a reply to: Psynic
“Yes I realize that the passengers board through the centre of the ring shaped pressure vessel, but that is not the purpose of the ring…”
This is not an “either/or” situation; it is a “both/and” situation. The toroidal shape of the passenger compartment provides BOTH a
structurally efficient pressure vessel as you suggest AND an efficient means of ingress and egress for the passengers as Zaphod points out. You and
Zaphod are not actually in disagreement.
You also wrote:
“…Blended wings are all about high lift to drag ratios, which translates into fuel efficiency. Something like 50% more efficient. ..”
Not exactly. About 10 years ago, I was leading a small NASA project to build a special purpose UAV for research purposes. The configuration we were
looking at called for a 60 degree delta fuselage with conventional outer wing panels. This was right about the time that the Boeing Phantom Works was
spending a lot of internal resources on their blended-wing-body (BWB) aircraft concept—they had all the computer analysis tools and specialist
engineers spooled up to go. When they heard about my project, they offered to donate an aerodynamic design because the configuration I was working on
was quite close to what they were already working with and they were eager to gain some acceptance for their work. So they did the design, gave me
the outer mold line data files and my team built a 2+ meter wingspan UAV and tested it.
Aerodynamically, the L/D ratio was very comparable to a conventional wing-and-tube type of construction; I don’t remember the exact numbers, but
they were not noticeably different. Why is that? It has long been known that the mathematically ideal span-wise lift distribution is elliptical.
You can’t do better than that. One way to achieve that lift distribution is by keeping the airfoil section constant along the span and making the
planform of the wing be an ellipse. This is what was done with the Spitfire, for example.
Another way to achieve an elliptical lift distribution is to vary the airfoil characteristics along the span such that all the pieces of the wing are
doing the same amount of lifting work that would be done by an elliptical wing of the same span. This would involve, for example, using different
airfoil sections at different locations, and operating them at different angles of attack. This is what the BWB design accomplishes. Even though the
planform of the aircraft is not an ellipse, the lift distribution is elliptical, and that’s what counts as far as the aerodynamic lift-to-drag ratio
is concerned. The wings of conventional wing-and-tube aircraft are also designed this way and approach an elliptical lift distribution as well.
Since both designs approach an ideal lift distribution, and have similar aspect ratios, they both approach the same L/D.
The theoretical advantage of the BWB is a more efficient (i.e., lighter) structure for the same amount of internal volume (i.e., a lower empty
weight). A lower empty weight means that a smaller amount of lift is required for a given payload weight. Given a fixed L/D ratio and a fixed
payload weight, less thrust is required for forward flight and therefore less fuel burn. This is where the supposed 50% improvement in fuel
efficiency comes from.
This theoretical improvement in structural efficiency comes from the short stubby design. A short stubby design reduces the total external skin area
(the “wetted area”) and reduces the skin thickness required to achieve a given strength or stiffness requirement. This advantage is greatest when
most or all of the internal volume of the aircraft is unpressurized—as in a cargo lifter or aerial refueler.
When you start adding large amounts of pressurized internal volume to carry passengers, the added weight of the pressure vessel itself starts to
overpower the weight savings you get from a short stubby external shell. It has long been known that the most efficient way to design a pressure
vessel is with predominantly circular cross-sections. The most efficient is a sphere, but spheres have lousy drag characteristics. The next best is
a long thin cylinder. A cylinder can be almost as light as a sphere and has much lower drag; that’s why it has become the most common design
solution. For some reason, the BWB design approach being studied 10 years ago envisioned an internal pressurized volume that did not use a circular
cross section pressure vessel. The NASA publication, NASA SP-2005-4539 [history.nasa.gov...
] discusses this problem:
“Finally, of all the disciplinary design challenges facing the BWB concept, perhaps none is as important as the design of a highly noncircular
pressurized cabin. The structural weight advantage of circular fuselage shapes for airplanes has been exploited since the earliest days of pressurized
I would surmise that this Airbus patent represents an effort to solve this problem by utilizing a circular cross section cabin (a toroid) that fills
the skin of a BWB design pretty efficiently.