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As of 2015, the Indian Air Force (IAF) is on the threshold of receiving the indigenously designed and built Light Combat Aircraft (LCA), also known as the Tejas. It is expected to be a far-superior aircraft compared with the existing fleet of Mig-21s in the IAF fleet. However, what remains unknown in the public domain is its performance characteristics. The following effort will aim to answer some questions of the LCA’s performance as a function of altitude, payload and range. Doing so will provide a reference data set with which the LCA can be compared to the existing fleet of IAF aircraft. The analysis is based on computationally evaluated aerodynamics and propulsion data developed by the author. Whilst the LCA is now available in the air-force single-seat fighter (LCA), two-seat trainer (LCA-T) and navy (LCA-N) versions, the following analysis restricts itself to the single-seat air-force version only citing the lack of available information on the other two variants. However, performance for the other variants can be extrapolated from the single-seat air-force version, on which they are all based. The LCA design as used for this analysis is shown below.
The realm of computational fluid dynamics
Evaluating aircraft aerodynamic performance using Computational Fluid Dynamics (CFD) methods is inherently time and resource intensive. As such, it remains the domain of large corporations and governmental agencies. However, such a detailed analysis is hardly required to determine aircraft performance. For an aircraft such as the LCA, suitable aerodynamic performance can be evaluated using surface-vorticity solvers such as FlightStream, developed by the Research in Flight Company (found here). The Author developed this software years ago as part of his academic research work and has applied this software to the LCA. In doing so, the primary aerodynamic data such as the force and moment coefficients can be evaluated as a function of angle-of-attack and altitude. The accuracy of FlightStream, for such an analysis, is acceptable only for the range of Mach numbers from low subsonic till the point of local supersonic flow over the wings and from small angles of attack till the point of flow separation over the wings. Working backwards from the available three-view data for the LCA available in the public domain, a surface mesh of the LCA was created using the open-source NASA software OpenVSP (found here). FlightStream has direct connectivity with OpenVSP and the surface mesh solution is evaluated for vorticity. The vorticity data is then used by the solver to generate aerodynamic loads data. Using this approach, the author was able to get reasonably close to the wind-tunnel data evaluated by the Aeronautical Development Agency (ADA) as shown below for the force coefficient matrix.
The force-coefficient matrix is plugged with a first-order propulsion module to simulate engine performance as a function of local atmospherics and altitude. The data generated by these routines are combined with the known mass properties of the aircraft to generate range and payload data which is then presented below for analysis. The said analysis is simplified to allow the reader to grasp the essentials of the performance results for the LCA.
Assumptions that went into the analysis include the empty weight of the LCA as being 6,500 Kg. The engine used was General Electric F404-GE-IN20 with a rated output of ~54 kN and a rated TSFC of ~0.77 lbm/lbf-Hr. The internal fuel capacity of the LCA is assumed to be 3,034 Liters and the aircraft is assumed to be capable of carrying a centerline external drop tank of 725 Liters as well as one 1,200 Liter drop tanks on pylon stations 1 and 2, respectively. The cruise speed of the LCA is evaluated for maximum range for each condition.
Performance comparisons
The performance of LCA are summarized in the form of payload and range plots. The payload is evaluated from 0 to 10,000 kg and is assumed to include the pilot weight and all auxiliary equipment excluding fuel. The vertical axis of the plots is range, measured in kilometers. The combat-radius of the aircraft is considered to be ~40% of the range. For example, a range of 1,000 km corresponds to a combat radius of ~400 km. Plots are provided for the LCA in three conditions: clean (internal fuel only), combat (internal + centerline drop tank) and ferry (internal + centerline drop tank + 2 x wing drop tanks). Additionally, three altitude conditions are provided: sea-level, 20,000 ft above-sea-level (ASL) and 30,000 ft ASL.
It is very clear that the design of the LCA allows for a dynamic performance matrix. In the ferry condition, flying at 20,000 feet ASL, the LCA can self-deploy to locations over 1,600 km away when carrying no payload. Carrying a light 1,000 kg payload, the LCA can ferry itself at high altitude to locations over 1,400 km away. Reviewing the altitude conditions suggest that there is an optimum high-altitude endurance regime for the LCA beyond which the benefits of altitude begin to wear off (as shown in the results for the 30,000 ft ASL plot above).
