Powering the World's Airliners

Engine Developments from the Propeller to the Jet Age

Reiner Decher

• Publisher: Air World
• Year: 2020
• ISBN:
  9781526759153
• Topic: Technology & Engineering
• Subtopic: Aviation
• Index: Technology & Engineering

From propellers to turbofans, this illustrated history of engines will be "of interest to modelers and aviation historians alike" ( AMPS Indianapolis ). The first efforts of man to fly were limited by his ability to generate sufficient power to lift a heavier-than-air machine off the ground. Propulsion and thrust have therefore been the most fundamental elements in the development of aircraft engines. From the simple propellers of the first airliners of the 1920s and 1930s, to the turboprops and turbojets of the modern era, the engines used in airliners have undergone dramatic development over a century of remarkable change. These advances are examined in detail by aeronautical engineer Reiner Decher, who provides a layman's guide to the engines that have, and continue to, power the aircraft that carry millions of travelers across millions of miles each year. Decher also looks at the development of aero engines during the Second World War and how that conflict drove innovation and explains the nature of wing design, from the early twentieth century to the present. To enable an easy understanding of this intriguing subject, Powering the World's Airliners is profusely illustrated, transporting readers back to the time of each major development and introducing them to the key individuals of the aero industry in each era. After reading this comprehensive yet engaging story of the machines that power the aircraft in which we fly, no journey will ever seem quite the same again.

Chapter 1

We are Flying!

You are sitting in a comfortable seat on your way to a destination a thousand, perhaps ten thousand, miles away. On take-off, each engine of your airliner takes in and ejects about one ton of air per second, perhaps two! In flight, you cruise above most of earth’s weather at 550 mph! You might argue about the seat comfort, but a nice person serves drinks and you have a chance to be with your thoughts, your own form of entertainment, or conversing in almost normal tones with your partner or someone new. You may even be enjoying the view of a beautiful world from your window seat. This is the mobility afforded by the modern aeroplane. Travelling was not always that easy: consider the travails of past travellers, explorers, and pioneers. The world has changed with modern air travel in many ways, not for everyone perhaps, but it has for those people active in the modern world’s economy who can take advantage of it.

Fig. 1: You are here, going somewhere.

The development of flight as a means of transport is a fascinating story that spans a little more than a century. Most of the important technical steps that take us from air travel as a novelty to the state we enjoy today took place in three or four decades of innovation, roughly the years from 1935 to 1970. To be sure, innovation does not rest and evolutionary improvements continue to this day.

This is the story of the increments of understanding involved in enabling us to sit in that airliner. Many breakthrough ideas are involved. When and by whom were they implemented? Why were they important? And, finally, why were they critical?

Speed!

Speed of movement has always been an important driving force motivating innovation for military and commercial advantage. It was generally advantageous for armies to be in a new place fast, at least faster than an adversary. Over land, we witnessed the historical development of the use of horses and other animals, including camels and elephants, carts, Roman chariots, trucks, engine-powered tanks, and the like. Over water, ship design followed a similar evolution, and in the air, speed considerations were a consistent theme.

In the commercial world, history displays similar patterns for the distribution of goods, and the conveyance of people is consistently made better if it involves less travel time. Before the aeroplane, significant journeys over land or on the seas took weeks or even months, often with considerable shortages of comfort. The aeroplane shortened the time first to days, then to hours and generally improved the comfort level.

In addition to the transport of people and goods, speed was always important in the transmission of information. From the legendary Marathon runner to the Pony Express, carrier pigeons, and airmail, it was always desirable to have news as soon as possible. As long as a human being or a physical object has been involved as the messenger or message, they needed to be transported. Nowadays with the invention of radio and the establishment of modern digital infrastructure, news travels at the speed of light, limited only by delays involved in the generation and reception of the message.

Thus, while information may not need an aeroplane, speed as a parameter describing flight is singularly important. The speeds quoted for airliners in this book will be cruising speed. Generally, that is the speed that leads to good fuel consumption performance. After all, range was always important and fuel wasted by going fast was not in the interest of an airline. Typically, aircraft have maximum speeds that are greater than cruising speeds, but maxima tend to be primarily of interest for military combat performance purposes, and to indicate to the airline pilot: do not exceed this speed!

