Working of F - 15

An Insight into the working of the F15s
(A Report Submitted by Tom Harris)
It didn't take long for the world to figure out the combat potential of airplanes. In 1911, only eight years after the Wright brothers got their creation off the ground, the U.S. military started dropping test bombs from above. A few years later, World War I troops were battling it out in the sky with machine-gun-packing fighter planes.
Things moved pretty quickly from there. Only 60 years later, the early single-engine propeller planes had evolved into sleek, powerful fighter jets that could make sharp aerial turns at more than 600 miles per hour (970 kmph).
In this article, we'll look at one of the most famous fighters, the F-15. This remarkable aircraft is getting up in years -- it has been around since the early '70s -- but it's still a crucial piece of the U.S. arsenal. According to the U.S. Air Force, it has a perfect combat record, with over 100 victories and zero defeats. As we'll see, its success is due to its phenomenal maneuverability, advanced electronic equipment and fearsome firepower.
What is an F15?
The F-15 Eagle is a small, highly maneuverable jet plane designed to fly combat missions in all weather conditions. Its primary mission is maintaining air superiority. In other words, its ultimate purpose is to defeat other planes in aerial combat.
(This is the figure showing an F15C preparing to refuel).



The United States Air Force commissioned the plane after they got a look at the MiG-25, a powerful fighter jet the Soviet Union unveiled in 1967. The MiG-25, commonly known as "the Foxbat," was far superior to the primary U.S. fighter jet at the time, the F-4 Phantom, and in the heart of the Cold War, the Air Force needed a comparable aircraft as soon as possible. McDonnell Douglas (now merged with Boeing) won the contract for the new project and delivered the finished F-15 a few years later. The company has introduced several variations on this plane since then, as technology and needs have changed. The current combat F-15 Eagle is the F-15C.
The Strike Eagle
The original F-15 Eagle was designed to handle only air-to-air targets (other planes). It wasn't built to bomb targets on the ground because the Air Force knew that the extra equipment would compromise the plane's aerial combat abilities. But when the Air Force needed a fighter bomber to replace the aging F-111 until the new stealth F-117 was ready, they decided to modify the F-15 for air-to-ground missions. The result was the F-15 Strike Eagle, designated F-15E.
(The figure on the left shows the F15 carrying air-to-ground weapons and on the right is the F15 loaded with air-to-air weapons).
The Strike Eagle is not a replacement for the original F-15, but a supplementary bomber plane. Surprisingly, the Air Force's temporary solution turned out to be one of the best fighter bombers ever made. In Operation Desert Storm, the Strike Eagle proved it could successfully fight its way past enemy planes, hit several ground targets, and then fight its way out of enemy territory.
Power and Flight
An F-15 has most of the elements found on an ordinary jet plane. It has two wings that generate lift, it has rear vertical and horizontal stabilizers and rudders that balance and steer the plane, and it has twin turbofan jet engines at the rear of the plane that generate thrust.
(This figure shows the various parts of the F15 fighter).




