Why are there no blended-wing passenger airplanes in operation?

Question

Blended wings are more beneficial when it comes to fuel efficiency increase and noise reduction. However, no manufacturer has decided to produce these airplanes for major airline operation so far. What are the inhibiting factors in the industry for not using this configuration?

From what I can guess:

• It is a complete new configuration with low experience on it. High risk.
• Aircraft will be in VLA sector (>500 passengers), a sector not very attractive nowadays.
• The 90 seconds evacuation limit needs to be observed.

Answer 1

All studies which so far showed an advantage for blended-wing bodies (BWB) were flawed.

The trick mostly used is to compare an existing airliner with a hypothetical BWB which uses equally hypothetical engines of improved efficiency, like what could be expected 20 years into the future. This masks the inefficiency of the BWB concept and makes the combination come out ahead.

The BWB will always have more surface area than a comparable conventional design. This translates into more friction drag and more skin mass, which more than offsets any advantage given by the bigger wing root (which helps to reduce wing spar mass). If you like real data, use the Avro Vulcan as an early BWB and compare it to its contemporaries. Note that design attempts for an airliner based on the Vulcan (type 722 Atlantic) went nowhere.

Why are these BWB studies published? The author gets more attention when he/she claims a "revolutionary breakthrough" than when he/she is more honest and admits that the concept is a dud. Even Boeing or Airbus like to publish BWB studies, so the public gets the impression they are ahead of the competition. It is sickening to read such academically dishonest studies - you need to spend time to dig to the bottom of the thing and to unravel the plot; however, once you have done this a few times, they all become alike. But compared to studies made 60 or 80 years ago, where the author factually lists what he did and why it didn't work out (which is the only way you can learn something), those modern studies are a waste of time.

Answer 2

Blended wings are more beneficial when it comes to fuel efficiency increase and noise reduction.

That has not been proven. There may be aerodynamic benefits, but unfortunately an aircraft needs to have a supporting structure as well. And if we look at a conventional configuration, we see a pressure vessel (the fuselage) supported by a lifting surface (the wing). Each has its own function and is optimised to do so:

• The fuselage has a round cross section, because that is the lightest form of pressure vessel we can make. Tilt it, and it provides some lift as well.
• The wing has the lowest surface area we can get away with, in order to minimise friction drag. Other factors in the optimisation equation are induced drag and structural weight.

Now combine the two functions. A blended wing or lifting body needs to be pressurised as well, how are we going to do that? Anything other than a pressure cylinder is going to be heavy, and extra weight carries a drag penalty. If we visualise a pressure cylinder inside the blended wing, we immediately start to wonder if the wing is not too large for the passenger payload required.

Start with a passenger payload, build a pressure vessel around them, provide lifting surfaces...I still come up with the conventional layout. Birds, bats and insects also have this layout, nature has not seen the advantages of a blended wing either. The answer to your question may simply be that the conventional layout is best.

Answer 3

Like any other engineering artifact, airplanes are a product of compromises between design, mission profile, aerodynamics, flight dynamics, structures, powerplant, systems, maintenance requirements, airport requirements, etc. As it turns out, Jack Northrop was right: Flying wings and blended wing designs are the most aerodynamically perfect solution for subsonic heavy aircraft with an added benefit of a very low radar cross section ideal for military applications.

When I was an intern at Boeing in Everett, WA back in 2000, we considered the idea of flying wings for civil transports. While the aerodynamics make them very appealing to design and build, there are several added concerns that, combined together, deter further development of a civil transport in this configuration.

EASE OF ASSEMBLY - It is easier to assemble a semi monococque fuselage and wing box as opposed to a single, large wingbox with a cabin inside.

EMERGENCY EVACUATION - The biggest drawback to large scale flying wings is that they have less surfaces available to construct walk-in entry and egress doors, which makes it difficult to evacuate the aircraft quickly in an emergency situation. As a yardstick, an A-380 can evacuate a total manifest of 700 people in a few minutes but the fuselage also accommodates no less than 16 emergency exits equipped with escape slides / rafts to do this. Said number of exits are not possible on a flying wing, making it more difficult and dangerous to conduct an evacuation in an emergency.

EASE OF MAINTENANCE - Conventional airliners contain their engines and system in easily accessed compartments along the lower parts of the fuselage and wing or tailcone pylons. On a flying wing, these systems are buried deep into the structure where they are not readily accessible.

AIRPORT INFRASTRUCTURE: Flying wings are going to have very large spans, which push the dimensional limits of existing runways, taxiways, ramp aprons, terminal gates, etc. placing limits on routes to airports which can accommodate these aircraft. This has a direct effect on operational flexibility for the airlines looking to maximize optimal routes, large amounts of paying passenger traffic, etc.

Answer 4

Internal volume to be pressurized is going to be rather large. Using a cylinder inside the Blended Wing Body (BWB) as pax compartment is doable. Weight penalty, structure increases for loads involved for pressurized compartments (passengers and critical cargo).

