Into the Unknown
Exploring how the possibilities offered by a few key technologies could unlock significant efficiency improvements for commercial aviation
Hello again! After the last post, where we explored the shape of aircraft today, I promised that we’d explore the future technology that could help make zero-/low-emissions aircraft work without fundamental breakthroughs in propulsion or energy storage.
All of the tech I’m going to discuss today comes with warning labels – there are gaps in the research, or large unknowns. Sadly there aren’t any super awesome technologies just waiting in the wings ready to go, just a lot of partially investigated concepts with a lot of work still needed! As always, a caveat before we dive in – I’m not an aviation expert, so might not convey all the nuances of these technologies quite accurately. Right, let’s get started!
Design
Let’s start with design, by which I mean the overall shape and form of the aircraft – the shape of the fuselage, the wing design, and how everything integrates. There have been many alternatives and evolutions of the classic tube-and-wing design proposed, including strut braced wings, C-wings, the ‘double-bubble’, laminar flow designs, the Flying V, and more. I may write another post in the future exploring some of these alternatives, but for now I want to focus on one design in particular.
The Blended Wing Body has been studied in some form or other for decades. The name encompasses a broad variety of shapes and concepts, but broadly, it is a design somewhere between a flying wing and a conventional airliner design, with the fuselage blended into the wing, hence the name. To my mind, if you asked what an airliner of the future looks like, chances are you’d come up with something like a BWB. I’m no blind techno-optimist, but if we want to inspire people about a decarbonised future, why not make that future look awesome? Many low-carbon aircraft feel like early hybrids – sure they are efficient, but they look terrible. The BWB feels like a Tesla competing against a field of Priuses.
OK, but other than looking cool, why is it so exciting?
Advantages
Let’s start with the good stuff. The big one is lift-to-drag ratio. All aircraft generate aerodynamic lift by moving through the air, but that movement also generates drag. The more lift and less drag you generate, the less energy you need, and the better performing it will be. Due to their blended, smooth shape, BWB designs can have a lift-to-drag ratio around 20% higher than a conventional aircraft, resulting in far greater efficiency, particularly in cruise.
The shape of such designs also gives much greater interior volume, which is great news should you want to lift bulky but light materials like, say, hydrogen. Most BWB designs also call for the engines/propulsors to be placed on the top of the body, which means noise is largely shielded from the ground, making operations significantly quieter. Another potential noise reduction may come from the high lift, which might mean that high-lift systems (flaps, slats) are not needed. This is both a noise improvement (flaps contribute significantly to aircraft noise) but also saves weight and complexity. That lift performance might also allow better short field performance (opening up more, smaller airfields) as well as steeper climb/descent, which would also reduce the noise impacts on communities around airfields.
Finally, the design provides an excellent platform for experimental propulsion systems, such as distributed propulsion or boundary layer ingestion, something I’ll circle back to later on.
Disadvantages
The first is simply complexity and lack of concrete research – after 20+ years of investigation, no design has progressed much beyond design studies. Virtually all papers are theoretical, with few companies ever having seriously considered a BWB design for practical use. Maybe a dozen designs have ever been flown, all as reduced scale models. Almost all investigations drop blended wing designs early on due to its high risk nature and uncertainties of the approach. Even something as simple as the overall shape is still not fully understood, with dozens of proposed concepts differing significantly in how they shape the aircraft.
Aside from that, flight control is a major unknown, and has been frequently highlighted as a problem area. BWB designs are much shorter than conventional designs, and lack the significant rudder and elevator lever arms to control the aircraft effectively manually. These problems are not insurmountable – the B-2 Spirit faced a host of similar issues, largely overcome by innovative design and sophisticated fly-by-wire computer controls – but the dynamics of flight with such a vehicle are poorly understood currently. Taking such a design to full type certification would be a tall order, and would likely require a very extensive program of flight testing and demonstration of safety in a wide variety of circumstances and conditions.
The shape itself also throws a few spanners in the work. Firstly, the fuselage is not round, which as I described last time is less efficient for a pressure vessel – more material is required to reinforce the corners of an edged shape. Here, though, there are promising advances in materials – the PRSEUS program developed a stitched composite structure specifically for BWB cabins, and tests of full scale pressure vessels showed that lightweight, strong cabins are possible.
Other studies also tried out PRSEUS for wings, and showed significant benefits both in terms of weight and failure modes over existing composite wings, so this could be very promising. However, PRSEUS has only been tested at a prototype scale, and productionising and certifying such a material is likely to prove complex, time-consuming and expensive.