The main difference between an F-15 and an ordinary jet is how these elements are balanced. The F-15's twin engines (Pratt & Whitney F-100-PW-220s or 229s) have a very high thrust-to-weight ratio, meaning they are relatively light for the amount of thrust they generate (they can generate almost eight times their own weight in thrust).
(This figure shows an Air Force Maintenance Squadron test out a Pratt & Whitney F100-PW-220e engine configured for the F15).
The plane body is relatively light, too, though it is extremely strong. The wing spars (the support structures inside the wings) are made of titanium, which is lighter and stronger than steel and most of the skin, is made of lightweight aluminum. According to the Air Force, each engine can generate between 25,000 and 29,000 pounds of thrust. The F-15C's normal weight is only 45,000 pounds, which means its thrust is actually greater than its weight! This lets it accelerate quickly, even while climbing in altitude.
The F-15 also has very low wing loading, meaning it has a lot of wing area for its weight. Greater wing area means greater lift, which makes the plane more agile. It can take off, ascend and turn much more quickly than an ordinary plane, which has much more weight per square foot of wing space.
(This figure shows the F15s high thrust-to-weight ratio and low wing loading as a result of which it can shoot off the ground @ a sharp angle).
Afterburner, Tanks and Banks
The engines are outfitted with afterburner nozzles, which can provide an extra kick of thrust when necessary. The afterburner simply injects fuel into the hot jet exhaust stream. It ignites, adding to the hot gases shooting out from the back of the engine.
At full force, the plane can get up to more than Mach 2.5 (approximately 1,854 mph / 2,984 kmph).
The high engine power does come at a price -- poor fuel economy. Of course, the F-15 was designed with this limitation in mind. In order to extend its un-refueled range, it was built with large internal fuel tanks in the fuselage (the main body) and in the wings. It can also carry three external tanks, as well as a pair of aerodynamic form-fitting tanks under the wings that generate some lift of their own. Fully fueled, the F-15C can fly 3,450 miles (5,550 km), and the F-15E can fly 2,400 miles (3,860 km).
(This figure shows the F15 carrying extra fuel in 3 large external tanks. One mounts under each wing and the other mounts under the fuselage).
The other problem with the engines is that they wear out pretty quickly. This is to be expected, given the amount of work they do. Fortunately, they're very easy to replace -- an Air Force ground crew can do it in less than an hour!
The F-15 doesn't just take off quickly, it stops quickly too. It has its own extendable air-brake, a hydraulically operated panel that dramatically increases the aircraft's drag to slow it down (just like a parachute).
(This figure shows an F15 extending its air brakes just before landing).

Electronic Control
The main thing that sets the F-15 and other modern fighters apart from their predecessors are their electronic systems. Early fighter pilots controlled their planes mechanically, by moving linkages, and they mainly used their own eyes to target enemy planes. In stark contrast, nearly every aspect of the F-15 is computerized.










The plane is essentially a robot. It has a central computer, which is connected to an array of advanced sensors. Based on input from the inertial guidance system (which contains highly sensitive gyroscopic sensors) and the pilot, the computer activates hydraulic actuators to adjust the wings and rear stabilizers. The pilot doesn't actually fly the plane directly: He or she gives instructions and the computer decides how to carry them out. The computer is constantly making flight adjustments on its own to improve flight performance -- the computer artificially creates a relatively smooth ride. The F-15 computer can make necessary adjustments in milliseconds, about a hundred times faster than a human being.


Sensor Array
The plane's main "eye" is its computer-controlled radar system, mounted in the nose. The radar's job is to locate other aircraft and generate ground maps. The dish is mounted on moving gimbals, so it can pivot to scan different areas or follow a moving target. The radar figures out which way targets are moving using the pulse-Doppler system -- essentially, shifts in the reflected radio wave frequency indicate whether the target is moving toward the radar system or away from it.
The F-15 Strike Eagle has additional scanning equipment called the low-altitude navigation and targeting infrared for night (LANTIRN) system. The LANTIRN system is housed in two pods mounted to the bottom of the plane, near the engine inlets.
The navigation pod holds another radar unit that is optimized to map the ground terrain, and a forward-looking-infrared (FLIR) night vision scanner that picks up the infrared heat energy from surrounding objects. Together, these sensors generate a detailed image of the ground below, allowing the pilot or computer to fly in total darkness.
(This figure shows one of the LANTIRN pods on an F15 strike eagle).





