Northrop YF-17 Cobra

Last revised July 20, 2009


Followed by the success of the F-5A in the world arms market, in 1965 Northrop began work on a new tactical lightweight fighter which would offer an even higher performance than the F-5E/F Tiger II which was at that time under development. Company studies, including new aerodynamic research, demonstrated that it would be possible to build a fighter with capabilities far exceeding the F-5. This project was known as the P-530 by the company.

The P-530 was to be powered by a pair of 13,000 lb.s.t. General Electric 15/J1A5 turbojets. The GE15 used a core that was scaled down from the F101 turbofan of the B-1 bomber, but was a twin-spool engine. There were ten stages of compression which generated a pressure ratio as high as 25:1. The GE15 was a low-bypass ratio turbofan, with a bypass ratio of only 0.25, and the airflow did little more than cool the rear casing of the engine. For this reason, the GE15 was often referred to as a "leaky turbojet". Because of the bypassed air, the engine bays required little cooling and could be made of lighter, lower-cost materials. The afterburning thrust of the GE15 was to be 14,300 pounds, similar to that of the J79 turbojet, but weighing only half as much and having much better fuel economy.

The wing planform was quite similar to that of the F-5. The sweepback value at the quarter-chord line was 20 degrees, and the trailing edge was unswept. The wing was initially shoulder-mounted and had an anhedral of 5 degrees. Over the next couple of years, however, the wing moved steadily downward on the fuselage until it ended up at the mid position. The area was 400 square feet as compared with 186 square feet for the F-5E. The wing had a variable camber, which was provided by by hinged flaps along the straight leading and trailing edges, those on the trailing edge stopping a little over half way to the tip in favor of conventional ailerons. In 1968, the leading edge flap was spilt up into front and rear sections to increase the lift coefficient in the depressed position.

The wing was provided with a leading edge root extension (LERX) that tapered into the fuselage on a level with the cockpit. The LERX made it possible to achieve post-stall maneuvering capabilities at angles of attack exceeding 30 degrees and later even 40 degrees. At high angles of attack the LERX added about 50 percent to the lift provided by the basic wing. Extending the LERX ahead of the engine inlets had the additional effect of guiding the airflow smoothly into the inlets and presented the engines with a full flow of relatively undisturbed air at high angles of attack, helping to prevent engine flameouts. In addition, a long axial slot was cut into each LERX adjacent to the fuselage ahead of the intakes, preventing a buildup of air ahead of the inlet while in supersonic flight. At low-speed and high angles of attack, these slots provided an escape for boundary-layer air which scrubbed across the fuselage ahead of the inlet. In 1968, the LERX were further enlarged, the forward portions continuing ahead as strakes almost to the nose.

The engines were fed by long ducts which admitted air from semi-circular inlets. These inlets originally were provided with a movable half-cone centerbody ahead of the wing. However, by 1971 it was concluded that Mach 2 performance was not all that important a design objective, and these conical centerbodies were eliminated. At about the same time, the inlets were made shorter and brought back under the LERX. That made the large LERX look a lot like the head of a cobra, so much so in fact that Cobra became the Northrop name for the P-530. The inlets were further refined throughout 1971-71, the final shape selected being a canted oval shape with a fixed, slightly-rounded edge, with the top located 4 inches below the underside of the LERX. The inlets were separated from the fuselage by a large rectangular splitter plate.

The tail of the P-530 was originally fairly conventional, with slab tailplanes mounted below mid-level. Originally, a single vertical fin was planned. The ability of the P-530 to fly at extreme angles of attack indicated that a single vertical tail would be inadequate, since it would be blanketed in the wake of the wing. To overcome this problem, the single centerline fin was replaced by twin fins, each about half as large as the original single surface, canted out at almost 45 degrees to put them in free-stream airflow. To reduce cross-coupling effects in roll, the rudders reached only halfway up the fins. In 1969, the canted fins were approximately doubled in size and moved forward to a position partially overlapping the wing. By late 1970, the tailplanes had been further enlarged and the outward cant of the fins had been reduced to only 18 degrees. At the same time, the horizontal tails were also enlarged and moved further reawards on the fuselage sides, as far aft as possible.

The aircraft was designed according to the principle of relaxed static stability, with the aircraft being longitudinally unstable with a tendency to pitch nose up. Air-combat maneuverability would be dramatically enhanced to the point where the limiting factor would now be the pilot. However, Northrop did not feel that 1960-vintage fly-by- wire control systems were sufficiently reliable, and retained conventional mechanically-signalled flight controls.