The air in which we fly

Since flight is in the Earth’s atmosphere, one cannot talk about the speed of aeroplanes without recognizing the limitations the atmosphere imposes. The atmosphere is a domain of varying air pressures and temperatures at various altitudes. For purposes of flight, the atmosphere consists of a lower layer where weather is active and variable, and a higher layer where the weather is less variable, because it is less affected by the influence of solar energy input. These layers are the troposphere and the stratosphere. The boundary between them, the so-called tropopause, lies as low as 20,000 feet in polar regions, and may approach 60,000 feet in the tropics. For a so-called standard atmosphere, the tropopause is fixed at 36,090 feet.

Fig. 2: The weather and visibility are better up here, but storm turbulence can easily penetrate this calm world.

Every aircraft has a ‘service ceiling’. This is the maximum practical altitude at which a particular aeroplane can fly. The aeroplanes of the 1930s had service ceilings of about 25,000 ft, and were unpressurized. But nowadays an aeroplane would never be unpressurized in service at that altitude. The passengers would surely object … if they made it to their destinations alive! The human body can tolerate the lower pressure at higher altitudes but is subject to a number of deleterious issues. These are numerous and complex; suffice to say that, if cabin air pressure or flight altitude is held to a level under 5,000 ft, the average passenger should be able to travel without ill effects. At that altitude, the normal temperature is some 27 degrees Fahrenheit lower than it is at sea-level. Heat must be provided or warm clothing worn by the passengers. In the early days of airline aviation, heights of 8,000 ft were routinely reached to cross the Rocky Mountains. Airlines provided oxygen for passengers and crews as needed.

Flight at lower altitudes is always subject to some influence by the weather. In the stratosphere, above the weather zone, the air is much less likely to be turbulent. Air pressure at 35,000 feet (near where modern airliners cruise) is one quarter of sea-level pressure. There are good reasons for flying high. Another is to avoid encountering mountains!

The Speed of Sound

In any medium, including the air, there is a fundamental speed at which pressure waves propagate. This is the speed with which information, pressure waves if you will, is passed on within the air to tell it to get out of the way of an approaching wing or propeller blade. Visually they may be compared to the waves that emanate from the bow of a ship. Very small pressure fluctuations are perceived as sound waves; hence we describe the air’s characteristic wave-propagation speed as a sound speed. The limited sound speed is one feature of Earth’s atmosphere that shapes the manner in which flight within it can take place.

Objects moving at speeds close to the speed of sound generate stronger waves. In turn, these waves have strong effects on the drag experienced by the moving object. This is true even for the propeller blades that powered all aeroplanes in the early years of aviation and limits the speed of today’s airliners to less than the sound speed.

When strong enough, the waves created by a moving object are shock waves. Air crossing a shock wave experiences a pressure increase. Shock waves affect the flow far from the body. That means that a lot of power is invested in creating and maintaining them. They also have a profound effect on the pressure over the wing or propeller blade. Briefly, they can cause the flow to fail to conform to the shape of the wing or blade and that results in a thick wake1 that should ideally be as thin as possible. The drag increase associated with increasing speed is commonly referred to as the ‘sound barrier’. This is not really a barrier but the reality that the power available may simply be insufficient to move the wing or propeller blade any faster.

The speed of sound in air depends solely on its temperature. It will therefore vary with altitude. At sea-level, on a so-called ‘standard’ day, the sound speed is about 760 miles per hour, and in the colder stratosphere it is a more uniform 660 mph. These speeds are good reference points for judging speed performance of aeroplanes in various periods. In practice today, speeds near that of sound are quoted as a fraction of the sound speed. This fraction is the Mach2 number, where movement at Mach one (M = 1.0) is at the speed of sound. The use of the Mach number frees the discussion from consideration of temperature conditions at the time and place an aeroplane is flying.

The Airline Business

The business of building and operating airliners is a technical and an economic challenge. Both aspects have quantitative dimensions that have to be considered. Numbers are critical and we have to take an interest in how they are determined. Airlines have to make a profit.

