Sunday, June 14, 2009
July 7, 2008
B.O.A.C de Havilland Comet
A-12 Avenger II
Royal Aircraft B.E.2
Boeing XB 15
Friday, June 12, 2009
When we think about what may appear in future aircraft designs, we might look at recent history. The look may be frightening. From first appearances, anyway, nothing has happened in the last 40 years!
There are many causes of this apparent stagnation. The first is the enormous economic risk involved. Along with the investment risk, there is a liability risk which is of especially great concern to U.S. manufacturers of small aircraft. One might also argue that the commercial aircraft manufacturers are not doing too badly, so why argue with success and do something new? These issues are discussed in the previous section on the origins of aircraft.Because of the development of new technologies or processes, or because new roles and missions appear for aircraft, we expect that aircraft will indeed change. Most new aircraft will change in evolutionary ways, but more revolutionary ideas are possible too.This section will discuss several aspects of future aircraft including the following:
1.Improving the modern airplane
3.New roles and requirements
Improving the Modern Airplane
Breakthroughs in many fields have provided evolutionary improvements in performance. Although the aircraft configuration looks similar, reductions in cost by nearly a factor of 3 since the 707 have been achieved through improvements in aerodynamics, structures and materials, control systems, and (primarily) propulsion technology. Some of these areas are described in the following sections.
Active flight control can be used in many ways, ranging from the relatively simple angle of attack limiting found on airplanes such as the Boeing 727, to maneuver and gust load control investigated early with L-1011 aircraft, to more recent applications on the Airbus and 777 aircraft for stability augmentation.
Reduced structural loads permit larger spans for a given structural weight and thus a lower induced drag. As we will see, a 10% reduction in maneuver bending load can be translated into a 3% span increase without increasing wing weight. This produces about a 6% reduction in induced drag.Reduced stability requirements permit smaller tail surfaces or reduced trim loads which often provide both drag and weight reductions.
Such systems may also enable new configuration concepts, although even when applied to conventional designs, improvements in performance are achievable. In addition to performance advantages the use of these systems may be suggested for reasons of reliability, improved safety or ride quality, and reduced pilot workload, although some of the advantages are arguable.
New Airfoil Concepts
Airfoil design has improved dramatically in the past 40 years, from the transonic "peaky" sections used on aircraft in the 60's and 70's to the more aggressive supercritical sections used on today's aircraft.
Continuing progress in airfoil design is likely in the next few years, due in part to advances in viscous computational capabilities. One example of an emerging area in airfoil design is the constructive use of separation. The examples below show the divergent trailing edge section developed for the MD-11 and a cross-section of the Aerobie, a flying ring toy that uses this unusual section to enhance the ring's stability.
Flow Near Trailing Edge of DTE Airfoil and Aerobie Cross-Section
Subtle manipulation of aircraft aerodynamics, principally the wing and fuselage boundary layers, can be used to increase performance and provide control. From laminar flow control, which seeks to reduce drag by maintaining extensive runs of laminar flow, to vortex flow control (through blowing or small vortex generators), and more recent concepts using MEMS devices or synthetic jets, the concept of controlling aerodynamic flows by making small changes in the right way is a major area of aerodynamic research. Although some of the more unusual concepts (including active control of turbulence) are far from practical realization, vortex control and hybrid laminar flow control are more likely possibilities.
Structural materials and design concepts are evolving rapidly. Despite the conservative approach taken by commercial airlines, composite materials are finally finding their way into a larger fraction of the aircraft structure. At the moment composite materials are used in empennage primary structure on commercial transports and on the small ATR-72 outer wing boxes, but it is expected that in the next 10-20 years the airlines and the FAA will be more ready to adopt this technology.
New materials and processes are critical for high speed aircraft, UAV's, and military aircraft, but even for subsonic applications concepts such as stitched resin film infusion (RFI) are beginning to make cost-competitive composite applications more believable.
Propulsion is the area in which most evolutionary progress has been made in the last few decades and which will continue to improve the economics of aircraft. Very high efficiency, unbelievably large turbines are continuing to evolve, while low cost small turbine engines may well revolutionize small aircraft design in the next 20 years. Interest in very clean, low noise engines is growing for aircraft ranging from commuters and regional jets to supersonic transports.
