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 posts 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 read my “why are plane wings angled backwards post”.
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 post, 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.
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