Structure of F-16 Fighting Falcon
Last revised March 19, 2000
80 percent of the airframe structure of the F-16 is of conventional aluminum alloy, and about 60 percent of the structural parts are made from sheet metal. An attempt was made to minimize the amount of exotic material used in the construction of the F-16 in the interest of saving cost. About 8 percent is steel, composites are 3 percent and titanium is 1.5 percent.
The F-16 is built in 3 major subsections, nose, center and aft. In order to save money, the fuselage structure is fairly conventional in overall configuration, being based on conventional frames and longerons. The forward manufacturing breakpoint is just aft of the cockpit, while the second is forward of the vertical fin.
The wing planform of the F-16 is effectively that of a cropped delta with a 40-degree leading edge sweep. The wing has 4 percent thickness/chord ratio, and the aerofoil section is 64A204. The wing structure incorporates five spars and 11 ribs. Upper and lower wing skins are one-piece machined components. From left to right, the wing gradually blends with the fuselage, making it impossible to tell where the wing begins and the fuselage ends. This wing/body blending made it possible to increase the internal volume, enabling more fuel could be carried. In fact, 31 percent of the loaded weight of an F-16 is fuel, accounting for the long range of the Fighting Falcon. Gradually increasing the thickness of the wing in the region of the root resulted in a stiffer wing than would have been possible with a conventional design. In forward-to-aft planform, the wing leading edge blends smoothly with the fuselage by means of leading edge strakes. At high angles of attack, these strakes create vortices which maintain the energy of the boundary air layer flowing over the inner section of the wing. This delays wing root stalling and maintains directional stability at low speeds and high angles of attack. Vortex energy also provides a measure of forebody lift, reducing the need for drag-inducing tail trim. By keeping the inner-wing boundary layer energized, the strakes allowed the wing area to be kept smaller, saving about 500 pounds in weight.
The wing trailing edges have a set of inboard "flaperons", which combine the duties of flaps and ailerons. The flaperons operate as conventional ailerons for controlling the aircraft during conventional flight. During takeoffs and landings, they can be drooped by as much as 20 degrees, operating as flaps. The outboard trailing edge wing surfaces are fixed.
Wind tunnel tests demonstrated the need for leading edge flaps to improve lift and directional stability at high angles of attack. Leading edge maneuvering flaps and trailing edge flaperon can be moved at up to 35 degrees per second to shape the wing aerofoil to match aerodynamic conditions. The moving flaps reduce the drag, maintain lift at high angles of attack, improve directional stability and minimize buffeting. The use of lift-increasing maneuvering flaps allowed a smaller wing of reduced span to be used.
The wing is only 1.5 inches deep at the point where the leading edge flap actuator is installed, so the design of this component was a significant challenge. In the spring of 1982, actuator failures caused the USAF to ground all F-16s that had exceed 200 hours flight time for an inspection of the wing leading edge flap. A routine inspection had turned up excessive wear in the actuation mechanism which controls the position of the leading-edge maneuvering flap. More than 40 aircraft required repair.
During the early development of the F-16, both single- and twin-vertical tail formats had been studied. Wind tunnel tests showed that vortices produced by the forebody strake or LEX generally improved directional stability but that certain strake shapes actually reduced stability at high angles of attack when twin tails were fitted. Consequently, it was felt that the use of the twin-tail format involved significantly greater development risks, and a single vertical tail was adopted. The disadvantage is that the single vertical tail now has to be sufficiently tall.
The single vertical stabilizer has a multi-spar and multi-rib structure made from aluminum, but the skins are made of graphite epoxy. The two ventral fins underneath the fuselage are made of glass fibre. There is a runway arrester hook underneath the rear fuselage.
Aft of the wing, the fuselage blends smoothly in cross-section into a side-body fairing that extends all the way to the rear of the aircraft. The all-flying horizontal tailplane is attached to the rear of this side body fairing. The air brakes are mounted inboard of each horizontal stabilizer at the end of the side body fairing, one set on each side of the rear fuselage. The air brakes are of the split type, the upper and lower sections opening through a maximum angle of 60 degrees.
The wings are far too thin to accommodate the main undercarriage units, so they are attached to the main fuselage and retract forward into wells in the lower fuselage. The nose gear is located just aft of the intake, so that debris thrown up by the nosewheel will not be ingested into the intake. The steerable nose landing gear retracts aft and rotates through 90 degrees to lie flat underneath the intake duct.
The air intake is located underneath the fuselage, at a point just below the cockpit. The ventral location of the air intake subjects it to minimal airflow disturbance over a wide range of flight conditions and aircraft maneuvers, since the forward fuselage tends to shield the intake from the full effects of aircraft maneuvers, minimizing the effects of sudden changes in the angle of attack on airflow into the engine. At an angle of attack of 25 degrees, for example, the air flows into the intake at an angle of only ten degrees with respect to the aircraft's longitudinal axis. The lower edge of the intake lip is only 38 inches above the ground, but, surprisingly, FOD problems caused by the ingestion of runway debris into the engine have been relatively minor.
The intake is of fixed geometry type, which saves on complexity, weight, and cost. A fixed-geometry boundary-layer splitter plate separates the upper lip of the intake from the lower fuselage. There is a separation strut mounted inside the intake for additional tunnel rigidity.
In the interest of saving in cost, a number of parts are interchangeable between port and starboard. These include the horizontal tail surfaces, wing flaperons, 80 percent of the main landing gear components, and many of the actuator units.