In addition to advances in disciplinary technologies, improved methods for integrating discipline-based design into a better system are being developed. The field of multidisciplinary optimization permits detailed analyses and design methods in several disciplines to be combined to best advantage for the system as a whole.
The figure here shows the problem with sequential optimization of a design in individual disciplines. If the aerodynamics group assumes a certain structural design and optimizes the design with respect to aerodynamic design variables (corresponding to horizontal motion in the conceptual plot shown on the right), then the structures group finds the best design (in the vertical degree of freedom), and this process is repeated, we arrive at a converged solution, but one that is not the best solution. Conventional trade studies in 1 or 2 or several parameters are fine, but when hundreds or thousands of design degrees of freedom are available, the use of more formal optimization methods are necessary.
Although a specific technology may provide a certain drag savings, the advantages may be amplified by exploiting these savings in a re-optimized design. The figure to the right shows how an aircraft was redesigned to incorporate active control technologies. While the reduced static margin provides small performance gains, the re-designed aircraft provides many times that advantage. Some typical estimates for fuel savings associated with "advanced" technologies are given below. Note that these are sometimes optimistic, and cannot be simply added together.
1.Active Control .............10%
3.Laminar Flow ..............10%
4.Improved Wing ...........10%
New Configuration Concepts
Apart from evolutionary improvements in conventional aircraft, revolutionary changes are possible when the "rules" are changed. This is possible when the configuration concept iteself is changed and when new roles or requirements are introduced.
The following images give some idea of the range of concepts that have been studied over the past few years, some of which are currently being pursued by NASA and industry.
The BWB design is intended to improve airplane efficiency through a major change in the airframe configuration. The thick centerbody accommodates passengers and cargo without the extra wetted area and weight of a fuselage. Orginally designed as a very large aircraft with as many as 800 passengers, versions of the BWB has been designed with as few as 250 passengers
The joined wing design was developed principally by Dr. Julian Wolkovitch in the 1980's as an efficient structural arrangement in which the horizontal tail was used as a sturcural support for the main wing as well as a stabilizing surface. It is currently being considered for application to high altitiude long endurance UAVs.
Oblique Flying Wing
One of the most unusual concepts for passenger flight is the oblique wing, studied by Robert T. Jones at NASA from 1945 through the 1990s. Theoretical considerations suggest that the concept is well suited to low drag supersonic flight, while providing a structurally efficient means of achieving variable geometry.
New Roles and Requirements
In addition to new configuration ideas, new roles and requirements for aircrafrt may lead to new aircraft concepts. Some of these are summarized below.
Pacific Rim Travel
As global commerce continues to increase, the need for passenger and cargo transportation grows as well. Many have speculated that growth in pacific rim travel may be the impetus for high speed aircraft development. The figure above suggests how the time required for flight from Los Angeles to Tokyo varies with cruise Mach number. (The somewhat facetious Mach 8 aircraft requires extra time to cool off before passengers can deplane.)
Supersonic transportation (Boeing High Speed Civil Transport Concept)
Vehicles designed for missions other than carrying passengers include military aircraft with new constraints on radar detection (low observables), very high altitude aircraft, such as the Helios solar powered aircraft intended for atmospheric science and earth observation studies, and vehicles such as the Proteus, designed as a communications platform.
Low Observables (B2 Bomber)
Autonomous Air Vehicles (Pathfinder: a prototype for Helios solar UAV)
Halo Autonomous Air Vehicle for Communications Services (an AeroSat)
Finally a new class of air vehicles intended to provide lower cost access to space is under study. The near-term future of such designs depends on the economic health of the commercial space enterprise and it presently appears that these concepts are not likely to be seen soon.
Access to Space
1. Improved understanding and analysis capabilities permit continued improvement in aircraft designs
2. Exploiting new technologies can change the rules of thegame,permitting very different solution
3. New objectives and constraints may require unconventional configurations
4. Future progress requires unprecedented communication among aircraft designers, scientists, and computational specialists
By: Thierry Dubois
Thursday, June 11, 2009
Wednesday, June 10, 2009
Tuesday, June 9, 2009
Various complex motions and forces and certain visual scenes encountered in flight can create illusions of motion and position. Spatial disorientation from these illusions can be prevented only by visual reference to reliable, fixed points on the ground or to flight instruments. Spatial disorientation is mainly associated with flight in instrument conditions, but they can happen in visual flying
ILLUSIONS IN FLIGHT
Runway Width Illusion:
A narrower-than-usual runway can create the illusion that the aircraft is at a higher altitude than it actually is. If you don't recognize this illusion, you may have a tendency to fly a low approach, risking a short landing. A wider-than-usual runway can have the opposite effect, with the risk of leveling out high or overshooting the runway. What is usual? Usual is what you are used to, so when you make your first landing at a new airfield, think about this illusion and deal with it.
