Structure of F/A-18 Hornet

Last revised April 17, 2000


The F-18 aircraft project was known as the Model 267 by the McDonnell Douglas prime contractor. The Model 267 retained the overall configuration of the Northrop YF-17 lightweight fighter prototype, with its two engines, its twin outward-canted vertical tail surfaces and its leading-edge wingroot extensions (LERX). However, the aircraft was structurally quite different from the YF-17 in order to make it capable of enduring the additional stresses and strains involved in carrier operations. Both the airframe and the undercarriage had to be strengthened for carrier operations, wing folding had to be incorporated and a tailhook had to be provided. Fuel capacity had to be increased to meet the Navy-specified mission radius.

As compared to the YF-17, the wing of the Hornet had 50 additional square feet of area (an increase from 350 to 400 square feet), with increases in both span and chord in order to improve the low-speed performance. The wing had a trapezoidal planform (swept on the forward edges but straight on the trailing edges) and incorporated variable camber. The variable camber is achieved by using full-span leading edge flaps and hydraulically-actuated single-slotted flaps on the inner trailing edges. These surfaces are all under computer control to manage extension and retraction, setting the surfaces to the most desirable angle in order to give optimal performance throughout the entire performance envelope. The ailerons on the outer portions of the wing trailing edges can double as flaps to enhance low-speed handling qualities, and differential operation of flaps and ailerons can be used for roll control. The outer wing panel is hinged at the inboard edge of each aileron for folding aboard carriers. One 96 US gallon fuel tank is installed in each wing, but most of the internal fuel is housed in the fuselage.

In order to provide more space for internal fuel, the width of the aft fuselage of the Hornet was increased by four inches over that of the YF-17, the engines were canted outwards at the front, and the fuselage spine was made significantly wider and taller. The swollen dorsal spine houses the aircraft's main fuel tanks (containing 426, 249, 200, and 530 US gallons of fuel). These tanks are installed in a row, beginning from just behind the cockpit and ending just forward of the engines. All tanks and fuel lines are self-sealing, with foam in the main tanks. There is a single retractable midair refuelling probe on the starboard side of the fuselage just ahead of the cockpit.

The simple undercarriage of the YF-17 had a track of 6 feet 10 3/4 inches. On the F-18, the track was increased to 10 feet 2 1/2 inches for greater stability during carrier landings. It was considerably strengthened to meet the 24 feet/second descent rate requirement that is needed for arrested carrier landings.

The all-flying horizontal tailplanes are of aluminum honeycomb construction with graphite epoxy skinning. They can be used in concert for pitch control or differentially for roll control, acting as "tailerons" for enhanced roll performance.

The twin vertical tails of the F-18 were necessary to offset the vortex flows coming off the leading-edge extensions of the wings. The twin tails are mounted far forward in order to close the aerodynamic gap between the trailing edge of the wing and the leading edge of the vertical tail. This results in a smooth and drag-free fuselage airflow. The forward position of the tails also reduced airflow interference around the engine nozzles and saved weight by eliminating the need for any major rear fuselage carry-through structure.

The intakes are set well back underneath the LERXes, the cobra-shaped extensions protecting the engine intakes somewhat from the disruption of the airflow caused by the effects of high angle of attack flight. Since there is no requirement for the Hornet to exceed Mach 2, the aircraft does not need sophisticated variable-ramp air intakes. The two-dimensional D-shaped intakes thus have a simple, fixed splitter plate mounted next to the fuselage. The only moving parts are two ducts cut into the top of the LERX which permit bleed air to be ejected upwards into the airflow generated by the LERX. The intake ramps/boundary layer splitter plates are solid at the front end, with perforations directly ahead of the inlet to permit sluggish boundary layer air to be bled away and dumped via spill ducts on top of the LERX.

The twin-hinged hydraulically-activated airbrake is mounted on the rear dorsal fuselage, between the vertical tail surfaces. This configuration gives the minimum pitch change when the airbrake is extended.

The main undercarriage units retract aft and rotate through 90 degrees so as to lie flat underneath the air intake ducts. The twin-wheel nose gear retracts forward into the nose.

The Hornet uses advanced composite materials for large portions of its structure. About half of the weight of the structure is made up of aluminum, while steel contributes about 16.7 percent of the weight. Titanium makes up about 12.9 percent of the structural weight, this metal being used for a considerable fraction of the wings, fin, and horizontal tail attachments as well as the wing-fold joints. About 40 percent of the aircraft's surface area is covered by graphite/epoxy composite material, this material making up 9.9 percent of the aircraft's weight. The remaining 10.9 percent of the weight is made up of various other materials (plastic, rubber, etc).

