Rafale - Design
The RAFALE features a delta wing with close-coupled canards. In-house research in computational fluid dynamics has shown the specific benefits of close coupling between the wings and the canards: it ensures a wide range of centre of gravity positions for all flight conditions as well as benign handling throughout the whole flight envelope. The choice of the close-coupled canards / delta wing configuration was decisive to ensure that the new fighter would offer the highest levels of performance during air-combats: even at high angles of attack, the RAFALE remains superbly agile, and its operational range for strikes at very long distances with incredibly heavy weapon loads is unmatched for such a compact design. Every effort has been made for the sake of tactical flexibility to obtain balanced performance between subsonic and supersonic regimes, either in heavy or lighter air-to-air configurations.
The Rafale configuration is in line with the family of delta wing aircraft which started with MIRAGE III aircraft and which, later, gave birth to MIRAGE 2000 aircraft then to the "canard + delta wing aircraft". This latter configuration dates from the "MILAN" aircraft which, in 1969 with its retractable "nose fins", was the first attempt within DASSAULT to decrease the relatively high approach speed of MIRAGE III aircraft (180 kts). Then in 1979 it was the MIRAGE 4000 aircraft and in 1982 the MIRAGE III NG aircraft. The MIRAGE 4000 aircraft is equipped with fixed canard fins, designed to improve its maneuverability, which can be disengaged in caso of multiple failure of the flight control system. This gives back stability to the aircraft and enables more traditional flying control.
The advantages of the delta moving canard configuration such as it is on the RAFALE are multiple. This configuration enahles excellent wing efficiency, especially at high angles-of-attack, due to deflection of the air flow on the wing by the foreplane, and extensive control of the aircraft's centre of gravity, thanks to the aerodynamic center effect created by the canard. It is the mastery of longitudinal balance that guarantees high maneuverability throughout the flight envelope. It has been proved in combat simulation that the negative static margin obtained, thanks to the fly-by-wire controls, which was optimum, depends on the optimum limit of maneuver. The selection of negative static margin thus made, a canard dimension linked to the selection of the aircraft c.g. position is obtained. A certain number of new FCS functions, as for instance gust alleviation, are decisive for multi-role aircraft. In fact the possibility of delaying the accelerations felt by the pilot at high speed and low altitude (penetration mission) makes possible the selection of larger wings which leads to an improvement of the aircraft qualities in the Air-to-Air dog fight (air superiority mission).
The semi-ventral pitot air intakes, which are of an entirely new design issuing from many computations and tests, meet specific technological requirements, such as improvement in air intake efficiency at high angle-of-attack thanks to the protection provided by the forward fuselage, and improvement in the quality of air supplied to the engines by increasing the stationary and unstationary homogeneity of the airflow. This permits maintaining a Mach 2 capability, while at the same time achieving simplicity no moving devices or bleeds. The design features complete separation of the right and left air intakes so that misfunctions of one does not affect the other engine, and also to allow sufficient space for installation of a forward retraction nose gear, leaving a large amount of space for carrying long under-fuselage stores.
The architecture of the aircraft air intakes, nose and main undercarriages gives the capability for a large store to be carried under the fuselage, which is essential to achieve certain Air-to-Ground missions. Certain configurations, such as those with air-to-air missiles conformal to the fuselage, have been designed especially to reduce drag and radar signature. Configurations were tested in the wind tunnel for under fuselage tanden-mounted missiles. The structural optimization enabled the installation of the missile ejectors inside the aircraft. This store carrying capacity, which is exceptional for an aircraft of this size, has been obtained by opting for a mid-fuselage wing location and desiqning a special linkaqe system for the rose gear that minimizes the space required under the front section for retraction and extension of the qear.
The wings have advanced mechanization: each wing panel has three elevons and a three-section leading-edge flap across its whole sweep, automatically controllable along with the elevons. The control system is a remote electronic one with fourfold redundancy. Depending on what is being carried on external mounts, it provides for the automatic limitation of handling according to the angle of attack, g-loading and the angular roll velocity.
DASSAULT AVIATION has long been recognised for designing sturdy airframes that sustain over 30 years of operation without heavy structural upgrades. Thanks to the DASSAULT AVIATION unique know-how in finite element modelisation, the RAFALE airframe fatigue is monitored with the same gauge-free concept which has proved its worth on the MIRAGE 2000 fleet.
