A Detailed Look At The Design Evolution Of India’s Naval-LCA Mk1 Fighter

September 2019 will go down as a seminal month in the history of Indian naval aviation, marked as it was by multiple milestones. On September 19, prototype NP-1 of the indigenous Light Combat Aircraft- Navy Mk1 (NLCA MK1) design became the first ever naval fighter aircraft developed in India to carry out an arrested landing. This feat took place at the Shore Based Test Facility (SBTF) in INS Hansa, Goa, where NP-1 would go on to successfully complete eight more arrested landings in the course of the month.  Rounding off a crucial period in Indian military aviation, on September 29, a second NLCA Mk1 prototype, NP-2, would complete another first for India by taking off from a ski-jump and then completing an arrested landing during the same test mission. What is more, NP-2 would accomplish the same feat later in the day. Indeed, it must be observed that such sortie rates are usually expected of operational aircraft, not prototypes in testing.  As such, these developments mark the coming of age of India’s naval fighter program and establish the credentials of the team which undertook perhaps the most difficult challenge of modern aviation – the development of a modern carrier-based fighter aircraft.

After all, India embarked on this journey (i.e. indigenous carrier-based aviation development) relatively late and somewhat half-heartedly, trying to piggyback on the development of the shore-based fighter program, the LCA Tejas. Consequently, as of today, India has not been able to develop a fighter that can fulfill all of the Indian Navy’s (IN’s) requirements, the recent successes registered by NP-1 and 2, notwithstanding. Nonetheless, the NLCA Mk1 project serves as a technology demonstrator (TD) program that can certainly be used to develop a fully specifications-compliant aircraft in the near future. NLCA Mk1 can also potentially be used as a Lead in Fighter Trainer (LIFT) at a significantly lower operating cost than what is incurred by using the MiG-29KUB in that role. It is therefore worthwhile to take a closer look at what the NLCA Mk1 brings to the table in terms of design pedigree.

Program Overview

The idea of developing a naval version of the LCA can be traced back to the Project Definition Phase (PDP) of the LCA in the late 1980s [1]. Ten years after the LCA project was sanctioned in 1983, the IN made preliminary inquiries with respect to the possibility of navalizing the baseline LCA design. The Aeronautical Development Agency (ADA), the LCA’s developer, having already anticipated the prospect, gave an enthusiastic response to the IN. Subsequently, a feasibility study was launched in 1995 as a result of which the LCA Navy program was sanctioned in stages: Project Definition Phase (PDP), Pre-Project Phase (PPP) and Full-Scale Engineering Development (FSED). The PDP was undertaken in collaboration with the Central Aero-hydrodynamic Institute (TsAGI), Russia to identify the key design drivers and preliminary design of the NLCA.

The earliest public depiction of the LCA design with Leading-edge vortex controllers (LEVCONs), which is a signature design feature of the NLCA Mk1 can be found in a 1995 research paper from ADA (See Fig.1 below). By this time, a sizable amount of the PDP-related work had already been done. The total expenditure on PDP was only about Rs 14 crores. The basic goal of the study was to develop a navalized design with minimum changes to the LCA Air Force version. Accordingly, the critical design drivers identified in the feasibility study were:

• Strengthening of landing gear (LG) for higher sink rate of flare-less landing on carrier deck
• Reduction in landing approach speed to match the maximum speed, any putative arrester gear system could handle
• Arrester hook and supporting structure design
• Over-the-nose vision during landing approach.
• Controllability of the aircraft after ramp exit until its airborne.

Figure 1: LCA MK1 with LEVCON configuration from 1995 ADA Publication

FSED-1 for the LCA Navy project was sanctioned in March 2003 by the Government of India with grant-in-aid seed funding of Rs 949 Crores and a planned completion date of December 2009. The IN contributed 40 percent of the development cost, with the rest being put up by the Defence Research and Development Organization (DRDO), which controls ADA. The NLCA Mk1, was envisaged as a tech demonstrator that would serve to germinate indigenous capability in the realm of carrier-based fighter aircraft design. In any case, the key objectives of FSED-1 are summarized below:

• Design and build two prototypes NP-1 (Two-seater Trainer) and NP-2 (Single-seater Fighter)
• One Structural Test Specimen (STS-Navy) for full scale structural strength testing.
• Shore Based Test Facility at INS Hansa, Goa
• Full range of test facilities specific to Naval requirements
• Flight testing for Carrier Compatibility

