Much of the basic design of the Eurofighter was derived from BAE Systems Experimental Aircraft Programme (EAP) and its preceding projects (P.106, P.110, TFK-90). However there do exist some notable differences between the current Eurofighter and its EAP cousin. For example while the EAP utilised a cranked delta layout the Eurofighter instead uses a standard delta configuration. Other differences include the inclusion of conformal recessed fuselage weaponry, a wide mouthed curved intake and a bubble type canopy.

Construction responsibility for the structure is split amongst the consortium. BAe manufactures the front fuselage, canards, starboard leading wing slats and flaperons, fin and centreline pylon. DASA constructs the centre fuselage, Alenia is responsible for the port wing, CASA and Alenia builds the rear fuselage and CASA and BAe build the starboard wing. Each nation maintains its own final assembly line thus ensuring local delivery times can be met but at a likely cost increase due to four-way shipping requirements.

 Structural Design

Structural details

The Eurofighter is a pitch unstable, delta-canard tail-less design with a 53° leading edge sweepback on the main wing. This configuration was found to give an optimal combination of lift and agility. With a wing area of around 50sqm it has a small loading in a typical combat situation which implies very good manoeuvrability. Pitch instability causes the aircraft to point its nose up during flight further increasing agility and helping to reduce drag.

With no tail the all-moving foreplanes, or canards impart pitch and roll control combined with the wing flaperons and rudder. In addition the canards can be used to trim the aircraft through different flight regimes minimising drag. The canards may also be used as an extra pair of airbrakes when landing by pointing them straight down maximising drag. Unusually the canards are mounted much nearer the nose than is typically found in similar aircraft. This increases the maximum achievable Angle of Attack (AoA). The drawback to this is a decreased view to the left and right of the pilot. Automatic slats are present on the main wing leading edges which ensure the correct wing camber is maintained across the flight envelope. A hydraulically operated air-brake is integrated behind the cockpit, moving into a near-vertical position to maximise drag when required.

Since the project began the Typhoon has undergone several increases in weight from the original 9750kg (21450lbs) to the current 10995kg (24239lbs). In turn this has resulted in the aircraft missing the intended final weight by a small margin, around 20 to 30kg. However it has resulted in a far more rigid aircraft with an improved multi-role capability. It has also allowed the amount of internal fuel to be increased by several hundred kilos.

Eurofighter Typhoon elevations Click images for larger view Typhoon Front Elevation © WAPJ Typhoon Plan View © WAPJ Typhoon Side Elevation © WAPJ

The air intakes are positioned below the aircraft (ventral mounting) much like the F-16. This ensures an uninterrupted airflow to the engines whatever the Angle of Attack or velocity. The intakes themselves are S-shaped along their length, this ensures that the engine turbine blades are not within frontal view of the intake, which in turn lowers the frontal RCS. In addition the intake box has a rounded bottom with sloping sides, improving air flow and further reducing the RCS. The intake is fitted with a variable position lower cowl, the upper cowl is fixed.

The particular combination and design of both the control surfaces and FCS combined with the high thrust output of the EJ200's imparts extremely good manoeuvrability throughout the flight envelope. According to comments from BAE Systems, the turning performance at supersonic velocities is superior to any of its contemporaries from either the West or East. The airframe itself is designed to last 6000 hours the equivalent of 30 years service. In September 1998 static testing of the airframe was completed with a simulated 18000 hours on the clock, or three times the expected life.

Ancillary structures

The ability to stay on station enhances any aircraft capabilities and to enable this the Typhoon is equipped with a standard retractable NATO refuelling probe. The system is mounted within a small starboard compartment just below the canopy. This gives the pilot excellent visibility during refuelling operations. In any emergency fuel can be jettisoned through a tail mounted ejector pipe.

One of the requirements imposed on the Eurofighter is the ability to operate from hastily prepared landing and take-off strips. To this end particular attention needs to be paid to the landing gear. The system used in the Typhoon is constructed by Dowty Aerospace. The gear is a relatively standard tricycle type with a single wheel on each unit. The wing units retract inwards while the nose unit retracts backward. Each wing, or main wheel measures 28" by 9.5" while the nose wheel measures 18" by 7.7". Actual control of the nose wheel is handled as a secondary function by the FCS. Additionally an emergency arrestor hook is fitted to the rear of fuselage.

 Materials

Cut-away of Eurofighter Typhoon structure © BAE Systems

Eurofighter benefits from advances over the past twenty years in the fields of metallurgy, polymer science and composites. Over 80% of the airframe is comprised of modern materials. This brings advantages not only in terms of the strength to weight ratio but also has implications for stealth. For example a far smoother surface finish can be obtained compared to metal structures which in turn can be a significant factor in reducing the radar cross section of an aircraft.