Running any business invariably involves keeping track of numbers. Some of the numbers cannot necessarily be controlled or accurately determined, so one has to employ estimates. As a result, proceeding into the future carries some risk. Fuel price is one dimension of the airline industry that has shaken it more than once. Further, the industry buys airliners with a period of years between order and delivery, and that new airliner should function economically for decades after it goes into service.

For example, an airliner might be designed to last thirty years. Cracks associated with metal fatigue of the structure are normally the critical life-determining issue. Take-off/landing events, together with violent weather encounters, tend to be the hardest loads on the airframe. Today, a long-range aeroplane might be designed for 40,000 cycles while a short-haul aircraft might be designed for 100,000 cycles. In either case, it flies for 10 to 20 minutes of every hour of its existence. That suggests a heavy time utilization of the aeroplane, and loading and unloading must also be taken into consideration.

If you think concerns for profitability over these long periods are difficult to manage, try to find answers to the questions that the airliner-maker will have to address before he launches a new aeroplane type. His thinking has to involve time periods even longer because the production run of the aeroplane will, hopefully, be many years long. He has to make sure that the enormous investment is repaid, with interest, and a profit made. If you glean that this industry is subject to economic turbulence, requires people with a high degree of risk tolerance and financial resources to back it, then you have a good idea of the environment in which this story is set.

A quick look on the Internet for ‘defunct airlines’ yields a rich history of airline names that are no longer. Some vanished airlines have been acquired by the large carriers that fly in service today. Others never did manage economic operation in the long run. In a curious way, many of the names that disappeared carried a regional aura: Western, Eastern, Southern, Northwest, Pacific, Mohawk, etc, while others reached across the US with their names: Continental and USAir, among them. Some even had a global aspirational reach: Transworld and Pan American. Finally, some names did not include geographical references and were associated with an individual owner. Hughes AirWest and Braniff come to mind. Only the older reader will be transported to the time when these airlines operated. They are now part of history.

Hindsight is an advantage that allows the historian to focus on the development avenues that bore the most fruit. Thus we can look back and concentrate on the elements that led to the present state of the airliner. As far as the technological issues are concerned, the principal actors are the aeroplane-manufacturers and the engine-manufacturers. The latter are included because it has always been true that propulsion plays the greatest role in determining an aeroplane’s design. It is therefore critical to include the development of engine technology because it leads the parallel evolutionary paths of aeroplane-engines and aeroplanes. There were, of course, other technical fields whose inventions and understanding are critical to the successful operation of today’s commercial aviation.

The Aircraft Engine Builders

The history of engines is complicated by the ever-changing business arrangements of the people involved. Many attempts have been made to establish successful enterprises around engine-manufacturing, and in the US these culminated in companies like Curtiss-Wright and Pratt & Whitney playing dominant roles in the technological development of reciprocating aircraft-engines. There were and are others, of course, Allison chief among them, but Curtiss-Wright and Pratt & Whitney were central to the large aeroplanes we will consider here. The history covered includes the Second World War in important ways. The military dimensions are relevant because the war effort underlies a lot of the motivation of engine development during the war. In the military, bombers and transport aircraft were demanding high performance and the associated technology is naturally passed on to civil airliners, later perhaps, but surely.

To be semantically correct we might note that a piston-engine driving a propeller is, strictly speaking, a ‘jet’ engine. The propeller’s function is to create a jet. To be clear with our descriptions however, we will adopt the normal convention and limit the label jet engine to the new-technology engine developed with continuous combustion and refer to the internal-combustion engines driving a propeller as piston or reciprocating engines.

During the war and the post-war years, the history of jet-engine development in the United States was dominated by two companies: General Electric (GE) and Pratt & Whitney. Curtiss-Wright was not successful in adapting to the Jet Age, at least in producing jet engines. The company made an attempt by licensing the British Armstrong-Siddeley Sapphire engine in 1950. It developed the engine and sold it to the US military as the J65 for a number of military applications. Allison and Westinghouse did, for a time, go on to be players in the jet engine industry. There were and are other jet-engine manufacturers who worked successfully in aviation, but the airliners of today employ primarily, though not exclusively, the large American engines made by GE and Pratt & Whitney. On the world stage, on which the airline industry performs, the British firm Rolls-Royce, the French firm SNECMA (now Safran) and others also have a strong hand in modern jet engines for airliners. The other manufacturers a...