One Step Ahead, Part I

In 1853- half a century before the Wright brothers’ Flyer winged aloft at Kitty Hawk- an English inventor, Sir George Cayley, built a somewhat similar contraption. After a lifetime of study, Cayley, who was then 80, had acquired a remarkable understanding of both the principles and the practices of aerodynamics, and that odd-looking assembly of cloth and wood embodied everything he knew. Too old for such strenuous adventures himself, he persuaded his coachman to act as test pilot. Then with great excitement, he mounted a hill and launched coachman and craft across a Yorkshire valley.

The flight was a success, and Cayley and his studies were vindicated. But he lacked the one thing that would have transformed his glider into an aeroplane: what he called a “first mover” and what we would call an engine. “The whole problem,” he sadly concluded, “is confined within these limits: to make a surface support a given weight by the application of power to the resistance of the air.” As Cayley understood so well, the key to flight can be summed up in one word – power.

Cayley had his disciples, and one of them, William Henson, actually patented a machine that looked very much like a modern aeroplane. It had twin propellers, flight controls, and fixed wings braced by strong spars. But though Henson experimented with light steam engines, he too could not come up with a satisfactory first mover. If he had, given his other knowledge, the age of air might have been advanced by 50 years: Flying might have become a Victorian sport, and instead of having cavalry for scouts, Lee and Grant might have had airplanes. There might even have been dogfights over Bull Run and Gettysburg. History, however, cannot be rushed. Neither Cayley nor Henson, nor anyone else, could fly until man’s ability to build lightweight but powerful engines had caught up with his knowledge of aerodynamics.

Engine and plane came together on December 17, 1903, and at least one historian claims that the hero of Kitty Hawk was not Orville or Wilbur Wright, but their unsung assistant, Charles Taylor. Just as Henson could not put the cumbersome steam engines of his day into his delicate airframe, so too the Wrights could not use the heavy motors that had been developed for the infant automobile. It was Taylor who was able to turn the Wrights’ engine design into a working lightweight gasoline engine: 152 pounds generating 12 horsepower, or 12. 7 pounds per horse power. That ratio – as important as the thrust-to-weight ratio is in today’s jets – rapidly improved, and by 1909 a French engine had cut the equation to 3. 3 pounds per horsepower. But so far as pilots are concerned, there is never too much power. That same year, the first man to fly the English Channel, Louis Bleriot, climbed out of his machine, saying, “More power! More power!” Like Cayley, he and the other pioneers realized that each time they left the ground, they were engaging in a never-ending contest between thrust and weight (plus drag, which is the natural resistance of the air to any object moving through it).

The contest soon became part of a much larger battle, World War I. A more powerful engine, combined with a well-designed airframe, meant the difference between victory and defeat in the skies. At first, speed was thought to be the single necessity. Gradually, the experience made clear that, as important as speed was, maneuverability was equally vital. Indeed, so long as the difference was not overwhelming, a somewhat slower, but more maneuverable plane usually had the advantage over a faster but less nimble opponent. The best fighter of the war, the German Fokker D VII, was, in fact, slower than its rival, the French SPAD XIII. But when the D VII became operational in 1918, it shot down virtually everything else in the sky. It gave the Allies such a fright that they demanded the surrender of all D Vlls in the Treaty of Versailles – the only aircraft they bothered to specifically mention.

Many factors go into making a plane maneuverable, but there are two that are crucial. The first is an aerodynamic concept called low wing loading, and it can be expressed by a simple equation: the weight of the plane divided by the area of the lifting surface. Expressed simply, that means that larger wings tend to make a plane more maneuverable; smaller wings tend to make it less so. But since large wings also offer more resistance to the air, thereby slowing the plane down, designers have often had to choose between maneuverability or speed. The best fighters, of course – like the Fokker D VII of World War I and the F-15 and F-16 of today – have struck a fine balance.

The second factor in maneuverability is an engine that has power in excess, an engine with enough extra push for the quick loops and turns of close combat. Drag increases in proportion to the tightness of a turn, and the only thing that can overcome it is power – over and above that which is necessary for level flight Everything else being equal, the plane with the more powerful engine will always have a vital advantage in any encounter. A strong engine inside an airframe that emphasizes low wing loading is what fighter pilots hope for when they train to fight the enemy.

