Flight is not a new idea – the concept has been around for millennia. Artefacts have been discovered in both Colombia and Egypt with remarkable resemblances to the aircraft of today, with the inclusion of engineered structures not found in nature (notably tailfins). Indeed, history is littered with myths of flight; Daedalus and Icarus of Ancient Greece, India’s vimana, and the ancient Chinese Emperor Cheng Tang Li, to name a few. Even some of the greatest minds in history were intrigued by flight. Leonardo da Vinci was inspired to design several mechanical wings and a few flying machines, most notably his Ornithopter and his famous aerial screw design, although he never built any of these ponderings.
The first recorded manned, untethered flight, however, wasn’t until 1783 when the Montgolfier brothers made a 9-minute flight over Paris in a paper balloon. The concept of hot air balloons is familiar to us all — a fabric balloon filled with hot air (some balloons were filled with helium or, often fatally, hydrogen) which rises through the air and allows flight to occur.
While air travel is not new, powered air travel is, still, relatively young. Conquering flight and the engineering challenges it presents has proved much more difficult than conquering land or water. While the general principle of flight is relatively unchanged over the last century or so, the way in which these machines are powered has changed dramatically. To compare the engine in the 1903 Wright Flyer with the engine found in the Lockheed Martin F-35 Lightning Fighter aircraft would be unfair. The former engine produced just 0.4kN of thrust. The latter produces nearly 500 times that – 191.3kN.
But it’s not just the power of the engines which has changed. The earliest aircraft engines were built of cast iron or, in some cases, aluminium to save weight. Most aircraft engines today are made with many different materials, including blades grown from a single crystal of titanium, engine casings made with fibre-reinforced composites and carefully mixed alloys containing numerous metals. Moreover, the overall design of the engine has changed rapidly over the past century. The earliest aircraft engines were either designed originally for use in automobiles or crude, custom-built reciprocating engines. It took only 30 years for jet engines to be designed and only another few decades for the turbofan to emerge. Nowadays ramjets are used in supersonic and hypersonic flight – speeds which were undreamable at the birth of modern, powered flight.
Engines have spearheaded an aerial revolution over the last century and the engines of today are barely recognisable as the engines of yesterday. The aim of this three-part editorial is to delve into the evolution of the aircraft engine and explore the factors behind its changes. Just why has the aircraft engine design changed so rapidly in comparison to the engines of other mechanical engineering disciplines? And will it continue that way – what will the engines of the next decades (or even centuries) look like? How has the design of the aircraft engine evolved, and what were the factors behind these changes?
During the early history of aviation most engines and powertrain units were adapted from other mechanical disciplines. Many engines were adapted from automobiles of the day due to their small, compact nature. This, surprisingly, didn’t present an obvious problem. In fact, many modern light aircraft, such as the Cessna 172 or the Piper PA-28 Cherokee, have Lycoming O-series piston engines not too dissimilar to those found in small cars (the two aforementioned aircraft’s engines are almost identical to the engine in the classic 1960s Volkswagen Beetle Type 1, notably lacking a gearbox). However, there are some major differences between the engines of two disciplines.
However, in cars, the demand of the engine is rather low. At cruising speed, a car engine works at roughly 10% power. In a plane, however, the cruising speed is much higher. Compared to the cruising speed of cars (60-70mph), the cruising speed of smaller aircraft is 105-140kts (roughly 120-160mph). This means that, naturally, the engine works at a higher strain. The cruising altitude of light aircraft (~1,500ft) means that there is a considerable change in air density (roughly 5% lower). The thinner air means that the engine needs to work harder than before for the same output energy to be achieved – if there is a lower density then this means that for the engine drawing in 10m3 of air at the same rate, the mass of the air is lowered, consequently lowering the efficiency of the engine. Moreover, a lot of an aircraft engine’s power is used in lift – the engine not only moves the aircraft through the air but it holds it up as well. As a result, cruising aircraft work the engine at up to 80% capacity.