Typically, it is easier to design a naval fighter first and then derive a shore-based variant from that baseline. This is evidenced by the development of the F-18 and Rafale families and is often called a two-aircraft design approach. Doing things the other way round is significantly more difficult. Russia and China certainly realized this while navalizing the Su-27 and the MiG-29. The resultant Su-33/J-15 and MiG-29K are virtually new designs. In India’s case, the decision was made based on funding, or the lack thereof. There was no scope for a dedicated naval fighter program counterpart to the shore-based LCA program which was already underway. The relative inexperience of India’s aviation designers also played a part. The end result was an expanded LCA program and a ‘Two-and-Half’ Aircraft design approach.

Under this approach, an Air Force fighter variant (now the Tejas Mk-1) and a Navy trainer would be progressed as ab initio designs. The Air Force trainer (a third variant) would essentially have the same design as the naval trainer albeit with stripped-down landing gear and supporting structure. Finally, the Navy fighter variant would be a modified version of the naval trainer. Accordingly, NP-1 was planned as a Navy trainer. And it was converted to a single-seater Navy fighter version in the form NP-2 with the second cockpit utilized for additional avionics and fuel. The Air Force trainer was designed keeping as much commonality with the Navy trainer as possible [3], [5]. All four variants can be considered to be ‘Mk1’ variants of the overall LCA-family.

Initially, ADA estimated that NP-1 would differ from the LCA Air Force fighter version by about 15 percent and that the incremental development involved could be wrapped up within 6-7 years. The designers tried to limit the changes to the mid-fuselage only, but as the detailed design work progressed the extent of changes burgeoned to as much as 40-45 percent [2]. Furthermore, it was evident that this constraint resulted in a sub-optimal and heavier than anticipated design. Consequently, Navy LCA (NLCA) Mk2 design was taken up in the FSED-II stage of the project, which was sanctioned in December 2009. With the experience of Mk1 behind them, ADA decided to design NLCA MK2 on an ab initio basis, with a view to optimizing it for naval applications from the ground up. It is this version which will have the required level of operational capability that the IN expects from the NLCA programme. Figure 2 below gives an overview of the LCA Navy program. FSED-II has the following key objectives:

• Two single-seater fighter prototypes (NP-3 & NP-4)
• Meeting all the mission objectives set out by the IN
• Better performance than LCA Mk1
• Integration of a full suite of weapons

In order to achieve these objectives, the following key changes were identified for the Mk2 aircraft configuration:

• Higher thrust engine
• Increased internal fuel capacity
• Increased wing area
• Area ruled and streamlined configuration
• Lighter Landing gear and supporting structure
• Improved layout for better safety and maintainability
• Reduced approach speed compared to Mk1 even with a landing mass that had increased by 2.5 tonnes.

Figure 2: Overview of LCA navy Program, FSED-1 and FSED-2 (Source – ADA)

It is noteworthy that, the IN has offered steadfast support to the LCA Navy program with manpower, materiel and monetary contributions right from its inception. For the long-term vision shown and pragmatic approach shown, Naval Headquarters (NHQ) needs to be applauded. The Navy’s commitment to NLCA is best summed up from the following passage from its ‘Maritime Strategy’ document:

“The ongoing Aerospace projects are bold and pioneering ventures into many esoteric fields like airframe and engine design, weapon system integration, flight-control development and evolution of new materials. It is inevitable that these projects will face many hurdles, impediments and delays, but IN will give them full support and backing. While the IN will demand quality from the DRDO, we will also extend financial and manpower support for vital projects.” [1].

Table 1: Prototypes of LCA Navy in FSED Phase I and II.

Table 2: Details of sanctioned funds for various LCA project phases and incurred expenditure till 31st March 2016 – Source ADA Annual Report 2015-16 (*Note – No authoritative data available on total sanctioned aid. The sanctioned funds are likely already revised and larger).

NLCA MK1

The NLCA Mk1 is designed to be compatible with the IN’s existing aircraft carrier, INS Vikramaditya and the under construction INS Vikrant, both of which feature a ‘Short Take-Off But Arrested (STOBAR) concept for aircraft operations. Accordingly, the NLCA Mk1 is capable of taking-off from a 14° ramp after a 200 m roll and being brought to a halt on deck by a three-wire arrestor system with 90 m wire pull out used for landing operations.