In the combat role, the LCA can execute either a high-altitude profile or a low-altitude one. The high-altitude profile corresponds with typical combat-air-patrols and long-range engagement of enemy fighters. In this role, carrying only internal fuel, the LCA has a combat radius of ~350 km when patrolling between 15,000 ft and 20,000 ft ASL and carrying 2,000 kg of air-to-air payload. At low-altitude, the LCA can execute close-air-support strikes or evade enemy air patrols at high altitude when executing strikes against enemy targets farther inside enemy territory. In this profile, the LCA has a combat radius of ~200 km when carrying 2,000 kg of air-to-ground munitions. A mix of high and low profiles will allow somewhat greater radius of action for the LCA and the numbers can be extrapolated from the plots above.
Flight time versus available fuel can be plotted using the above data to extrapolate time on station for each type of mission profile. The flight-time/available fuel plot is provided below for the LCA cruising at 20,000 ft ASL. The clean, combat and ferry conditions are illustrated. It is clear that the LCA will not be a long endurance fighter under any conditions. The maximum flight time for the LCA on its own fuel capacity in the ferry condition is around ~110 minutes from take-off to empty fuel tanks. The performance for combat conditions is much lower at ~60 minutes when carrying a 1,000 kg payload beyond the fuel.
Conclusions
The LCA is, by definition, a light fighter. Its flight performance metrics confirm this design feature. It is not meant to be a long-range bruiser like the Su-30MKI. Instead, it will replace existing short-range Mig-21 type aircraft in the IAF. In that role it is very aptly suited. This analysis was conducted using the F404-GE-IN20 engine. It is possible that more fuel efficient engines may be used in the future. In that case, the performance of the aircraft can be further enhanced, but only within limits. Further, weight reduction will play a key factor in the final iteration of the LCA for the IAF. The analysis above was conducted assuming an empty weight of 6,500 kg. However, there are varying numbers for the empty weight of the aircraft in the public domain. It is no secret that the increasing use of composites in the LCA is designed to bring the overall weight of the aircraft down. Even if the empty weight of the LCA is brought down by 5-10%, the improvements in the flight performance will be noticeable. Whether that is achieved, remains to be seen.
Dr. Vivek Ahuja
References
1. Jebakumar, S. K., “Aircraft Performance Improvements: A Practical Approach”, Center for Military Airworthiness and Certification (CEMILAC), March 2009, Bengaluru, India
Source The Beta Coefficient: LCA Tejas versus F-16 in combat (Part-I)
The realm of computational fluid dynamics
Evaluating aircraft aerodynamic performance using Computational Fluid Dynamics (CFD) methods is inherently time and resource intensive. As such, it remains the domain of large corporations and governmental agencies. However, such a detailed analysis is hardly required to determine aircraft performance. For an aircraft such as the LCA, suitable aerodynamic performance can be evaluated using surface-vorticity solvers such as FlightStream, developed by the Research in Flight Company (found here). The Author developed this software years ago as part of his academic research work and has applied this software to the LCA. In doing so, the primary aerodynamic data such as the force and moment coefficients can be evaluated as a function of angle-of-attack and altitude. The accuracy of FlightStream, for such an analysis, is acceptable only for the range of Mach numbers from low subsonic till the point of local supersonic flow over the wings and from small angles of attack till the point of flow separation over the wings. Working backwards from the available three-view data for the LCA available in the public domain, a surface mesh of the LCA was created using the open-source NASA software OpenVSP (found here). FlightStream has direct connectivity with OpenVSP and the surface mesh solution is evaluated for vorticity. The vorticity data is then used by the solver to generate aerodynamic loads data. Using this approach, the author was able to get reasonably close to the wind-tunnel data evaluated by the Aeronautical Development Agency (ADA) as shown below for the force coefficient matrix.
The force-coefficient matrix is plugged with a first-order propulsion module to simulate engine performance as a function of local atmospherics and altitude. The data generated by these routines are combined with the known mass properties of the aircraft to generate range and payload data which is then presented below for analysis. The said analysis is simplified to allow the reader to grasp the essentials of the performance results for the LCA.
