This concept was used to curb turbulence which a normal horizontal tail plane is limited by (Kundu 21). The strategic positioning of the engine makes it safe for work around the plane to be done when the engine is on and also protects the engine from foreign ground materials that would otherwise damage the engines. However, care should be taken to position the wings at the back of the fuselage to maintain balance. The tail region has to considerably strong to support the engines and the T-tail.
2. The second conventional configuration is the most commonly used design today. The engines are mounted below the wing which makes them fairly easy to maintain and replace, and accessible easily in case of engine fire. Landing gears can be stored in the wings, but precaution must be taken to avoid the interaction of hot exhaust gases from the engine and the flaps. The wing should be robust enough to accommodate the weight of the landing gear and the engines. This aircraft has the advantages of optimizing pressure since there is low vibration on the fuselage as engines are fitted on the wings. The modern day passenger aircrafts use the second conventional design.
3. The Blended Wing Body (BWB) design is a blueprint for future airplanes in which initial tests have been done. The engines are ‘blended’ in at the top of the wing and the tail piece is eliminated. The passenger airline is estimated to carry between 190 and 800 passengers depending on the overall construction. It complies with all aircraft requirements in terms of range, cruise altitude, speed, take-off, and landline lengths. In the Blended Wing Body configuration, the fuselage carries part of the lifting load. The blended wing body design features lower drag coefficients and higher lift capabilities compared to modern day conventional passenger aircrafts. This model could carry up to 800 passengers while using 20-25% less fuel.
The unconventional hybrid wing body which is the next generation reduces noise by 42 dB, nitrogen oxide emissions (NOx) by 75%, fuel utilization by 40% and take-off field length by 50%. Alternatively, morphing wings could be incorporated to cater for various stages and conditions of flight since they significantly reduce drag and turbulence due to the smooth flow of air by blending in the co-flow jet on the wing. These wings eliminate the presence of excessive mechanical parts. The use of carbon fiber-reinforced composites to design the wings into desired shape reduces the overall weight of the aircraft. Using the shape memory alloy, the required design is easily achieved and less power is needed. Making the winglets out of graphite composite materials and aluminum saves approximately 60 pounds as compared to if it would be made out of aluminum only. Graphite in the construction of aircrafts has proved effective in reduction of overall weight. Graphite composite floor panels (or aluminum alloys) and structural carbon brakes on the main landing gear wheels (which also improve energy absorption properties and friction resistance) save an estimated 1,800 pounds compared to previous aircraft brakes.
The BWB prototype introduces wide airfoil shaped body which promotes laminar airflow. It enables the aircraft contribute to lifting off the ground thus reducing fuel consumption. The flattened aircraft body integrates the engines and wings into a single unit lifting surface thereby increasing the overall aircraft performance and efficiency. Aircraft manufacturers are using the concept for future aircrafts comprising of a blended wing body architecture that incorporates an embedded twin engine technology making it suitable for changes in high level design properties. The baseline is designed for 4,000 nautical miles range, Mach 0.85 during cruise, about 30,000 feet cruise altitude and approximately 12,500 feet take-off field length. In the design of the wings, it is important to note that high aspect ratio; that is larger span (length) than chord (width), improves lift capabilities and lowers drag at subsonic speeds. The blended wing provides the effect of achieving a larger wingspan without necessarily outgrowing the standard airport slot, and durability is guaranteed. It combines fixed wide airfoil-shaped fuselage with a greater aspect ratio wings and embedded engines with a common integrated nacelle. The aircraft also provides a wide double deck passenger and cargo compartment due to the wing which provides a grater payload volume. Furthermore, the wing extension reduces fuel burn and improves the aircraft’s range.
Current versions of the Boeing 767-200 can accommodate a maximum of 290 passengers occupying a 7-abreast double-aisle configuration. The BWB seeks to fuse two aircraft bodies together to also create a 7- or 6-abreast double-aisle with more passengers seated next to the window or the aisle for maximum comfort. The D double bubble design is an advanced idea of the conventional blended wing body design which conceptualizes the idea of merging two bodies. In relation to its carrying capacity, the BWB is divided into four regions; the front most part just after the cockpit allows for 6-8 seat 2-aisle sitting arrangement just before the wing section expands to accommodate most passengers in the coach class. This allows the tube portion of the fuselage for business and first class. In a double-decked BWB interior, the 150 inch wide cabin sections allow at least 74 inch high upper decks and 84 inch high lower decks. Windows are located along the front edge of each cabin bay. Cargo compartments are located on the outer section of the passenger cabin with pressurized fuel tanks around them. The amount of cabin noise is greatly reduced because the engines are mounted on the rear.