The canopy of the P-530 was a large frameless moulding with a bulged cross section that provided the pilot with unobstructed 360-degree vision and good sightlines over the nose and downwards on each side.

The maximum takeoff weight of the P-530 was estimated at 40,600 pounds, and a maximum speed of Mach 2 was expected. The armament was to be a single 20-mm M61 rotary cannon mounted on the centerline underneath the nose, and a Sidewinder missile could be carried on each wingtip. An array of missiles and bombs could be carried on external weapons pylons, three under each wing and one on the underfuselage centerline.

Potential customers for the P-530 Cobra were Western nations who wanted and who could afford a fighter aircraft with a higher performance than that of the F-5A/B. Prime targets were every user of the F-104 Starfighter, as well as several Middle Eastern countries, including Iran. It was estimated that the P-530 program development costs would be 350-400 million dollars, and the flyaway price would be two million dollars apiece, assuming a total procurement equivalent to that of the F-5.

On January 28, 1971, Northrop revealed their P-530 program to the world. A model of the P-530 was constructed and more than 5000 hours of wind tunnel testing was carried out. However, there were no buyers.

As early as 1965, the USAF had begun concept formulation studies of new high-performance fighters. These included the F-X, a heavy interceptor/air-superiority fighter, and the lightweight Advanced Day Fighter (ADF). The F-X was to be in the 40,000-pound class and was to be equipped with advanced, sophisticated long-range radars and armed with long-range, radar-guided air-to-air missiles. The ADF was to be in the 25,000 pound range and was to have a thrust-to-weight ratio and a wing loading intended to better the performance of the MiG-21 by at least 25 percent. The general concept behind the ADF was much the same as the reasoning which had led after the Korean War to the Lockheed F-104A Starfighter.

The appearance of the Mach 2.8-capable MiG-25 Foxbat in 1967 frightened Defense Department analysts and prompted a redirection in USAF fighter plans, with high performance being once again the primary concern. The F-X concept was later to emerge as the McDonnell Douglas F-15 Eagle, a twin-engined fighter with advanced avionics and long-range missiles. The ADF was temporarily shelved.

The ADF concept was kept alive by former fighter instructor Major John Boyd and by Pierre M. Sprey, a civilian working in the office of the Assistant Secretary of Defense for Systems Analysis. They both disliked the F-X concept as it then existed, and preferred a much simpler design. In the late 1960s, they proposed a 25,000 pound aircraft designated F-XX, which was to be a dedicated air superiority fighter with a high endurance, minimal electronics, and no long-range missiles. Later studies brought this weight down to 17,000 pounds. The concept met with much opposition, since some considered it a threat to the existing F-X project. However, the Pentagon decided to continue the project at a low level just in case the F-X (i.e. F-15) program got delayed.

In 1969, a Pentagon memorandum suggested that both the Air Force and the Navy adopt the F-XX as a substitute for the F-15 and F-14 respectively, since both these planes were becoming increasingly expensive. Both services vigorously resisted these moves, and both the F-14 and F-15 projects surged ahead.

Deputy Defense Secretary David A. Packard (who came in with the new Nixon Administration in 1969) was a strong advocate of returning to the concept of competitive prototyping as a way of containing the ever-increasing costs of new weapons systems. During the 1960s, under Secretary of Defense Robert MacNamara, the Total Procurement Package philosophy had been adopted, in which an aircraft was committed to production even before the first example had flown and without any competitive flyoff against rival designs. This had led to such controversial aircraft as the Lockheed C-5A Galaxy and the General Dynamics F-111, which had both encountered expensive and time-consuming developmental problems and extensive cost overruns. Under the new competitive prototyping philosophy, Air Force Secretary Robert C. Seamans drew up a set of ground rules in which the initial funding of a new weapons project would be limited, with initial performance goals and military specifications being kept to a minimum. By 1971, Boyd was working for the Air Force Prototype Study Group. He was able to push the concept at a time when the idea of competitive flyoffs was coming back into fashion.

A Light Weight Fighter (LWF) program came into being under Packard's watch. A Request For Proposals (RFP) was issued to the industry on January 16, 1971. The RFP called for a high thrust-to-weight ratio, a gross weight of less than 20,000 pounds, and high maneuverability. No attempt was to be made to equal the performance of the MiG-25 Foxbat, the emphasis being on what was thought to be the most-likely conditions of future air combat--battles at altitudes of 30,000-40,000 feet and speeds of Mach 0.6 to Mach 1.6. Emphasis was to be on turn rate, acceleration, and range rather than on high speed. A small size was stressed, since the small size of MiG-17 and MiG-21 had made them difficult to detect visually during combat over North Vietnam. The RFP specified three main objectives. The aircraft should fully explore the advantages of emerging technologies, reduce the risk and uncertainties involved in full-scale development and production, and provide a variety of technological options to meet future military hardware needs.