As such, NLCA Mk1 retains the key elements of the aerodynamic configuration of the LCA-Tejas Mk1 namely wings, fin, air intakes, nose cone, air data system and the engine. This allowed the NLCA team to leverage the already well-understood aerodynamics of the Tejas Mk1 and build upon the well-validated digital Fly-by-Wire (FBW) Flight Control System (FCS) of the design while focusing on the special control law modes needed for carrier take-off and landing (CTOL). All the control surfaces along with their actuators and the Digital Flight Control Computer (DFCC) used in the Tejas Mk1 have also been also retained.

Table 3: Naval LCA MK1 Specifications (*values estimated from available information, no official data is released for these parameters)

The most striking change in NLCA is the considerably beefier main landing gear (MLG) (compare Figure 3 and Figure 4). This MLG is designed to absorb five times the energy that its Air Force counterpart is designed for. This allows the aircraft to handle a flare-less landing at more than twice (7.1 m/s) the sink rate of that of the Airforce variant (3 m/s). In addition, the track (distance between the two wheels) is significantly larger on NLCA. This provides more stability during arrested landings and the usage of a common restraining gear system (RGS) during launch. A design choice was made to retain MLG attachment points from the Airforce variant. This choice played a major role in increasing the NLCA’s MLG design complexity. Each landing gear now has two arms, the first protruding outwards from the attachment point and the second extending downwards to meet the wheel. This two-part landing gear is structurally weaker mandating the need to significantly beef up its construction. This has resulted in an MLG that is significantly heavier to its counterpart on the Airforce version. Another aspect of naval landing gear design is the sudden expansion of the oleo struts, the moment the aircraft leaves the ramp. During a lecture at Aero India 2013, the chief test pilot cited examples where naval aircraft have lost their wheels due to violent oleo expansion [2].

Figure 3: Main Landing Gear of Naval LCA Mk1

Figure 4: Main Landing Gear of LCA MK1, as seen from the back (Source – ADA/Deb Rana)

Figure 5: Comparison of Main LG design for Naval LCA with that of the Airforce LCA version (source – ADA)

The support structure of the NLCA Mk1’s MLG is accordingly strengthened to take up the higher loads of deck landing without passing the shock to line-replaceable units (LRUs). Provision for ‘lashing’ i.e. tying of the aircraft on the deck securely is also incorporated in the mid-fuselage.

The rear fuselage was modified to incorporate the arrester hook system (AHS) and the actuator-cum-damper assembly. The internal structure was strengthened to handle the tremendous loads encountered during arrested landings that can be as high as -4.5 g. A twin-engine aircraft has a natural attachment point for the arrester hook in the keel beam running between the two engines. But for the single-engined NLCA, the design is tricky, especially when the original internal structure of Tejas Mk1 was not designed keeping in mind the structural loads encountered during arrested recovery on a carrier deck. The designers have placed the AHS on a platform that is externally mounted to the rear fuselage. The internal structure has also reinforced.

Figure 6: The arrester hook system along with the actuator-cum-damper assembly installed on the underbelly of NP2 (Source – ADA)

One of the key design drivers for NLCA Mk1 was the required approach speed. The approach speed for landing is limited by the AGS, as the hook speed with which it engages one of the wires is a limiting factor. The ‘design landing speed’ of the Tejas Mk1 is around 170-200 knots (depending on various parameters like weight, altitude, temperature etc.). This is too high for the AGS installed on IN Carriers which can at best handle a touch down speed of 130 knots with respect to the deck.The landing speed for the NLCA Mk1 therefore had to be reduced to about 130 knots [7].

To maintain the necessary lift at this reduced speed, the NLCA Mk1 has to approach for landing at a higher angle of attack (AoA). Further, the degree of increase in AoA is higher in NLCA Mk1 vis-à-vis other naval fighters because it has the lowest aspect ratio and highest sweep angle with respect among naval fighter designs. However, this higher AoA during approach conflicts with the requirement of over-the-nose vision for landing. Accepted safety margins dictate that the pilot should have a line of sight greater than 4.78 degrees below horizontal. With this restriction in place, the original nose design did not allow the AoA required to generate the necessary lift (measured in coefficient of lift or C _L).

