- 2 January 2006
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[pictures replaced with higher resolution copy below]
[pictures replaced with higher resolution copy below]
These pictures depict the McDonnell Douglas Model 279-3 concept for a Stovl fighter. A 9.2 per cent scale model f top,) is about to begin windtunnel testing at Nasa's Ames facility. The proposed aircraft ( centre) uses 41 per cent composites and is powered by an advanced Rolls-Royce Pegasus in four-poster configuration producing 36,900lb thrust with fan stream burning. Maximum speed is Mach 2.0 with acceleration from Mach 0.8 to Mach 1.6 in 70sec. Model 279-3 is one of four Stovl concepts now being evaluated for further work by Nasa
Powered by a Pratt & Whitney STF561-C2 turbofan producing 34,000lb with plenum chamber burning, the McDonnell Douglas
Model 279-3 can reach Mach 2. VTO weight is 29,840lb for a 103 n.m. combat radius
Not surprisingly, the McDonnell Douglas submission is a vectored thrust design based on the proven Harrier/Pegasus principle a single 1: 1 bypass-ratio turbofan fitted with rotating nozzles which divert fan and core air downwards for take-off and landing, rearwards for forward flight, and anywhere in between for STO or manoeuvring.Add reheat to the forward, "cold" nozzles and the concept is suitable for supersonic aircraft. Plenum chamber burning (PCB) brings with it design problems, but they can be identified more clearly at this stage than those of any other approach. The engine, with its large-diameter fan, must be located close to the e.g., resulting in an unfashionably fat fuselage. This must be stretched, adding weight, to achieve reasonably low supersonic drag. So much fiery air expelled close to the ground can pose both exhaust reingestion and ground-erosion problems.
The McDonnell Douglas Model 279-3 is a wing/canard design with a 48,800lb gross weight for a ski-jump take-off. Powerplant is a PCB-equipped Pegasus-type engine by Pratt & Whitney, associate to Rolls-Royce on the US Marine Corps AV-8B Harrier II programme. Instead of the raked intakes associated with supersonic flight, the Model 279-3 has short inlets of the type used successfully on the AV-8B, but with half-cone centrebodies.
The all-moving canard destabilises the aircraft, so a digital fly-by-wire flight-control system is used to restore stability artificially. Relative position and output of the front and rear nozzles is governed by the flight-control system for pitch control and vectoring in forward flight: Vtol pitch attitude is controlled by modulating thrust from the forward nozzles, while differential deflection of front and rear nozzles helps lift the nose when wing high-lift flaps are lowered.
McDonnell Douglas pursues Harrier concept
McDonnell Douglas is fully committed to the four-poster vectored thrust concept for its supersonic Stovl fighter design. Windtunnel testing of a 9.2 per cent scale model at Nasa's Ames facility is imminent, and will last 1,500hr. The Model 279-3 is designed to Navy requirements issued 2 years ago. The aircraft uses 41 per cent composites by weight, and draws on technology developed for the AV-8B, F-15, and F-18, all of which are now in production at St Louis. Model 279-3 is a Mach 2.0 aircraft, capable of pulling 6-9g at Mach 0-65 and accelerating from Mach 0.8 to Mach 1.6 in 70sec at 35,000ft. "It has F-15 manoeuvre characteristics combined with vertical take-off and landing," says Dick Martens, senior advanced V/Stol programme engineer at McDonnell Douglas.
Low risk is the theme of MD's work. The company is throwing a lot of effort into developing an idea it is confident could work. Its plenum-chamber burning design aims to equalise all four nozzle temperatures, possibly using independent vectoring front and rear. Integration of engine and airframe would draw on ideas now being developed by the company for its Advanced Tactical Fighter design, due to be submitted to the USAF soon. Windtunnel testing will concentrate on solving hot gas re-ingestion (HGR) and ground effects problems. Like Rolls-Royce, it suggests sizing the engine upwards to reduce PCB temperatures. Dick Martens notes that in any case the need for good acceleration to supersonic speed dictates a larger engine than currently used. Another point, he says, is that a surprisingly small reduction of thrust in such an engine results in a useful drop in PCB temperatures. Hot gas-re-ingestion trials are being Pegasus on full PCB at Shoeburyness. PCB would boost the thrust of today's Pegasus II to more than 30,000lb carried out using water and a 4.1 per cent scale model. Dyed water will be pumped through oversized front nozzles and illuminated by laser so that effects of toeing in the nozzles, auxiliary engine inlets and retractable deflectors can be assessed. Extra MD funding covers research into ground and suckdown effects, looking in particular at the use of lift improvement devices (Lids). "It's all inexpensive testing," says Martens.
