Northrop YF-17 - Design and Development

Design and Development

The aircraft's main design elements date to early 1965, from the internal Northrop project N-300. The N-300 was itself derived from the F-5E, and features a longer fuselage, small leading-edge root extensions (LERX), and more powerful GE15-J1A1 turbojets, rated at 9,000 lbf (40 kN) each. The wing was slightly elevated to increase ordnance flexibility. The N-300 further evolved into the P-530 Cobra, which uses 13,000 lbf (58 kN) GE15-J1A5 engines, with a very small .25 bypass ratio leading to the nickname "leaky turbojet". The bypass effectively was only a cooling stream for the rear of the engine, allowing the engine bay to be constructed of lighter, cheaper materials.

The P-530's wing planform and nose section was similar to the F-5, with a trapezoidal shape formed by a sweep of 20° at the quarter-chord line, and an unswept trailing edge, but was over double the area, with 400 sq ft (37 m2) as opposed to 186 sq ft (17.3 m2) of the F-5E. Initially shoulder mounted, the wings were gradually shifted down to the mid position. Its most distinctive new feature were the LERXs that tapered into the fuselage under the cockpit. They enabled maneuvering at angles of attack exceeding 50°, by providing about 50% additional lift. The extensions also trapped airflow under them at high angles of attack, ensuring airflow into the engines. The resemblance to the head of a cobra lead to the adoption of the nickname "Cobra", often unofficially used for the YF-17.

The YF-17 was primarily constructed of aluminum, in conventional semi-monocoque stressed-skin construction, though over 900 lb (408 kg) of its structure were graphite/epoxy composite. The small nose contained a simple ranging radar. The cockpit sported an ejection seat inclined at 18°, a bubble canopy, and a head-up display (HUD). The thin wings carried no fuel, and in areas such as the leading and trailing edge and the LERX, were composed of a Nomex honeycomb core with composite facesheets. The rear of the aircraft featured twin all-moving stabilators of aluminum over a honeycomb core, and twin vertical stabilizers of a conventional construction. Like the wings, the leading and trailing edges were constructed of composite facesheets over honeycomb core. A composite speedbrake was located above and between the engines.

The aircraft was powered by a pair of 14,400-pound-force (64 kN) General Electric YJ101-GE-100 turbofans, a development of the GE15, mounted next to each other to minimize thrust asymmetry in the event of an engine loss. For ease of maintenance, the engines are mounted in a steady-rest that allows removal from below the aircraft, without disturbing the empennage controls. Each engine drove an independent hydraulic system. Unlike the P-530, the YF-17 sported a partial fly-by-wire control scheme, formally called the electronic control augmentation system (ECAS), utilizing ailerons, rudders, and stabilators for primary flight control.

Studies showed a single vertical stabilizer was insufficient at high angles of attack, and it was changed to twin vertical stabilizers, canted at 45°, resulting in a "relaxed longitudinal stability" design, which enhances maneuverability. Northrop was not yet confident in fly-by-wire controls and retained mechanically-signaled flight controls. The resulting aircraft, unveiled on 28 January 1971, advertised a maximum weight of 40,000 lb (18,144 kg) and maximum speed of Mach 2, but stirred little interest among foreign buyers.