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F-15 Eagle

Design

The McDonnell Douglas F-15 Eagle emerged from the complex and extensive set of requirements established by the USAF. Configuration of the twin-engine aircraft is characterized by a high-mounted wing, twin vertical tails mounted at the rear of the short fuselage, and large, horizontal-ramp variable- geometry external-compression inlets located on the sides of the fuselage ahead of the wing. The horizontal-tall surfaces are mounted in the low position on fuselage extensions on either side of the exhaust nozzles.

Propulsion of the F-15 is supplied by two Pratt & Whitney F100-PW-100 afterburning turbofan engines of 23,904/14,780 pounds thrust each. Developed especially for the F-15, these high-pressure-ratio engines are reported to have much improved efficiency over earlier engines for fighter aircraft.

The wing planform of the F-15 suggests a modified cropped delta shape with a leading-edge sweepback angle of 45. Ailerons and a simple high-lift flap are located on the trailing edge. No leading-edge maneuvering flaps are utilized, although such flaps were extensively analyzed in the design of the wing. This complication was avoided, however, by the combination of low wing loading and fixed leading-edge camber that varies with spanwise position along the wing. Airfoil thickness ratios vary from 6 percent at the root to 3 percent at the tip.

To succeed in the air-to-air role, a plane needs the right airframe in combination with strong powerplant and avionics. The plane's designers understood this and stretched technology to the limits. It was determined that a very low wing loading combined with heavy thrust from the engines would be required. US fighter aircraft of the period were going faster (Mach 2 plus), but were heavy and lacked maneuverability compared to their Soviet counterparts. When combined with a capable airframe, better maneuverability can be achieved by maximizing thrust, thereby maximizing energy. The Pratt & Whitney F100 Turbofan engine provides the needed thrust. Each engine is capable of producing 15,000 pounds of thrust at maximum power, and 25,000 pounds of thrust in afterburner. This gives the Eagle a total of 50,000 pounds of thrust. In other words, a nominally loaded F-15 Eagle of 48,000 pounds has a thrust-to-weight ratio of 1.04 pounds of thrust to each pound of aircraft weight. Thrust of this caliber allows an F-15 to accelerate while going straight up! A specially modified F-15A Eagle known as the "Streak Eagle" was able to outclimb a Saturn V Moon Rocket to almost 60,000 feet. This same aircraft flew to 98,430 feet (30,000 meters) in 207.80 seconds (less than 3 minutes and 30 seconds).

The wing loading of the F-15 is significantly lower and the thrust loading much greater than corresponding values for earlier fighter aircraft. At the lower weights to be expected during combat, wing loadings as low as 55 pounds per square foot and static thrust-to-weight-ratios of as much as 1.35 might be expected. (As the Mach number increases at a given altitude, the thrust of the afterburning turbofan also increases. For example, the thrust of the F-15 engine at sea level and Mach 0.9 is nearly twice the sea-level static value.) The values of these parameters represent a significant departure from previous fighter design philosophy and resulted from the energy-maneuverability concepts employed in specifying the aircraft.

To understand the design of the F-15 and its unique capabilities, some insight into the meaning of maneuverability and its relation to several aircraft design parameters is necessary. The maneuvering capability of an aircraft has many facets, but one of the most important of these is its turning capability. In a combat situation between two opposing fighters flying at the same speed, the aircraft capable of turning with the shortest radius of turn without losing altitude usually has the advantage. This assumes equality of many other factors such as aircraft stability and control characteristics, armament, and, of course, pilot skill.

In steady, turning flight the lift developed by the wing must balance not only the weight of the aircraft but the centrifugal force generated by the turn. (The term "balance" is used here in a vector sense; that is, the lift vector must equal the sum of the weight and centrifugal force vectors.) The load factor is defined as the ratio of the lift in the turn to the weight of the aircraft and is usually expressed in g units, where g is the acceleration due to gravity. Thus, a 2-g turn is one in which the wing must develop a lift force twice the weight of the aircraft. The value of the load factor is uniquely defined by the aircraft angle of bank. For example, 2-g and 5-g turns require bank angles of 60 and 78.5 respectively. Finally, for a given bank angle and thus load factor, the turning radius varies as the square of the speed; for example, doubling the speed of the aircraft increases the turning radius by a factor of 4. It would then appear that two different aircraft flying at the same speed would have the same turning radius; however, this conclusion is not necessarily correct. The maximum load factor and associated turning radius may be limited by wing stalling. For a given speed and altitude, stalling occurs as a function of the wing maximum lift coefficient and the wing loading in straight and level flight. Clearly then, the turning capability of different aircraft types may vary widely.

Two other important aircraft physical parameters may also limit turning performance. First, at a given speed and altitude, the aircraft drag increases rapidly with lift coefficient; as a consequence, the available thrust may not be sufficient to balance the drag at some load factors that the wing can sustain. In this case the aircraft loses altitude in the turn, an undesirable situation in combat. As for maximum lift coefficient, the drag rise with increasing lift depends upon the wing design and Mach number, as well as upon the added drag required to trim the aircraft at high lift coefficients. Finally, the turning performance may be limited by the control power available in the horizontal tail for trimming the aircraft at the high maneuvering lift coefficients.

These ideas are embodied in a technique for describing and specifying fighter aircraft maneuverability. Known by the term "energy maneuverability," the technique involves the specification of desired aircraft climb and/or acceleration capability for various combinations of speed, altitude, and turning load factor. The quantity specified for each of these combinations is labeled "specific excess power" Ps and is simply the excess power available per unit aircraft weight as compared with the power required to maintain constant altitude in the turn.