A BWB would also have a shorter, wider cabin, which makes for a wildly different passenger experience to a conventional airliner. Given the placement of the cabin deep within the fuselage, windows would be at a premium, and the layout would be more akin to a movie theatre than today’s tubular designs.
Finally, the centrebody is designed to be an airfoil, to generate lift and reduce drag. However, if an airfoil is made too thick, it becomes less efficient, so there are lower limits on the minimum length of a BWB fuselage. Most early studies focused on extremely large BWB designs (500-800 pax) for this reason – it allowed the use of slim airfoils in the body, whilst retaining room for a full passenger cabin and cargo bay/landing gear beneath.
However, recent studies have suggested that there are ways to compress the design. Notably, DZYNE Technologies created a BWB business jet concept for a NASA competition that was far smaller than previous designs, whilst retaining the core benefits of the BWB design. Whilst most aircraft place the main landing gear roughly under the centre of gravity, DZYNE’s approach moves the main gear right to the rear of the aircraft, with a patented tilting method to allow for a mostly conventional take-off. There are other approaches that could also solve this take-off issue, but again, a topic for another time!
Overall, the BWB is a high potential design, but there’s a huge amount of design work to figure out all the many missing pieces. It feels too early to write off BWB as a whole, but many of the ‘will it work’ questions simply can’t be answered yet – so much more research is needed.
Propulsion
Now let’s take a look at propulsion. Here, it’s worth dividing things between energy production and the conversion of that energy into thrust.
Energy generation can go a host of different ways. If zero or ultra low emissions is our goal, hydrocarbon fuels are more or less out – SAFs offer the promise of offset emissions, but come with too many trade-offs to rely on. As I mentioned last time, battery-electric and hydrogen (either fuel-cell or turbine) are our key ingredients, but even with just these options there are a number of options as to how we combine them.
Hybrid
You’ve probably encountered hybrids in cars – systems that combine an engine with a motor and battery. The idea here is that the two architectures work in tandem, supporting each other. The engine is able to operate at peak efficiency at all times, providing a constant base level of power, whilst the motor and battery work to manage peaks and troughs, storing power when there’s surplus, and drawing it when extra is required. There are a few different ways that hybrids can arrange their component systems, but all share this common guiding principle.
Hybrids make a lot of sense for vehicles, because they tend to stop and start, to accelerate and slow. This forces the engine to mostly operate outside of its peak efficiency range, and hence the gains for a hybrid system are significant. Airliners are rather different however – they generally spend most of their time at cruise, and jet engines are optimised to be most efficient at this design point. There are still gains to be made, but the difference is far less stark than in road vehicles. Another topic to expand upon later!
Turbo-electric
Turbo-electric is, more or less, hybrid but without the battery. It has been used in ships and trains and other heavy equipment for decades, using an engine to power motors that provide the motive force required. This allows the engine to provide the raw power, whilst the motors handle the output requirements. Motors can be designed to perform at many different loads and particularly handle high torque, low RPM loads much better than engines, eliminating the need for complex (and heavy) gearboxes. A turbo-electric aircraft potentially offers the range of conventional fuel with the efficiency of electric, and without the extra weight of batteries. A number of initial studies suggest gains over the hybrid architecture for aircraft, but the system dynamics are complex, and a lot more study is needed.
Thrust
This is how the power of the engine or motor is converted into a force that pushes the plane along. For example, a propeller accelerates air, providing a force against the air, whereas a turbojet pulls in air and combusts it, which makes it expand, and then forces those expanded hot gases out of the back. Modern turbofans are a combination of both, using the thrust gases for a small fraction of their thrust, with the rest coming from the giant front fan.
When we consider electric aircraft, we’re basically considering motors to power the aircraft (there is some very early stage plasma-based electric thrust research, but that’s likely decades out). Our options therefore are propellers and fans to generate thrust. To compete with turbofan aircraft, we need to operate at high subsonic and transonic speeds (Mach 0.8-0.85), eliminating propellers for efficiency reasons. So, what can we do with motors and ducted fans?
Distributed Propulsion
Electric motors scale almost linearly with size, and so lack the scale penalties inherent to turbines (not to mention that megawatt scale lightweight motors are a fair way off yet). Motors also require less infrastructure than turbines – power, and maybe cooling – offering far more flexibility in placement. These two combined give rise to distributed propulsion – using a number of smaller propulsors spread across the airframe for greater efficiency. The possibilities are many – wingtip motors to reduce losses from tip vortices, leading edge motors for a blown wing/blown flap, and much more.