Runway and Terrain Slopes:
IllusionAn up sloping runway, up sloping terrain, or both can create the illusion that the aircraft is at a higher altitude than it actually is. A low approach can result if the pilot allows this illusion to convince him/her that the aircraft is high. A down-sloping runway will have the opposite effect causing the pilot to flare or round out too high.
Featureless Terrain Illusion:
A pilot landing in a featureless area such as a dry lake bed or desert, will tend to fly a lower than normal approach, thinking he/she is to high.
Rain on the canopy will give the illusion of greater height. Haze will give the illusion of being a greater distance from the runway.
VISION IN FLIGHT
Light from the low sun levels, reflecting off the canopy or other surfaces can create a hazard when it obstructs other aircraft from you view. This is the perfect excuse to convince your significant other that a new pair of good sunglasses are needed. Sunglasses for protection from glare should absorb at least 85 percent of visible light (15 percent transmittance) and all colors equally (neutral transmittance), with negligible image distortion from refractive and prismatic errors. In other words, you need the good ones.
Scanning for Other Aircraft:
Scanning is the key factor in collision avoidance. In order to scan, your eyes have to be looking out of the cockpit. We have more and more sophisticated equipment in our aircraft and each piece of electronic wizardry demands a certain amount of attention. It doesn't make a bit of difference if you are perfectly on course when you're on a collision course with a twin engine airplane.While the eyes can observe an approximate 200 degree arc of the horizon at one glance, only a very small center area called the fovea, in the rear of the eye, has the ability to send a clear image to the brain. The rest of the area will be of less detail, in fact, an aircraft at a distance of 7 miles which appears in sharp focus within the center of vision would have to be as close as 7/10 of a mile in order to be recognized. Because of this physical limitation, one must scan a series of regularly spaced horizontal movements that bring successive areas of the sky into the central visual field. Break the scanning area up into 10-degree segments and stop and observe a few seconds at each area. When you stop to observe the area, look out and then back toward the aircraft. A successful scanning pattern is a very personal thing, and with practice, it will become a positive habit that will keep you safe as well as increase your flying enjoyment.
Empty-feild Myopia is a condition that occurs when flying on hazy days. The haze provides nothing specific to focus on and this causes the eye to focus 10 to 30 feet in front of your aircraft. So while you are looking, you are not seeing. An effective scan will help you avoid Empty-field Myopia. Look out in front of the aircraft and focus on something on the ground, then raise your eyes up to and above the horizon. This will force your eyes to focus beyond the 10-to-30 foot distance.
JUDGMENT ASPECTS OF COLLISION AVOIDANCE
Use the horizon as a reference point. If an approaching aircraft is above the horizon it is probably above you, if it is below the horizon it should be below you.Taking Appropriate ActionBe familiar with the rules on right of way.
Collision Course Targets:
Any aircraft that appears to have no relative motion and stays in one scan quadrant is likely to be on a collision course. If the target shows no lateral or vertical motion, but increases in size, take immediate evasive action.
Recognize High Hazard:
AreasAirways, especially near navigation radio stations like a Very High Frequency Omnirange Station (VOR), and instrument approach courses at airports are areas to avoid. Knowing the locations of instrument approach courses at your local flying field and avoiding them is a must. Having a radio to monitor the common traffic advisory frequency (CTAF) is a must for aircraft operations and especially on an airport with different types of traffic. Take time to talk with the locals when first flying at a new field. Get the lay of the land and any particular traffic procedures for that field.
Keep it clean. This is often overlooked; however, a dirty canopy or wind screen can greatly reduce a pilot's ability to avoid other aircraft.
Be aware that smoke, haze, dust, rain and flying into the sun can greatly reduce your ability to avoid other aircraft.