The lightly loaded airframe is combined with an equally impressive flight control system. A hydraulically actuated, mechanically controlled flight control system is augmented by an electronic system known as the Control Augmentation System (CAS). This system takes the stick inputs from the pilot and deflects the flight controls in the proper direction at the proper rate for optimal aircraft handling. This system allows the pilot to fly the aircraft to the limits of its capabilities without losing control of the aircraft. The CAS can also actuate the flight controls via pilot input if the hydro-mechanical system is damaged.

In order to win air-to-air battles, the pilot must be able to see, shoot, evade, and destroy the adversary first. The Eagle has an impressive array of weapons and avionics which allow it to get the advantage. The APG-63 and 70 radars allow crews to see targets that are as far away as 100 miles. These "Eyes" are able to ferret out the targets even if the targets are flying at high speeds at low altitudes. A Tactical Electronic Warfare System (TEWS) lets the aircrew know if any threat is present. The Heads-up-Display (HUD), and the Hands on Throttle and Stick (HOTAS), allow the Pilot to select, track and shoot the adversary without having to look back into the cockpit.

The F-15's versatile pulse-Doppler radar system can look up at high-flying targets and down at low-flying targets without being confused by ground clutter. It can detect and track aircraft and small high-speed targets at distances beyond visual range down to close range, and at altitudes down to tree-top level. The radar feeds target information into the central computer for effective weapons delivery. For close-in dog fights, the radar automatically acquires enemy aircraft, and this information is projected on the head-up display.

In addition to the F-15C AESA, Raytheon is developing AESAs for the F/A-18E/F Super Hornet. The Boeing Phantom Works unit led a team that received a $250 million contract to install the AESA radar, upgrade the aircraft's environmental control systems and install an advanced identification friend or foe system. Honeywell Aerospace and BAE Systems, respectively, provided the latter systems. The Air Force F-15 System Program Office's Projects Team at Wright-Patterson Air Force Base, Ohio, managed the program for the U.S. government.

An inertial navigation system enables the Eagle to navigate anywhere in the world. It gives aircraft position at all times as well as pitch, roll, heading, acceleration and speed information.

The F-15's electronic warfare system provides both threat warning and automatic countermeasures against selected threats. The "identification friend or foe" system informs the pilot if an aircraft seen visually or on radar is friendly. It also informs U.S. or allied ground stations and other suitably equipped aircraft that the F-15 is a friendly aircraft.

The Fiber Optic Towed Decoy (FOTD) provides aircraft protection against modern radar-guided missiles to supplement traditional radar jamming equipment. The device is towed at varying distances behind the aircraft while transmitting a signal like that of a threat radar. The missile will detect and lock onto the decoy rather than on the aircraft. This is achieved by making the decoy's radiated signal stronger than that of the aircraft.

A variety of air-to-air weaponry can be carried by the F-15. An automated weapon system enables the pilot to perform aerial combat safely and effectively, using the head-up display and the avionics and weapons controls located on the engine throttles or control stick. When the pilot changes from one weapon system to another, visual guidance for the required weapon automatically appears on the head-up display.

The Eagle can be armed with combinations of four different air-to-air weapons: AIM-7F/M Sparrow missiles or AIM-120 Advanced Medium Range Air-to-Air Missiles on its lower fuselage corners, AIM-9L/M Sidewinder or AIM-120 missiles on two pylons under the wings, and an internal 20mm Gatling gun (with 940 rounds of ammunition) in the right wing root.

The current AIM-9 missile does not have the capabilities demonstrated by foreign technologies, giving the F-15 a distinct disadvantage during IR dogfight scenarios. AIM-9X integration will once again put the F-15 in the air superiority position in all arenas. The F-15/AIM-9X weapon system is to consist of F-15 carriage of the AIM-9X missile on a LAU-128 Air-to-Air (A/A) launcher from existing AIM-9 certified stations. The AIM-9X will be an upgrade to the AIM-9L/M, incorporating increased missile maneuverability and allowing a high off-boresight targeting capability.

Low-drag, conformal fuel tanks were especially developed for the F-15C and D models. Conformal fuel tanks can be attached to the sides of the engine air intake trunks under each wing and are designed to the same load factors and airspeed limits as the basic aircraft. Each conformal fuel tank contains about 114 cubic feet of usable space. These tanks reduce the need for in-flight refueling on global missions and increase time in the combat area. All external stations for munitions remain available with the tanks in use. AIM-7F/M Sparrow and AIM-120 missiles, moreover, can be attached to the corners of the conformal fuel tanks.

Conformal Fuel Tanks [CFT] are carried in pairs and fit closely to the side of the aircraft, with one CFT underneath each wing. By designing the CFT to minimize the effect on aircraft aerodynamics, much lower drag results than if a similar amount of fuel is carried in conventional external fuel tanks. This lower drag translate directly into longer aircraft ranges, a particularly desirable characteristic of a deep strike fighter like the F-15E.

As with any system, the use of CFTs on F-15s involves some compromise. The weight and drag of the CFTs (even when empty) degrades aircraft performance when compared to external fuel tanks, which can be jettisoned when needed (CFTs are not jettisonable and can only be downloaded by maintenance crews). As a result, CFTs are typically used in situations where increased range offsets any performance drawbacks; for example, F-15s flying alert missions out of Keflavik, Iceland often encounter unforseen weather changes forcing them to fly more than 500 miles to an alternate landing field in Scotland or Norway. In the case of the F-15E, CFTs allow air-to-ground munitions to be loaded on stations which would otherwise carry external fuel tanks. In general, CFT usage is the norm for F15Es and the exception for F-15C/D's.



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