A number of studies have been performed on this form of propulsion, but given the huge number of possibilities, there’s still a lot of unexplored potential in this approach, with lots of investigation still to be done. Many of the supporting systems required for such an architecture, such redundant power and control systems for large numbers of motors, are still very immature at present.
Boundary layer ingestion
Air is a fluid. As it flows over and around surfaces, the air closest to the surface is slowed by viscosity, causing drag. Naturally, we want to decrease drag in any way we can. One proposed solution is known as boundary layer ingestion – using aft-mounted propulsors to ingest this slow moving layer of surface air and re-energise it.
This has been studied on and off numerous times and ways over the years, and research so far suggests it could yield non-trivial gains. Electric propulsion could combine well with such approaches, as the benefits of BLI scale with the proportion of the boundary layer you can ingest. More, smaller fans can cover a greater amount of the surface of the aircraft, increasing the gains. As always, there are still a lot of open questions about the technology, including optimal fan placement and layout, dealing with the fan stresses caused by operating in turbulent flow, and more.
These two technologies – distributed propulsion and BLI – offer significant gains alone, but also pair well with the BWB design. Its broad body airfoils and flat shape offer plenty of space to exploit boundary layer effects across much of its surface, and indeed a few studies have looked at exactly this combination. However, few have managed much depth, mostly due to the compounding complexities involved – multiple unknown technologies with many possible variations means that it’s hard to explore much in detail. As a result, it seems likely that the best combinations and possibilities of these technologies (and possibly some key downsides too) have yet to be discovered.
Supporting Systems
Modern airframes are increasingly moving to composite, and indeed, for forming complex 3D shapes, composites are likely the best solution (indeed, the possibilities offered by PRSEUS suggest that as a promising direction). However, composites are complex to work with, requiring a lot of tooling and expertise, and offer difficulties with maintenance and repair. These challenges are not insurmountable, but add extra complexity. These are issues that face the incumbents today, with the A220, A350 and B787 all featuring heavy use of composite materials.
MEA
Large aircraft have a host of different supporting subsystems to operate and support flight operations. Aside from the main propulsion, there’s generally a backup power unit (APU), electrical systems for everything from lights to flight computers, hydraulics for landing gear and flight surfaces, pneumatics for brakes and landing gear doors, bleed air for HVAC and de-icing, the list goes on. Generally, each system has been chosen as the optimal solution given the dynamics of the situation it operates in. However, carrying multiple redundant versions of multiple types of power system is weighty, and conversion losses are also non-trivial.
As a result, a growing movement in aviation design is that of MEA – More Electric Aircraft. This sees systems replaced by electrical equivalents wherever possible, simplifying systems and saving weight. The B787 is the best known example of this, lacking a bleed air system and moving many internal systems to electrical power. However, with a largely electric propulsion architecture, it makes even more sense to move to this type of system. Even here, however, there are complexities – designing systems to be effective and redundant is tricky. Naturally, any steps in this direction need to ensure that their solution is at least as safe and effective as what it’s replacing!
Radical Ideas
Lastly, there’s space for the radical ideas, the far out concepts. Maybe these only deserve brief attention, but it’s worth considering the more unusual ideas just in case something has been missed. For example, can long-distance power transfer overcome its inherent inefficiencies to power electric aircraft? Could aerial recharging/battery swaps be a workable concept for aviation? Could some parallel to an aircraft carrier catapult system save on take-off power and increase range? These ideas are wild, and I’m sure you can already see countless potential pitfalls. But if we’re to radically innovate, it’s worth giving these more unusual ideas some credence too – you never know what may turn out to be surprisingly achievable!
Onwards, and upwards?
So there we have it, a whistle-stop tour through some of the possibilities for a future aircraft that could reduce the work that batteries/hydrogen have to do. Maybe there’s a single possibility in amongst all this, or perhaps there are many viable solutions for different aircraft missions.
I should reiterate, that this isn’t intended to supplant work on SAFs, or synfuels, or other alternative approaches to decarbonising aviation. Nor is it meant to avoid the very real debate about the role of aviation in a decarbonised world (I spy another upcoming post!). However, it is a potential option, and as with everything in the climate crisis, we need to explore all possibilities – there is no silver bullet to decarbonising the global economy.
Now, the real work begins – we have to figure out how to design and test and validate these ideas. But how do we even begin to design an aircraft, and figure out if it’s feasible? Next time, we’ll dive into preliminary design, and look at how aircraft are designed now, and to what extent existing design methodology can be repurposed for investigating novel aircraft.