=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.)