The 15,000 lb.s.t. General Electric YJ101 turbofans which powered the YF-17 were replaced by their F404-GE-400 derivatives, rated at 16,000 lb.s.t. with afterburner. The F404 is a low-bypass turbofan, with a bypass ratio of 0.34, which makes it a true turbofan rather than a "leaky" turbojet as was the YJ101. It has essentially the same thrust as the J79 turbojet, but weighs only half as much. The engine has a three-stage titanium fan, with one row of fixed inlet guide vanes and one row of variable guide vanes. The compressor has seven stages, with the first three stages having variable stators. There are single-stage high and low pressure turbines.

The F404 engine is fairly simple, with relatively few moving parts. As compared to other recent turbofans, the F404 has experienced relatively few developmental problems. In particular, it is extremely resistant to compresssor stalls even at high angles of attack. Even if a stall does occur, the problem corrects itself very quickly, with engine and afterburner relighting themselves automatically. The engine is remarkably responsive, being able to accelerate from idle to full afterburner in only four seconds. However, the time taken to accelerate from Mach 0.8 to Mach 1.6 was originally longer than the required value. Although some progress has been made in improving this response time, this problem has persisted in spite of numerous attempts to fix it.

Since the Navy wanted all-weather capability and the ability to carry and launch radar-homing missiles such as the AIM-7 Sparrow, the small radar of the YF-17 had to be replaced with a more powerful installation. At the end of 1977, the Hughes AN/APG-65 digital multi-mode radar was selected over its Westinghouse competitor. This installation required an enlarged nose shape for the 28-inch radar dish needed to met the Navy's weapons system search range requirement of over 30 nautical miles.

The Hornet has a total of nine external weapons hardpoints--one at each wingtip, two underneath each wing, one on each corner of the fuselage just aft of the air intakes, and a centerline ventral underfuselage station. The F/A-18 retained the wingtip-mounted Sidewinder infrared-homing air-to-air missiles and the 20-mm M61 cannon of the YF-17, but since the Navy had specified compatibility with the AIM-7F Sparrow semi-active radar homing missile, the F-18 incorporated a corner station underneath both sides of the fuselage to carry Sparrow missiles when the Hornet is operating in the intercept mode. These stations carry FLIR and laser designation pods when the Hornet is operating in the attack mode.

The F-18 incorporated a quadruply-redundant digital fly-by-wire flight control system, the first of its kind to be installed in a production aircraft. It works by having stick and rudder inputs being directed into a computer which interprets them and issues the appropriate commands to the various control surfaces. The FBW system will not allow the pilot to overstress the airframe. The system operates by the principal of majority vote. If one of the four systems disagrees with the other three, this is interpreted as a failure, and the dissenting system is ordered to shut down. FBW redundancy is such that should a second system fail, the remaining two systems can still operate the controls so long as they remain in agreement. In the unlikely event of all four systems failing, there are electrical backups for all control surfaces. There is even a direct mechanical backup for the horizontal tail surface which will give the pilot some degree of pitch control in an extreme emergency.

Guided by experience from Vietnam, duplicate hydraulic systems were fitted, which were routed separately to the degree possible. This arrangement would, it was hoped, prevent the aircraft from being disabled by a single hit.

The F-18 had always been intended as a single-seater, so a lot of attention was paid to reducing the pilot workload by the extensive use of automation. The F-18 has what has become to be known as a "glass" cockpit, with many of the dial-type instruments being eliminated and the information that they provide being displayed on cathode-ray tubes similar to computer monitors. It incorporates a heads-up display, and the control panel is dominated by two multi-function cathode-ray tube displays and a single horizontal situation cathode-ray tube display. The pilot is provided with a hands-on throttle and stick (HOTAS), with all the controls required for combat being located on either the throttle lever or control column for easy access. This means that the pilot does not have to take his eyes off his target during the stresses of combat.

The pilot sits on a slightly reclined Martin Baker US10S (SJU-5/6) zero-zero rocket-assisted ejector seat.

Sources:


  1. Hornet, Robert F. Dorr, World Air Power Journal, Spring 1990, p. 38.

  2. McDonnell Douglas Aircraft Since 1920: Volume II, Rene J. Francillon, Naval Institute Press, 1990.

  3. Vespidae Varius--Recent Variations in the Hornet Family, Paul Jackson, Air International, December 1993, p. 301

  4. The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.

  5. F/A-18 Hornet, Lindsay Peacock, Osprey Combat Aircraft Series, Osprey, 1986.