One of the ways to mitigate this problem was to increase the nose droop angle by about 4 degrees [2]. The other primary change to the front fuselage was to modify it to accommodate the larger single wheel Nose Landing Gear (NLG) instead of the pair of smaller wheels on the Tejas Mk1. The larger wheel size allows NLCA Mk1 to trample over the arrester wires during landing without issues. It has a 72-degree nose wheel steering capability to manoeuvre on the deck, where space is at a premium. The NLG was also redesigned to handle significantly higher loads.

Figure 7: Increase in max Angle of Attack during approach achieved with drooped nose while maintaining over-the-nose visual requirement (Source – ADA)

The second and more significant remedy was to increase the C_L at a given AoA. This increase in C_L is achieved through the introduction of LEVCONs to the Tejas Mk1 wings. These LEVCONs increase the total wing area by about 4 percent, thereby increasing lift at a given AoA. The LEVCONs change the vortex patterns over the wing in a favourable way. When deflected upwards by 30 degrees, the LEVCONs increase the intensity of the leading edge (LE) vortices, thereby increasing C_L. They also anchor the vortices at the apex of the wing, which pushes the Centre of Lift (CL) upstream with respect to the Centre of Gravity (CG), resulting in an increase in the ‘pitch up’ tendency of the aircraft.

Figure 8: Impact of LEVCON on the Leading Edge vortex, shown with static pressure contours from CFD simulation (Source – ADA)

The flight control system (FCS) compensates for this pitch up tendency by deflecting elevons downwards for trimming the aircraft. Since the elevons act like flaps when deflected downwards, this additional deflection serves to enhance lift. With these changes, the designers were able to achieve the lift required during approach with the maximum AoA permitted with the drooped nose and the maximum velocity that AGS can handle.

Figure 9: LEVCON increase the pitch up tendency when deflected upwards during the approach. This require larger downward deflection of Elevons for trimming, which helps increase wing lift (Source – ADA)

LEVCONs are also expected to result in a much flatter weight vs approach speed curve. This means that the landing speed is expected to remain relatively constant with respect to the landing weight. The FCS is expected to use the LEVCONs in accordance with the landing weight in order to maintain a similar approach speed on all occasions and alleviate pilot load during approach.

Figure 10: Summary of how required lift was achieved during landing approach using drooped nose and LEVCON (Source – ADA)

LEVCONs offer another aerodynamic advantage, which though not being utilized at the moment, is nevertheless earmarked for future testing and flight envelope expansion. When deflected downwards, LEVCONs suppress vortex formation on the inboard LE and direct the airflow over the wing. This reduces the amount of air from the inboard section getting entrapped in the LE Vortex. Though, this also seems to reduce LE vortex strength and causes early vortex burst, the attached airflow on the wing inboard even at high AoA more than compensates for the loss of lift (see Figure 11 and Figure 12). And the resulting redistribution of lift ensures that the pitch up tendency seen for the baseline (Air force version) is reduced. In fact, for 20-degree downward deflections, computational fluid dynamics (CFD) studies suggest complete elimination of the pitch-up tendency (see Figure 13). Further downward deflection does not seem to have any more positive impact, hence downward deflection is curtailed at 20 degrees. There is a component of lift produced by LEVCON in the forward direction which acts as an additional element of thrust. Essentially it reduces the overall drag of the wing. Slightly higher lift and lower drag increases the lift to drag (L/D) ratio, which is a measure of wing aerodynamic efficiency, across the range of AoA. The transonic manoeuvrability of the aircraft is improved as a result of this enhanced L/D ratio. It is proposed that the FCS will use LEVCONs as an active control surface for the entire flight envelope in order to take advantage of this increased L/D ratio in the future.

Figure 11: Impact of downward deflection of LEVCON on NLCA’s L/D ratio (Source – ADA)

Figure 12: Streamlines showing how increasing downward deflection of LEVCON reduces airflow from the apex region from mixing into the LE vortex core and helps maintain attached air flow on the inboard wing airflow even at high AoA. The improved airflow near centreline of the aircraft also increases effectiveness of the fin and the directional stability of NLCA (Source – ADA).