An advanced digital “Fly-by-Wire” (FBW) Flight Control System (FCS) provides for longitudinal stability and superior handling performance. The FCS is quadruple redundant with three digital channels and one separately designed analog channel, with no mechanical back-up: design independence between channels is key to avoiding simultaneous anomalies on all channels. Design independence between channels is pivotal in preventing anomalies simultaneously affecting several channels. This is a unique feature, the result of Dassault Aviation's extensive in-house experience in FBW design: in over one million flight hours with full fly-by-wire (i.e. without any mechanical back-up), not a single accident has ever been caused by the Flight Control System.
The Rafale flight control system (FCS) is a full authority digital system in a longitudinally unstable aerodynamic configuration. The FCS was designed to control an aerodynamic configuration that was optimized from aerodynamics, buffet, lift, drag, structural loads, and control surface loads considerations. The FCS functionality provides 6 axis stability augmentation on ground and in the air including gust alleviation. Handling qualities are optimized to give carefree handling and structural protection by controlling incidence sideslip, normal acceleration, roll rate and roll acceleration. Autopilot functions are included, which utilize novel inceptors such as brake pedals to provide input commands. The FCS processes all the air data sensor inputs and is integrated with the avionic and weapon delivery systems in the provision of general navigation, terrain following, auto approach and landing (for both land and carrier based operations), and auto weapon delivery functions. The FCS was designed with the safety specification requirement for a failure rate better than 10-6 per flying hour. This requirement included the FCS, hydraulics, and electrical supply system. The redundancy of the system is either quadruplex, triplex, or duplex depending upon the criticality of the subsystem. For example, quadruplex functions include inertial sensors, actuator-loop control, and moding status whereas air data is triplex.
The RAFALE is safe and easy to fly in all flight regimes, featuring the same precise, yet benign handling performance in all load-out configurations throughout the flight envelope. The flight control system of the RAFALE offers auto flight in terrain following mode in all weather conditions, allowing the RAFALE to fly unobserved in the opponent’s airspace: an important survivability factor in a high threat environment.
Moreover, Terrain Following modes allow the RAFALE to automatically fly unobserved at very low altitudes whatever the weather conditions, optimising its survivability in a high threat environment. Minimising the radar cross section has also been a design driver in order to make stealth tactics possible. Most of the stealth design features are classified, but some of them are clearly visible, such as the serrated patterns on the trailing edges of the wings and canards. Dassault Aviation has a long praised tradition of designing sturdy airframes that sustain over 30 years of operation without heavy structural retrofit. Thanks to a Dassault Aviation unique know-how in finite element modelisation, the RAFALE airframe fatigue is monitored with a gauge-free concept, a now proven concept which, day after day, has demonstrated its relevance on the Mirage 2000 fleet.
Composite materials are extensively used in the RAFALE and they account for 70% of the wetted area. They also account for the 40% increase in the max take-off weight to empty weight ratio compared with traditional airframes built of aluminium and titanium.
The M88-2 incorporates advanced technologies such as integrally bladed compressor disks (“blisks”), a low-pollution combustor with smoke-free emissions, single-crystal high-pressure turbine blades, ceramic coatings, and composite materials. It also features the latest advances in reducing electromagnetic and infrared signatures. In short, the M88-2 is a very compact powerplant, offering a high thrust-to-weight ratio and exceptional controllability, especially in terms of acceleration.
The M88-2 powerplant is rated at 10,971 lbs dry and 16,620 lbs with afterburner (also reported as 11,250 dry to 17,000 lb of thrust with afterburner). It is equipped with redundant “Full Authority Digital Engine Control” (FADEC), which provides for carefree engine handling anywhere in the flight envelope : the throttle can be slammed from combat power to idle and back to combat power again, with less than three seconds from idle to full afterburner.
Launched in 2008, the M88 TCO (“Total Cost of Ownership”) programme was initiated to further improve engine durability and bring support costs down. Capitalising on the ECO project, SNECMA was able to upgrade the high-pressure compressor and the high-pressure turbine of the M88-2 : cooling is ameliorated and stronger components have been introduced, boosting durability by up to 50%. Life expectancy between overhaul has been considerably expanded for a number of modules, helping further minimise the impact of planned maintenance on engine availability.
The M88 is the subject of a constant improvement effort by SNECMA, leading to the latest M88-4E version, which builds on the TCO program. To further ameliorate performance and increase the life of some critical components, Snecma (Safran Group) engineers designed a new M88-2 variant called "4E". This variant is fitted with an upgraded high-pressure compressor and with an improved high-pressure turbine. With these new modules, engine durability and availability will be significantly increased while operating and maintenance costs will be brought down.. From 2013 RAFALE aircraft will come out of the production line with M88-4Es fitted on them. The M88-2 was designed from the outset for high dispatch reliability, along with easy maintainability and lower operating costs, to reduce the overall cost of ownership.
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