The targeting pod houses a powerful laser and another FLIR scanner, mounted to a swiveling turret. The laser works as a range-finder, calculating the distance to targets based on how long it takes a laser beam to bounce off of them, and also as a target designator, marking targets for laser-guided missiles. The targeting system is designed to pick out ground targets, but it can also be used in air-to-air combat.
The central computer processes data from the radar and the LANTIRN system and presents targeting and navigation information to the crew.
Inside the Cockpit
The original F-15 was designed for a single-person crew. The pilot flies the plane and targets enemy aircraft at the same time. The F-15 Strike Eagle has an additional station in the back of the cockpit for a weapons systems officer, or WSO (pronounced "wizzo"). In the Strike Eagle, the WSO is in charge of selecting and eliminating ground targets while the pilot concentrates on maneuvering the plane and fighting enemy aircraft. Both stations are housed in a sturdy "bubble" canopy on top of the plane. This canopy design gives the crew a full 360-degree view of their surroundings.
(This figure shows the F15 bubble canopy which gives the crew a wide view of the sky).



The pilot's station is designed to make flying and targeting as easy as possible. The computer presents most relevant information on the heads-up display (HUD), a monitor that projects an image onto a transparent screen at the front of the cockpit canopy. With the heads-up display, the pilot can monitor the flight data and the radar information while keeping an eye on the sky. This is crucial in combat -- a pilot can't keep looking down at gauges and instruments while evading or chasing enemy fighters. The Air Force is planning to eventually replace this system with a helmet-mounted monitor that projects flight data onto the pilot's visor.
(This figure shows an infra red terrain image displayed on the F15s heads up display).


Controls
The pilot's controls are also fairly straightforward. The pilot steers the plane with a control stick located in the center of the cockpit, and controls the engine with the throttle on his or her left. Both controls have several buttons and switches that operate the radar equipment, select options on the heads-up display, and target and fire the weapons.
The controls are designed with the hands-on throttle and stick (HOTAS) system. In the HOTAS system, every switch and button on the controls has a different shape and texture. This way, the pilot can control all the major aspects of the plane without ever looking down into the cockpit.
(This figure shows an additional cockpit in the F15 stationed for the weapons system officer).

The WSO, by contrast, doesn't spend much time looking outside the cockpit. He or she monitors radar, LANTIRN and flight data on four multi-function displays (MFD) -- cathode ray tube monitors surrounded by buttons (sort of like the display on an automatic teller machine). The WSO position has a full set of flight controls, but this is only a back-up provision -- normally, the WSO doesn't help fly the plane. Both the pilot and the WSO sit in high-tech ACES II ejection seats, which launch them clear of the plane in an emergency.
All of this expensive equipment serves one basic purpose: It is designed to deliver various missiles, bombs and bullets, known in military circles as ordnance, to enemy targets.

Missiles
The F-15 Eagle is loaded up with weaponry that can take out almost every aircraft in existence. It sports eight air-to-air missiles of different designs. It can carry various combinations of AIM-120 advanced medium range air-to-air missiles (AMRAAMs), AIM-9L/M Sidewinder missiles, or AIM-7F/M Sparrow missiles.
(This figure shows 2 F15s ejecting AIM-7F/M Sparrow Missiles during a training exercise)
All three missile types are designed to actively seek out their target. The AMRAAM and Sparrow missiles are both radar-guided. The AMRAAM has its own radar unit and flight control system. Before firing the missile, the F-15 computer transmits radar information specifying the intended target, and the missile's radar unit locks on. After the missile launches, its one goal is to steer itself (by adjusting flight fins) toward that target.
The Sparrow missile works on a similar principle, but it doesn't have its own radar transmitter. The pilot has to keep the plane's transmitter aimed at the target, to "paint" it for the missile.
The sidewinder missile uses an infrared sensor to pick up on an enemy plane's hot engine exhaust. The flight controls simply steer the missile toward the hottest area in sight.
Other Weapons
The F-15 also has a built-in machine gun, an M-61 20-mm 6-barrel cannon, mounted inside the starboard (right) wing. The gun has an efficient Gatling gun design that can fire about 6,000 rounds per minute. It never gets the chance, however, because its magazine only holds 940 rounds. It can empty its entire magazine in less than 10 seconds!
(This figure shows airmen loading ammunition for the F15s 20mm canon).
The pilot selects a different targeting display on the HUD for each weapon. The machine gun display, for example, consists of a funnel shape. The pilot maneuvers the plane so that the target is in the center of the funnel and then opens fire.
The F-15 Strike Eagle has everything the F-15 Eagle has, and it can also carry just about any air-to-ground missile in the Air Force arsenal. It often carries guided munitions, such as the GBU-15 bomb. All in all, it can carry approximately 23,000 pounds (10,430 kg) of ordnance.
(This figure shows an F15 Strike Eagle dropping Mark 84 laser guided bombs during a training exercise).