Figure 13: Surface static pressure distributions showing how increasing downward deflection of LEVCON helps maintain lift on the inboard section even at high AoA, thereby compensating loss of lift due to vortex burst. The resulting distribution of lift also reduces pitch up tendency seen with zero LEVCON deflection (Source – ADA).

Figure 14: Coefficient of pitch moment plot, showing LEVCON with 20deg downward deflection almost completely eliminates the pitch up moment seen in the baseline case with LEVCON un-deflected at moderate to high AoA (Source – ADA).

Beyond these aerodynamic changes, LRU-level changes have been incorporated in the hydraulic and electrical systems, avionics and the environment control system(ECS). The brake-chute has been suitably modified for naval operations. A fuel jettisoning system was added, since it is a critical requirement for a naval fighter during an emergency landing situation, wherein it would have to dump most of its on-board fuel in order to bring down its overall weight within landing limits. NLCA Mk1 was also the first Indian designed fighter to have demonstrated hot refueling capability. A slight offset in T5 (engine exhaust temperature) was also achieved to provide slightly higher thrust at take-off. In addition to these system and sub-system level changes, there are a number of minor changes e.g. canopy shape, a larger MLG retraction bay and so on. A few special materials/alloys have also been incorporated, so that the airframe can better withstand the more corrosive saline environment that naval fighters are usually subjected to, as compared to their shore-based counterparts.

Some of the changes which could not be included in NP-1 were included in NP-2. NP-2 features an air intake with three auxiliary air intake doors to improve performance at take-off. The avionics were upgraded to a full Navy specified suite. The internal fuel was also increased giving it the most fuel capacity among all LCA Mk1 variants. A fuel auto-transfer functionality and an Attitude Compensated Fuel System were added to better manage the CG during flight which has direct consequence for drag and manoeuvrability. The on-board Multi-Mode Radar (MMR) was augmented with a data link facility. The Data-link functionality was demonstrated in conjunction with the Sea Harrier. Integration of the Derby missile beyond visual range air to air missile (BVRAAM) and a self-protection jammer (SPJ) pod were accomplished and the ECFM systems were updated.

NLCA Mk1 was designed keeping in mind air superiority as its primary mission along with limited air-to-ground and anti-ship strike and reconnaissance capability. Currently, it is integrated with the R-73 close combat missile (CCM), besides the Derby BVRAAM. It also inherits the GSh-23 gun from the Airforce version. NLCA Mk1 can easily be outfitted with additional weapons meant for the Airforce Tejas Mk1 and its future variants.

Figure 15: NLCA Mk1 in Air Superiority weapons configuration (Source – ADA)

In ADA General Body Meetings during 2010-11, it was concluded that the two Mk1 prototypes are grossly inadequate for flight testing. Three more Mk1 prototypes, namely, a twin-seater trainer (NP-5) and two single-seater fighters (NP-6 and NP-7) were sanctioned. As per the last authoritative information available, NP5 is under construction and will be an improved version of NP-1. It is also deemed to be the production version for the Naval Trainer aircraft of the LCA family. It is likely that there will not be a separate NLCA Mk2 Trainer version. As of now, the IN has ordered a total of 8 NLCA Mk1 aircraft which includes 4 Trainers and 4 Fighters [5].

In a future article, we will discuss the overall test experience of NLCA Mk1 prototypes and discuss the roadmap outlined for the NLCA Mk2.

References:

1. ‘The LCA Navy – Is it Ready for Sea?’, Former CNS Admiral (rtd) Arun Prakash, Vayu, IV/2012.
2. “Why Navy’s rejection of Naval LCA is wrong”, Cmde CD Balaji, Indian Defense Review, 12 April 2017
3. “India’s Naval Light Combat Aircraft [LCA-Navy] – Unique Features & Flight Testing”, Capt. Maolankar, Aero India Seminar 2013.
4. “Design of Control Laws for Carrier Operation – LCA Navy Experience”, Aero India Seminar 2019
5. “Preliminary design of LCA Mk 2 to be ready by next month”, Interview of P. Subramanyam, Director ADA, AeroMag Asia, Vol IX Issue 1, January – February 2015
6. MoD Annual Reports
7. DRDO Technology Focus, February 2011
8. ADA LCA Brochure 2019
9. ADA Annual Reports
10. ADA Research Publication

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