The materials used have as with all parts of the aircraft been developed jointly by the consortium, here EADS/CASA is the project leader. It addition to new materials, new reliable and efficient methods for fabricating and bonding them, such as Super Plastic Forming, Diffusion Bonded (SPFDB) Titanium have been developed. These new techniques not only speed production they also improve the materials physical properties in various ways.

Most of the aircraft shell, >70% is comprised of Carbon Fibre Composite (CFC), namely; the outer fuselage, wings (including in-board flaperons) and rudder. Additionally a significant proportion of the structural members are also constructed from CFC. The canards, out-board flaperons and engine nozzles are subject to large stresses and/or high temperatures and thus are made from SPFDB Titanium. The SPFDB process yields a far more rigid structure resulting in an improved strength to weight ratio compared to normal, machined Titanium. The wing leading edges, fin leading edges, rudder trailing edge and wingtip DASS/ECM pods are made from a Lithium-Aluminium alloy imparting superior strength to weight than standard aluminium alloys. Additionally these areas are also coated in Radar Absorbent Materials (RAM). The canopy seal surrounds are manufactured from a Magnesium alloy.

The radome is comprised of a complex layered Glass Fibre Reinforced Plastic (GFRP) structure manufactured using very high tolerance automated processes. Since the material used to construct the radome must be transparent to microwave energy it is an obvious source of Radar Cross Section (RCS) reduction problems. To overcome this, BASE, British Aerospace Systems and Equipment who supply the radome structure have developed various Frequency Sselective Surface (FSS) materials which have been subsequently put to use in the Typhoon's radome. FSS materials are composed of a precisely defined array of metallic elements contained within a conducting frame. The use of these materials (when laid up in the correct fashion) results in a reduction in the transmission of all out of band frequencies. Therefore the radome can be designed to be transparent only to those frequencies and polarisation's used by the aircraft's own radar. This of course should lead to a reduction in the aircraft's radar cross section, from all frontal aspects at least.

Overall only 15% of the Eurofighter shell is metal while 40% of the structural weight comprises CFC.

 Reduced Observability

The Eurofighter Typhoon cannot and is not classed as a stealth fighter (see fact box). However the consortium did take measures to reduce the aircraft's radar cross section. Many of these Reduced Observable (RO) features were tested over the years at BAE Systems covered radar signature range at BAe Warton near Preston, NW England. Some examples of this design include; the intakes which are shaped so as to hide the engine compressor blades, the sloped intake sides, the fuselage recessed medium range weapons, the wing hardpoint placement and design, radome construction, etc. In addition Radar Absorbent Materials (RAM) developed primarily by EADS/DASA coat many of the most significant reflectors, e.g. the wing leading edges, the intake edges and interior, the rudder surrounds, strakes, etc.

The actual radar cross section is of course classified, it is however set out for the RAF in SR(A)-425. According to the RAF the Eurofighter's RCS more than exceeds these requirements. More recent comments from BAE seem to indicate the radar return is around four times less than the Tornado. During a recent press event BAE Systems stated that the Typhoon's RCS is bettered only by the F-22 in the frontal hemisphere and betters the F-22 at some angles. Although the later comment is very questionable it still indicates a real attempt to reduce the Typhoon's radar signature. This should enable a Eurofighter pilot to remain undetected by his enemy until he his significantly closer than he may otherwise be able to achieve.

Other issues

It is important to remember there is more to being stealthy than simply having an aircraft with a low RCS. Most current radar and radio sets for example can be relatively easily detected by enemy aircraft support measures and warning receivers and used to plot position. At this time Eurofighter's ECR-90 radar falls into this category of being relatively easily to detect. However thanks to the integration of both on-board and off-board sensors the Typhoon can somewhat circumvent this problem. Of these the single greatest asset is PIRATE and its Infra-Red detector which is entirely passive in nature and thus impossible to detect. The secure radio systems as well as the MIDS datalink (providing the off-board target information) are said to incorporate Low Probability of Detection and Exploitation (LPI) features. In addition the aircraft features automatic EMission CONtrols, or EMCON. Although these precautions do not prevent an opposing aircraft from detecting Electro-Magnetic (EM) emissions from the Typhoon they should limit the likelihood of such interception or their subsequent utilisation.

The webmasters would like to express their thanks to BASE and particularly Chris Tear for providing information on the radome.

Sources :

[1] : BAE Systems On-Line.
[2] : World Air Power Journal, Various Issues
[3] : Janes All the Worlds Aircraft 1996/97
[4] : BAE Systems, UK
[5] : Airforces Monthly, November 1997
[6] : John Turner, BAe press briefing, March 3rd 1999 (reported in Air International)
[7] : British Aerospace Systems and Equipment, Devon, UK
[8] : Flight International, 16-22 June 1999