WHEN the whistle of the jet engine was first heard in 1939, it was a clear but unrecognized commentary on a major reversal in design process. Prior to that time, airframe development had been limited by engine development; every new operational requirement was keyed to the often tortuous delays occasioned by the introduction of a new engine of greater horsepower. Oftentimes airframe designers were too optimistic and anticipated greater power than was actually realized; as a result, outstanding airplanes like the Boeing XB-15 and the Douglas XB-1 9 were underpowered and thus not brought into production. The basic reason was simple: the design of more powerful reciprocating engines was both more expensive and more time-consuming than the design of airframes that could employ them.
This dependence an engine power can be traced in the serial development of famous fighters like the German Messerschmitt Bf 109 or the British Supermarine Spitfire. The initial prototypes of these aircraft flew, respectively, with the Rolls-Royce Kestrel V engine of 695 horsepower and the Rolls-Royce Merlin “C” of 990 horsepower. The Messerschmitt quickly switched to a German engine, of course, and successive requirements for increased performance were met by introducing new subtypes of the Junkers Jumo and Daimler-Benz liquid-cooled V-12 engines. The last variant of more than 33,000 Bf 109s built, the K-6, was powered by a 1550-horsepower Daimler-Benz DB 605 engine that could, with methanol injection, reach 2000 horsepower for short periods. The Spitfire, of which 20,334 were built, had in its Mark 22 version a 2050-horsepower Rolls Royce Griffon. As an American yardstick for comparison, the North American XP-51 flew with a 1150-horsepower Allison, while the last version, the P-51H, had a 2218-horsepower Packard Merlin.
Thus, in the roughly ten years between the first flights of the European prototypes and the end of the war, conventional fighter demands were met by tailoring airframes to engines that had just about doubled in power.
More powerful piston engines were being brought into production in every country. Through greater volume, increased supercharging, and vastly greater complexity, the goal was to increase the horsepower limit. In England the Rolls-Royce Eagle, a 24-cylinder “H” style engine a was bench run in 1944 and ultimately achieved 3450 horsepower. In Germany, a 3900-horsepower BMW 803 engine was bench run; it was a 28-cylinder aircooled, four-row radial, similar to the Pratt & Whitney R-4360 in the United States. The latter was flown in a Goodyear F2G Corsair before V-J Day and ultimately, of course, became a workhorse engine in the Convair B-36, Boeing B-50 and other multiengine aircraft.
The largest piston engine ever built, however, the Lycoming XR-7755, was a liquid-cooled, 36-cylinder, four-row radial engine that was intended to generate 5000 horsepower. Not even bench run until after World War II, the XR-7755 represented a peak in reciprocating aircraft engine power but was never required, for which maintenance crews were undoubtedly very grateful.
As the piston engines increased in power, so to a greater degree did their mechanical complexity, weight, size, maintenance requirements, fuel consumption, and cost. By unusual engineering achievement, the jet engine arrived on the scene at a horsepower equivalent to where the reciprocating engine was peaking out. In addition, the jet engine had a relatively simple construction that did not require the same investment in heavy machinery and was relatively lightweight and low in cost. While initial fuel consumption was high and reliability low, the jet engine improved rapidly in both these areas.
Perhaps even more important, from the standpoint of increasing absolute speeds, the jet engine eliminated the requirement for a propeller, with its inherent complexity and limitations.
Given the terrible urgency of wartime conditions, it is a tribute to both Sir Frank Whittle and Dr. Hans von Ohain that the inspired courses they pursued in the invention of the first jet engines were tolerated in their respective countries. At the time they were advocating the radical new style of power plant, the upper limit of piston engine development was not clearly perceived, while the need for thousands of more powerful engines was. Their genius attracted sufficient backing to enable the jet engine to come into being at exactly the time the reciprocating engine had reached its developmental limit.
The number of pioneers in the turbine engine field was very small; besides Whittle and von Ohain, the only contributor of comparable stature was Dr. Franz Anselm, who developed the axial-flow Junkers Jumo 004 used in the Messerschmitt Me 262, the world’s first operational jet fighter.
When the war ended, the piston engine fighter was still predominant, but the future was clearly signaled with the Me 262, the Arado Ar 234, the Gloster Meteor, and the Lockheed P-80.
After the war the situation changed dramatically; the piston engine was abandoned by designers first for fighters and then bombers; it was not long before transport and utility aircraft would also be turbine-powered. Engine and airframe designs were in abundance. Designers became encouraged by the fact that for the first time engine power was becoming available in greater increments, over a shorter development time, than ever before; engines and airframes could be designed almost in parallel.
The situation was exploited, and there was a flowering of designs in numbers that probably will never be seen again. Jet engines appeared to be relatively simple to manufacture in terms of machine capability, and everyone sought to get into the act. Allison, Curtiss-Wright, General Electric, Lycoming, Marquardt, Pratt & Whitney, Westinghouse, and others competed in what seemed to be virgin territory. Soon, however, the list began to dwindle as manufacturers found that the degree of engineering skill necessary to reach new levels of power and reliability was difficult to muster.