Light emitting diodes are fitted together with an interior décor that seeks to make it sophisticated and appealing. Although overall interior space will be increased for passenger and crew movement, lavatory space and overhead cabins for carry-on luggage, the load located within the interior is checked such that materials making the panels is made as light as possible and simple items such as carpets and magazines are eliminated. The BWB aircraft’s gross weight is estimated at about 310,000 pounds.
Advanced propulsion engines on the current market provide substantially high speeds but have greater fuel burn. The Pratt & Whitney JT9D-7R4E turbofans proves to be the most effective by providing a 50,000 pound thrust per engine. Boeing 767-200 which uses twin Pratt & Whitney JT9D-7R4E turbofans engines carries 100 more passengers than its predecessor and take-off and landing field lengths are much shorter. Currently, two models of both the Pratt & Whitney JT9D and the General Electric CF6 turbofan engines are being used in passenger aircrafts. Both engines provide 48,000 to 50,000 pound thrust and have bypass ratios between 4.5 and 5.0 and compressor pressure ratios between 25 and 30 which are highly effective in meeting NASA requirements. Specific fuel consumption of these engines, expressed in pounds of fuel per pound of thrust per hour, is between 20 and 25 percent lower than that of the Pratt & Whitney JT3D engine. Incorporation of these two engines to the BWB’s advanced technology will revolutionize aircraft manufacturing.
The BWB incorporates these aspects and the much larger payload capacity and more efficient engines make the new aircraft more efficient in terms of cost per seat mile for long, medium and long range routes. It has advanced aerodynamic performance and is beneficial in that there is room inside the wing that is reserved for fuel, cargo and engines; and this space is also crucial for pressurization of the fuselage because it maintains circularity. With the assumption that the aircraft will be capable of performing rudder actions by raising drag or creating a variety of engine thrust, the tail is omitted in the design (Anderson 73). The BWB airliner is notably strong in that it absorbs both cabin pressure and wing bending loads. The design reduces weight on the outer wing section airfoils and the centrally aligned chord provides significant strength for sectional lift coefficient. Since the demand for lift strength is reduced, the center body is allowed to hold passengers and cargo. Due to its shape and architecture, oncoming airflow is supersonic and behind the aircraft, the air is slow with high pressure allowing the aircraft to increase speed easily.
The engines of the aircraft are half-embedded into the wings to optimize airfoil efficiency and reduce the amount of noise on the ground. The aerodynamic shape is significant for improving fuel consumption. The overall structural weight is reduced and speed is improved. Wing bending relief is made possible. The BWB outperforms all current conventional aircrafts whereby there is increase of lift-drag ratio by 56%, 10% decline in operating-empty weight, 20% decrease in fuel consumption and twice the carrying capacity of Boeing 747-400. The amount of harmful emissions per passenger mile are reduced and the manufacturing, purchase and maintenance cost are reduced while at the same time improving aircraft performance and flexibility. BWB’s advanced systems are designed with fly-by-wire flight controls which enable the pilot to fly any other aircraft of the same family. If constructed, it seeks to reduce 70% fuel burn, lower noise, reduce nitrogen oxide emissions, and take off and land on shorter runways. The wings have a larger span and utilize the triangular blended hybrid wing body technology. The name originates from the concept of fusing two airplane cylinders together side by side. Fitting the engine at the rear of the fuselage ensures that air is propelled into the engine thus resulting in reduced fuel consumption with thrust being maintained.
Jet engines emit pollutants including carbon dioxide, nitrogen dioxides, sulphur oxides, soot and even water vapor. Carbon-dioxide and nitrogen oxides are the major constituents of air pollution today. About 5% of air pollution is blamed on aircrafts where 70% of an aircraft’s exhaust gases is CO2, and this is set to rise as air traffic increases. As part of the solution, aircraft engine exhaust gases should be mixed with air and fuel for re-combustion. A secondary combustion chamber draws in air from the suction surface of the wings and exhaust from this chamber exits over the wing and the fuselage surfaces.
The use of bio-fuel in aircraft engines could significantly reduce air pollution and also save on fuel cost. Also turning off of auxiliary power units when the airplane is on the ground and using ground power could prove effective, as well as cleaning the engine blades for better fuel efficiency. On the other hand, aircrafts propelled by hydrogen could fly at incredible speed capabilities of over 2700 mph without emitting carbon pollutants compared to the current aircraft that flies at 600mph using gasoline. Use of single rotating rotors both ducted and unducted that are gear-driven, multiple fans, fuel cells, hydrogen fuel and constant combustion produces greater thermal efficiency and low emissions.
