by Indranil Roy & Nilesh Rane

With the Indian Air Force’s (IAF’s) MMRCA program getting serially delayed and recast more than once, there was a feeling in various quarters that the Tejas Mk2 design should perhaps evolve further than what was initially envisaged to provide an indigenous option for the IAF’s requirements. Thus, the IAF and the Aeronautical Development Agency (ADA) sat down to redefine the Tejas Mk2 with more elaborate modification such that it could function as a medium weight fighter for ground attack roles while continuing to be nimble in the air to air (A2A) role. In fact, the version of the Tejas Mk2 currently envisaged has been rebadged as the Medium Weight Fighter or (MWF) and is being designed as a replacement for the Mirage 2000 with a view to surpassing its capabilities in almost every respect.

The most eye-catching change is the addition of canards. Although, ADA had considered Light Combat Aircraft (LCA) configurations with canards as one the short-selected design concepts in the 1980s, it decided to drop the canards after careful wind tunnel studies. At the time, the advantages offered by the canard configuration were deemed to be minor in comparison to the added complexities of having an extra control surface. After all, this was the first time that a flight control system (FCS) was being designed for a fighter within India, let alone a highly unstable one. And SAAB’s experience with the Gripen in the 1990s has shown that this was a wise decision. Instead, ADA went with the iconic double delta wing with lower sweep inboard section.

Figure 1: Comparison of LCA Mk2 (MWF) fuselage CAD image with LCA MK1. Red dotted profile of Mk1 is superimposed on MK2’s fuselage. Approximate measurements highlighting changes in the fuselage length and some key features

However, now that a stable and robust FCS has already been designed, tested and validated, the addition of the canards is an incremental development which ADA is confident of undertaking. The canards in MWF are positioned below the avionics bay cover, just behind the cockpit. They are in close-coupled configuration and are positioned slightly ahead and above the wing plane for optimal aerodynamics. Close-coupled canards significantly affect the wing aerodynamics by helping to stabilise the wing leading edge (LE) vortices for medium to high angle of attack (AoA), thereby improving lift capability. In addition, they also produces significant lift by decreasing the wing loading, and provide better area ruling for reduced wave drag. In the air, they can act as an extra control surfaces for pitch and roll control, and on the ground, as air-brakes. In contrast, long coupled canards (as seen on Eurofighter Typhoon) are only meant to be control surfaces and they do not contribute much to the overall lift.

Figure 2: Front fuselage section showing Close-Coupled canards adopted for LCA Mk2 (MWF). The front fuselage is elongated by approximately 1.5m using two plugs and has increased height. The width remains same as that in MK1

In order to overcome the internal space constraints of Mk1, the MWF has been lengthened to 14.65 m, a sweet spot for a modern single engine multi-role fighter. This allows the fighter enough internal space for carrying the necessary systems internally while having enough fuel for the range, endurance and performance requirements. This increase in length is achieved using two plugs, one in the nose, and another behind the cockpit. As both of these plugs are ahead of the wing, the centre of gravity (CG) shifts forward with respect to the centre of lift, reducing the static stability margin, or in general terms, the manoeuvrability of an aircraft. Canards help to compensate for this by shifting the centre of lift forward proportionally to maintain the same static margin.

The canards also help smooth out the discontinuity in area ruling curve ahead of the wing that exists for Mk1 (see Area curve in Figure 2). By employing a canard and fatter spine, the MWF no longer needs as bulged a canopy as recommended by earlier studies. While those studies predicted a 6 percent supersonic wave drag resulting in a 20 percent improvement in transonic acceleration and 2 percent improvement in maximum speed, the MWF is expected to have even greater transonic and supersonic performance given a near perfect area ruling through the changes such as the addition of a nose plug, elongated and fattened front fuselage, optimised canopy shape and rear fuselage. The canards lower the trim drag across the flight envelope, further enhancing overall aircraft performance. As a matter of fact, the MWF is expected to have a top speed of Mach 1.8 in level flight with two CCMs, which is a 12.5 percent increase over its existing performance.

The shape of the canard was chosen after carefully studying a variety of geometries. Based on published CFD studies, the LE sweep is expected to be equal to 50°. At this angle, the LE sweep provided an optimal increase in the lift coefficient with a smooth and desirable linear variation in the pitching coefficient at high AoA regimes.

In summary, a pair of closed coupled canards offer the following advantages to the Tejas Mk2 MWF:
Maintain low wing loading by generating additional lift from canards
Improved wing lift and better aerodynamic stability of wing vortices
Reduced trim drag
Better area ruling for reduction in transonic and supersonic wave drag
Additional control surface for longitudinal control
Allows considerable increase in fuselage length, which is one of the key changes helping MWF completely fulfil the IAF’s original ASQR.
Could be used as air brakes during landing reducing landing roll. Canards also help reduce Take-off distance (Short Take-Off and Landing capability)
Figure 3: CFD Study showing effect of Canard LE sweep angle on overall aerodynamic performance of LCA. A canard with 50° sweep provides optimal lift increase and more desirable pitch moment characteristics in the high AoA regime

The MWF retains the main wing from MK1. The aerofoil design, dimensions and the upper interface with the fuselage also show no noticeable changes. On the other hand, the lower wing join shows better wing body blending which should lead to lower interference drag. The wingspan of the design has been increased slightly to 8.5 m on account of the addition of wingtip mounted CCM pylons (see Figure 4). The wing starts further aft of cockpit due to the addition of the two fuselage plugs in the front fuselage section. On the other hand, the air intakes have been slightly moved aft, proportional only to the significantly smaller nose plug. This has resulted in a configuration where the intakes are no longer shielded by the wing, as is the case with the Mk1. However, the introduction of the canards should provide this shielding effect which helps straighten and redirect airflow to the intakes during high angle of attack manoeuvres, to some extent.

