Basic Assesment
The most likely assessment of this aircraft is that as long as it lacks a modern jet engine, the aircraft does not represent a threat in terms of performance at the moment due to the lack of a proper engine type, the contradiction of its aerodynamics and stealth does not make it as capable as F-22, it might represent a threat in aerodynamics for F-35, but not for Su-57, F-22 or Rafale in terms of maneuverability or rate of climb, however if it has the proper avionics and weapons it might surpass F-35.
However this does not mean, it is not an aircraft that does not represent a threat for air forces in the west pacific region, it does represent a threat undoubtedly, however its level of threat in terms of aerodynamics is unlikely it will be better than Rafale or Su-57, but as a stealth machine it does represent a very difficult target.
As it evolves with better engines it might even eliminate the ventral fins and even modified the wing to become a pure Delta tailess fighter interceptor.
An ideal wing leading dge extension (LEX) requieres the following aspects
Work in combination of a less sweptback wing part (better low-speed properties, greater flap effectiveness) and delta wing (better stall characteristics)
Leading edge can be straight or curved
Must always have a sharp leading edge
Small aspect ratio
High sweep angle
This shows the LEX on J-20 is not the ideal one, a long sharp leading edge as the one of F-16, F-18 or Su-27 are the ideal LEX, but it does help its delta wing.
At low angles of attack, the LEX has little effect
At higher angles of attack a vortex, formed from the leading edge of the LEX, flows over the wing.
The vortex helps to energize the upper surface boundary layer, delaying separation.
LEX vortex stabilizes wing leading edge vortex and prevents it from separating
LEX vortex and wing leading edge vortex exist side by side and support each other
Higher CL AoA capability
Higher CL max
Better maneuverability, especially during turns in aerial combat
Smaller wing for same lift
YF-17 showed 50% increase in max lift for just 10% more wing area
F-16 was able to reduce wing size and save about 500 lbs in weight
Reduced transonic lift center shift, giving lower supersonic trim drag at high g
"All F-5 aircraft have included a wing-root leading edge extension (LEX) in their aerodynamic design. The original W1 LEX (designated by its wind tunnel model component number) was added to the F-5A/B wing design as a fairing over the leading edge flap actuator. Even though the W1 LEX was less than 2% of the wing area, it provided a 10% increase in the maximum lift capability of the wing, resulting in improved maneuverability. When the F-5E aircraft was developed, Northrop engineers increased the size of the LEX to approximately 5% of the wing area. This new LEX was designated W4 and provided a further 25% increase in maximum lift. " Tiger Century Aircraft
The increase of size of the LEX from the F-5 to the F-17 shows the importance of the LEX size, the early J-20 also had an increase of LEX size mirroring the F-5 case.
The application of 2-D nozzle technology to fighter design offers a number of potential advantages: increased survivability through reduced IR and RCS signatures, thrust-minus-drag improvement due to more favorable nozzle/airframe integration, and the expanded aircraft maneuvering capability offered by thrust vectoring and reversing. At this moment J-20 does not have these F-22 features
"Northrop applied a large highly swept LEX to the YF-17 prototype aircraft to enhance lift and stabilize the flow over the YF17 main wing at high angles of attack. The formidable task of converting the land-based YF-17 lightweight day fighter into an all-weather fighter-attack aircraft capable of carrier operations with heavy ordnance loads required significant changes from the earlier configuration. The required approach speeds for carrier landings resulted in modifications to the wing and LEX surfaces of the YF-17 configuration to provide more lift. McDonnell Douglas consulted the Langley research staff, and several individuals participated in the analysis of wind-tunnel tests that had been conducted at NASA Ames Research Center and McDonnell Douglas facilities. As a result of the analysis, changes were made to the aircraft configuration. The geometric shape of the YF-17 LEX was extended farther forward on the fuselage and the plan view of the LEX was modified to produce additional lift while retaining the good high-angle-of-attack characteristics exhibited by the YF-17. " Langley Contributions to the F/A-18
J-20 similarly increased its LEX size from the early prototypes to the late prototypes
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