by Tom Clancy
An illustration of the four primary forces on a powered aircraft: thrust, drag, lift, and weight.
Jack Ryan Enterprises, Ltd., by Laura Alpher
In the world of combat aircraft design, the engine's raw propulsion power is expressed as its thrust-to-weight ratio. This ratio compares the amount of thrust that the engines can produce to the weight of the aircraft. The higher the ratio, the more powerful the aircraft. For most combat aircraft, this ratio is around 0.7 to 0.9. However, really high-performance models, like the F-15 and -16, have thrust-to-weight ratios greater than 1.0 and can accelerate while going straight up!
LIFT
Lift is the force that pushes an object up due to the unbalanced movement of air past it. In an aircraft, the unbalance comes from the different curvature of the upper and lower surfaces of the wings (the upper surface has more curve than the lower), and the movement of air is provided as a consequence of the engine's thrust. When the moving air comes in contact with the leading edge of the wing, the air separates. Part of the flow passes over the top of the wing, and the remainder below. Given the shape of an aircraft's wing, the air stream on top has to travel a greater distance than the stream below. If both air streams are to arrive at the trailing edge at the same time, then the air stream above the wing must have a higher speed.
In aerodynamics, there is a simple, but neat, relationship between the speed of a gas and its pressure: The faster a gas travels, the lower its pressure and vice versa. This principle is called Bernoulli's Law, in honor of the 18th-century Italian scientist who first investigated it experimentally. So if the air stream above the wing is moving faster than the air stream below the wing, air pressure above the wing will be lower than below the wing. This difference causes the air below to push upward and "lift" the wing up. As the speed of an aircraft increases, the pressure difference grows and produces more lift. This wing's angle, called the angle of attack (AOA) of the aircraft, can have a significant effect on lift.
Initially, lift increases as AOA increases, but only up to a certain point. Beyond this point, the AOA is too large and the air flow over the wing stops. Without the air flow, there is no pressure difference and the wing no longer produces lift. When this situation occurs, the wing (and the aircraft) is said to have stalled. Now, a high AOA isn't the only thing that will cause an aircraft to stall. If an aircraft's speed gets too low, the air no longer moves fast enough over the wings to generate adequate lift, and again the aircraft will stall — and any pilot will tell you that stalls can be really bad for your health.
DRAG
Drag is the force that wants to slow the aircraft down. In essence, drag is friction; it resists the movement of the aircraft. This is a tough concept to grasp, because we can't see air. But while air may be invisible, it still has weight and inertia. We've all taken a walk on a windy day and felt the air pushing against us. That is drag. As an aircraft moves through the air, it pushes the air out of its way, and the air pushes back. At supersonic speeds, this air resistance can be very significant, as a huge amount of air is rapidly pushed out of the way and the friction generated can rapidly heat the aircraft's body to temperatures over 500deg F/260deg C.
There are two types of drag, parasitic and induced. Parasitic drag is wind resistance associated with the various bumps, lumps, and other structures on an aircraft. Anything that makes the aircraft's surface rough or uneven, like bombs, rivet heads, drop tanks, radio antennae, paint, and control surfaces (rudder, canards), increases the aircraft's wind resistance. Induced drag is more difficult to understand because it is directly linked to lift. In other words, if lift is being generated by the wings, so too is induced drag. Since drag is unavoidable, the best that can be done is to minimize it and understand the limits it places on the aircraft's performance. And the limits are significant. Drag degrades the aircraft's ability to accelerate and maneuver and increases fuel consumption, which affects combat range/radius. Therefore, a good understanding of drag is needed not only by aircraft designers, but by aviators as well.
WEIGHT
Weight is the result of gravitational attraction of the Earth, which pulls the mass of the aircraft toward the Earth's center. As such it is in direct opposition to lift. Of all the forces involved with flying, gravity is the most persistent. To some extent, we can control the other three. But gravity is beyond our control. In the end, it always wins (unless you're riding a spacecraft fast enough to escape the Earth's gravity entirely — about 25,000mph [40,000 kph]!). Thrust, lift, and drag are all accounted for in the design process of the aircraft. But when thrust or lift become insufficient to maintain the aircraft aloft, gravity will bring the plane down.
