How the F15 Works

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.

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 (see below). The current combat F-15 Eagle is the F-15C.

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 Strike Eagle is not a replacement for the original F-15, but a supplementary bomber plane. Surprisingly, this temporary solution turned out to be the pride of the Air Force inventory, and 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.

In the next section, we'll see how these two planes are put together and find out how they dive, climb and dodge so gracefully.

An F-15 has most of the elements you'll find 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.

F-15 Models

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).

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.

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 the back of the engine (see this Question of the Day for details on afterburners). At full force, the plane can get up to more than Mach 2.5 (approximately 1,854 mph / 2,984 kph).

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, additional 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 form-fitting, exterior conformal fuel 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).

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).

The main thing that sets the F-15 and other modern fighters apart from their predecessors is their flight control 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.

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 (see How Radar Works for more information.)

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.

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 automated weapon system enables pilots 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. In the next section, we'll look inside the cockpit to see how the crew accesses this information, flies the plane and targets the enemy.

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.

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.

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.

The WSO, by contrast, doesn't spend much time looking outside the rear 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. In the next section, we'll find out what the F-15 is actually packing when it goes to war.

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.

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.

That's not all for air to air weaponry either. 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!

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.

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 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.

The Raptor takes everything on the F-15 to a whole new level, with an increased maximum takeoff weight, improved acceleration and maneuverability, and an upgraded central computer. It's also designed for stealth flying, just like the F-117 and B-2 bomber.

For much more information about fighter aircraft, including the F15, the F-22 and other military aircraft, check out the links on the next page.