Throughout this 3-part editorial, I have explored the key developments of the aircraft engine and what has influenced this evolution. Upon reflection, one can see two major overlying themes behind the factors; Firstly, the desire for speed, and secondly, the desire for efficiency. These two factors may seem obvious, but I feel that they can sometimes be overlooked.
I feel that engineers have an incessant need to build to the superlative. Can I make the fastest? Can I make the biggest? Can I make the strongest? It is this mindset which I feel dominated the development of many engines. Firstly, the turbojet was influenced hugely by the desire to go faster – Sir Frank Whittle had seen the natural limitations of the reciprocating engines and tried to engineer his way around them. He did, indeed, succeed. But everything was triggered by his will to overcome the speed limit of aircraft. Indeed, if you look towards the German side of the development there is a similar story. Hans von Ohain was employed by Heinkel to make the gas turbine engine, but Heinkel themselves were a primary manufacturer for the German Luftwaffe. Heinkel, therefore, had a huge amount of pressure on them to make aircraft faster than British or American or Russian manufacturers. Both Whittle and von Ohain found a shared solution to a shared problem and as a result of these developments aircraft were indeed able to develop much more rapidly – the top speed of turbojet-powered aircraft quickly became supersonic, and are now hypersonic.
Similarly, the ramjet was born out of a desire to go even faster – the ramjet has powered aircraft (such as the unmanned HTV-2 Falcon) to speed well beyond Mach 15. Indeed, the factors behind the development of the ramjet are very similar to those behind the Heinkel developments – the US government funded the development of the engine (whose possibility was known about for several decades previous) to construct a plane simply to out-run missiles.
From these two previous points, one might assume that the military has a major role in the development of the aircraft engine. After all, the military has a reputation of funding development of the fastest – the forefront of almost all technological frontiers (of relevance) are found in the military. However, everything can be categorised into either military or civilian. As my final points will explore, the role of civilian aviation and its demands and priorities were of equal, if not greater importance in the evolution of the aircraft engine.
In civilian aviation, aircraft efficiency is much more prevalent. While performance is an important factor, the manufacturers of engines were focused on the efficiency. Accelerating beyond the sound barrier brings many problems in other aspects of aircraft design (notably airframe and structure design), thus decreasing the overall competence. However, neither the turbojets nor the reciprocating engines available at the dawn of the civil aviation industry were efficient enough; the reciprocating engine was rather inefficient at all speeds but most efficient at very low speeds, whereas the turbojet engine is inefficient at the subsonic speeds of civilian airliners. This, I feel, is the main reason for developing both the turbofan engine and the turboprop engine.
The turbofan engine offers a very good compromise between efficiency and performance for the high subsonic speed of almost all airliners and most civil aircraft of today. Its bypass ratio decreases the performance at supersonic speeds, but massively increases its efficiency and decrease the exhaust noise. These two factors were both vital in the civilian aeronautical scene and were key in the development of bypass engines, such as the turbofan. Similarly, the turboprop was also born out of a striving for efficiency. The turboprop is, after all, widely regarded as the most efficient engine, although performance is heavily sacrificed.
Overall, therefore, the two outstanding factors forming the backbone of engine evolution is the need for an increase in performance and for an increase in efficiency. As I briefly look into the future of aircraft engines you will see that it is further efficiency which has been reflected upon.
Accurately predicting what might eventually happen in the future is very, very difficult. In this section, I will outline a few possible futures of the aircraft engine.
Of course, none of these changes may ever happen. It might be that the current turbojet engines may be incorporated into supersonic airliners. Flying at speeds greater than Mach 1, as previously explained, would indeed mean that turbojets would be much more efficient than the current turbofans found on airliners. Maybe the foreseeable future may have no further technological breakthroughs in store, allowing us to design a different type of engine. New engines will be made, doubtless, as the design of the current engines are refined and improved and, indeed, the engineering and manufacturing techniques improve.
Pratt & Whitney have detailed where they believe engine design is headed. Their panel predicts that new technology will result in adaptive bypass ratios which will optimise the performance of aircraft whilst cruising. They also predicted that propulsive and thermal efficiencies would improve – the materials used for the fan in turbofan engines would shift to one side of the composite-or-metallic argument. But these changes simply detail the refinement of the currently existing engines.
As well as altering the design of the actual engine, Pratt & Whitney are also looking at changing the way it is implemented. There is a large emphasis on aircraft designers to reduce the noise of aircraft and the expense of fuel and Pratt & Whitney have come up with a rather novel solution. They have designed a prototype engine wherein the core is mounted backwards and at an angle and the engine is separated into parts. The reason for this is to power what has been heralded as the next generation of aircraft design. Aircraft like NASA’s D8 Double Bubble has been designed to combat the aforementioned dilemmas. Fitting the engine at the rear of the fuselage as opposed to underneath the wings can battle these problems, and Pratt & Whitney’s engine design can be integrated into these aircraft, whereas a traditional turbofan engine cannot.
However, one new method of propulsion which does look viable is electric propulsion. With the revolution of eco-friendly devices in the last decade or so many industries have looked at electrically powered machinery compared to fossil-fuel burning. This is shown largely in the automotive industry where nearly 1% of all new cars are now fully electric with many more hybrid cars. There are around 2,400 all-electric cars sold every month now, an increase of over 200% compared to just 2014. The electric car industry, then, is growing rapidly. The technology developed in this area is now being applied to the aerospace industry.
Innovations such as LEAPTech’s (a company formed by NASA and Joby Aviation) HEIST wing are looking as though it could be the breakthrough. With a wingspan of 31 feet and made from a carbon composite, this wing has 18 electric motors, each powered by a lithium-ion phosphate battery, similar to the battery in most Smartphones. It might sound ridiculous, but the Tecnam P2006T on which it will be incorporated has a reported top speed of 200 mph. With much development, electric power really could be a viable alternative to both reciprocating engines and, eventually, gas turbine engines.
Due to the pressure of consumers to decrease emissions and use of fossil fuels, I believe that in the next few decades we will see first the dawn of aircraft like the D8, but later fully electric aircraft. While the infrastructure is in place to develop fully electric aircraft I do not believe that the technology is not sufficient enough to build one today. But I do believe that fundamental limitations such as battery life, which have been a major problem in the electric car market (most notably losing power after prolonged usage), will limit the future of electric aircraft. Reducing emissions, albeit not to zero, is a very viable solution. I believe, therefore, that engines such as Pratt & Whitney’s angled, reversed turbofan engine are the engines of the future.