When we look back over the last century of
innovation in flight it’s sometimes hard to believe how far we have come. The Wright’s first flight in 1903 was at
best a proof of concept; only managing to fly 37 metres before falling ungracefully
from the sky. We often look back at this historic event
and see it as the spark that ignited a century of human flight, the truth is, the event barely
registered in national media and most questioned the legitimacy of the news. It took another 3 years of incremental improvements
and public test flights before the international community began to accept their achievements
and by that stage others had begun to catch up and even surpass their designs. By 1910, Louis Blériot had flown across the
English Channel, Georges Chavez soared over 2 kms to clear the Alps and Glenn Curtis began
to testing planes as a platform for weapons and his biplane became the first to take off
from the deck of a ship. This marked a trend for the next 35 years
of aviation history, which was dominated by war and by the time World War 2 came to a
close giant companies had been formed who were mass producing planes capable of transporting
humans across the world. These companies were not going to simply vanish
as the war ended and instead set their sights on building a new commercial civilian transport
industry. In the final year of World War 2 over 4 thousand
Douglas DC-3s had been built and many of these would go on to be converted for civilian use. The DC-3 is still the most produced airliner
in history with over 16,000 built and some are even still in service across the world,
but it’s slowly being caught up by the Boeing 737, which has sold so many units that at
any single point there is an average of 2000 737s in the air. The 737 made it’s debut in 1968 and it’s
design has essentially become the template for which most jet airliners have been built
on since. The initial design of the 737 had the engines
mounted on the tail, similar to the DC-9, which the 737 was competing with, but placing
the engines here reduced the amount of space available towards the rear of the cabin and
mounting the engine pods tight against the underside of the wing freed up space at the
back of the cabin for more passengers, which was important for this narrow and short body,
short haul plane. It also reduced the bending load on the wings,
counter-acting the upward bending load caused by lift. The success of this design has allowed the
737 to stay in service for over half a century with incremental improvements and today it’s
so popular that most budget airlines like Ryanair and Southwest airline use no other
plane. It’s engines have got gradually larger and
more powerful. It’s cabin got larger as traffic increased,
wingets were introduced to the wing to reduce induced drag and later this year the latest
iteration of the 737, which has already sold over 3400 units, will make it’s debut with
new split winglets, more efficient engines, an improved flight deck and the modern cabin
interior developed for the 787 dreamliner. This theme of incremental improvements in
the airline industry happens for a reason. Introducing a totally new plane design is
an incredibly risky business. We need to look no further than the failed
Concorde for proof of that, but even introducing a new plane series like Boeing’s 787 can
cause massive losses in revenue. The plane was plagued with delays, originally
slated to arrive in 2008, but actually made its first commercial flight in 2011 and only
recently has hit it’s stride in manufacturing and sales. New designs are simply a risky business decision
and in general companies will play it safe and not break the mold. On top of this a plane’s service life is
a huge part of its selling point. Airlines want to buy planes that maintain
their value over the years and can last them a significant amount of time with minimal
maintenance, so manufacturers have made effort to increase the service life of these planes,
which in turn has increased the cycle times between new iterations of planes. Making progress even slower again. With the current status quo of the airline
industry. We aren’t likely to see much change any
time soon, BUT what if a new industry disrupter emerged. One that could shake up the duopoly of Boeing
and Airbus to force competition and new designs? We have seen this happen in other industries
recently. The energy sector is being revolutionised
by cheap solar panels, Tesla was the first successful car start up in America in over
a century and composite materials are set to continue replacing metals in many every-day
applications. These disruptive technologies combined with
rising air traffic could raise the pressure to innovate. In this new series of videos I am going to
break down a number of future aircraft and the design challenges they need to overcome
to become a reality. Let’s first take a look at the D8, nicknamed
the Double Bubble, developed by Aurura, MIT and with the help of NASA. The current template of plane design at the
moment consists of a tubular fuselage. This shape is primarily there to resist the
internal pressurisation, allowing the fuselage to expand without creating dangerous stress
concentrations. As long as we pressurise the inside of our
planes this design aspect won’t change, but we can create fuselages with multiple
interconnecting tubular sections. This is exactly what the D8 does, with it’s
double bubble fuselage. So let’s look at how they came up with this
design and the theory behind their design choices. To design this concept they actually started
off with a 737 and performed a morphing study by gradually introducing their design goals
to the current design. They started by first optimising the airframe
of the current 737-800 airframe with current generation improvements. They then changed the fuselage to feature
the double bubble. This shortened and widened the fuselage considerably. The wider body and shaped nose allows the
body of the aircraft to generate more lift, particularly at the nose. This allowed the wings to get thinner and
thus reduce the drag they generate, but it also meant that the tail wing could decrease
in size too. The primary purpose of the tail wing is to
generate downforce at the rear of the plane, which keeps the nose of the plane up, an important
stability characteristic, but when the nose generates it’s own lift, the importance
of the tail wing is diminished and it can decrease in size, which again reduces the
drag. They then reduced the cruise speed of the
plane from 0.80 mach to 0.76 mach, which may seem like a step backwards, but remember the
primary goal of this future design are to improve efficiency. This allowed the wing sweep of the plane to
decrease, if you don’t understand this go ahead and watch my “why are plane wings
angled backwards video”. In the next iteration they reduced the cruise
speed again to 0.72, essentially removing the wing sweep altogether. Reducing the speed of the plane reduces the
thrust requirements of the plane, which reduces it’s fuel consumption, reducing the sweep
reduces the wing area, which again reduces the drag. So reducing the speed by just 10% results
in a much larger percentage of in fuel savings. Consider that if you were flying on a 3 hour
flight this would increase your flight time by just 18 minutes and this increased transit