It was just such a combination that made the Spitfire such a formidable weapon during the Battle of Britain. Its unique elliptical wings, which gave it the graceful look of a gull in flight, made it easy to turn, and its Merlin engine gave it sufficient power to make its turns with little loss of speed. The Merlin, indeed, was perhaps the most advanced engine of the time, far superior to any operational engine possessed by the Germans. The Merlin, manufactured by Rolls-Royce, brought together 20 years of research into fuels, design, and metallurgy. By extending the range of alloys, metallurgists produced forgings, castings, and parts that were stronger and lighter at the same time. The Merlin’s new duralumin crankcase was almost twice as strong as the aluminium alloy previously used, and it weighed only one-tenth as much.

The equality of airframe and engine finds no better example than the American P-51, a top piston-powered fighter of World War II. The airframe design was a success from the beginning, but the Allison engine with which it was fitted limited its performance, particularly at high altitudes. As an experiment, the British Merlin was substituted for the Allison. It so improved the Mustang’s capability that the Army ordered over 13,000 Merlin-equipped craft. Soon, long-range P-51s were escorting bombers over Europe, enabling the Allies, for the first time, to strike deep inside Germany without suffering prohibitive losses. Goering is said to have remarked that he knew the war was lost when he first saw Mustangs guarding B-17s over Berlin.

Toward the end of World War II, however, even the P-51 found a nemesis, as the precarious balance between maneuverability and speed was upset by an entirely new kind of fighter – the jet. The plane was the Me 262, and inside it the Germans had placed an engine more powerful than any yet seen. The difference was that this new engine had no propellers in front of it. The plane was driven by the air itself. Cruising at 500 miles an hour, the Me 262 had one-fourth again as much speed as the P-51. Luckily for the Allies, the new fighter, like the Fokker D VII of World War I, came too late to alter the course of the air war.

The Americans and the British were also experimenting with new technology, and the introduction of jet power seemed to spell the demise of the traditional fighter, a line that extends from the Sopwiths, Spads, and Fokkers to the Spitfires, Mustangs, and Messerschmitts. The new planes flew too fast, military planners reasoned, for the quick turns and intricate maneuvering that are required in the dogfights fighters had always been built for. The Soviet Union’s mastery of nuclear weaponry also affected military thinking after World War II. Planners demanded, on the one hand, fast interceptors that could meet attacking bombers and defend the North American continent, which, for the first time in history, had become vulnerable from the air. On the other hand, they wanted fighter-bombers that could supplement American bombers and carry nuclear bombs themselves. Fighters whose primary mission was to engage other fighters were deemed superfluous in the age of jets and nuclear weapons.

American defence was – and still is – based on a policy of nuclear deterrence. An attack from either side, postwar strategists argued, would guarantee an immediate response and the certain destruction of the attacker. War, therefore, would be suicidal for both and unthinkable for both. During the Cold War, the United States built air bases in countries on the fringes of the Soviet Union, and whole generations of heavy bombers were developed to take off from their runways. The 8-50, a more modern version of the 8-29, and the giant B-36 were the principal aircraft charged with this strategic mission after World War II. Later came the 8-47 and 8-52. The 8-52 fleet, which still undergoes continual modernization, will be a mainstay of American defence in the 1980s. Until the mid-60s, fighters were expected only to protect bombers or to attack them. Air-to-air combat – the dogfight – belonged to an earlier age. Or so it was thought.

The F-86 Sabre, which first flew in 1947, was one of those planes that was not expected to see a dogfight. The war in Korea, however, did not adhere to the rules, and despite all predictions, the F-86 was forced into combat with the Soviet-built MiG-15. The Sabre proved more than equal to the task. The craft not only had enough speed and altitude to meet enemy bombers; its design was also exceptionally maneuverable, with low wing loading (or relatively large wings in proportion to its weight). In the early months of the war, the MiG-15 had gained a tactical advantage over the slower P-51s, F-80s, and F-84s. The introduction of the F-86, in November 1950, reversed that equation. By the end of the war, the Sabre had racked up an exchange ratio of better than 8-to-1 over the MiG, and it was the mount of 39 of the conflict’s 40 Allied jet aces. The F-100 Super Sabre was designed during the Korean War as a supersonic follow-up to the F-86, but its primary mission was also as an interceptor, rather than as a dogfighter. As the Tactical Air Command’s responsibility expanded to include nuclear deterrence, however, later C and D versions of the F-100 were modified to include room for nuclear bombs. The F-105 Thunderchief, the F-100’s successor, was the first American plane designed from the beginning as a nuclear-bombing fighter aircraft.

