Winging it in aircraft design

In the future, passenger jets may have just one wing instead of two. Some of them may even have four. Recent American researching has focused on radical approaches to the problem of transporting more people further distances for less fuel. Two of the more advanced concepts could see the most significant changes to airframe design since we did away with biplanes.

The idea most relevant to commercial air travellers is the Blended Wing Body (BWB) aircraft being investigated by NASA, in conjunction with various academic and industrial parties. The BWB is an offshoot of the flying wing pattern, the most famous example of which is the B2 ‘stealth’ bomber. Continuing a project pioneered by Lockheed Martin, the Seattle-based aviation giant Boeing is exploring the possibilities of ‘box wing’ aircraft. This design employs four wings set in a diamond pattern to provide increased lift and in-flight refuelling points.

The design is intended to boost an aircraft’s performance by using its whole surface to provide lift. With so much lift available, less energy is needed to keep the plane in the air. As for carrying capacity, the surface area to volume ratio of the aircraft is far superior to traditional layouts. In building the B2, the possibility of doing away with the irregular surfaces of a conventional airframe was also a factor in choosing a flying wing arrangement.

With lift though comes drag, and the complexities of designing the much thicker airfoils of the flying wing has meant that to date the concept has seen little application in the commercial world, especially for very large aircraft. The NASA BVB project is the first time that a serious investigation has been made into using a flying wing as a civilian passenger carrier.

NASA is intending its BVB aircraft to be capable of carrying 800 passengers over a distance of 11,000 km at a speed of approximately 900 kph. With a wingspan just a few metres wider than a 747, that’s almost twice the passenger capacity and still more than the biggest A3XX variant planned. Fuel economy would be about 30% better than any current wide-body and the uniform shape and improved efficiency would also reduce noise output. Passengers would be housed in long bays within the body of the aircraft (meaning window seats would be very limited).

With this scale of BVB aircraft though, the drag problem will be extreme. The need to sandwich two decks of people and their luggage into a flat space will result in leading edges that are metres thick at some points. Solving this dilemma is the chief obstacle to the BVB concept. The increasing power of computer-modelled flow dynamics will probably be the deciding factor.

Another issue with the flying wing layout has always been cabin pressurisation. The classic cylinder shape of regular airliner fuselages presents few engineering challenges: its circular cross-section is easy to build strongly using only light materials. The much more irregular BVB cabin is more problematical.

Air pressure differences over such wide, flat areas would oblige an interior hull of battleship-like thickness. Current thinking is to use 10 ribs running the length of the plane to connect the upper and lower wing skins. This would separate the passenger compartment into 10 separate bays on each deck, exacerbating the claustrophobic effect of having no windows, but providing massive airframe strength and pressurisation redundancy.

A box wing layout is a modern re-working of the biplane. Two sets of swept wings extend forward and back from each end of the fuselage. Some designs have the wings on the same geometrical plane, in which case they meet at a right angle at the aircraft’s mid-point. The Boeing / Lockheed Martin idea though has the two sets of wings set at different heights with a vertical end-plate joining their tips together.

As well as increasing lift, these additional wing surfaces could be used to mount extra refuelling booms. Supplying two or more booms to a single aircraft would allow the same amount of aerial refuelling to be performed by fewer tankers in a shorter time. The cost-saving implications are clear. In a further attempt to reduce the R&D money involved, the company is talking about producing these future strategic aircraft in a modular fashion, meaning optional systems like the box wing layout could be added or removed from a standard fuselage and avionics package.