YF-17 Cobra

The McDonnell Douglas F/A-18 Hornet traces its direct ancestry to the Northrop Cobra, a twin engine multimission fighter design developed for the export market in the late 1960s. The YF-17 was a fighter aircraft prototype developed to demonstrate advanced technology applicable to air combat. Distingwishing features include the moderately swept wing, with the large highly swept leading edge root extentions, differential area ruling of the fuselage, underwing inlets with wing root slots for fuselage boundary layer diversion, twin vertical tails, and twin jet engines.

In the Vietnam War, the lack of maneuverability of US fighters at transonic speeds provided key advantages to nimble enemy fighters. Industry, the Department of Defense (DOD), and NASA were all stimulated to sponsor research to achieve unprecedented transonic maneuverability while maintaining excellent handling qualities. In the early 1970s the Air Force pressed for development of a new generation of lightweight fighters-single-seat jet aircraft "optimized" for agility and air combat maneuvering, with high thrust-to-weight ratios (above 1 to 1), and good acceleration. Out of this interest came the so-called "Lightweight Fighter" program.

On 06 January 1972, the Air Force issued a request for proposals (RFP) for a Light-weight Fighter (LWF) Program. Participants were told to tailor their specifications toward the goal of developing a true air superiority lightweight fighter. General Dynamics and Northrop were asked to build prototypes, which could be evaluated with no promise of a follow-on production contract. These were to be strictly technology demonstrators. The two contractors were given creative freedom to build their own vision of a lightweight air superiority fighter, with only a limited number of specified performance goals. Northrop's entry was derived from the Cobra design. Northrop produced the twin-engine YF-17 using breakthrough aerodynamic technologies and two high-thrust General Electric YJ101 engines. General Dynamics countered with the compact YF-16, built around a single F100 engine.

In the early 1960's, the Northrop Company had noticed an improvement in the maximum lift of the F-5 aircraft because of a small flap actuator fairing that extended the wing root leading edge. This phenomenon spurred interest in the effects of inboard vortex flows and led to a cooperative NASA and Northrop study. The cooperative study of hybrid wings centered on the use of relatively large, highly swept wing extensions at the wing-fuselage intersection, which promoted strong beneficial vortex-flow effects. The scope of the study included parametric studies to maximize the lift- and stability-enhancing effects of the wing extension concept, which became known at Northrop as the leading-edge extension (LEX). Studies were also directed at cambering the leading edge of the LEX to suppress the vortex at low angles of attack, and thereby minimize drag at cruise conditions. Northrop applied a large highly swept LEX to the YF-17 prototype aircraft to enhance lift and stabilize the flow over the YF-17 main wing at high angles of attack. The integration of the large LEX surfaces and the placement of the twin vertical tails provided exceptional tail effectiveness at high angles of attack. The small strakes added to the forward fuselage nose by Northrop resulted in extremely high directional stability at high angles of attack.

Northrop placed a high priority on superior high-angle-of-attack characteristics and a high degree of inherent spin resistance for the YF-17. The company had also placed priorities in these areas during the development of the F-5 and T-38 aircraft, which had become known for outstanding resistance to inadvertent spins. To provide superior handling qualities at high angles of attack for fighter aircraft, Northrop provided the airframe with the required levels of aerodynamic stability and control characteristics without artificially limiting the flight envelope with the flight control system. This approach proved to be highly successful for the YF-17 [and was adopted by McDonnell Douglas (now Boeing) and used in all variants of the F/A-18 aircraft]. Handling characteristics at high angles of attack were excellent. The YF-17 could achieve angles of attack of up to 34 deg in level flight and 63 deg could be reached in a zoom climb. The aircraft remained controllable at indicated airspeeds down to 20 knots. Northrop consequently claimed that their lightweight fighter contender had no angle-of-attack limitations, no control limitations, and no departure tendencies within the flight envelope used for the evaluation.

The basic wing plan form combined with the lead edge root extension, is ideitified as a hybrid wing. The vortex flow generated by the extension significantly increases lift, reduces buffet intensity and drag, and improves handling qual1ities. Leading edge and trailing, edge flaps are used to vary the wing's camber for maximum maineuvering performance.

The aircraft configuration is area-ruled to achieve at smooth overall area distribution. However, within the smooth overall area distribution, the area ruling above and below the wing is apportitoned to create favorable supersonic lift interference (increased lift at a given angle of attack) for minimum drag-due-to-lift with a small increase in zero-lift drag.

The horizontal tail is located below tbe wing to provide increasing longitudinal stability at high angles of attack approaching maximum lift, and to reduce buffeting from the wing wake at high-G flight conditions. The tail is sized larger than the minimum required for stability and control in order to lower trim drag and increase supersonic sustained maneuvering performance.