Northrop believed that it had the basis for an entry in the LWF contest in its P-530 Cobra project, which had so far failed to attract any customers. Its entry was given the company designation of P-600. Externally, the P-600 was almost identical to the 1971 form of the P-530 Cobra. However, the P-530 had always been envisaged as a multi-role aircraft with a significant air-to-ground capability, whereas the P-600 was to be purely an air-to-air demonstrator with no armament except a gun and a Sidewinder missile at each wingtip.

The P-600 was to be powered by a pair of General Electric YJ101-GE-100 turbofans rated at 15,000 lb.s.t. each with afterburning. The J101 was a development of the GE15 engine that had been proposed for the P-530. The two engines were mounted close together to minimize the asymmetric effects in the event of an engine loss. The maximum takeoff weight of the P-600 was initially only 21,000 pounds, but it soon grew to 23,000 pounds. The landing gear was much simpler than that of the P-530, saving considerable weight. A much higher proportion of the structure was of graphite fibre, including the LERX, ailerons, flaps, airbrake engine doors, fin leading and trailing edges, rudders, and many access doors.

An inflight refuelling receptacle was installed above the nose. The M61 cannon was relocated to the upper part of the nose instead of underneath.

Full fly-by-wire controls were adopted, Northrop management finally concluding that these systems were now sufficiently reliable to warrant their incorporation. The tail circuits were quadruply redundant, but the ailerons were simplex because the aircraft could always be controlled in roll by the tailplanes.

The cockpit of the P-600 was generally identical to that of the P-530. An inertial navigation system (Litton LN-33) was planned, but Northrop at USAF request did not plan for a large and expensive multi-mode radar and the designers retained a constricted nose with a pointed conical form. However, in April of 1974 Northrop contracted with Rockwell for a compact radar with a phased-array antenna that could fit inside the narrow nose.

Four other manufacturers submitted proposals--Boeing, General Dynamics, Ling-Temco-Vought, and Lockheed. In March of 1972, the Air Staff concluded that the Boeing Model 908-909 was the first choice, with the General Dynamics Model 401 and the Northrop Model P-600 as close seconds. The Vought V-1100 and Lockheed CL-1200 Lancer had been eliminated.

The Source Selection Authority, after further work, rated the General Dynamics and Northrop proposals ahead of the Boeing submission. The General Dynamics Model 401-16B and the Northrop P-600 were chosen for further development on April 13, 1972, and contracts for two YF-16s (72-1567/1568) and two YF-17s (72-1569/1570) were awarded. Rather than the "X" (experimental) prefix being used, the "Y" (development) prefix was used in order to indicate that a mixture of off-the-shelf and experimental technologies were being used. The YF-16 was to be powered by a single Pratt & Whitney F100 turbofan, whereas the YF-17 was to be powered by a pair of General Electric YJ101-GE-100 engines. The "cost plus fixed fee" contracts covered the design, construction, and testing of two prototypes, and a year of flight testing. At the time, the Air Force was still very much committed to the F-15 fighter, and visualized the LWF program as more of a technology-demonstration project rather than a serious effort for a production aircraft. At the same time, contracts were given to Pratt & Whitney for a version of the F100 turbofan specially adapted for single-engined aircraft and to General Electric for the new and smaller YJ101 engine.

The first YF-17 (72-1569) was rolled out at Hawthorne, California on April 4, 1974, and was trucked to Edwards AFB. The first YF-17 took off on its first flight on June 9, 1974 from Edwards AFB, with test pilot Hank Chouteau at the controls. On June 11, the YF-17 became the first American fighter to exceed the speed of sound in level flight without the use of an afterburner. The second YF-17 flew for the first time on August 21, 1974. The two prototypes carried out a series of 288 test flights totalling 345 hours.

The flyoff began as soon as flight testing started. There was an attempt to get as many pilots as possible to fly both the YF-16 and YF-17. The Lightweight Fighter prototypes never flew against each other, but they did fly against all current USAF fighters as well as against MiG-17s and MiG-21s that had been "acquired" by the USAF and operated at the Nellis AFB complex.

In the meantime, the governments of Belgium, Netherlands, Denmark, and Norway had begun to consider possible replacements for the Lockheed F-104G Starfighter. They formed the Multinational Fighter Program Group to choose the successor, and agreed that all four countries would purchase the same aircraft. The prime candidates were the Northrop YF-17, the Dassault Mirage F.1, the SAAB JA37 Viggen, and the General Dynamics YF-16. The winner of the ACF contest in the USA would probably be the favored candidate, but the MFPG wanted to see if the USAF was going to buy the plane for itself before they made any commitment to an American candidate.

