Since I have read all 800+ pages, permit me to summarize some of the technologies that may be incorporated into a clean sheet Boeing 737 (and 757) replacement.
Both Boeing and Northrop did similar type studies and arrived at similar conclusions on technology that may be incorporated into new airliners. Both Boeing and Northrop looked at number of aircraft configurations but in the end decided that conventional twin engine configuration like the B-737 was the preferred configuration. Both Boeing and Northrop looked at the Blended Wing Body (BWB) but did not see any advantages over the conventional configuration for B-737 replacement aircraft. Both Boeing and Northrop preferred high by-pass turbine engines rather than open rotor engines.
Based upon the Boeing and Northrop studies, I list the technologies that could be incorporated in the Boeing 737 (and 757) replacement aircraft, more or less in the order of importance.
Install CFM Leap X and P&W GTF engines for a 16% reduction in fuel burn.
Advanced composite structures for lighter weight. Ultra high performance composite fibers can permit higher aspect wings. Very tough composites – resin systems with greatly reduced suspectibility to impact damage and reduced curing temperature to support lower costs..
Natural laminar flow wings to reduce drag. The designers will probably eliminate leading edge slats to achieve laminar flow but also have wings with lower wing loadings to maintain or improve the takeoff field length.
Riblets on the fuselage. Same aerodynamic effect as dimples on a golf ball.
Relaxed static stability to reduce the size of the horizontal stabilizer.
Active/passive aeroelastic for load control so that you do not have to build the wings as strong.
Boeing would probably keep the 118 foot wingspan on a B-737 replacement, but lower the wing sweep to approximately 20 degrees, reduce the cruise speed to M.70 or M.75 and reduce the wing loading to compensate for the leading edge slats being removed. The lower wing loading means that the B-737 replacement can cruise at least 3000 feet higher than the B-737NG thus saving on the fuel burn. The new wing would increase the Lift to Drag ratio by 10% and have a higher aspect ratio.
The overall design improvements should reduce the Operating Empty Weight (OEW) by 10% and reduce the Maximum TakeOff Weight (MTOW) by 15% as less fuel is required for the same mission.
The new design would improve the design integration of the nacelles for less drag
Carbon nanotube electrical cables and bundle together advanced subsystem technologies for less weight.
Improvements in the Air Traffic Management system to permit more direct flying between cities and improvements in the interior of the aircraft (such as twin aisles) to permit faster turnaround times are expected to offset the slower cruise speed on the new aircraft.
MIT D8 Study
MIT designed a replacement for the Boeing 737-800 model which they called the D8.1 model. The MIT D8 is a double bubble fuselage lifting body fuselage with twin aisles and eight abreast seating. The D8 has the following physical features:
Double bubble fuselage cross section
Nearly unswept wing for M.72 cruise at 40,000 feet.
Rear mounted engines with boundary layer ingestion to direct the exhaust into the low pressure zone behind the aircraft.
Lightweight Pi tail – two vertical stabilizers with a horizontal stabilizer mounted on top of the vertical stabilizers.
The MIT D8 configuration is said to offer the following advantages compared to the B-737-800.
The D8.1 fuselage is slightly draggier than the B-737-800 but it enables:
A lighter wing
Smaller lighter tail
Enables Boundary Layer Ingestion
Smaller lighter engines
Shorter, lighter landing gear
Based on a change in airframe configuration but using the same engines and systems technology as a Boeing 737-800, MIT is claiming that the MIT D8.1 configuration results in a 49% fuel burn reduction compared to the Boeing 737-800. Pretty impressive if it is true.
Since I am intrigued by the lifting body fuselage and Boundary Layer Ingestion as envisioned by MIT, I think one viable Boeing 737 replacement may be a wide oval fuselage design with 7 abreast seating and twin aisles like the Boeing 767. The wide oval fuselage would be 16’ 6’ wide (same as B767) and 14’ 6” high (as compared to 17’ 9” in the B767). The wide oval fuselage would have about 85% of the frontal area of a B767 fuselage. The wide oval fuselage cross section would have two LD3-45 cargo containers side by side in the lower lobe. The front to rear cross section of the fuselage could be shaped liked a supercritical airfoil so that the fuselage generates lift. Or the fuselage could incorporate a lifting body nose to generate lift. The aerodynamics of a lifting body fuselage are well above my competence so I will leave the design to the experts.
The wide oval aircraft will have a low wing with two primary high by-pass turbofan engines hung from the wings. The engines mounted on the wings will reduce wing and fuselage weight and promote better access for the mechanics to service the engines.
To gain the benefits of boundary layer ingestion, I would install two or three electric ducted fans on the rear fuselage between the twin vertical stabilizers. The electric ducted fans would be designed to ingest the boundary layer and eject the exhaust into the low pressure zone behind the fuselage thus reducing form drag. The electric ducted fans would be designed to produce several thousand pounds of thrust – just enough to ingest the boundary layer – and their source of electric power would be the generators on the primary engines and/or the APU.
The electric motor to power to ducted fan could be a large diameter motor similar to a wheel motor. The electric rotor could be installed on the outer perimeter of the fan blades and the stator could be imbedded into the structural ring that holds the ducted fan in place.
Since Rolls Royce Liberty Works (formerly Allison) and Remy International (a noted electric motor manufacturer) are both in Indiana, they could joint venture to design the electric ducted fan.
The wing on the Boeing 737 replacement would have the 118’ wingspan so that the replacement aircraft can readily fit into the existing B-737 gates. Wing sweep would depend upon the cruise speed required by the airlines. The Boeing 757 replacement would have a stretched fuselage and new very high aspect wing. The wingspan may be 160 feet and the aspect as high as 16 to permit the aircraft to cruise at 41,500 feet. In their study, Boeing indicated that such a high aspect wing could produce another 19% reduction in fuel burn compared to the 118’ wing.
It will be interesting to see what designs Boeing actually brings forth in the next year or two as they begin their cleansheet B-737 replacement program.