Visual Obstructions in the Cockpit:
Become aware of blind spots in different aircraft. Always move your head and look around potential blind spots. You may even need to drop or raise a wing or maneuver the aircraft to clear your flight path.
Hemoglobin in red blood cells transports oxygen to body tissue. Anything that adheres to hemoglobin takes up space on the cell and limits the amount of oxygen that gets to body tissue. Smoking deposits carbon monoxide on the hemoglobin and literally takes up space that should be carrying oxygen. If you smoke, realize your susceptibility to Hypoxia is heightened. A leaky exhaust system can certainly raise carbon monoxide levels in the cockpit to dangerous levels.
As the glider cockpit pressure decreases during ascent, the expanding air in the middle ear pushes the Eustachian tube open, allowing the air to escape down the nasal passages, equalizing the middle ear chamber pressure with the outside pressure. However, on descent the pilot must periodically open the Eustachian tube to equalize pressure. This can be done by swallowing, yawning, tensing muscles in the throat or by doing the Valsalva Maneuver. The Valsalva Maneuver is done by closing your month, pinching your nose shut and attempting to blow through your nostrils. An ear block can produce severe pain and loss of hearing that can last for several hours. If an ear block does not clear shortly after landing, a physician should be consulted.
A sinus block can produce the same excruciating pain as an ear block. Again, don't fly with a cold, sinusitis, or a nasal allergic condition.
Decompression Sickness After Scuba Diving:
Pilots should allow their body to rid itself of excess nitrogen absorbed during diving. The recommended waiting time before going to flight altitudes of up to 8,000 feet is at least 12 hours after diving which has not required controlled ascent, and at least 24 hours after diving which has required controlled ascent. The waiting time for flights above 8,000 feet is 24 hours. Flying too soon after scuba diving could allow nitrogen gas bubbles to form around joints and muscles causing severe pain.
Hyperventilation in Flight:
It is an abnormal increase in the volume of air breathed in and out of the lungs. This can occur during a stressful situation. During hyperventilation, the pilot blows off excessive carbon dioxide from his body. This can cause lightheadedness, suffocation, drowsiness, and tingling in the extremities. Incapacitation can result from disorientation and painful muscle spasms. A pilot can stop hyperventilation by breathing into a paper bag or simply recognizing the symptoms and making a conscious effort to slow down his/her breathing. Do you carry a paper bag, I don't. But singing works well also. Singing forces you to breath normally. It might not be audibly pleasant for your passengers, but you passing out might make them a little more uncomfortable.
To prevent hypoxia above 10,000 feet during the day and 5,000 feet at night, pilots are encouraged to use supplemental oxygen.According to Federal Aviation Regulations (FARs) require that the flight crew use supplemental oxygen after 30 minutes of exposure to pressure altitudes between 12,500 and 14,000 feet. Above 15,000 feet, crew and passengers have to use supplemental oxygen.
Monday, June 8, 2009
Air entering the lungs at sea level enters at a pressure of 760 mm Hg. This results in a partial pressure of oxygen in sea level air of about 160 mm Hg. (that's about 21% of 760 mm). The blood flowing through the lungs isn't exposed to atmospheric air though. Blood comes in contact with alveolar air -- the air mixture contained in the tiny air sacks of the lungs -- which is only 14% oxygen. (This is because of the addition of water vapor to the air you breath in plus the carbon dioxide that has diffused from the blood returning from the tissues.) The partial pressure of oxygen in alveolar air is about 14% of 760 mm Hg or 106.4 mm Hg. Carbon dioxide, which is 5.5% of alveolar air (as contrasted to less than 1% in the atmosphere) exerts a pressure of 41.8 mm Hg.
In a normal, healthy individual, sea level pressure is sufficient to cause the blood leaving the lungs to be almost totally (97%) saturated with oxygen. At 10,000 feet the saturation has dropped to almost 90% -- still sufficient for nearly all usual life functions. An oxygen saturation of 93% is considered by medical folks to be the low limit of normal functioning. On top of Pike's Peak (about 14,500 feet and 438 mm Hg atmospheric pressure) the oxygen saturation has dropped to about 80%. Many people, if left in this rarefied air for some period, will develop mountain or altitude sickness: vertigo, nausea, weakness, hyperpnea (increased breathing), incoordination, slowed thinking, dimmed vision and increased heart rate. At 25,000 feet the oxygen saturation is only 55% and consciousness is lost. (Note that the partial pressure of oxygen in alveolar air at 25,000 feet is 14% of 281.8 mm Hg or 39.5 mm Hg -- slightly less than that normally found in venous blood returning from the tissues. Which way do you think the oxygen will diffuse at altitudes above 25,000 feet?)