Within the Air Force staff, there was a strong institutional bias against the LWF, since they perceived it to be a threat to the F-15 program. However, the prospect of a big European order for the LWF whetted the appetite of certain Air Force brass, who now regarded the project as something more than just a technology demonstration program. To try and convince the F-15 lobby that the LWF program was not a threat to them, the LWF program was renamed Air Combat Fighter (ACF) by the Defense Department.

In September of 1974, Defense Secretary James R. Schlesinger announced that that he was considering production of the winner of the LWF contest to satisfy USAF, Navy, and export requirements. Up to that time, the LWF/ACF program had been largely an academic exercise for the USAF, but the possibility of a large European order led the USAF to change its mind and envisage large-scale service for the aircraft. However, the design emphasis would be changed to that of a multi-role aircraft rather than simply an air-superiority fighter. It was agreed that the ACF would complement rather than supplement the F-15 Eagle in USAF service, easing somewhat Air Force fears that it would somehow sidetrack their Eagle program. The production form of the LWF (by now known strictly as the ACF in Defense Department press releases) would have a larger radar antenna, giving the aircraft some BVR capability. On September 11, 1974, the USAF announced plans to buy 650 ACFs, with the possibility that this could be increased to 1400 or more. This move was made to assure the potential NATO customers that the USAF would stand firmly behind the new fighter.

Although the Northrop contender demonstrated remarkable handling qualities and was actually superior in certain areas, on January 13, 1975, Air Force Secretary John McLucas announced that the YF-16 had been selected as the winner of the ACF contest. The YF-16 was a little faster than the YF-17, and its F100 powerplant was a proven engine that was in use in other warplanes already in service. The J101 engine was a new, relatively untried powerplant which would require enormous investment in tooling, spare parts, and documentation. In addition, the J101 was considered somewhat underpowered and was not a true turbofan like the YF-16's F100, and suffered from a lower specific range. In addition, the YF-16 had a better specific range than the YF-17 and was considerably less expensive.

That might have been the end of the line for the Northrop design, were it not for the US Navy's desire for a new fighter. In August of 1974, the US Navy's own VFAX program had been cancelled by Congress in favor of NACF, which instructed the Navy to choose its fighter from between the two ACF rivals. Northrop decided to team up with McDonnell Douglas to promote a version of its YF-17 as an entry in this contest. This design was eventually to emerge as the F/A-18 Hornet, which was ordered by the Navy on May 2, 1975. The second YF-17 was earmarked for development work as the "F-18 prototype" (even though the true F/A-18 did not fly until November of 1978).

During the LWF/ACF program, the two YF-17s logged 345.5 flying hours (including 13 hours supersonic). A sustained angle of attack of 63 degrees was achieved at a speed of 50 knots.

In 1976, YF-17 72-1569 was sent to NASA's Dryden Flight Research Facility at Edwards AFB for base drag studies for the US Navy.

72-1569 is currently on display at the Western Museum of Flight in Hawthorne, California. 72-1570 is currently on display at the US Naval Aviation Museum in Pensacola, Florida.

Serials of Northrop YF-17:

72-1569/1570 		Northrop YF-17 

Specification of Northrop YF-17:

Engines: Two General Electric YJ101-GE-100 turbojets, 15,000 lb.s.t. each with afterburning. Performance: Maximum speed: Mach 2.0 (1320 mph) at 40,000 feet. Service ceiling 60,000 feet. Maximum range 2800 miles. Weights: 21,000 pounds empty, 23,000 pounds gross, 30,630 pounds maximum takeoff. Dimensions: Wingspan: 35 feet 0 inches, length 55 feet 6 inches, height 14 feet 6 inches, wing area 350 square feet. Armament: One 20-mm M61A1 cannon. One AIM-9 Sidewinder infrared-homing air-to-air missile could be carried at each wingtip. Stores could be carried on one ventral and four underwing pylons.

Sources:


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

  2. Combat Aircraft F-16, Doug Richardson, Crescent, 1992.

  3. United States Military Aircraft Since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian, 1989.

  4. F-16 Fighting Falcon--A Major Review of the West's Universal Warplane, Robert F. Dorr, World Airpower Journal, Spring 1991.

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

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

  7. Northrop YF-17: The Fighter They Didn't Want, Bill Gunston, Wings of Fame, Vol 1, 1996.