The vertical tails are sized and located to provide positive directional stability beyond the maximum trimmed angles of attack across the speed range. The twin vertical tails are canted outboard for proper placement relative to the vortex flow generated by the wing leading-edge extensions. The forward location of the verticals was selected to preclude reduction of horizontal tail effectivness caused by interference withi the canted vertical tailis, provide low supersoonic drag through the favorable influence on the area distribution of the aircraft, and integrate more effectively the vertical tail sutpporting structure and horizonta tail actuators into the fuselage design.

Two General Electric YJ101 continuous-bleed, afterburning turbojet engines in the 15,000-pound-thrust class are installed in the aft fuselage. The location of the engine inlets under the wing minimizes flow angularity to the inlet and places the inlet in a positzioen to take advantage of the compression effects of the wing leading edge root extension, thus decreasing inlet flow Mach number and increasing pressure rececovery at angles of attack. The key feature of airframe/inlet integration is a longitudinal slot through the wing root which allows passage of fuselage boundary layer air through the slot and over the top of the wing. Thickening of the boundary layer is prevented and a narrow fuselage boundary-layer gutter can be used, resulting in a low-drag installation while maintaining high-quality airflow to the engine inlet.

Outstanding visibility is achieved by the canopy shape and location. The pilot has full aft vision at eye level and above. Armament consists of one M61 20mm cannon and two wingtip-mounted AIM-9E missiles. External store pylons are located at wing stations 78 and 138 (left and right) and on the fuselage centerline.

The YF-17 was an advanced technology prototype fighter aircraft whose flight control system was designed to comply with MIL-F-9490C except for variations as allowed by the procurring activity for prototype aircraft. The YF-17 control system includes a control augmentation system (CAS) using both electronic and mechanical elements in the pitch axis, a fly-by-wire control augmentation system for aileron control, and mechanical control with electronic stability augmentation (SAS) in the yaw axis. Also, this aircraft had a mechanical rolling tail control and electronically controlled maneuvering leading-edge and trailing-edge flaps. Both digital and analog computations are utilized. The actuators are electrohydraulic and electromechanical.

Midway down the development path the stakes changed; what had been a technology demonstration became a Department of Defense competition for a new fighter for both the Air Force and Navy, and for allied nations as well. In April 1974, the LWF Program changed from a technology demonstration program to a competition for an Air Force Air Combat Fighter (ACF), and the flight-test programs for the YF-16 and YF-17 were rushed through in a few months instead of the planned 2 years. First flight of the YF-17 was in June 1974. By this time, the Air Force had decided to proceed with Air Combat Fighter (ACF) Program, based on flight testing of the YF-16 and YF-17 prototypes. The Navy was also initiating a program to develop a new VFAX in this time period--a strike fighter to replace both the F-4s and A-7s in its carrier air wings.

When the competition was completed early in 1975, both the YF-16 and the YF-17 showed great promise. The two prototypes performed so well, in fact, that both were selected for military service. The Air Force selected the F-16 to be produced for the Tactical Air Command, and the Navy was directed by Congress to base the VFAX on either the YF-16 or YF-17 designs. The Navy, unhappy with the outcome, proceeded independently with a derivative of the YF-17 Cobra, this evolving into the Navy's Northrop F-18 Hornet fighter program. To meet Navy requirements, considerable improvements in areas such as combat radius and radar capability were incorporated, in addition to carrier suitability features. The resulting redesign was extensive and, when the McDonnell Douglas design was selected as winner in 1976, it was assigned the F-18A designation.

After sitting briefly in storage, the two YF-17 prototypes flew again, this time as development aircraft for the proposed F-18. At the request of the Navy, Dryden flew the first YF-17 for base drag studies and to evaluate the maneuvering capability and limitations of the aircraft. NASA pilots-all of whom got at least one flight in the plane-and engineers examined the YF-17's buffet, stability and control, handling qualities, and acceleration characteristics.

From May 27 to July 14, 1976, the Dryden Flight Research Center, Edwards, California, flew the Northrop Aviation YF-17 technology demonstrator to test the high-performance U.S. Air Force fighter at transonic speeds. The objectives of the seven-week flight test program included the study of maneuverability of this aircraft at transonic speeds and the collection of in-flight pressure data from around the afterbody of the aircraft to improve wind-tunnel predictions for future fighter aircraft. Also studied were stability and control and buffeting at high angles of attack as well as handling qualities at high load factors. Another objective of this program was to familiarize center pilots with the operation of advanced high-performance fighter aircraft. During the seven-week program, all seven of the center's test pilots were able to fly the aircraft with Gary Krier serving as project pilot.

In general the pilots reported no trouble adapting to the aircraft and reported that it was easy to fly. There were no familiarization flights. All 25 research flights were full-data flights. They obtained data on afterbody pressures, vertical-fin dynamic loads, agility, pilot physiology, and infrared signatures. Average flight time was 45 minutes, although two flights involving in-flight refueling lasted approximately one hour longer than usual. Dryden Project Manager Roy Bryant considered the program a success. Center pilots felt that the aircraft was generations ahead of then current active military aircraft.