Despite the experience of the Korean War, the Navy was also convinced that supersonic speeds and air-to-air missiles had largely ended the need for the dogfighter. In the 1950s, it developed a carrier-based interceptor capable of meeting and downing enemy bombers far from the fleet: the F-4 Phantom. Incorporating incredibly sophisticated avionics, the F-4 had such advanced radar that the pilot could attack the enemy without ever actually seeing him. This plane was basically a swift platform for radar-guided, air-to-air missiles, which would do the maneuvering that planes had done in preceding generations. Capable of high speed (Mach 2. 4) and flight at high altitude (60,000 feet), the Phantom captured a basketful of world records in its early years. The Air Force also ordered it in quantity.

The air war in Vietnam also defied prediction. Dogfighting once again broke out, and pilots found themselves locked in fierce combat over Hanoi and Haiphong. Good as it was for its assigned mission as an interceptor, however, the F-4 was not as good in air-to-air combat. To give it its high speed, its builders had decided on a high wing loading configuration that would reduce drag. But that speed was bought at a price – maneuverability. Unfortunately, The F-4’s chief opponents, the subsonic MiG-17 and the supersonic MiG-21 were both highly agile. The kill-loss ratio for American aircraft (which also included the Air Force’s F-104 and F-105 and the Navy’s F-8) declined from 3-to-1 at the beginning of the war to less than 1-to-1 at the end. Ironically, the F-8 Crusader, which had been designed with low-wing loading in an earlier era, was credited with 56% of all kills by Navy fighters.

To the frustration of American fliers, none of the F-4’s advanced features proved of much help in close combat. Powered by the General Electric J79, it was faster than its Russian-built adversaries and it could fly higher. But in Vietnam, 85% of all engagements took place below 30,000 feet and at speeds less than Mach 1. 2. And in that area the Phantom, with its high wing loading, was at a disadvantage. The F-4’s radar-directed missiles were also of less use than expected. In crowded skies, they could not tell friend from foe, and on more than one occasion they shot down another friendly aircraft rather than the MiG they were intended for. Radar, too, had its limitations, and a pilot usually needed visual identification before firing weapons. Vietnam made clear that speed alone was not sufficient to ensure mastery of the air. A new generation of fighters was needed that would give Americans the command of the air they had had with the P-51 and F-86.

Actually, that need was felt in Washington even before the Vietnam conflict and military thinking had already shifted from the theories of the 1940s and 1950s. When President Kennedy took office in 1961, the policy of all-out nuclear response-or massive retaliation, as it was sometimes called-was modified in favour of one that would allow a more flexible response. The United States should be able to meet limited challenges, Kennedy said, with appropriately limited responses. For both the Air Force and the Navy that meant, among other things, a non-nuclear, maneuverable fighter that could prevail in dogfights with other fighters in the conventional warfare of a Korea or a Vietnam.

A New Generation

In the early 1960s, even before the dogfights over Vietnam, the Air Force had begun thinking of a new kind of jet fighter FX, or Fighter Experimental. Formal studies were launched in December 1965, and a few months later the plane had been tentatively outlined. The new craft was to be an interceptor as well as an air-to-air fighter. It was to weigh 60,000 pounds, a little more than the Phantom; it was to be propelled by two engines; and it was to have a variable-sweep wing, like that of the F-111. This early version of the FX was never to leave the drawing boards, however, and in the end, all that remained of the original concept was that the fighter should have two engines.

By the spring of 1967 there was already a new FX concept, with a third less weight. Then, that July, FX thinking was further altered by something that took place in Moscow. At the Domodedovo Airshow, the Soviets unveiled no fewer than twelve new planes, or new versions of old ones. One of the new ones was the MiG-25, a high-performance machine that Western intelligence soon dubbed “Foxbat. ” The FX study was urgently revised a month later.