Both F-15 models also have a number of high-tech defenses. They have radar warning receivers, which detect enemy radar from ground stations, planes or guided missiles, and an advanced radar jammer to confuse these radar units. They also have a chaff dispenser, a device that shoots out a cloud of metal strips. Enemy radar picks up the chaff and temporarily loses its lock on the F-15.
The future
The F-15's combination of high maneuverability, sophisticated electronics and powerful weaponry have made it a hugely successful weapon in the United States arsenal (and a number of other countries' arsenals, as well). But now it's approaching the end of its run. Boeing and Lockheed Martin have already developed its replacement, the F-22 Raptor.
(This is the figure of the F22 Raptor, the F15s high tech replacement).





The Raptor takes everything on the F-15 to a whole new level, with much greater acceleration, maneuverability and computer power. It's also designed for stealth flying, just like the F-117 and B-2 bomber. When the F-22 enters service in 2005, the Air Force will phase out the F-15 Eagle. The F-15 Strike Eagle will keep flying for the foreseeable future.

6 Stroke technology

The capacity for original thought is a mark of genius. In the past century, it has delivered us countless advances. Often, they have been achieved against the odds, in the face of reactions ranging from disinterest to scorn: almost every engineer has to deal with dismissive responses to new ideas, usually on the basis that something so different obviously can’t be good. On that basis, we should all be riding single-cylinder side-valve motorcycles. Daring to be different is obviously essential. Malcolm Beare, a 53-year-old Australian wheat farmer is indeed an original thinker. Designing a better motorcycle engine has been his mission since 1981, when he built his first prototype. Beare has created an innovative hybrid design of the internal combustion engine, combining a two-stroke top with a four-stroke middle section of sorts. An exciting V-twin Ducati-based BEARS/Sound of Thunder racer incorporating Malcolm’s self-built frame, fitted with a 90deg V-twin version of the Beare six-stroke engine was the first multi-cylinder application of six-stroke technology. Beare began working 23 years ago on a stratified charge opposed-piston two-stroke, he recalls, “but after building a couple of prototypes I realized flow control was a problem, with too many ports going everywhere.” He thus simplified it into a six-stroke, with the objective of improved efficiency by overcoming the drawbacks of poppet valves, with rotary valve application (commonly used in two-stroke engines). This overcomes the problems of poppet valve inertia that inhibits flow, especially the exhaust valve hotspot within the combustion chamber. While various rotary valve designs made in the past were advantageous because they didn’t restrict flow or absorb as much energy as poppet valves, they had significant overheating problems, with associated drawbacks in sealing, lubrication and oil consumption. So Beare decided to produce the best of both worlds, taking the basic components of a rotary disc-induction two-stroke engine, and mating it with a four-stroke motor. “After a great deal of work and experimentation since building my first six-stroke prototype in 1990, I truly believe we’ve refined the concept, proved its reliability and established its worth, so now we have it fully patented — even in the USA — and ready for commercial use in the variety of applications it obviously lends itself to —including motorcycles, stationary engines and propeller aircraft engines. Below the cylinder head, everything is conventional. So one advantage is that the Beare concept can be transplanted on existing engines without redesigning. But above there, he’s thrown away the cylinder head, complete with poppet valves. To replace the camshaft and associated valves, Beare has retained the cam drivebelt and fitted an ultra short-stroke upper crankshaft complete with its inverted piston, the belt driven at exactly half engine speed. This piston slides up and down in a sleeve; past inlet and exhaust port cavities set into the cylinder wall, very much like on a two-stroke. Continuing the two-stroke analogy, the Ducati-based V-twin is fed fuel-air mixture via a reed valve, thus preventing exhaust gas