Sumeet wrote:Su 57 is not a proper Stealth design due to these and other reasons (Gaps between panels, rivets, rear side stealth compromised by engines, engine blades visible via duct, not stealthy if someone is looking at it from below [not flat/smooth like F-22], it has spherical ball shape IRST above and below). Comparatively AMCA the way it is planned is much much better.
Who ever told you that Flat Surface is good for Stealth ? Try to bring a Flat Surface near a RF source at 90 degree and try to measure the RF returns and then try to move the Flat Surface in different direction , The least RF returns comes when your Flat Surface is pointing at the RF source at the edge of the diagional at 90*
But you cannot design a fighter aircraft without flat surface due to aerodynamic reason hence using RAS and RAM is the only option.
Flat under surface for aircraft is also bad for another reason because of its large Wet Surface area which is draggy.
The Y Duct is an old design approach , Y Duct takes too much internal space and during High Angle of Attack they would starve the engine of air which can lead to flame out, The Only reason Y Duct was adopted because its simpler to design.
A better approach is to design a Blocker which does not compromises the Air Intake at any angle of Attack and Hides the Engine Blade but that is a more sophisticated approach which PAK-FA has adopted.
The only western LO aircraft using Blocker Approach was Boeing X-32, Again its one thing to Design a Blocker and another thing to Design a Sophisticated one that does not Starve the Engine of Air at Any Angle of Attack, PAK-FA Blocker is a Dynamic one that adapts itself to AOA and there is a patent information available on the net if you search
The Rear Engine thing is taken care by Serated Nozzle , I have posted the picture above and another option is 2 D Nozzle , Both option are presented to RuAF and its upto them to decide which one they want , 3D Nozzle has the advantage that it will alllow the Nozzle Movement in any direction and give aircraft manouveribility while a 2 D engine can have just pitch and yaw movement and better for LO but compromised manouveribility.
Indranil had already dealt on Stealth Aspect of PAK-FA many times and even he came to conclusion LO of PAK-FA is no better or worse than F-35 or F-22.
With 11.5 Tons of Internal Fuel , 1500 km of Super Cruise Range ( Supersonic without AB ) and more than 3500 in subsonic range , With Supermanouverbility out classing even Su-35 , PAK-FA will be more than competitive to any flying aircraft.