They will likely be very similar at the end of the day.

Agreed. While components like the engine, avionics suite, and weapon systems can be re-used and re-configured to suite specific requirements, redesigning the airframe from scratch is simple unnecessarily costly, not to mention the time needed to re-do all the test flights and the extra risk of malfunctioning. I would expect the difference between the Russian version and the Indian version to be no greater than that between, say, F-35A and F-35C.

Ok, there is a long discussion on lifting body judging only the section profile of the fuselage…and overlooking:

– the fact that the fuselage is not a 2D wing section and is subject to complicated boundary layer and vortexes;
– interaction with the primary lifting device (the wing);
– interaction with secondary lifting device (canard, LERX, tail, etc);
– interaction with engine inlet and exhaust;
– interaction with other parasites (landing gear doors, weapons bay doors, etc)

That is to say, without making a representative model and putting it in a wind tunnel, there’s not much we can judge about how lifting-body like a particular fuselage is…

At best we can say, that a flat, integrated fuselage works better at generating body lift. In fact, given enough engine power and proper flight control, any flattish object can fly. An F-15 with its flat body creates more body lift than a MiG-21 (remember that Israeli F-15 landing with the starboard side wing chopped off?!). But that’s it, any discussion beyond this qualitative generalisation is, quite frankly, baseless.

The Flanker and T-50 are known to integrate the fuselage and the wing to achieve wave riding, not because they look like so, but because TsAGI did all those wind tunnel testing and they said so.

F-22 is known to use its forward fuselage and integrated inlet to generate vortex, minimise wet surface area, and minimise radar signal return, not (just) because it looks like so, but because Lookheed and NASA did all those wind tunnel testing and you can even find NASA paper on F-22 high alpha boundary control.

Whether the belly or the back should have more curvature to contribute the most body lift is just a non-question. (Unless, of course, if you are talking about a flying wing, in which case 3D wing data applies to the entire aircraft. But even in that case, trimming the aircraft itself poses a lot more constraint on the wing cambre than just simply coming from the standpoint of generating enough lift.)

A lifting body is an aerodynamic body where lift is provided almost exclusively by the fuselage and wing area is minimized or nonexistent. Plenty of aircraft generate “body lift” but that does not make them lifting bodies, as they still primarily generate lift via their wings. The Wikipedia article you cited could have told you this.

That’s right! The amount of body lift contribution also varies by AoA and Mach number. The F-16’s fuselage generates very little lift in level flight, but as much as 30% of total lift at alpha > 9deg and Mach 0.9 (forgot altitude). This is due to the strong vortex generated by the LERX at high alpha interacting with the blended wing-body upper surface. The F-22 and F-35 would generate even more body lift with forward fuselage shaping and integrated inlet-fuselage design.

Flanker and the T-50 would do a lot better with their flat fuselage, and it was said the engine mount + flat integrated lifting structure provides extra lift during supersonic flight due to an effect known as wave riding.

The problem with vortex lift from LERX/forward fuselage is the lift is generated ahead of the CG, hence producing significant pitch up moment at high alpha before deep stall. The pitch-up moment, if not countered, would tend to increase the AoA of the aircraft, creating a positive feedback (hence negative static longitudinal stability). Modern flight control system at this time would droop the elevator/elevon to counter the pitch-up moment.

If the pitch control device cannot produce the required pitch-down moment at high-alpha, the aircraft is simply not controllable. The F-16’s tail at AoA > 36deg cannot generate enough pitch down moment to return the aircraft to level flight. A technique known to the F-16 pilots is to keep pitching UP the aircraft post 70deg AoA, then back down and use the inertia to get the F-16 down below 36deg AoA. F-22 and T-50 pilots won’t have to do this trick since they have vector control.

The rationale behind J-20’s design was, according to early designer Song’s paper, to use the canard as a lifting device, and to dump that canard generated lift at high alpha when the elevons just simply aren’t able to generate more aft-body lift to counter the pitch-up moment. Obviously thrust-vectoring wasn’t in his mind when the paper was written.

Now we are done with the body lift topic, can we go back to S-duct? The J-20’s forward fuselage isn’t wide enough to cover the area of two engine faces, therefore there must be line-of-sight coming from the engine face to the intake unobstructed.

There is nothing in this thread that proves anything you said. :rolleyes:

If you are blind enough not to see significant horizontal and vertical difference between the intake and engine face, you are beyond help. 😮

Horizontal displacement is constrained by trough and weapons bay between the engines. There is a slight bending to cant the engine axis outwards, but the angle is very gentle and since the bending occurs almost immediately ahead of the engine compressor faces, it does not help hiding the compressor from frontal view.

Vertical displacement is more obvious. There is a downward displacement immediately aft of the intake lip, and the duct then tilts upward slightly. This displacement can hide a portion of the compressor face from frontal view, but not entirely. At maximum, the vertical displacement is no bigger than that on the F-16.

In comparison, the so-called “S-duct” featured on the F-22 and the F-35 has a **dramatic** turns, first upward and towards the centre line to clear the side and belly weapons bays, then turns again to align with the engine axis (merging into one in the F-35 case).

Even ignoring the frontal night shot of the T-50 that was purportedly tempered, the rest of the images showing the bottom, sides and rear clearly shows that the inlet duct does not use bending for stealth purposes.

Some people misread this argument as the equivalent of claiming the T-50 is not stealthy. They need to realise that no one is make that claim simply because T-50 does not feature an S-duct. T-50 does feature a shock ramp and likely a radar blocker–both can effectively reduce the radar energy return from the engine.

Whatever Sukhoi has come up with to deal with the stealthiness of the T-50, it does not include an F-22 style S-duct. They have tested the S-duct on the Su-47 and made the decision it is not worth pursuing the same design for the PAK FA. Sukhoi is confident to do it their way to show not every 5th gen fighter has to be an F-22 clone. If Sukhoi is ready to let go of the S-duct, maybe some of us in the forum can, too.

They may have internal structures that are similar shaped to S-ducts. Or maybe they use a blocker. I’m sure Sukhoi knows about the well known stealth concerns surrounding the inlets of fighter aircraft- after all the Su-47 did have S-ducts. But all will be revealed in time, so all we can do for now is speculate until we have better info.

I too believe Sukhoi knows all the pluses and minuses of an S-duct. We are also sure that it is not used on the T-50, based on the test flight photos released so far. Remember aircraft design is not about picking features. It is about balancing all aspects to increase overall efficiency. Sukhoi in this case opted for the lifting wing-body configuration, similar to the Flanker (as opposed to integrated-inlet-fuselage design exemplified by the Raptor and the Berkut). This means two parallel inlet ducts protruding from the wing-body structure to produce a trough which regulates supersonic airflow.

Whatever measure Sukhoi has planned for the inlet duct, be it a blocker, plasma or any other esoteric concepts we can think of, would be just part of the overall performance balancing.