This article illustrates contribution of mechanical engineering in the aviation industry. The most obvious role of the mechanical engineer involves the design of engines. From the Wrights’ four cylinders, 12-horsepower engine, aircraft propulsion has evolved into today’s high-bypass turbofans developing over 90,000 pounds of thrust in some instances. The most visible contribution of mechanical engineers to aviation, engines are far from their only contribution. Changes in the design, construction, and capabilities of increasingly modern aircraft challenged the mechanical engineering in many other regards. The introduction of gas-turbine power required a concurrent revolution in manufacturing, test, and maintenance facilities and techniques at the engine builders. As advancements in aircraft construction and power opened the door to higher and faster flight, virtually every system within the airplane had to become more sophisticated, and new ones had to be devised. Air conditioning systems also changed, both to better suit the gas turbine prime mover and to accommodate wider external temperature extremes.
The 20th century has accurately been called the century of flight. No other machine of man's invention has done as much to make the world accessible to so many people, so it is no surprise to find the airplane listed among the great achievements of this century. When Orville Wright guided the first successful powered airplane into the air for 12 seconds at Kitty Hawk, N.C., on Dec. 1 7, 1 903, neither he nor his brother Wilbur could have imagined that thousands of airplanes with wingspans greater than the length of that first 1 20-foot flight would routinely take to the air less than 60 years later.
A flying machine is clearly a work of engineering, but the vital role of mechanical engineers in the story is not always appreciated. While it is true that aeronautical engineering began as a specialty within mechanical engineering, the story doesn't end there. The direct contributions of M.E.'s have made possible many of aviation's greatest advances. As we celebrate the airplane as one of the great engineering achievements of the 20th century, it is useful to examine, and appreciate, some of those contributions.
The most obvious role of the mechanical engineer involves the design of engines. From the Wrights’ four-cylinder, 12-horsepower engine, aircraft propulsion has evolved into today’s high-bypass turbofans developing over 90,000 pounds of thrust in some instances. In the process, the whole concept of how best to power airplanes has been revised a time or two, and the quality and performance of all types of aircraft engines have advanced in substantial ways.
While many planes still use piston engines and propellers, these power plants have matured into reliable machines used for a variety of applications. The Wrights’ engine was a simple, in-line Four. Over the years, mechanical engineers developed new designs, including larger inline, vee, rotary, radial, and opposed-cylinder engines to suit an increasingly wide range of military and civilian applications. These engineers took hundreds of steps along the way, introducing numerous modifications, including supercharging, turbocharging, and turbocompounding to enhance performance. The largest of these engines generated well over 4,000 hp, and they made possible large, intercontinental bombers and transports.
More Efficient Props
J. Lawrence (Larry) Lee, P.E., is the immediate past chair of the ASME History and Heritage Committee. A doctoral candidate in the history of technology at Auburn University, he is currently writing his dissertation on the history of American wind tunnels.
Getting this power to the air efficiently required great innovation in the field of propellers. Conditions at takeoff and at cruising altitude and speed became very different as airplane design improved. Fixed-pitch propellers could not hope to efficiently satisfy the needs of all flight regimes.
Research by Frank Caldwell and others showed that varying the pitch, or angle, of the blades could solve the problem, but developing a reliable mechanism to accomplish this was no easy task. Several designs of variablepitch propellers, including constant-speed, were used, but Hamilton Standard’s Hydromatic propeller, an ASME Historic Mechanical Engineering Landmark, is probably the best known. Variable-pitch propellers were a vital ingredient of the “design revolution” of the 1930s that produced commercially viable airliners like the Douglas DC-3 and a host of high-performance military aircraft.
What historian Edward Constant calls the “turbojet revolution” began during World War II with the introduction of the first turbojet engines of Hans von Ohain in Germany and Frank Whittle in England. Here was a radically different way to propel aircraft, one that promised vast increases in speed. Unprecedented speeds were realized in the Messerschmitt Me-262 and Gloster Meteor fighters, and these planes paled in comparison with the supersonic fighters that became common a decade later.
But speed was only part of the jet story. What neither Whittle nor von Ohain foresaw was the potential for increases in size and power that could drastically reduce unit operating costs by enabling bigger planes that could carry larger payloads. Piston engines were reaching their practical size and power limits in the 1950s, but as engineers refined jet engine designs, they began to develop the power and efficiency to double airliner speeds while cutting the seat-mile cost of operation.
Further engineering work led to the high-bypass turbofans that were central to the wide-body airliner designs, such as the Boeing 747, which reduced the seat-mile cost still further— to the point that, even with rising fuel costs, air travel became a commodity rather than a luxury by the 1980s. In the process, safety and reliability also advanced significantly. Where over-water flight was once the province of the foolhardy, we now routinely trust hundreds of lives to only two of these modern engines on intercontinental flights thousands of miles long. In the military arena, jets designed for particular functions continue to expand the range of the possible, including the Pegasus vectored turbofan engine—another ASME landmark—that made the Hawker Siddeley Harrier Jump Jet possible and the latest vectored-thrust designs that offer new concepts in aircraft maneuverability.