exit through the inlet port. Beare technology recently has been paired with a new form of fuel injection known as the Pulse Fuel Injection system, which has been tested by the US Naval Air Warfare Center, among other international authorities. Unlike conventional fuel injectors, compressed air is used to atomize liquid fuel and pre-mix it with air, increasing efficiency and reducing emissions. Interestingly, this nozzle technology is being developed for emission control systems in diesel exhausts for Ford Motor Company.At the other end of the upper crankshaft is a two-stroke type rotary disc valve that regulates exhaust timing, cutting off the exhaust flow at the appropriate time to stop gas returning into the cylinder, and thereby creating the inlet cycle suction. This being its only function, the valve bears a light load, reducing lubrication and sealing problems. The Ducati I rode uses hardened, anodized aluminum rotary disc valves, which work well. Then, during the compression and expansion strokes, the upper piston seals both ports, leaving pressure between the two pistons. The lower piston is a conventional flattop three-ring design, while the conical upper one (so shaped to aid gas flow during both inlet and exhaust cycles, by guiding it towards the ports) has two rings — one compression, one oil. In the combustion phase, twin sparkplugs provide ignition. Not only does the engine run on pure petrol (no need to add oil, because it is positively lubricated, in spite of the application of two-stroke technology), it’s very happy on low octane unleaded fuel — a boon for the Indian market. OK, so the Beare six-stroke has impeccable credentials but that just gives a hint at the commercial and technical advantages Malcolm seems justified in claiming. This new technology has some strong backing — Sir Jack Brabham, the renowned three-time World F1 Champion, keenly supports the Beare ideology and his eponymous firm has gone to the world market along with CEO Alan Casey. The obvious benefit in eliminating poppet valves on a four-stroke is that it removes any chance of valve float, which in turn implies a far higher safe rev limit for the six-stroke — an eye-popping 28,000rpm in theory, given the half engine-speed operation of the upper crankshaft, and the fact that GP reed-valve two-strokes peak at 14,000rpm. But Malcolm Beare says the rev limit, primarily depends on what the crankshaft can bear and so he’s arbitrarily limited the Ducati-based 6S-V2 to 9000rpm, at which point he says 86bhp is delivered at the rear wheel. Instead of absorbing about 10 percent of engine power in driving the camshafts, the cam belts now deliver about nine percent net power back to the main crankshaft, via the upper, conical porting piston. If you factor in a 1997 model Ducati 900SS delivered 73bhp at the rear wheel in stock form, that’s quite an impressive claimed power increase. But there are other, much more significant apparent spin-off benefits from the Beare design. It is these that make it seem so very viable for a demanding Indian market, the most important being the ubiquitous fuel economy. Beare claims his engine is 35 percent more economical at low revs/throttle openings than the equivalent conventional engine, and 13 percent less thirsty at high rpm/full throttle. Therefore, there are reduced hydrocarbon emissions, because you use less fuel to achieve the same performance. Next, there’s improved torque at lower revs proven on all prototypes, which will surely help in the stop-and-go nature of congested Indian traffic. Beare discovered the six-stroke version produced the same torque as the four-stroke conventional motor 1000rpm lower down the rev scale, as well as making exponentially more torque as revs rose. But in a commercial application, perhaps the most attractive benefit is the significantly reduced number of moving parts, compared to a four-stroke — not as few as a two-stroke, but what you appear to get is improved performance and torque, coupled with four-stroke inherent advantages. Fewer moving parts also mean reduced manufacturing costs. My riding impression of the 6S-V2 revealed that while the Beare V-twin may appear a little rough at the edges, it’s obviously built to a budget as well as acting as a rolling test bed for continuous modification. So let’s ignore the looks and worry about the sizzle. For starters, it has