A demonstrator, if funded, would also be a low-cost machine, with forward cockpit and fin from the F/A-18, canard from the A-7 (tail), and an AV-8A wing. Co-ordination of the work being done by McDonnell Douglas and other US manufacturers with that under way in the UK is the aim of a Memorandum of Understanding that should be signed before long by the two Governments. The idea is to formalise the supersonic Stovl effort, rationalise the many concepts being proposed, and concentrate resources. In the USA the job of co-ordination is more difficult than in the UK, where research means the Royal Aircraft Establishment, and the industry is British Aerospace and Rolls-Royce. Nasa has already received proposals from the many interested airframe and engine companies and is working now to identify critical technologies thought suitable for further study. The hope is that the USA and UK will be able to exchange technology plans before the Farnborough Air Show, so that final discussions on a joint technical plan for each of the four main concepts can be completed around show time. The four concepts are: PCB Harrier (McDonnell Douglas); ejector (General Dynamics); remote augmented lift system (Grumman); and tandem fan (Vought). A detailed description of each concept appeared in our last Military Propulsion special, published on January 15, 1983.
The concepts studied by McDonnell-Douglas (MCAIR) is a canard/wing design with swiveling nozzles forward and aft of the aircraft center of gravity. The four poster configuration, MCAIR model 279-3, is seen in Figs. 69 and 70; 69 depicts the vertical flight configuration, and the cruise flight mode is shown in Fig. 70. References 51 and 58 give details of this concept.
Configuration. Model 279-3 features a close-coupled canard and side-mounted half-axisymmetric inlets to provide air to a single engine with modulated fan-stream augmentation. Four swiveling nozzles provide thrust vectoring capability for vertical flight as well as for in-flight maneuvering. Fan air flows through the forward nozzles and the engine core flow exits through the aft nozzles. Modulation of the fan stream and engine speed provides the capability of trimming center of gravity travel associated with fuel burnoff and store loading. This modulation can also provide a portion of the pitch maneuvering control or can be used as a backup system. The location of the aft nozzles near the wing trailing edge offers the potential of enhanced circulation, translating into increased maneuverability and STOL performance. Thrust vectoring can increase the sustained load factor of Model 279-3 by 0.2 g and the instantaneous load factor by 2.0 g's at 0.6 Mach number at an altitude of 3,048 m (10,000 ft). As shown in Fig. 69, the main landing gear of Model 279-3 are located fore and aft on the fuselage in a bicycle fashion with outriggers in pods on the wing. Three views of the MCAIR concept are shown in Fig. 71, and a dimensional summary is given in Table 9. The wing has an aspect ratio of 3.0, a leading-edge sweep of 45 ° , and 9° of anhedral. The close-coupled canard is mounted high on the inlet sides and has 0 ° of dihedral, a leading-edge sweep of 50 °, and an aspect ratio of 3.0. The exposed area of the canard is 20% of the wing reference area. The single vertical tail is mounted on the aft fuselage.The configuration has a vertical takeoff wing loading of 3.34 kN/m 2 (69.7 Ib/ft 2) and a tropical-day, vertical-takeoff, thrust-to-weight ratio of 1.15 with full fan-stream burning.
Aerodynamic Surfaces. Pitch control is provided by the all-movable, close-coupled horizontal canard; roll control by the differential ailerons; and directional control by the rudder. The wing leading and trailing edge flaps and also the canard are deflected as a function of angle of attack and Mach number to maximize maneuvering capability. The leading edge flaps also are used supersonically as decamber flaps to reduce drag. The trailing edge flaps, which are plain flaps at small deflections, become single slotted flaps at large deflections, for high-lift operation. These flaps, which are close to the aft nozzle, increase the STOL lift. The location of the forward nozzle under the wing, rather than at the leading edge, also improves lift during STOL.The wing planform selection is based on a compromise between subsonic and supersonic performance. Subsonic emphasis is on high sustained maneuverability requiring low drag due to lift. Supersonic emphasis is on lower-lift-coefficient maneuvering conditions during which the minimum drag coefficient CDo is equally important.The wing airfoil camber increases outboard on the wing. There is no twist at the wing-fuselage juncture, but there is leading-edge-down twist at the wing tip.