Considering maintenance, composite structures inspections and repairs of surfaces with compound curves and critical surface fineness leaves a lot to be desired. One of those long term costs: fuel consumption. Airlines pay attention to this as it is the largest long term expense. May be a plus if the seat mile costs prove out.

Insurance. No existing comparable aircraft to compare it to. What do you think the coverage will cost? Accountants and statisticians are not fools. Historically, aircraft design has always outstripped the power plants abilities. Going to have to consider designs with existing power plants in mind. SR-71 first flew with J-75s because the J-58 wasn't there yet. Don't even consider what will be a power plant in twenty years.

Dispatch reliability needs to be taken into account for all phases of design. Mounting the engines high on top of the BWB is going to make quick engine changes let alone preflight and post-flight inspections very much down on my list of favorite things to do. Ground service infrastructure will need a lot thought.

Where do you propose to store fuel in the structure? Next to the passenger compartment? Self loading cargo won't like that.

Marginal control surface effectiveness is going to narrow down the allowable weight and balance envelope. Movement of fuel to accommodate the movement of self loading cargo and placement of non-self loading cargo increases complexity. not acceptable. Airlines live by the KISS Principal.

Airframe manufacturers and airlines of all types have been considering BWB aircraft for twenty years and have yet to put one in service. Ask yourself why.

Answer 5

Normal single or even twin rudder won't work with a flying wing. This is due to a length/wingspan ratio. Normal aircraft have a length/wingspan ratio of more than 1. Flying wing on the other hand have the ratio of less than 1. To solve this problem, the designer either have to enlarge the rudder or add more rudder. Up to 4 in YB-49 for example. But this enlarge or added rudder also provide added drag, thus negating the low-drag advantage the flying wing are suppose to give.

For rudderless design, the problem is yaw stability. to provide yaw control, rudderless aircraft use some sort of differential braking system. The problem is, this system doesn't provide static stability like a rudder provide. To create some semblance of yaw stability, the differential braking system have to be adjusted regularly for the entire duration of a flight. Done manually, this would've tire the pilot quickly. So a rudderless aircraft require some form of autonomous FCS to handle this. This automated system account for the extra cost and added an extra point of failure. This limit the usage of this kind of design to a military only and not the cost conscious commercial airliners.

Answer 6

The answers above are all very good, but I suspect that the most important reason for the lack of blended-wing passenger aircraft in service may be the simplest: flyers don't appear to like them very much.

"Boeing was going to develop a blended wing commercial aircraft in their 20 year commercial plans, but found in their initial testing of the design that passengers didn’t like it at all. The theater design of the seating just didn’t generate a favorable result and that caused Boeing to drop all commercial applications for the blended wing design, but not military applications." [emphasis added]

http://occupytheory.org/boeing-797-hoax-debunked/

All of that being said, I'd be interested to see if passengers could be induced to change their minds on that by a combination of:

1) Improved display and lighting technology in the years since this design was last tested. (Assuming the problem was claustrophobia caused by lack of windows).

2) Perpetually shrinking seat width and pitch on existing designs for economy class. If a blended-wing design allows for even a tiny bit of extra room (I realize there's analysis above to suggest that it won't, but if), budget travelers who are quite literally feeling the pinch might be won over.

3) If blended wing designs are measurably faster than their wing-and-fuselage counterparts, that's golden. If you ask a focus group participant if they like windows, they'll naturally say yes; but if you ask them to rank the value of windows vs. shaving a few hours off their flight, you might get a different response.

Answer 7

The first step would be a BWB military transport. The X48B / X48C scaled drones helped understand many issues associated with control. The B2 bomber cost was mostly a result of early stealth materials, early large scale usage of composites. If a B2 were to be re-engineered with current stealth materials and reduced composite costs it would cost a fraction of that value to build. The remaining substantial challenge with large BWB aircraft is pressurization. The tube and wing shape is much easier to pressurize (wing isn't pressurized tube is one of the easiest shapes to pressurize). BWB aircraft appears to have larger surface, but that's not true compared to an aircraft with the same cargo/pax capacity. Also what matters isn't total surface area but rather total lift vs total drag. A high lift body with less drag : lift ratio allows for higher altitude cruising which would greatly reduce fuel requirements. BWB airliner proposed is about the same as a A380, with a much shorter length, much wider cabin, ability to carry cargo further out into the wing. An A380 has 4 fairly large turbofan engines. A BWB the same size would have 3 similarly sized turbofans in the aft fuselage.

Answer 8

The first, in depth study of the Lifting Fuselage Configuration in 80 years from David Singg at the University of Toronto shows the LFC has twice the "fuel burn reduction" of the BWB/HWB compared to the tube and wing. Vindication of the work done by Texas born Vincent Burnelli from 1921 to 1964. Google Burnelli aircraft.

Report link... http://oddjob.utias.utoronto.ca/~dwz/Miscellaneous/ReistZinggJofA2016.pdf

The BWB was a good step forward from the tube and wing configuration but now, the most efficient and useful design has emerged. Even this will not guarantee production. Just as the BWB has not been accepted because it's too radical a design, the Lifting Fuselage will take years to win acceptance. More research to come.