While perhaps the most visible contribution of mechanical engineers to aviation, engines are far from their only contribution. Changes in the design, construction, and capabilities of increasingly modern aircraft challenged the M.E. in many other regards.
Early airplanes were constructed of wood and fabric for the most part. After World War I, engineers began to think about all-metal construction, but the conversion was anything but simple. While some early designs essentially exchanged metal for wood, it soon became clear that a good metal airplane was not simply a wooden design built out of metal. The characteristics of the two materials were decidedly different.
It took nothing short of a sea change in thinking about the design of aircraft, a change that drew on the best understanding of mechanical engineers in such areas as structures, material selection, and manufacturing techniques, to effect change. Out of this came such modern features as stressed skin, monocoque construction, and multicellular wing designs that formed the basis of lighter, stronger airframes.
As materials became more exotic, involving difficult-to-process materials like titanium and graphite composites, mechanical engineers led the way in devising practical fabrication methods—an essential process of innovation that continues. The introduction of gas-turbine power required a concurrent revolution in manufacturing, test, and maintenance facilities and techniques at the engine builders. Once again, the responsibility for developing them fell largely to M.E.’s, and this, too, is an ongoing effort with no end in sight.
As advancements in aircraft construction and power opened the door to higher and faster flight, virtually every system within the airplane had to become more sophisticated, and new ones had to be devised. Such systems—most of them the realm of the mechanical engineer—are often unseen or unheralded, but modern flight would be impossible without them.
Large planes have correspondingly large control surfaces needing forces to move them that are well beyond human capability, but powered control systems furnish both the necessary force and the precision positioning needed. A multitude of flaps, slats, spoilers, ailerons, elevators, rudders, and trim tabs all require such actuation systems, and they must function with near-perfect reliability.
Retractable landing gears must have fail-safe extension/retraction mechanisms and reliable, high-performance brake and suspension systems. High-altitude flight is practical because of cabin pressurization systems introduced in such planes as the Boeing 307 Stratoliner and Lockheed 049 Constellation. More than simply a device to pump air into the cabin, pressurization also in-volves the utmost in design of thin-wall pressure vessels, for that is what the modern airliner fuselage is.
In addition, heating and air conditioning systems are necessary to maintain comfortable temperatures inside, while outside temperatures range from more than 100°F on the ground to less than —20°F at altitude. With the introduction of jet transports, cruising altitudes increased, and pressurization systems became more complex and critical, requiring emergency oxygen systems that automatically deployed in the rare event of cabin depressurization.
Air conditioning systems also changed, both to better suit the gas turbine prime mover and to accommodate wider external temperature extremes. Fuselage doors became more than mere plugs in the cabin wall. They are now often complex mechanisms in their own right, which must reliably accommodate dynamic loads and maintain a positive pressure seal while enduring thousands of open/close cycles. Many have elaborate, powered mechanisms to open and close them.
From the baggage bin mechanisms to the seats, and from sanitary systems to the beverage cart, there is little in the modern airliner that has not been shaped by the mechanical engineer. In recognition of all these mechanical systems and manufacturing techniques, the Boeing 367-80, prototype for the famous 707, joined the roster of ASME landmarks in 1994.
Perhaps the most clearly mechanical flight of all is that of helicopters, for here is a machine that depends on active mechanisms to create lift as well as control the flight. They require a complex marriage of mechanical systems to produce power, generate lift, and maintain stable control, so it is not surprising that the road to a successful helicopter was long and winding.
Numerous attempts were made during the first part of the 20th century, but it was Russian émigré Igor Sikorsky who first put the complete package together, in 1939, with the Vought-Sikorsky VS-300. This historic aircraft, also an ASME landmark, established the pattern for most of the helicopters that have followed. Even more than with fixed-wing aviation, mechanical engineers have played leading roles in the evolution of these versatile flying machines.
This is but a Small sampling of the myriad ways that mechanical engineers have participated in and shaped the evolution of the airplane during this century, and it doesn’t really do justice to these innovations. It would take a large volume to cover all of them as completely as they deserve. Yet, even a thorough analysis of these engineering achievements would fail to explain the effect the airplane has had on modern civilization.
Truly a product of 20th-century technology, the airplane has had a vast impact on the domestic and international scene by dramatically reducing travel times over long distances—by several orders of magnitude, in some cases. The airplane actually enabled a redefinition of the concept of travel, since the route and speed limitations of land and sea did not apply. Similarly, it allowed—some would say forced—military planners to redefine their ideas of strategic force projection and defense.
Perhaps just as important to the engineering profession, the airplane redefined the relationship between the traveling public and the engineers who designed and built the planes, as millions of travelers entrusted their lives to engineered products as never before. As a result, more people have been able to travel farther than they would have ever dreamed, and the entire world has become accessible. This century has witnessed a steady improvement in aircraft performance, safety, and efficiency as engineers—mechanical engineers, in particular—have continued to refine the airplane and its supporting systems.
Since the quest for safety and efficiency shows no hint of faltering, one can only wonder what advancements 21st-century engineers will create. After all, they will be inspired by an amazing legacy.