time would be even less of an issue when you factor in the reduced boarding times that
the double aisle configuration facilitates. The next design iteration moved engines from
under the wing to the rear of the plane and mounted the engines flush with the fuselage,
but this requires some future tech that isn’t quite ready. With the current configuration, engines are
placed far from the body of the plane and so the air entering them is undisturbed and
uniform. This is ideal for the engine designers because
each of the blades in the compressor experience the same air pressure and speed through each
cycle. But if we move the engines tight against the
back of the plane the engines have to ingest the boundary layer air-flow, which is the
slow moving layer of air that builds up on the surface of the plane. This type of engine is called a boundary layer
ingestion engine and it has been a topic of great interest for NASA and other aerospace
companies, because it reduces the loss of kinetic energy of the aircraft greatly. In a normal plane this boundary layer of slow
moving air simply rolls of the back of the plane and mixes with the fast moving air. This causes vortices and a low pressure zone
behind the plane, which creates drag. The idea behind the BLI engines is that they
take this slow moving air and speed it up and thus eliminate some of that drag. It’s a nice idea that is far from being
ready. The first problem we face is that non-uniform
air entering the engines. The air entering the engine furthest from
the fuselage of the plane is moving faster than the air entering the engine near the
surface. This creates a discontinuity of stress, as
discussed before in my dreamliner window video, cycling high and low stresses is VERY bad
for any part, as it results in fatigue of the part and when your part is rotating through
those high and low stresses a few thousand times per minute...your part isn’t going
to last very long and that’s just problem number one. The next big problem is stall. Airflow normally moves uniformly through a
jet engine, but when it’s distorted as it enters the engine, there’s a high risk of
compressor stall. Compressor stall works similarly stall on
a wing, where the speed and angle of attack of the wing can result in flow separation
behind the wing. This prevents the wing from generating lift
and thus stall occurs. Non-uniform, turbulent air makes this far
more likely to occur. When this happens in a compressor it can lead
to a chain reaction of stall, as the localised stagnated air travels with the blade it stalled
on, but lags behind slightly allowing it to come in contact with other blades, which then
stall too. Compressor stall may just result in localised
areas of stall that affect the engine's performance or it can result in a complete flow reversal
where the incoming air is not being compressed enough to work against the previously compressed
air which results in an explosive flow reversal with air coming out the inlet of the engine. For these embedded engines to ever make their
way onto a commercial aircraft significant leaps in airflow prediction and engine design
& control will be needed. Although there are technical challenges, their
use could offer significant reduction in fuel consumption over the current generation of
podded engines. All of these technologies combined in the
D8 have been calculated to have a potential fuel savings of nearly 50% over conventional
technology and with the continual rise of fuel prices. This plane could be making it’s way to an
airport near you sooner than you may think.
Thanks for posting OP! This entire video, excluding the script, was made in 2 days. Feels very rushed, hope it stands up to reddits high standards.
If ye have any questions feel free to ask here or over on /r/realengineering
Airline executive would sacrifice their own children for 1% fuel savings, if this thing crashed twice as often as current airplanes we would see congressional directed rule changes at the FAA to allow for it.
/u/TaytoCrisps at 5:56 you probably meant to say "Why are plane wings angled backwards" instead of "Why are plane windows angled backwards" :)
I had the opportunity to fly in one of the brand new 737's two weeks ago. They are great IMO. Taller aisle makes it nice for us tall people when walking around, it's bright and airy when boarding, and the seats were more comfortable although that could have just been the configuration of that particular layout. Also had individual screens for each seat with USB and (IIRC) AC charging ports and headphone jack. The screen could play music or videos from your mobile device too.
Stickers!
Interesting video and it looks like there is a fair amount of development work remains before it could even be considered viable.
So that brings me to question- since a lot of the work involves engine development, what is the motivation for a major airplane engine developer (GE, RR) to divert precious resources from R&D in improving "conventional" engines and focus on these hard problems for a radical engine design with no airframe to immediately slot into?
Or if the onus to mature this technology is to fall to the university/small scale company level is it likely to succeed at all?
I remain skeptical. %50 fuel savings is such a round figure it sounds like someone did it on a napkin. It also sounds too good to be true. I'll believe it when they fly the same route as a 737 or an A321 and it actually uses half the fuel when adjusted for thrust/weight/air resistance/etc. I don't see how the drag has been so drastically reduced, since even if you reduce the profile of the wings you've radically increased the width and profile of the cabin, even if you're producing lift with it. It doesn't seem like that would reduce drag, just shift where it is. Maybe I'm missing some fluid principle that just reduces overall drag, but it seems to me like the as long as the profile is the same size, the drag will always be constant, like energy can't be lost, only transferred. Also (hopefully I'm not being too simplistic) but a slower plane is naturally going to use less fuel, but over a greater length of time, so how does it end up using less fuel? And then there's the engines, the problems endemic to it (ie compressor stall, high stress on components, and just poorer quality air for the engine to breath from a fluid point of view) seem to greatly outweigh the benefits, as I don't see how it reduces drag that much. How exactly does the boundary layer create more drag once the slow moving air has "rolled" off the back of the plane? And how does the engine cancel or reduce that drag? Also, I looked it up and there are no working prototypes, just one mock up model from NASA. I'm as dubious of it as the EM drive. Wouldn't a larger, irregular cabin be more susceptible to torsional forces and the pressure of altitude, and have less usable space because of the gap in the middle above people's heads? There's probably a lot at play here that I don't understand here, since I don't have an engineering degree, and I want to be wrong, but it just seems like a lot of this is unfeasible and the benefits are overstated. I'm all for a more efficient plane though, they are a major source of pollution, and I want to be proven wrong!
Also RealEngineering, I really enjoy your videos, especially on architecture and buildings. Very well produced and professional, always interesting.