The pilot's view from the cockpit of the F-16 is superlative, and is unmatched by just about any other fighter aircraft. The pilot sits underneath a clamshell-type canopy whose forward and center sections are made of a single piece of polycarbonate. The windshield arch normally fitted to the cockpit canopies of most jet fighters is absent on the F-16, offering the pilot an excellent forward view. Visibility covers a full 360 degrees in the horizontal and from 15 degrees down over the nose through the vertical and back to directly behind. The sideways view extends down to a depression angle of 40 degrees. The optical quality is high, and the curved surfaces offer minimal optical distortion.
The transparent part of the canopy is 0.5-inches thick, and was designed to resist the impact of a 4-pound bird at 350 knots. However, even if the canopy happens to fail under the impact of an especially large bird, the heads-up display is sufficiently robust to provide additional back-up protection for the pilot.
The elimination of the normal windshield arch improves the forward view, but this means that the entire transparency has to be as thick as the front portion, which is designed to survive birdstrikes. This imposes a substantial weight penalty. Another disadvantage is that the canopy must be jettisoned before the pilot can escape, since the polycarbonate transparency is too thick for him or her to eject through it.
The inside of the canopy is covered with a thin gold film which dissipates radar energy to reduce the radar cross section, especially from the front. A redundant safety lock ensures that the canopy cannot be inadvertently opened. The canopy is normally operated by electrical motors, but there is a manual crank as a backup.
The pilot sits on a McDonnell Douglas ACES II (Advanced Concept Ejection Seat) rocket-powered ejection seat equipped with a vectored-thrust pitch control system. It is capable of zero-zero performance, and is cleared for use up to a height of 50,000 feet and a speed of 600 knots. The seat is tilted backwards at an angle of 30 degrees, and the pilot's knees and legs are raised in order to provide some extra physiological tolerance to high-g maneuvers. However, it is debatable whether a 30-degree seat inclination really does increase the g-tolerance of the pilot. It certainly does make it more difficult for the pilot to turn his/her head to check the six.
The conventional joystick control column is replaced by a sidestick controller located on a cockpit console at the pilot's right hand. Left-handed pilots, however, appear to be able to use the sidestick without difficulty. The sidestick fitted to the first YF-16s did not move at all, operating strictly on the amount of force applied by the pilot to determine the desired pitch or roll rate. On production aircraft, it was found suitable to introduce some artificial "feel" into the system, and the sidestick now moves up to 3/16 of an inch aft, 3/32 of an inch left and right, and less than a hundredth of an inch forward (since pilots under negative g tend to give more forward stick than needed).
Under the Multinational Staged Improvement Plan (MSIP) approved in February 1981, a series of improvements were developed for the F-16. The intention was to evolve the F-16s capabilities in such a way as to minimize risks during technology developement while ensuring the aircraft's continued value and worth. Among these were modifications of the structure and wiring of the wings to carry the AMRAAM and the provision of hardpoints on the intake sides to carry the LANTIRN electro-optical system.
A new horizontal tailplane of increased area was introduced under Engineering Change Proposal 426. It provides greater control forces needed to cope with heavier munitions loads. The revised tail was easier and less expensive to produce.
The vertical fin can be modified to allow the fitting of a braking parachute if the customer so desires. Norwegian F-16s were all fitted with this feature, since Norway has many short airfields which are often covered with ice and snow, making the use of wheel brakes impractical.
The F-16A/B employed an all-electronic fly-by-wire (FBW) flight control system instead of the traditional hydromechanical systems with linkages and cables. The system is a four-channel analog system, the F-16A/B having been designed too early to accommodate the quadruplexed digital system that was provided on the Space Shuttle and on the F/A-18 Hornet. The FBW system makes it possible for the F-16 to fly safely with its center of gravity behind the center of pressure, thus providing the aircraft with an inherent instability that makes it highly responsive to the controls and to use relatively modest amounts of tail deflection during high-g maneuvers. The use of relaxed stability enabled a smaller tail to be used, since less force was needed to alter aircraft attitude. The General Dynamics team was the first to take the bold step of eliminating mechanical backups to the FBW system, trusting the aircraft completely to electronics.
Experience with a triplex digital system on the AFTI/F-16 gave GD the confidence to abandon the proven analog FBW system of the earlier Fighting Falcon and adopt the quadruplex digital FBW system for the Block 25 and beyond F-16C/D.
An inflight refuelling socket is mounted on the top of the fuselage just ahead of the fin leading edge. It is normally covered by an inward-hinged door when not in use. The receptacle can accommodate the rigid boom used by USAF aerial tankers, or it can have a probe fixed into it for use with drogues.
Sources:
- Combat Aircraft F-16, Doug Richardson, Crescent, 1992.
- General Dynamics Aircraft and their Predecessors, John Wegg, Naval Institute Press, 1990.
- The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.
- United States Military Aircraft Since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian, 1989.
- F-16 Fighting Falcon--A Major Review of the West's Universal Warplane, Robert F. Dorr, World Airpower Journal, Spring 1991.
- The World's Great Interceptor Aircraft, Gallery, 1989.
- Modern Military Aircraft--F-16 Viper, Lou Drendel, Squadron/Signal Publications, 1992.
- Lockheed F-16 Variants, Part 1, World Airpower Journal, Volume 21, Summer 1995.
- E-mail from Ben Marselis
Ja melkein kaikki muukin hävittäjiin ja ilmailuun liittyvä asia.