The effects of hypoxia upon flying skills and the symptoms of its onset are the same no matter what the cause of the hypoxia. It is useful, however, to look at some varying causes of this condition so we can be alert to its possible onset when of one or more of these factors is present.
Hypoxic hypoxia occurs because there is a smaller and smaller pressure differential between the pressure of oxygen in the inspired air in the lungs and the pressure of the oxygen in the blood and tissues. Remember that the combining power of hemoglobin and oxygen is influenced by this pressure differential. The greater the differential, the more efficient the hemoglobin becomes. As this pressure differential lessens, it becomes harder and harder for the hemoglobin to pick up and transport the oxygen.
Any condition that would cause a reduction in the number of healthy, functioning red blood cells (anemia or reduced production of red blood cells, blood loss, deformed blood cells, disease, etc.) will impair the blood's ability to supply the tissues with oxygen. Remember the old advertisements warning about "iron poor blood?" Iron is the functional part of the hemoglobin molecule and it is the iron which renders the hemoglobin absolutely indispensable for life. In addition to a reduction in the number of red blood cells available, anything that would interfere with the ability of hemoglobin to transport oxygen or anything that would displace the oxygen that is bound to the hemoglobin will affect the oxygen available to the cells.
The most common impairment to oxygen transport by the hemoglobin is carbon monoxide. Carbon monoxide combines with hemoglobin 200-300 times more readily than does oxygen and once bound is extremely hard to eliminate. Smokers will find that the carbon monoxide bound to their hemoglobin will lower their altitude for onset of hypoxic symptoms by 2000-3000 feet. This effect is not limited to smokers, however. Anyone exposed to a smoky atmosphere will suffer somewhat. (Remember this next time you volunteer to go along as a designated driver for a group of drinkers. Just sitting in that smoky bar for several hours is going to affect your performance the next day, even without alcohol and fatigue!) Other chemicals, among them sulfa drugs and nitrites (found in food preservatives) can have an adverse effect on the ability of hemoglobin to combine with and transport oxygen.
Now, we are all aware of the hazards associated with alcohol and flying and I'm not suggesting that any true professional would knowingly violate these rules and guidelines. Many pilots, however, may be impaired by alcohol at the cellular level and not be aware of the problem -- or its cause. Remember the iron poor blood mentioned earlier? Be cautious of the "tonics" or "elixirs" offered as remedies. Carefully read the labels on any over-the-counter medications or nutritional supplements you propose to ingest. Although many more manufacturers are eliminating or reducing the alcohol content of the liquid medications, you may be surprised at the percentage of alcohol some still contain. One popular vitamin supplement for "iron poor blood" contains 12% alcohol!
A simplified view of how a jet engine works.
Before World War II, in 1939, jet engines existed only as laboratory items for test. But at the end of the war, in 1945, it was clear that the future of aviation lay with jets. The new engines gave great power and thrust, but were compact in size. They also were simple in their overall layout.
A jet engine, down to the present day, pulls in air by using a compressor. It looks like a short length of an ear of corn, but instead of corn kernels, the compressor is studded with numerous small blades. The compressor rotates rapidly, compressing the air.
The compressed air flows into a combustor. Here fuel is injected, mixed with this air, and burned. This heats the air to a high temperature. The hot, high-pressure air then passes through a turbine, forcing it to spin rapidly. The turbine draws power from this hot airflow. A long shaft connects the turbine and compressor; the spinning turbine uses its power to turn the compressor.
The jet-engine principle was known early in the twentieth century. However, jet engines work well only at speeds of at least several hundred miles per hour. Racing planes were the first to reach such speeds, with a British seaplane setting a record of 407 miles per hour (655 kilometres per hour) in 1931 and an Italian aircraft raising this record to 440 miles per hour (708 kilometres per hour) in 1934.