So there is an obvious problem with using engines designed for automobiles in aircraft – these engines simply aren’t designed to withstand the continued load demanded of it. The Lycoming O-series engine mentioned before is, of course, designed for aircraft and this increased workload. During the typical 2,000 hour service, a car engine can’t survive this heavy, sustained use. Whilst very similar in appearance and mechanics, the engineering of early plane engines is much more thorough and built to very fine tolerances.
In 1902 Wilbur and Orville Wright asked several engine manufacturers to construct an 8-hp gas fuelled engine that would weigh no more than 200lb (90kg). No company took up the challenge. The pioneering brothers ended up designing their own piston engine, with the help of Glen H. Curtiss. Previous aviation pioneers such as Hiram Maxim and Clément Ader used steam-powered engines in their designs. Samuel Langley did use a gasoline-powered engine in his much larger Aerodrome aircraft.
These steam-powered are very similar to an ancient design by Greek mathematician Heron of Alexandria. His aeolipile was a hollow sphere mounted on an axis and turned by steam which was ejected through tubes. Dating from the 1st Century AD, it is the first record of transforming steam into rotary motion and since has powered many mechanical devices throughout history. Steam engines successfully powered Robert Stephenson’s famous locomotive Rocket and many early motorcars. But when it came to aircraft steam power simply wasn’t powerful enough.
The basic design of the Wright Brothers’ 1903 engine is very similar to that of modern, four-stroke, four-cylinder car engines. To save weight the crankcase was made of aluminium. It is, of course, much more primitive. However, their engine did include a magneto for ignition, combustion chambers, a carburettor and a system of belts to keep a defined firing order. Their engine produced a magnificent 12-hp, far beyond their target of 8-hp. Consequently, most aircraft engines (and indeed most car engines) were based on an internal combustion engine very similar to this design.
There are other reciprocating engines such as rotary engines, radial engines and Wankel engines which have some advantages and, of course, some disadvantages over piston engines and were originally developed at similar times. All of these reciprocating engines work based on the same, fundamental principle of combusting a compressed mixture of air and fuel and converting it to motion; the engines simply have different techniques to execute this. Indeed, the Wankel engine has been notably used in Mazda’s RX-7 and RX-8, while the legendary Pratt & Whitney Wasp aircraft engine series has been used in many series of aircraft.
So why is the piston engine so good? Well, one of the biggest factors was that over the many decades through which it was so popular, designers refined the design and improved it. Modern engines have many more pistons (such as the impressive fourteen found in the Bristol Hercules) and modern machining allows the engines to be machined to very fine tolerances. Moreover, full-authority digital engine controls (FADEC) have revolutionized the way planes fly through the introductions of computers. Almost every conceivable aspect of an engine and of the entire aircraft can be measured and monitored many thousands of times a second.
But development aside, the piston engine is a very reliable engine. Compared to rotary engines, the piston engine offers an efficient performance across a wide operating range. Typical rotary engines simply do not deliver a suitable amount of torque at lower operating ranges compared to piston engines. However, a piston engine has some fundamental limitations. Due to the lower density of air at altitude, aircraft with piston engines have a natural ceiling. The propeller itself has a fundamental limitation – during flight at altitude the pitch of the propeller (the angle at which the blades are set to the air) must be great. A low pitch it would mean that maximum speed is compromised; the excessive rate of rotation wouldn’t propel the aircraft to as great a speed as it otherwise would achieve. During low speeds, however, such as during take-offs or landings the pitch needs to be very low to ensure sufficient power is produced at these low speeds. One widely used solution to the latter problem is the variable pitch propeller (controllable pitch propeller). Now used in both aviation and marine technology, this allows the pitch of the propellers to be altered during their use. This was a solution to the problem of power and torque at different speeds, but it didn’t solve the limit of aircraft ceilings.
This is part 1 of a 3-part editorial.