The use of fuel cell propulsion gives up to 70% efficiency such that there is high efficiency at low power and low efficiency at high power. There are significantly low emissions with the LH2 fuel. LH2 fuel cells have the potential of reducing energy consumption by 20% to 30% and eliminating harmful emissions such as NOx. In models that eliminate the presence of the fuselage, liquid hydrogen LH2 replaces the upper passenger deck, or is situated in the central section beside passenger decks for larger tanks or smaller tanks share the center section with passenger decks. Although this is effective in reducing overall aircraft weight and proper utilization of space, it creates a very complex design that is difficult to construct and reduces the amount of volume for payload. According to Kundu (45), “validation of potential fuel burn benefits will require extensive full scale testing. The principal challenges lie in the overall structural integrity of the oval pressure vessel, integration of propulsion ad airframe, emergency evacuation of passengers on land and water, passenger acceptance, and airport compatibility” ().For greater fuel efficiency to be achieved, electricity-generating fuel cells are used to run the aircraft’s advanced electrical systems. Battery technology is estimated to produce a hybrid turbo-electric propulsion system and use both fuel to propel the engine’s core, and electricity to turn the turbofan when the core is powered down produces impressive results. To add to that, low speed of cruising especially for long range trips at about Mach 0.7 at higher altitudes, saves on fuel. (Deidrich 131) argues that blended winglets that are currently available can be further modified to stand at 10 feet, which will reduce fuel burn by decreasing vortex drag.
In order to reduce noise pollution, the new age architecture is set to use advanced turboprop engines sport low-noise propellers and further mitigate noise by providing thrust sufficient for short take-offs and quick climbs. Most modern conventional aircrafts incorporate a hush kit in an existing engine to reduce noise. A device called the multilobe exhaust mixer mixes the exhaust gases from the engine core with external air and bypass air which reduces the forward high-pitched noise caused by the fan. Although the hush kit is very effective in overall noise reduction, it adds significant amounts of weight to the aircraft which in turn increases the amount of fuel consumption. For instance, Boeing 727 aircrafts’ hush kits add about 900 pounds causing a 0.5% increase in fuel usage. Instead future models should design sizeable fans in the engine for the purpose of noise reduction. In high-bypass turbofan engines, large fans should be mounted in front of the engine core. If the size of the fan is considerably larger than the core, the turbines required to rotate the large fan effectively lower the speed of engine exhaust, which in turn reduces noise.
The dominant source of noise is the high velocity exhaust generated by the turbojets. Reducing the speed of air exhausted through the use of turbofans significantly decreases noise produced. Higher bypass ratios generate slower but larger jets of exhaust air. Mixers located inside the engine further quiet these jets of air and acoustic liners absorb some of the noise before it is passed out. The landing mechanism produces a large amount of drag and noise when approaching the runway. Although this drag is required for the aircraft to decelerate, descend and eventually halt, the drag could be produced in a quite manner whereby the landing gear is faired, less noise will be generated and the pilot has more control over the drag process. Boundary layer ingesting inlets have the potential to improve fuel economy by eliminating the boundary layer from a surface and ingesting this air into the engine inlet. The decreased efficiency of the engine is balance or overweighed by the increased aerodynamic performance of the blended wing. This mechanism also reduces the noise which would have been otherwise generated by the thick boundary layer leaving the trailing edge of the wing.
According to (Deidrich 11), NASA is currently researching on designs that will aid in meeting the N+2 goals, which include; non-conventional aircraft architectures that enable simultaneous achievement of noise, Landing Take Off (LTO) NOx and fuel burn metrics in the N+2 timeframe, drag reduction through laminar flow, advanced propulsion architectures (open rotor, geared and direct drive turbofans), advanced composite structural concepts for weight reduction, low NOx, fuel-flexible combustors, and propulsion and airframe integration for noise reduction and fuel burn improvements. All these features are incorporated to a great extent in the BWB design which is proving to outdo all current and previous aircrafts in overall performance and efficiency.
Anderson, John. Aircraft Performance and Design. McGraw Hill Publishers, 1999. http://www.nasa.gov/topics/aeronautics/features/future_airplanes.html
Deidrich, Adam. The multidisciplinary design and optimization of an Unconventional, Extremely Quiet Transport. Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2005.
Kundu, Ajoy.Aircraft Design. Cambridge University Press, 2010.
Siuru, Bill,and Busick, John. Future flight: the next generation of aircraft technology. 2nd ed. McGraw Hill Professional, 2003.