A young German physicist, Hans von Ohain, was in the forefront. He started by working on his own at Gottingen University. He then went to work for Ernst Heinkel, a plane builder who had a strong interest in advanced engines. Together they crafted the world's first jet plane, the experimental Heinkel He 178, which first flew on August 27, 1939.
Building on this work, the German engine designer Anselm Franz developed an engine suitable for use in a jet fighter. This airplane, the Me 262, was built by the firm of Messerschmitt. It was the only jet fighter to fly in combat during World War II. But the Me 262 spent most of its time on the ground because it used too much fuel. It was a sitting duck for Allied attacks.
Two Jumo 004 engines powered the Me 262. This was the first jet fighter to fly in combat and probably broke the sound barrier first. Because the Germans had not secured a source of chromium, the blades would stretch after a few hours making engine life very short indeed.
In England, Frank Whittle had no knowledge of Ohain's ideas but invented a jet engine completely on his own. The British drew on his work and developed a successful engine for another early jet fighter—the Gloster Meteor. Britain used it for homeland defence but it did not see combat over Germany because it lacked high speed. The W.1 turbojet engine used to power the Gloster E28/39 aircraft. It was designed to produce a static thrust of 1,240 lbs at 17,750 rpm. This engine was also the basis of the design of the General Electric I-14 turbojet engine used to power the Bell XP-59A twin engine experimental fighter.
The British shared Whittle's technology with the United States, enabling the engine-builder General Electric (GE) to build jet engines for America's first jet fighter, the Bell XP-59. The aircraft company Lockheed then used a British engine in the initial version of its Lockheed P-80, America's first operational jet fighter, which entered service soon after the war's end. The British continued to develop new jet engines that used Whittle's designs, with Rolls-Royce initiating work on the Nene engine during 1944. Rolls sold Nenes to the Soviets, and a Soviet-built version of the engine subsequently powered the MiG-15 jet fighter that fought U.S. fighters and bombers during the Korean War.
The surrender of Germany, in 1945, unlocked a treasure trove of wartime discoveries and inventions. General Electric and Pratt & Whitney, another American engine-builder, added German lessons to those of Whittle and other British designers. Early jet engines, such as those of the Me 262, gulped fuel rapidly. Thus, an initial challenge involved building an engine that could give high thrust with less fuel consumption.
The J-31 (also known by its company designation, I-16) was the first turbojet engine produced in quantity in the United States. It was developed from the original American-built jet engine, the General Electric I-A, which was a copy of the highly secret British "Whittle" engine.
Pratt & Whitney solved this problem in 1948 with its "dual spool" concept. This combined two engines into one. The engine had two compressors—each rotated independently, with the inner one giving high compression for good performance. Each compressor drew power from its own turbine; hence there were two turbines, one behind the other. This approach led to the J-57 engine, which entered service with the U.S. Air Force in 1953.
The turboprop used power from a jet engine to drive a propeller. Additional turbines, placed near the exhaust, tapped this power and spun rapidly. An attached shaft delivered this power to a gearbox. Turboprops drew attention between 1945 and 1960 but lost out because jet aircraft were faster.
This was one of the outstanding post-war engines. It powered U.S. Air Force fighters, including the F-100, the first to break the sound barrier without going into a dive. Eight such engines powered the B-52 bomber. Commercial airliners—the Boeing 707, the Douglas DC-8—flew with it. This engine also saw use in the U-2 spy plane, which flew over the Soviet Union and photographed its military secrets.
Twin-spool jet engine (top) compared with a conventional design (below). Note that the twin-spool version has two compressors, each driven by its own turbine. This arrangement gave more thrust with better fuel economy.
The dual-spool engine represented an important step forward, but engine designers soon wanted more. As they reached for increasing performance, they ran into the problem of "compressor stall." This meant that at certain speeds while in flight, the compressor would pull in more air than the rest of the engine could swallow. Compressor stall produced a sudden blast of air that rushed forward within the engine. The engine lost all its thrust, while this air blast sometimes caused severe damage by breaking off compressor blades.
