BWB aircraft

=aircraft

 

 

A recurring theme of posts on this blog is that some technological thing that many people are excited about is completely unviable. That's not the case here: blended-wing body (BWB) aircraft are significantly better than current conventional configurations. Development costs are justified by:

- fuel savings per payload-km typically estimated at 1/3 (here's a classic paper by Liebeck)
- greater internal volume per payload, so perhaps passengers could actually sleep
- lower noise from the ground if engines are top-mounted
- potentially lower radar cross-sections for military aircraft
- much better aesthetics, which remind me of eagle rays

 

So, Airbus is seriously pursuing BWB development for passenger aircraft. But despite having been considered extensively, BWB aircraft are rarely used, so there must be some issues. What are they?

 

stability

 

A BWB aircraft is kind of like a canard aircraft. The center of lift of airfoils is generally towards their front, and the body of a BWB has a center of lift ahead of the wing part. This separation makes pitch control with ailerons on the wings possible.

Historically, canard aircraft use a highly-loaded canard, because you want the canard to stall before the main wing so that if the aircraft starts stalling the pitch decreases instead of increasing further.

Because the body of a BWB is a very large airfoil, it thus has lower loading (lift per area) than the wing part, and pitching up will increase lift from the body more than the wing, making the BWB aircraft inherently unstable.

If you don't need passive stability, then this doesn't matter: "control canards" can have low loading, and a tail with positive or zero lift is also flyable with active stabilization. Aircraft that aren't passively stable have been used in military applications, but haven't been used for passengers due to safety concerns; this is one reason why BWBs have been more common in military applications.

Those safety concerns have now been mostly solved by trying to avoid mistakes and not worrying about it: Airbus now makes passenger aircraft that are fly-by-wire with no mechanical backups. Aircraft have been shifting towards using self-contained electrically-controlled actuators, such as electrohydraulic and planetary roller screw linear actuators.

Automatic pitch control isn't particularly complex, so if flight is dependent on electronic control already, then passive stability isn't very important. All you need to add is really reliable computer systems and software, and aircraft are already relying on software now, so that should be easy enough, right? (Well, we already know the main things that lead to bugs in software, but getting people to use bounds checking for arrays in security-critical functions is just too difficult, I guess.)

 

cabin pressure

 

Passenger aircraft that fly at high altitudes must be pressurized. The circular cross-section of conventional fuselages contains pressure better than the flatter surfaces of a BWB body.

Military aircraft are often unpressurized; this is one reason why BWBs have been more common in military applications.

In theory, it's not a significant problem to make the skin a bit thicker and add some internal bracing, but doing that cost-effectively would probably require some new composite manufacturing techniques. (That's a topic I'm not going to post about because of potential military applications and other strategic significance. Yes, posting something like that here is unlikely to matter much, but it's the principle of the thing.)

 

windows

 

A passenger BWB aircraft would have fewer seats near possible windows.

These days, it's not a problem to give everyone a screen that can show video of outside the aircraft. I think that's good enough.

 


 

There are also some concerns that people have had which are actually non-issues:

- Evacuation would probably be faster with a BWB because it's shorter, not slower.
- Passengers near the sides would go up and down when the aircraft turns, but this isn't a big enough effect to be a problem.

 


 

Some BWB aircraft designs use podded engines on the top, and others use boundary layer ingestion (BLI), with engines inside the body.

Under-wing podded engines are the most common kind in current aircraft, despite engines in the wing root having lower drag and top-mounted engines being less likely to ingest debris during takeoff, mainly because maintenance is easier. Podded engines also make upgrading models to use bigger turbofans easier. But removing internal engines on the top of a BWB should at least be easier than removing engines in the wing root, because the cover on them could be non-structural.

Air slows down (relative to the aircraft) from friction with the aircraft body, and using that slower air for propulsion increases efficiency. Podded engines would make maintenance somewhat easier, but the bigger reason why podded engines are proposed over BLI is concern about turbine blade fatigue. If there's a speed difference between the air going into the top of a turbine and the air going into the bottom, then the forces on turbine blades change with each rotation, which obviously causes fatigue.

Newer aircraft engines are starting to use carbon fiber for the front blades, and carbon fiber is much more resistant to fatigue than metals. This helps with that problem somewhat, but there's actually a fairly simple solution to it, which I suppose Airbus or NASA will probably figure out eventually. (At least, I hope they do, because the podded engines are kind of ugly.)

 

 


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