ENGINES
Once you understand the physics of flight, and you can build a sufficiently lightweight power plant, getting an aircraft into the air is a relatively simple matter. But operating high-performance aircraft in the hostile environment faced by today's military aircraft is quite another thing. These machines are anything but simple.
With complexity comes problems. The heart of a good aircraft is a good engine — the thing that makes it go! More fighter programs have been plagued by engine troubles than by any other source of grief. So, what's the big deal in making a good jet engine, you might ask? Well, try and imagine building a 3,000-to-4,000 lb./1,363.6-to-1,818 kg. machine that produces over seven times its own weight in thrust and is made with tolerances tighter than the finest Swiss watch. It has to operate reliably for years, even when pilots under the stress of combat or the spur of competition push it beyond its design limits.
To give you a better picture of how exact these engines are made, look at a human hair. While it may look pretty thin to you, it would barely fit between many of the moving parts in a jet engine. That's what I mean by tight tolerances! Now, let's spin some of those parts at thousands of revolutions per minute and expose a few of them to temperatures so high that most metal alloys would melt instantly. One can now begin to appreciate the mechanical and thermal stresses that a jet engine must be designed to handle every time it runs. Should even one of the rapidly rotating compressor or turbine wheels fail under these stresses and come into contact with the stationary casing, the resulting fragments would shred the aircraft just as effectively as missile or cannon fire.
Since a combat aircraft's performance is so closely tied to its propulsion plant, the limits of engine technology are constantly being pushed by designers and manufacturers. Their goal is to design an engine that is lighter than its predecessors and competitors, but produces more thrust. To accomplish this, an engine designer almost always has to bet that a new emerging technology or two will work out as anticipated. Occasionally, this means taking some pretty big risks. Risks that usually turn into problems that get widely reported in the media. For example, engine-development problems in the mid-1950s almost wrecked major aircraft companies, when airframes like the McDonnell F-3H Demon and Vought F-5U Cutlass had to wait months — or even years — for their engines to be developed. So, just how far has jet engine performance come along in the past forty years? Let's take a quick look.
In the mid-1950s, the U.S. Air Force began operating the North American F-100 Super Sabre, nicknamed the "Hun." Powered by a single Pratt & Whitney J57-P-7 engine, an axial-flow turbojet generating up to 16,000 lb./ 7,272.7 kg. of thrust, and aided by the newly developed afterburner, it was the first supersonic fighter, achieving a top speed of Mach 1.25. With confidence growing in the axial-flow turbojet engine, new fighter designs quickly showed up, and in 1958 the first McDonnell F-4 Phantom II flew. In the world of combat aircraft, the F-4 is legendary. During the Vietnam War it proved to be a formidable fighter bomber, and it still serves in some air forces. Powered by two giant General Electric J79-GE-15 turbojet engines, each generating up to 17,900lb./8,136kg. of thrust, the Phantom, or the "Rhino" as it was affectionately called, could reach speeds up to Mach 2.2 at high altitudes.
To illustrate the axial-flow turbojet, consider the J79 engine and its
five major sections:
A schematic cutaway of a typical turbojet engine, such as the Pratt & Whitney J57.
Jack Ryan Enterprises, Ltd., by Laura Alpher
At the front of the J79 is the compressor section. Here, air is sucked into the engine and compacted in a series of seventeen axial compressor stages. Each stage is like a pinwheel with dozens of small turbine blades (they look like small curved fins) that push air through the engine, compressing it. The compressed air then passes into the combustor section, where it mixes with fuel and ignites. Combustion produces a mass of hot high-pressure gas that is packed with energy. The hot gas escapes through a nozzle onto the three turbine stages of the engine's hot section (so-called because this is where you find the highest temperatures). The stubby fan-like turbine blades are pushed by the hot gas as it strikes them. This causes the turbine wheel to spin at very high speed and with great power. The turbine wheel is connected by a shaft which spins the compressor stages which compact the air flow even further. The hot gas then escapes out the back of the turbojet and this flow pushes the aircraft through the air. When the afterburner (or augmentor) is used, additional fuel is sprayed directly into the exhaust gases in a final combustion chamber, or "burner can" as it is known. This provides a 50 % increase in the final thrust of the engine. An afterburner is required for a turbojet to reach supersonic speeds. Unfortunately, using an afterburner gobbles fuel at roughly three to four times the rate of non-afterburning "dry"-thrust settings. For example, using full afterburner in the F-4 Phantom II would drain its tanks dry in just under eight minutes. This thirst for fuel was the next problem the engine designers had to overcome.
