Challenges In Redesigning Tejus To Naval Role

© Aeronautical Development Agency The success of Tejas fighter programme for Indian Airforce, built the confidence of Indian Navy to e...

The success of Tejas fighter programme for Indian Airforce, built the confidence of Indian Navy to entrust Aeronautical Developmental Agency (ADA) with the design and development of naval version of LCA Tejus for operation from aircraft carriers.

Aerodynamic enhancements to improve low speed performance for carrier operation, addition of arrestor hook for deck recovery, need for a stronger undercarriage and cockpit redesign for naval operations have made the LCA Navy development programme an immense challenge.
LCA Navy is the only fourth generation light weight carrier borne aircraft in the world. The first Naval Prototype NP1 made its maiden flight in Apr 2012. The aircraft is designed to operate in saline and humid conditions.

N-LCA is designed to perform Ski jump Take Off, But Arrested Recovery (STOBAR), from the indigenously developed Vikrant class aircraft carriers. US Navy employs Catapult Assisted Take off (CATOBAR) on its Nimitz class aircraft carriers. N-LCA requires a 14 degree Ski jump take off with a 200 metres deck run and landing in 90 metres using arrester hook, resulting in a longitudinal deceleration of upto 4.5gs, the corresponding landing and take off ground, roll distance for IAF variant is more than 1000 m and 600 m respectively.

Drooped Nose photo © Aeronautical Development Agency

Development of the Naval LCA by modifying the IAF LCA variant with minimum changes was a challenging task involving redesigning for improved low speed performance and visibility during landing, redesigning of the undercarriage including its retraction mechanism for a sink rate of 7.7 m/s as against a value of 3.05 m/s for IAF variant, introduction of a arrestor hook, cockpit redesign and changes in flight control laws.

N-LCA has a drooped nose and LEVCON system for better vision during landing on a carrier deck. LEVCON ensures low landing speeds and good controllability at landing and consists of hinged surface at the wing root leading edge operated by a wing concealed rotary actuator.
The articulated landing gear was redesigned to the telescopic landing gear concept for higher sink rates. The landing gear had to withstand landing loads of about 20,000 kg and 32,000 kg on the nose and main landing gear respectively as against corresponding load of about 5800 kg and 10200 kg on the IAF variant. The vertical travel also increased from 250 mm to 270 mm for the nose and main landing gear to 440 mm and 500 mm respectively. Landing gear for the prototype aircraft NP-1 weighed 995 kg against the corresponding weight of 400 kg for IAF variant. Redesign is currently being done to reduce weight by about 300 kg. As increase in the size of the landing gear bay drastically reduced the space available for other parts, relocating Line Replaceable Units (LRU) and rerouting of system pipelines/cables etc led to redesigning of all the aircraft systems. The wheel track was also increased from 2.2m to 3m for improved stability.

The arrestor hook was designed for a approach speed of 240- 260 km/hr. Three wires, 12m apart on the ship deck engage the hook with wire pull out of 90m generating limit loads of about 495 KN. The rear fuselage structure houses the removable engine, bay door and end cone which made the task of integrating the arrestor hook without making engine removal more difficult in the absence of any fixed structure. Additional testing and recertification of many systems and LRUs was required due to the increased loads at landing and changed operating environment. The changes required were infact equivalent to the development of a new aircraft.
The main features of Naval LCA in addition to relaxed static stability, fly by wire flight control system, integrated digital avionics system with glass cockpit, multi mode radar and composite skin are:

Dropped nose for better cockpit visibility during deck landing.
LEVCON (Leading Edge Vortex Control) for reducing landing speed and altitudes for precise deck landing.
Redesigned landing gear including its retraction mechanism for higher deck loads.
Arrester hooks for arrested recovery.
Strengthened fuselage structure to withstand higher landing shocks from landing gear and arrester hooks.
Auto Throttle Function during critical phase of carrier landing.
Fuel Dump System for reducing weight to perform an emergemcy landing immediately after launch from carrier.