Recently, some Request for Information (RFI) documents were issued related to the manufacture of assembly jigs for the fore, mid and aft fuselage sections of the MWF. Figure 4 shows a composite diagram obtained by joining three sections from the tenders.

Figure 4: LCA Mk2 fuselage CAD images from reference. Images of the three fuselage sections are scaled correctly and fused together

Figure 1 shows the above composite CAD image scaled with respect to an image of Tejas Mk1. This image allows us to compare feature of the MWF with respect to the Mk1. As stated above, the length of the fuselage has been increased to 14.65 m. The spine is slightly more bulged for better area ruling. The vertical tail sits on a lifted spine, increasing total height by an estimated 0.25m. The tail itself is expected to be extended by about 0.25 m due to elongated fuselage. Hence the total height has increased to 4.86 m.

Small strake or leading edge root extension (LERX) has been added ahead of the wing which extends till the point where the wing starts with respect to the cockpit in Mk1. The addition of LERX has a positive impact on the wing aerodynamics stability. Coupled with the canards, aerodynamic refinements, and enhance engine power, the MWF is designed to reach the IAF’s Mk1 ASQR requirement of a sustained turn rate (STR) of 18 degrees per second. The splitter plate as well as the air intake are canted backwards. This is expected to result in an optimal shock structure in the local vicinity and better intake performance at supersonic speeds. The improved intake cowl contours and the new 3-door auxiliary intake design will also be incorporated. All these modifications improve intake aerodynamics by improving pressure recovery and better uniformity of the flow at low speed, high Angle of Attack (AoA) regimes. This will lead to augmented thrust and reduced chances of engine stall. The low energy boundary layer flow separated by the splitter plate is redirected under the fuselage now. The slot which redirects some of this air over the wing in Tejas Mk1 is eliminated in the MWF. Other drag reduction features in the aft fuselage as envisioned in the original Mk2 design can also be seen on the optimised fuselage of MWF.

MWF will be a multi-role aircraft capable of carrying R-73 (and possibly ASRAAM) CCMs, Derby and Astra BVRs, 250 kg and 500 kg dumb and laser guided bombs, heavy precision glide bombs of standoff ranges, India’s New Generation Anti-Radiation Missile besides lightweight cruise missiles. It will sport an active electronic scanned array (AESA) radar with an integral Unified Electronic Warfare Suite (UEWS) and a dual color Missile Approach Warning System (MAWS) along with an upgraded glass cockpit with larger MFDs. With enhanced network centric warfare capability, the MWF represents a multi-fold increase in capability over the Mk1.

Specification LCA MK1 LCA Mk2 Mirage 2000 Gripen E
Length 13.2 m 14.65 m 14.36 m 15.2 m
Height 4.4 m 4.86 m 5.2 m 4.5 m
Wingspan 8.2 m 8.5 m 9.13 m 8.6 m
Wing Area 38.4 m2 38.4 m2 41 m2 31 m2
Empty weight 7040 kg 7500* kg 7500 kg 8000 kg
Take-off Clean Weight 10300 kg 12,000* kg
Internal Fuel 2300 kg 3300 kg 3200 kg 3400 kg
Hard points 7 + 1 11 9 9 + 1
Max Take Off Weight 13,500 kg 17,500 kg 17500 kg 16500 kg
Max payload capacity 3910 kg 6300 kg 6300 kg 6000 kg
Engine F404-IN20 F414-INS6 M53-P20 F414
Max Thrust 84kN 98kN 98kN 98kN
Max Speed 1.6M 1.8M 2.2M 2.0M
G limits +8/-3.5 +9/-3.2 +9/-3
Ferry Range 1750 km 3500 km 3335 km 4000 km
Table 1: LCA MK2 (MWF) specifications compared with those of LCA-MK1, Mirage-2000 and Gripen E. (* estimated or expected)

The expected features of the MWF are listed in Table 1 above and compared to that of Tejas Mk1, Mirage 2000 and Gripen E. The payload capacity of MWF is anticipated to be over six tons as compared to the Mk1’s 3.9 tons. To carry this increased payload, the number of pylons have been increased from 8 to 11. The gun has been moved to a shoulder mounted position which has freed up space below the right intake for an additional pylon. Each wing also features four stations instead of the current three. ADA is also developing multi-rack pylons for carrying two BVR AAMs. The addition of the nose plug has also afforded space for an infra-red search and track (IRST) system, and the fuselage plugs facilitate the availability of space for an internal self-protection Jammer and significantly higher internal fuel. Cumulatively, these changes not only enhance mission capability of the aircraft, but add greatly to its flexibility.

Every successful fighter aircraft till date has evolved and refined over many tranches and iterations to reach its final optimised version. The process of development itself leads to an increase in the knowledge and confidence of the designers and the associated manufacturing group. This allows the shortening of development time of not only the next iteration of the aircraft, but also for next generation aircraft as well. The Tejas story is no exception to this fundamental fact and on its shoulders stand the development of India’s future generation of fighter aircraft. Therefore, the evolution of the Tejas can rightly be called the evolution of India’s fighter aircraft industry.