Airframe developers followed a similarly diverse course. The path of fighter progress was marked by a curious set of factors. Although the rapid development of engines enabled designers to overcome some discouraging new aspects of the fighter aircraft business, the specter of available power caused military requirements to be increased to levels that would have been considered absurd just a few years before. This had the effect of vastly increasing the development time necessary to bring an aircraft from concept to flightline because of the ever-increasing size, cost, and complexity. This combination of factors meant that not only would older fighters have a much longer service life than had been anticipated but that newer fighters would be procured in far smaller numbers than ever before.
To utilize the thrust expected to be available and meet the increased requirements, aerodynamicists were forced to evolve a whole series of new airframe innovations, almost always of greater and greater sophistication and complexity.
Thus, while sweptwings were adopted to enable aircraft to approach mach 1, it was necessary to apply the formulations of Whitcomb’s area rule to design airframes to slip smoothly through the supersonic region without excessive drag buildup. In a similar way, the need to combine long-range, good load-carrying capabilities, and high speed with reasonable takeoff and landing distances led to the development of variable-geometry aircraft. Other practices ranged from the subtle change of wing airfoil and camber to aerial refueling to the inclusion of a second crew member, always a problem in fighter pilot psychology. With these new advances came problems of structural strength, fatigue, corrosion, training, repair, etc.
One can trace this pattern of increased power, size, and complexity in the aircraft delivered to the United States Air Force. The Lockheed P-80, first operational USAF jet fighter, led to the F-94 Starfire, and ultimately to the F-104 Starfighter with its razor-thin wing, The sweptwinged North American F-86 was improved through a long series of design changes before being replaced by the far larger and heavier supersonic F-100. Convair entered the field with two much-advanced fighters, the delta-winged F-102 and F-106, before developing the controversial F-111, the first swing-wing aircraft in the USAF inventory. Northrop achieved success with the F-89 Scorpion before turning, in advance of all of the other manufacturers, to a lightweight fighter in the form of the F-5.
McDonnell Aircraft, after years of being a Navy supplier, evolved the long-range, supersonic F-101 Voodoo and followed this with the immortal F-4 Phantom II, perhaps the most important jet fighter in history.
Republic (subsequently a division of Fairchild Industries) created the F-84 almost in parallel with the P-80, and the design matured into a long line of rugged, successful warplanes. From these evolved the immortal Thud, the indefatigable F-105 that carried a major burden in the air war over North Vietnam.
These fighters were the workhorse aircraft that provided the USAF with a worldwide capability from Korea to Vietnam, and they represent the main lines of development in response to the increased power of turbine engines. Interspersed with these aircraft were others designed to fill special niches. For various reasons, they failed to achieve operational status. Among the more interesting of these were the last fighter from Curtiss, the four-engined F-87 Blackhawk; the improbable,-looking XF-85 Goblin, designed to be carried in the belly of a B-36; the mixed-power, inverse taper-wing Republic XF-91; and the fast, capable, humpbacked North American F-107.
Two other revolutions in aircraft design, both quite as important as the development of the jet engine, were also going on, but their effects have somehow been generally overlooked because they were so much slower in coming to maturity.
First was the almost painful evolution of the effective air-to-air missile. Expectations had been high for the rocket-powered missile ever since the first LePrieur rockets were launched from Nieuport l7s during World War I. Somehow, missiles never reached their full potential until Vietnam, but even there their utility was vastly limited by the rules of engagement. Not until the most recent generation of missiles and fighter tactics did the concept of the missile-equipped jet fighter reach maturity.
The second revolution was in the multiple application of computers, not only to onboard use but also to the design of the aircraft and its systems. Airborne computers were not “user friendly” even through the McDonnell Douglas F-4s. Space, weight, and the crew inputs necessary for optimum use were all excessive by today’s standards. Perhaps even more important was the fact that only in the post F-4 generation of fighters, in the General Dynamics F-16 and the McDonnell Douglas F-15 and F-18, has there been sufficient use of computers in the basic design process.
As a result of these two revolutions, airframe design has for the first time entered the jet age and caught up with the jet engine in development potential. One can assume that computers of the future will enable simultaneous development of airframes, engines, and missiles that will avoid the timing mismatches of the past.
The evolution of fighter aircraft since World War II has been a fascinating process. From the straight wings of the P-80 through the sweptwings of the F-84F, past the swing wings of the F-111 and beyond the melded body and wings of the F-16, one can look to a future that might include such things as vertical takeoff, vectored maneuverability, and so on. The fighters of the future will undoubtedly be neither so numerous nor so diverse as the fighters of the past, but they will embody successive developments and will depend, as always, on capable crews that fly them for ultimate success.
Courtesy Courtesy Air & Space Power