During the early 1950s, Pratt & Whitney rode merrily along with its J-57. Its competitor, GE, had a good engine of its own: the J-47, which powered the F-86 fighter and B-47 bomber. Still, GE's managers wanted something better. They got it from the engineer Gerhard Neumann, who found a way to eliminate compressor stall. Neumann introduced the "variable stator." This was a set of small vanes that protruded into the airflow within the compressor. Each such vane was like your hand that you stick into the outside air when you ride in a car. Like your hand, each vane could turn as if mounted to a wrist. When the vanes faced the airflow with their edges forward, they allowed the flow to pass them freely. But when the vanes were turned to present their broad faces to the flow, they partially blocked it. These vanes then reduced the amount of flow that was passing through the compressor, and kept it from gulping too much air.
Jet fighters gained speed by burning fuel within an afterburner. This was a tube fitted to the end of the jet engine. Exhaust from that engine contained a great deal of hot air and allowed fuel to burn within the afterburner, for more thrust.
This invention led to an important GE engine, the J-79. It became the first true engine for supersonic flight. With it, the Lockheed F-104 fighter flew at twice the speed of sound. In May 1958, U.S. Air Force pilots used this airplane to set a world speed record of 1,404 miles per hour (2,260 kilometres per hour) and an altitude record of 91,249 feet (27,813 meters). With supersonic flight in hand, the next frontier in jet-engine progress called for engines of very great power, suitable for aircraft of the largest possible size. The key concept proved to be the "turbofan," also called the "fanjet."
General layout of a turbofan engine. Note that a separate set of turbines drives the front fan, as in a turboprop. The term "high-bypass" means that most of the air in the exhaust comes from the fan and flows past the rest of the engine, rather than flowing through it.
The "jet" of a jet engine is the hot stream of exhaust that blasts out the back to produce thrust. However, that exhaust carries power as well as thrust, which the turbines use to run the compressor. By using a larger set of turbines, it is possible to tap off still more of this power. The big turbine then turns a fan, which somewhat resembles an airplane propeller but has many long blades set closely together. The fan adds its thrust to that of the jet. This arrangement yielded the turbofan. It more than doubled the thrust of earlier engines. It also further improved fuel economy. In addition, turbofan engines were relatively quiet, in contrast to earlier jets that produced loud shrieks and screams. GE and Pratt & Whitney both built turbofans after 1965, with Rolls-Royce, offering versions of its own. All truly large airliners have used them, starting with the Boeing 747. These engines have also powered large U.S. Air Force cargo planes, including the C-5A and C-17.
The first aircraft to use these large engines was the Lockheed C-5, which entered development in 1965 and first flew in 1968. A key to its design was the engine—the GE TF-39 turbofan. It had a dual-spool layout as well as a variable stator, with its big fan providing 85 percent of the thrust. The dual-spool arrangement gave the fan its own turbine for power, separate from the rest of the engine. The compressor had 16 stages, or rows of blades.
These three design principles—dual-spool layout, variable stators, and the turbofan—remain in use to this day. All three can even appear in the same engine, as with the TF-39. The dual-spool design gives high thrust with good fuel economy. Variable stators allow efficient operation at all flight speeds. The big forward fan reduces noise, further improves fuel economy, and produces much of the thrust. In turn, the thrust of engines continues to increase. Germany's engine for the wartime Me 262, the Jumo 004, delivered 2,000 pounds (8,900 Newtons) of thrust. The J-57 was rated at 13,500 pounds (60,000 Newtons) of thrust. The J-57 was similar in thrust but weighed considerably less, which made it much speedier. Early turbofans, around 1970, came in around 40,000 pounds (180,000 Newtons) of thrust. But GE's new GE 90 turbofan is rated at close to 90,000 pounds (400,000 Newtons) of thrust! That is why today's planes fly fast and are very large.
In the early 1990s, GE developed the GE90 turbofan engine to power the large, twin-engine Boeing 777. The GE90 family, with the baseline engine certified on the 777 in 1995, has produced a world's record thrust of 110,300 pounds in ground testing, has the world's largest fan at 123 inches in diameter, composite fan blades, and the highest engine bypass ratio (9:1) to produce the greatest propulsive efficiency of any commercial transport engine.
In this engine, air is sucked in from the right by the compressor. The compressor is basically a cone-shaped cylinder with small fan blades attached in rows (eight rows of blades are represented here). Assuming the light blue represents air at normal air pressure, then as the air is forced through the compression stage its pressure rises significantly. In some engines, the pressure of the air can rise by a factor of 30. The high-pressure air produced by the compressor is shown in dark blue.