Many minds worked on the projected plane, but two men had a key role: Air Force Major John Boyd, and Pierre Sprey, a staff member working for the assistant secretary of Defense for Systems Analysis. A former fighter instructor, Boyd had spent a lot of time thinking about what makes a plane maneuverable. He knew, as did other experts, the basic factors of low wing loading and power. But he did not know and neither did anyone else up to that time-how to translate them into quantifiable measures. The notion of what is now called energy maneuverability first came to Boyd while he was discussing air-to-air combat with a group of thermo-dynamics students

“I started thinking about how I could talk to them in their language. So I said: Let’s pretend that maneuverability is an energy problem. When you go to maneuver an airplane you need energy. On the other hand, you lose energy either in gaining altitude, or airspeed both. Normally you lose energy in turning. What happens; that your drag exceeds your thrust, and at that point, you have a negative energy rate. That negative rate has to come out of altitude, airspeed, or out of a combination of the two. You reach a point, even with your military afterburner, at which drag is greater than thrust. That means you have a negative vector. As a result, you get a net drag rather than a net thrust. Then, of course, you multiply net drag by velocity, and the answer tells you how much energy you’re going to have to pump up.”

Even as he was speaking, Boyd realized that he had discovered a new way of looking a fighter’s performance. For several days he continued to mull over his ideas and scratch down different equations. Yet all the while he felt that he was probably doing little more than wasting his time: The theory that was forming inside his mind was so simple, he told himself, that somebody else must already have put it into writing

“I started laying it out and looking at it, comparing designs. Of course the emphasis shifted. Rather than thinking about maneuvers, I began thinking about the existing aerodynamics of structures, engines, and combinations of the two. I thought: How can we put them in better combinations to get a better airplane, using energy as a criterion? In other words, you just turn the problem of maneuverability around and look at it from a different viewpoint. And the result of that was obvious right off the bat: The right thrust-to-weight ratio could give you an important edge on your adversary.”

Boyd’s theory of energy maneuverability was, just as he thought, startling in its simplicity. What it meant was that the performance of an airplane could be measured at various altitudes, airspeeds and maneuvering situations in terms of what aeronautical engineers call “specific excess power,” or Ps for short. The simplicity of the theory did not make it any less valuable, however. In one respect only was Boyd wrong: No one else had yet thought of it.

Using Boyd’s equation of energy maneuverability, The FX study went forward. As the craft began to take shape, the Air Force dignified it with a name of its own F-15 Eagle – and gave its designers a more precise definition of its mission:

“Develop and acquire an advanced tactical fighter aircraft capable of gaining and maintaining air superiority, through air to-air combat with non-nuclear weapons against potential enemy fighters in the post-1975 time period.”

What was needed to fulfil that mission, Air Force analysts finally decided, was a plane that had low wing loading, like so many of the fighters of the past, together with a high thrust-to-weight ratio, like that of the Phantom. (Thrust-to-weight ratio is the thrust of the engine, or engines, divided by the weight of the combat-loaded airplane. But to achieve that ratio a radically new engine was required, one with an extraordinarily high thrust-to weight ratio of its own (the thrust of the engine divided by its own weight). Even before airframe plans were completed, the Air Force announced that it wanted a propulsion system with a thrust-to-weight ratio of 8:1. If achieved, that would mean that the new system would produce 25% more thrust per pound of engine weight than the best previous engine, the Pratt & Whitney TF30, which was already powering the F-111 and which was soon to be powering the F-14. It meant, moreover, that with two of the new engines, the F-15 would be the first plane in full operation to have more thrust than weight -over 5,000 pounds more. McDonnell Douglas was chosen to build the airframe after stiff competition, and the first Eagle flew from Edwards Air Force Base on July 27, 1972, Though its maximum speed was Mach 2. 5 at over 40,000 feet, it performed best at Mach . 9 and 30,000 feet, the top speeds and altitudes of almost all air-to-air combat in Vietnam.

Concerned by shrinking budgets and rising costs, the Department of Defense also began considering a less expensive, less sophisticated companion for the F-15. The projected plane was also to have low wing loading and a high thrust-to-weight ratio, but its light weight, about half that of the F-15, would enable it to achieve the necessary thrust with only one of the powerful new engines being built for the F-15. By reducing the top speed requirement of the F-15, as well as lowering its radar and structural specifications, most of the F-15’s other measurements of performance could be maintained and in some cases even improved by the smaller craft. After a fly-off between two prototypes, the General Dynamics YF-16 on January 13, 1975. Six months later a consortium of four NATO countries – Holland, Belgium, Denmark and Norway – chose it as their common fighter, with each country to share in production of both airframe and engine. The NATO consortium was favourably influenced by the F-16’s relatively low cost. It is the first plane ever purchased by the U. S. Air Force that costs less than its predecessor.