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Dr. Shyam Chetty was the Former Director of CSIR - National Aerospace Laboratories, Bangalore, and held additional responsibilities as Director CSIR-4PI, Bangalore, Project Director of the National Control Law team for LCA, and Chairman of the Systems Engineering Cluster of NAL. He has over 40 years of experience in the field of Aircraft Flight Mechanics & Control. His research interests include Flight Control System Design & Development, Aircraft Simulation & Modelling, Handling Qualities & Aircraft Pilot Coupling, Computer Aided Flight Control Design & Rapid Prototyping Techniques.

He also has the distinction of having served as the Chairman / Technical Expert of Review Committees on most of the Major National Aerospace Programmes of ISRO, DRDO and ADA. He has won several awards, prominent among them being the Jawahar Lal Nehru Memorial Award in 1976, Sir C.V. Raman Distinguished Young Scientist Award in 1998, DRDO Award for Path Breaking Research in 2002 and many more.

CSIR-NAL has also provided significant value-added inputs to all the Indian national aerospace programmes. Its contributions over the last five decades have enabled it to create a niche for itself in advanced aerospace research and technology development.

 

 

In the Interview with Dr. Chetty, we take you through many important technologies developed over the years by CSIR-NAL who have developed many critical technologies for Tejas and continue to support the programme.

Question 1: It was over 25 years ago when Dr. APJ Abdul Kalam took the decision to form the National CLAW team with scientists & engineers mainly chosen from ADA, NAL, IAF, DRDO & HAL. The CLAW team was responsible for the design and development of the Fly-by-wire Digital Flight Control Laws& Airdata Algorithms for the Tejas.
Tell us how did it all start and what was it like working as a team together along with some of India’s top scientists on the LCA Project?

Though preliminary Design & Development activities for realising a modern Light Combat Aircraft to replace the Mig-21’s was initiated in mid 80’s with the formation of the Aeronautical Development Agency to spearhead the programme, it was only after Dr. Kalam took over as Head of DRDO & DG,ADA  in 1991, that the programme got the necessary impetus and sizable initial funding. Since there had been no follow on fighter aircraft programme in the country after the HF-24 development which happened at HAL in the sixties it was also considered prudent by the government to initiate  LCA development as a Technology Demonstration programme by building two prototypes. 

The four major technologies that had to be demonstrated  on these prototypes were  Design, Development, Testing & Certification of a Digital Fly-By Wire Flight Control System for controlling the aerodynamically unstable Airframe, Computer controlled utility and stores management system, a modern glass cockpit driven by a modular avionics suite and extensive use carbon epoxy composites to realise the weight optimised high strength structural components for the fuselage, wet wing and empennage.

Dr. Kalam adopted a two pronged strategy, he set up high level committees to assess the technology status within the country in some of these areas & in order to help speed up the activities global tenders were also floated seeking joint development  support from established international aircraft OEM’s and System Houses. Responses were received  for the development / supply of the FBW FCS Sensors, Electrohydaulic actuators, design of the Quad redundant DFCC hardware, firmware and embedded software.

However, no agency was willing to help in the design & development of the flight control laws and airdata algorithms. The high level Committee  on Digital flight control headed by Dr. I.G. Sharma & Dr. Vidyasagar based on their evaluation of the  technology readiness level of various Aerospace R&D / academic institutions and HAL came to the conclusion that no single institution in the country was capable of undertaking this onerous & challenging task of CLAW development. After several rounds of high level discussions it was finally decided to form a National team drawing talent from ADA,  CSIR-NAL, DRDO, HAL & IAF. Due to technology denials a similar approach was also followed for the development of the Carbon Composite Wing.

CSIR-NAL was given the lead responsibility  for both these critical activities and two National teams were set up at NAL under the leadership of Dr. Srinathkumar & Dr. K.N. Raju respectively for indigenous development of the flight control laws and composite wing.

Coming to the second part of the question, since modelling simulation and control law synthesis needs extensive design data and information from not only aerodynamics but also from all other onboard systems, the control law team members had to interact on a regular basis with other LCA design and flight test team members and consultants. This not only helped in broadening their  knowledge base but was also  found to be very interesting and exciting though many a times extremely stressful due to the stringent time targets set by the programme management. To be very honest since a fourth generation aircraft was being developed for the first time the project management team too did not have the ability to estimate the quantum and complex nature of work involved in designing the  digital flight control system.

Question 2: What are the Control Laws(CLAW) built into the Tejas and what do they do?

Flight Control Laws very simply put are similar to decisions taken and signals generated by the human brain based on visual and other sensory inputs to the drive the neuro-muscular organs of the body for performing a given task say for example riding a two wheeler.

Flight control laws or CLAW as it is popularly known, consists of multiple sets of complex mathematical algorithms coded into the onboard quad redundant FCC which receives and processes inputs from the pilot’s stick, pedal,  throttle and several other flight motion and airdata sensors  to generate output commands for the  control surface actuators  which in turn move the primary and secondary control surfaces to maneuver the aircraft as desired by the pilot. The flight control laws have multiple functions and therefore structured in a hierarchical manner.

LCA is designed to be Aerodynamically unstable in the longitudinal axis at subsonic Mach Nos, this is essential to provide the aircraft with the desired levels of agility and maneuverability at Supersonic flight  conditions as the aircraft becomes more stable under these conditions due to rearward shift of the lift vector in relation to the centre of gravity. 

The aircraft in flight constantly changes its dynamic characteristics and hence 80 times a second, the algorithms based on the measurements of the airdata parameters and control surface positions identify the airframe model at that instant, determine the deficiencies of the airframe including the level of instability, and correct it by feeding back processed inertial and airdata sensor signals to provide the pilot an “ideal” aircraft which is easy and comfortable to fly.  In the manual control mode, the inner loop control laws recover the longitudinal stability and  improve the flying qualities i.e. reduces the mental and physical workload of the pilot for accomplishing a given flight task, thereby allowing him to concentrate on the actual combat mission. The control law also incorporates carefree maneuvering safety features which includes boundary limiting, autolevel and auto low speed recovery functions thereby ensuring that the aircraft never violates the design flight envelopes in spite of accidental mishandling of the aircraft controls by the pilots.

In addition to manual flight  control, there are several autopilot control modes which are basically pilot relief modes. It consists of basic hold modes, advanced coupled  / navigation modes, and Weapon delivery modes.  The Navigation modes for example  allows the pilot to program the entire flight plan for a particular mission prior to takeoff. Once programmed all he needs to do after takeoff & reaching the safety height is press a button or set of buttons and the control system  will automatically navigate and guide the aircraft to the selected destination as per the flight plan and flight levels  cleared by the air traffic control.