shiv wrote:For all the unquestioning and patriotic support the entire nation gave the LCA can anyone answer why the choice of cranked double delta was made?
Hmm. Delta is easy to answer. Low wave drag, brilliant performance and supersonic speed and at altitude, structurally very efficient , light for it's strength, has space for a boatload of fuel. A great choice of wing form, especially if you have decided to go FBW. In fact with FBW /Unstable layout, the disadvantages of traditional deltas disappear and you can have the cake and eat it too!.
Crank. I think is just like a notch/dogtooth . Creates a vortex where the crank is and helps keep the flow glued to the wing, so increases AoA over what you could get with a straight/ordinary delta.
Twist/Washout. Increases AoA again by having control over stalls by making the root stall first before the tips, so that there is no sudden wing drop or something I guess.
Point is, there was careful design of the wing and plan form from all I can see. I really don't have any quarrels with the basic design. But if you argue at a Philosophical level and "Strategee" like Prof Das and propose radical alternatives , then it takes it to a different plane (pardon the pun) altogether.
Vina - I realize that you did not arrive at the decision to make a cranked delta with FBW. Having made that choice what has been achieved is creditable. No existing aircraft in the world has this configuration. Certainly not the Vulcan.
The LCA's design was arrived at in the 1980s at a time when the world had not yet moved out of dedicated single-role aircraft and arriving at multi-role compromises. In the early 80s most of the dedicated "fighters", "bombers" and "attack aircraft" of the 60s and 70s were still active. In that era the US came out simultaneously with two competing designs - the YF 16 and YF 17. Unusually both designs were taken forward and made into successful aircraft still flying today. Both had FBW. However the US, having put the variable geometry design into production with the F-111, F-14 and B-1 moved out of that line. Europe was reeling with the inability to compete with the US. Panavia was created and they came up with the Tornado with FBW, variable geometry and huge box intakes. Until the MiG 29 arrived in the scene as an air superiority fighter Russia was playing with variable geometry MiG 23 and 27, Su 17, Su 24, as well as tailed delta with Su 15.
But apart from the Panavia Tornado which I think was envisaged as "multirole" most countries were still designing single role aircraft with a limited secondary role. The US had its B1 and the USSR was still making its attack bombers. And each country was responding to the other's bomber threat by creating fighters like the MiG 25 and F-15 - which again had a similar design. The US created the A-10 - an aircraft unsuited for any role other than what it was designed for (close support, battlefield interdiction, anti-tank), and the USSR came up with the similar Su-25. Only the Tornado was the MRCA in this stable. The spectacular appearance of the MiG 29 was well known as a great development in aerodynamics and perhaps the limits of that possible without FBW or TVC. The tailless delta had no role anywhere here.
It was only the French who almost moved out of the Mirage III's delta with the Mirage F1 and Mirage G, came back to the delta design for the Mirage 2000. Again the Mirage 2000 which has now panned out into multirole was a pure fighter with secondary attack role in the beginning.
Here is a description of the choices made for the Mirage 2000http://www.vectorsite.net/avmir2k.html
Using the delta wing configuration seemed to many like a backward step. Dassault had used that configuration on the Mirage III and 5, but abandoned it for the Mirage F1. A delta wing tends to be a good choice in terms of high-speed flight characteristics, simplicity of aircraft construction, relatively low radar signature, and internal volume. It tends to be a poor choice in terms of maneuverability, low-altitude flight, and length of take-off and landing run.
Dassault engineers managed to bring the delta wing up to date by moving the aircraft's center of lift to the front of its center of gravity, giving the fighter a degree of instability that enhanced maneuverability. This also reduced the takeoff run: in the older Mirage deltas, lowering the elevons on takeoff to increase lift would push the aircraft's nose back down, but shifting the center of lift eliminated this problem.
The aircraft featured a redundant "fly-by-wire automatic flight control system (FBW AFCS)", providing electronic "smarts" to compensate for the aircraft's inherent instability; two-piece elevons on each wing; and automatic, full length, two-segment leading-edge slats. This gave the Mirage 2000 a level of agility that the Mirage III and 5 lacked, and in fact the new machine would acquire a reputation for superb handling. An airbrake was fitted on top of each wing, the arrangement being very similar to that of the Mirage III. A noticeably taller tailfin allowed the pilot to retain control at higher angles of attack, assisted by small strakes mounted along each air intake.
With the Mirage 2000 the French were looking for export as well as a competitor to the F-16.
What was the thinking in India? India (the Air Force) certainly understood the meaning of "multi-role". Hunters ended up being used in air combat and MiG 21s for ground attack. But there were very few multi-role choices available to us in the 1980s. Also - all air combat in India's wars had taken place at subsonic speeds at low altitude. In fact the 80s were an era of low altitude penetration for radar avoidance and precision bomb delivery. The Jaguar had been designed for that as was the Tornado. The delta wing design of the LCA would have been no good for that, nor would it have been very maneuverable at low altitude. India threat perception did not include high flying supersonic bombers. The Delta Dagger and Mirage III were designed to take on those supersonic bombers.
The choice of delta wing for LCA in the 1980s had the following drawbacks:
1) There was no threat perception or IAF experience requiring a high altitude interceptor
2) The delta wing configuration was not a good choice for low level "below the radar" terrain following attack aircraft role.
3) The delta by nature was never going to be as nimble in tight turns at low level where all air combat had been taking place.
4) FBW would be essential to reduce the drawbacks of the delta design of the LCA and India had no experience designing FBW. Failure of FBW meant failure of the entire project.
Given these issues why on earth did the Tejas take the shape it has today? What sort of logic was followed?