Control System. The Model 279-3 has a digital fly-by-wire control system, which is necessary to augment the subsonic longitudinal instability. This active control system also makes possible (1) engine/fan-stream augmentation/reaction-control-system integration, (2) augmented thrust-vectoring control, and (3) coupled flight/propulsion control. A three-axis reaction control system (RCS), operating on engine bleed air, provides control moments independent of dynamic pressure. During VTOL operation it provides the complete maneuvering control. The pitch RCS is located in the aft fuselage and the forward lower mold line of the inlet, just forward of the nose gear. The lateral RCS thrusts both up and down in opposite wing tips. The directional RCS, thrusting laterally in either direction, is located in the aft tip of the fuselage. During VTOL operation the thrust center is positioned by varying the engine speed and the fan-stream augmentation, using the flight controller. Decreasing the forward nozzle thrust moves the thrust center aft, with the level of thrust maintained by increasing the engine speed. This provides the static trim during VTOL; transient control is provided by the pitch RCS. Additional control is provided by the engine nozzle thrust-vectoring control (TVC). The fore and aft nozzles are symmetrically deflected a small amount for rapid load-factor changes, with rapid turns plus deceleration followed by acceleration. Differential deflection of the fore and aft nozzles is used for STOL control to augment the canard deflection in controlling the high-lift flap pitching moment.
Propulsion System. A single, advanced Pratt and Whitney thrust-vectoring engine (STF 561-C2) with fanstream augmentation serves as the propulsion system. It has a twin-spool turbofan gas generator utilizing a two-stage fan and a five-stage low-aspect-ratio high-through-flow axial compressor with a single-stage, high pressure turbine and a two-stage, low-pressure turbine. The bypass ratio is 1.16, the overall pressure ratio is 25.0, and the fan pressure ratio is 3.50. Table 10 gives additional propulsion system characteristics.The forward, side-mounted nozzles incorporate fan-stream burning augmentors. There is no engine-core augmentation associated with the aft nozzles. The half-axisymmetric, side-mounted inlets have fixed 16.5° half-conical spikes.
Structure. Composites are used extensively in the Model 279-3. The structural weight consists of 41% graphite epoxy, 21% aluminum, 13% titanium, 8% steel, and 17% other materials. Graphite epoxy is distributed as follows: wing 50%, canard 52%, vertical tail 65%, fuselage 46%, and the engine section 55%.
Mission Performance. MCAIR sized the configuration to the vertical-takeoff, supersonic, DLI mission defined in Fig. 72. Weapons and ammunition are retained throughout the mission. To accomplish this mission and remain within the guideline vertical takeoff gross weight of 13,606 kg (30,000 Ib), the aircraft has a mission radius of 191 km (103 nmi.) and a vertical takeoff gross weight of 13,535 kg (29,840 lb). With full internal fuel [gross weight = 14,161 kg (31,220 lb)] and a rolling takeoff of less than 15 m (50 ft), the radius of the DLI mission is increased to 296 km (160 n. mi.) A weight summary for the vertical takeoff supersonic DLI mission is given in Table 11.
Performance of the Model 279-3 and NASA guideline performance are shown in Table 12. As indicated, all performance requirements are met or exceeded.
The STO characteristics of the Model 279-3 with full internal fuel have been determined by MCAIR for both a flat deck and a 12 ° ski jump. For a 122-m (400-ft) flat-deck run'with zero wind over the deck, the Model 279-3 has an STO gross weight of 18,960 kg (41,800 Ib) as shown in Table 12. With this same takeoff run, the STO weight is increased 17%, to 22,135 kg (48,800 Ib) using the ski jump.
Mark Nankivil said:Good Day All -
I recently picked up the aviation collection of a friend who passed away earlier this year. Most of the collection is made up of 35mm slides but in the prints I came across the following artwork. When I saw this, I realized it matched many of the features of a small McDonnell Douglas model we have in the Museum collection. Any ideas on a Model #?
Enjoy the Day! Mark