Assumptions that went into the analysis include the empty weight of the LCA as being 6,500 Kg. The engine used was General Electric F404-GE-IN20 with a rated output of ~54 kN and a rated TSFC of ~0.77 lbm/lbf-Hr. The internal fuel capacity of the LCA is assumed to be 3,034 Liters and the aircraft is assumed to be capable of carrying a centerline external drop tank of 725 Liters as well as one 1,200 Liter drop tanks on pylon stations 1 and 2, respectively. The cruise speed of the LCA is evaluated for maximum range for each condition.
Performance comparisons
The performance of LCA are summarized in the form of payload and range plots. The payload is evaluated from 0 to 10,000 kg and is assumed to include the pilot weight and all auxiliary equipment excluding fuel. The vertical axis of the plots is range, measured in kilometers. The combat-radius of the aircraft is considered to be ~40% of the range. For example, a range of 1,000 km corresponds to a combat radius of ~400 km. Plots are provided for the LCA in three conditions: clean (internal fuel only), combat (internal + centerline drop tank) and ferry (internal + centerline drop tank + 2 x wing drop tanks). Additionally, three altitude conditions are provided: sea-level, 20,000 ft above-sea-level (ASL) and 30,000 ft ASL.
It is very clear that the design of the LCA allows for a dynamic performance matrix. In the ferry condition, flying at 20,000 feet ASL, the LCA can self-deploy to locations over 1,600 km away when carrying no payload. Carrying a light 1,000 kg payload, the LCA can ferry itself at high altitude to locations over 1,400 km away. Reviewing the altitude conditions suggest that there is an optimum high-altitude endurance regime for the LCA beyond which the benefits of altitude begin to wear off (as shown in the results for the 30,000 ft ASL plot above).
In the combat role, the LCA can execute either a high-altitude profile or a low-altitude one. The high-altitude profile corresponds with typical combat-air-patrols and long-range engagement of enemy fighters. In this role, carrying only internal fuel, the LCA has a combat radius of ~350 km when patrolling between 15,000 ft and 20,000 ft ASL and carrying 2,000 kg of air-to-air payload. At low-altitude, the LCA can execute close-air-support strikes or evade enemy air patrols at high altitude when executing strikes against enemy targets farther inside enemy territory. In this profile, the LCA has a combat radius of ~200 km when carrying 2,000 kg of air-to-ground munitions. A mix of high and low profiles will allow somewhat greater radius of action for the LCA and the numbers can be extrapolated from the plots above.
Flight time versus available fuel can be plotted using the above data to extrapolate time on station for each type of mission profile. The flight-time/available fuel plot is provided below for the LCA cruising at 20,000 ft ASL. The clean, combat and ferry conditions are illustrated. It is clear that the LCA will not be a long endurance fighter under any conditions. The maximum flight time for the LCA on its own fuel capacity in the ferry condition is around ~110 minutes from take-off to empty fuel tanks. The performance for combat conditions is much lower at ~60 minutes when carrying a 1,000 kg payload beyond the fuel.
Conclusions
The LCA is, by definition, a light fighter. Its flight performance metrics confirm this design feature. It is not meant to be a long-range bruiser like the Su-30MKI. Instead, it will replace existing short-range Mig-21 type aircraft in the IAF. In that role it is very aptly suited. This analysis was conducted using the F404-GE-IN20 engine. It is possible that more fuel efficient engines may be used in the future. In that case, the performance of the aircraft can be further enhanced, but only within limits. Further, weight reduction will play a key factor in the final iteration of the LCA for the IAF. The analysis above was conducted assuming an empty weight of 6,500 kg. However, there are varying numbers for the empty weight of the aircraft in the public domain. It is no secret that the increasing use of composites in the LCA is designed to bring the overall weight of the aircraft down. Even if the empty weight of the LCA is brought down by 5-10%, the improvements in the flight performance will be noticeable. Whether that is achieved, remains to be seen.
Dr. Vivek Ahuja
References
1. Jebakumar, S. K., “Aircraft Performance Improvements: A Practical Approach”, Center for Military Airworthiness and Certification (CEMILAC), March 2009, Bengaluru, India
Source The Beta Coefficient: LCA Tejas versus F-16 in combat (Part-I)