The axial flow turbojet became the dominant aircraft propulsion plant in the late 1950s because it could sustain supersonic flight for as long as the aircraft's fuel supply held up. The term "axial" means along a straight line, which is how the air flows in these engines. Up until that time, centrifugal (circular) flow engines were the military engines of choice — they were actually more powerful than early axial flow turbojets. But centrifugal flow engines could not support supersonic speeds.
Instead of a multiple stage compressor, centrifugal flow engines used a single stage, pump-like impeller to compress the incoming air flow. This drastically limited the pressure (or compression) ratio of the early jet engines, and therefore the maximum amount of thrust they could produce. The comparison between the air pressure leaving the last compressor stage of a jet engine and the air pressure at the inlet of the compressor section is how the pressure ratio is defined. Because the pressure ratio is the key performance characteristic of any jet engine, the axial flow designs had more growth potential than other designs of the period. Therefore, the major reasons why axial flow engines replaced centrifugal flow designs was that they could achieve higher pressure ratios and could also accommodate an afterburner. Centrifugal flow simply could not move enough air through the engine to keep an afterburner lit. By the mid-1960s, it became apparent that turbojet engines had reached their practical limitations, especially at subsonic speeds. If combat aircraft were going to carry heavier payloads with greater range, then a new engine with greater takeoff thrust and better fuel economy would have to be designed. The engine that finally emerged from the design labs in the 1960s was called a high-bypass turbofan.
At first glance, a turbofan doesn't look all that much different from a turbojet. There are, in fact, many differences, the most obvious being the presence of the fan section and the bypass duct. The fan section is a large, low-pressure compressor which pushes part of the air flow into the main compressor. The rest of the air goes down a separate channel called the bypass duct. The ratio between the amount of air pushed down the bypass duct and the amount that goes into the compressor is called the bypass ratio. For high bypass turbofans, about 40 % to 60 % of the air is diverted down the bypass duct. But in some designs, the bypass ratio can go as high as 97 %.
A schematic cutaway of a typical turbofan engine, such as the Pratt & Whitney F-100.
Jack Ryan Enterprises, Ltd., by Laura Alpher
I know this doesn't appear to make a whole lot of sense. Don't you need more air, not less, to make a jet engine more powerful? In the case of turbofans, not so. More air is definitely not better. To repeat, pressure ratio is the key performance characteristic of a jet engine. Therefore the designers of the first turbofans put a lot of effort into increasing this pressure ratio. The result was the bypass concept.
If an engine has to compress a lot of air, then the pressure increase is distributed, or spread out, over a large volume. By reducing the amount of air flowing into the compressor, more work can be done on a smaller volume, which means a greater pressure increase. This is good. Then the designers increased the rotational speed of the compressor. With the compressor stages spinning around faster, more work is done on the air, and this again means a greater pressure increase. This is better. The bypass duct was relatively easy to incorporate into an engine design, but unfortunately, a faster spinning compressor proved to be far more difficult.
There were three major problems: 1. Getting more work out of the turbine so that it could drive the compressor at higher speeds. 2. Preventing the compressor blades from stalling when rotated at the higher speeds. 3. Reducing the weight of the compressor so that the centrifugal stresses would not exceed the mechanical strength of the alloys used in the compressor blades.
Each problem is a formidable technological challenge, but mastering all three took some serious engineering ingenuity.