an absolutely unique engine sound, like nothing else I’ve ever heard in my life. You clearly hear the ring-ding two-stroke signature above the trademark four-stroke V-twin boom from the twin megaphone exhausts. An uncanny two engines in one. Malcolm claims there’s much reduced mechanical noise from the engine, because of fewer moving parts. Without any tappet noise, it’s remarkably silent — and that’s in spite of being air-cooled. Heat dissipation is not a problem, so while it’d be easy to water-cool the engine, there’s honestly no need for the extra complication on grounds of noise or thermal efficiency. Riding the bike for a dozen laps showed up three significant assets for the six-stroke engine and one negative. The question mark hung over the power curve: while there’s no doubt it revs reasonably high by four-stroke standards, there seemed to be no direct benefit in doing so — there was no more power up high at, say, 8000rpm than there seemed to be 2000 revs lower. One thing Beare certainly doesn’t need to worry about is torque, a vital ingredient for the Indian rider, which together with the crisp, responsive throttle pickup and the reduced vibration are the three strong points of the six-stroke motor. The Beare engine is unbelievably muscular in terms of torque, and importantly from very low revs. Ironically, you appreciate this because those of a tube-bender do not match Malcolm’s evident talents as a bush engineer: the square-tube chassis was pretty awful in terms of handling. But the fact that the bike jumps around over bumps and doesn’t steer very well actually helps you appreciate the engine’s responsiveness, because you need to back off the throttle in, say, the middle of a chicane to allow the chassis to recover. But when you get on the gas again — get ready for a surprise. Not only is there instant response when you twist the wrist, there’s tractor-pulling torque at seemingly whatever revs the engine’s turning over at — even as low as what felt like 2000-3000rpm or so. There’s also no transmission snatch at very low revs, indicating a responsive, torquey motor, while the manner in which it pulls hard from way down low bears all the hallmarks of a long-stroke engine design, whereas the opposite is in fact the case: the lower ‘four-stroke’ crankshaft assembly has 86 x 57mm dimensions, while the upper ‘two-stroke’ measures 60 x 25mm. Yet the inherent nature of the Beare motor is one of a torquey slugger, belying these measurements. And allied with the so-responsive pickup and a wide spread of usable power, makes this bike ridiculously easy to ride. You hardly need to use the gearbox, just park it in top gear and ride. Even backing off the throttle in the middle of a turn doesn’t require hooking down a gear — just crack it open when you’re ready and feel the front wheel start to aviate on you. My hands-on assessment of the Beare six-stroke leads to some inescapable conclusions. What we have here is a hybrid engine design with the advantages already recounted above, allied with good squish, the ability to run radical bore/stroke ratios, quiet mechanical operation, and no exotic materials such as ceramic coatings or costly alloys required, nor complicated machining operations.As such, the Beare design surely seems worthy of serious consideration for powering low-cost transportation in developing countries like India, where expense of manufacture, fuel economy and torque (delivering carrying capacity for people and/or goods) are all paramount factors, yet where emissions matter too. The industry trend away from cheaper two-stroke power in favor of costlier but cleaner four-stroke engines in both Europe, Japan and South East Asia makes a concept like the Beare six-stroke, which offers the best of both worlds, project a strong case towards volume manufacture. At the time of going to print, Autocar India had learnt of a six-stroke small capacity single-cylinder prototype being in the pipeline for production by a leading motorcycle manufacturer in China, but few details are available. What is known is that a 150cc six-stroke Taiwanese single-cylinder bike from Yingang is ready to roll. The questions of how the technology will work in actual practice, whether it does make it into production reality at all in India, and how reliable it proves under harsh and demanding use remain to be answered. Watch this space.