Getting more work out of a turbine is basically a metallurgy problem: To produce the hotter gases needed to spin the turbine wheels faster, the engine must run hotter. Next, if the turbine's weight can be reduced, more useful work can be extracted from the hot gases. Both require a stronger, more heat-resistant metal alloy. But developing such an alloy is a difficult quest. In working with metals, you don't find high strength and high heat resistance in the same material. The solution was found not only in the particular alloy chosen for the turbine blades, but also in the manufacturing technique.
Traditionally, turbine blades have been constructed from nickel-based alloys. These are very resistant to high temperatures and have great mechanical strength. Unfortunately, even the best nickel-based alloys melt around 2,100deg to 2,200degF/1,148deg to 1,204degC. For turbojets like the J79, in which the combustion section exit temperature is only about 1,800degF/982degC, this is good enough; the temperature of the first stage turbine blades can be kept well below their melting point. But high bypass turbofans have combustion exit temperatures in the neighborhood of 2,500degF/1,371degC. Such heat turns the best nickel-based turbine blade into slag in a few seconds. Even before the blades reached their melting point, they would become pliable, like Silly Putty. Stretched by centrifugal forces, they would quickly come into contact with the stationary turbine case. Bad news.
Nickel-based alloys still remain the best material for turbine blades. So improvements in strength and heat resistance depend on the blade manufacturing process. The manufacturing technology that had the greatest effect on turbine blade performance was single-crystal casting.
Single-crystal casting is a process in which a molten turbine blade is carefully cooled so that the metallic structure of the blade forms a single crystal. Most metallic objects have a crystalline structure. For example, you can sometimes see the crystal boundaries on the zinc coating of new galvanized steel cans, or on old brass doorknobs etched by years of wear. When metal objects are cast, the crystals in the metal form randomly due to uneven cooling. Metal objects usually break or fracture along the boundaries of crystal structures. To melt a crystalline object, the heat energy must break down the bonds that hold the crystals together. The bigger the crystals the more energy it takes. If these crystalline boundaries can be eliminated entirely, a cast metal object can have very high strength and heat resistance, qualities highly desirable in a turbine blade.
The first step in forming a single crystal st
ructure is to precisely control the cooling process. In turbine blade manufacturing, this is done by very slowly withdrawing the mold from an induction furnace. This works like your microwave oven at home, only a lot hotter. Controlled cooling by itself, however, will not produce a single crystalline structure. For that you also need a "structural filter."
So the molten nickel alloy is poured into the turbine blade mold, which is mounted on a cold plate in an induction furnace. When the mold is filled, the mold/cold plate package is slowly retracted from the furnace. Immediately, multiple crystal structures begin to form in a crystal "starter block" at the bottom of the mold. But because the cold plate is withdrawn vertically, the crystals can only grow toward the top of the starter block. At the top of the block is a very narrow passage that is shaped like a pig's curly tail. This pigtail coil is the structural filter, and it is only wide enough for one crystal structure to travel through. When the single crystal structure reaches the root of the turbine blade, it spreads out and solidifies as the blade mold is slowly withdrawn from the furnace. Once it is completely cooled, the turbine blade will be a single crystal of metal with no structural boundaries to weaken it. It now only requires final machining and polishing to make it ready for use.
A cutaway of the molding process for a modern turbofan engine fan blade.
Jack Ryan Enterprises, Ltd., by Laura Alpher
While single-crystal turbine blades are very strong and heat resistant, they would still melt if directly exposed to the hot gases from the combustion of a turbofan engine. To keep molten turbine wheels from dribbling out the back end of the engine, a blanket of cool air from the compressor is spread over the turbine blades. This is possible because complex air passages and air bleed holes can be cast directly into the turbine blades. These bleed holes form a protective film of air, which keeps the turbine blades from coming into direct contact with the exhaust gases, while simultaneously allowing the turbine blades to extract work from those gases. Earlier non-single-crystal turbine blade designs had very simple cooling passages and bleed holes that were machined out by lasers or electron beams, and didn't provide as much thermal protection.
