The USA has had partial success with conversion of a contemporary fighter to a SEMI STEALTH fighter with F15SE...
The russians and chinease simply designed brand new fighters
Which incurred a lot of expense.
The reason why the F-15 is successful -- and I use that word cautiously -- is because of its inherent features, such as twin vertical stabs. Canting them is easier and would eliminate the dreaded 90 deg corner reflector.
The above is a corner reflector created by the F-22's rear quarters. In shaping for RCS control and reduction, the rules regarding corner reflectors are that avoid them completely, but if you cannot and a lot of the time you will not be able to, then avoid the 90 deg type. The F-15's twin vertical stabs can be re-positioned to eliminate the 90 deg type. We cannot do the same for the F-16 without incurring a lot of cost. Technically feasible? Yes. But financially effective? Dubious. If 'cost is no object' kind of mentality, then start anew, do not bother with trying to modify an existing airframe.
The corner is one example.
Another example is external stores. The F-15's conformal storage was originally designed for fuel and the system worked. It was only inevitable that they would be redesigned to carry weapons, but at the cost of reduced armament per sortie. We have no such for the F-16 because its fuselage design and shape. So far, the only places for the F-16 to carry anything enclosed are the dorsal spine and the two very oddly shaped conformal fuel tanks on the back, hardly safe for weapons.
The F-18 Super Hornet have a smaller RCS than the previous Hornets, but the SH is practically a different aircraft because it is a larger design and have some features like the F-15 that are conducive for RCS reduction.
Here is what I think Pakistan could do for the JF-17 if finance allows, but not 'no object' kind of mentality:
- Get rid of the single vertical stab. Do what the Iranians did for their F-5s.
- Install absorber on all leading edges.
- Re-shape all panels, such as using those 'saw tooth' patterns wherever gaps are likely to be in
DIRECT contact with an impinging radar signal. Note: In RCS control, we call such physical shaping 'geometric absorber', which is contextually different than how a material like a sponge 'absorb' water. From the seeking radar perspective, if its signals actually impact a body but somehow the echoes are affected in any way, then as far as the seeking radar is concerned, the echoes have been 'absorbed'. Denial is considered 'absorption', hence the phrasing 'geometric absorber'. Not absorbed like a sponge.
The above is how much work is involved on panel gaps on an existing
SHAPE, even a redesigned one like the F-18 SH. The F-22 is no different but just a lot less because it was designed from paper to have less of these small RCS related geometric issues. There were a lot of conflicts, mostly finance related, during the F-18SH design process, because of the desire to install those 'saw tooth' geometric absorber versus leaving the panel shapes as is and use the more maintenance intensive 'zip strips' methods to cover up those surface discontinuities. Guess who won the arguments? If it was not easy for US, what make anyone think it will be easy, financially and technically, for anyone else?
- Re-shape all tip diffraction generators like air data probes. This is not physically difficult but dangerous for avionics because they rely on
CONSISTENT air data flow across their ports. Raw air data is not a problem but
CONSISTENCY in receiving them is. Re-shaping the air data probes could adversely affect that availability, especially during maneuvers, and that will create all sorts conflicting flight control commands.
- Enclose all external stores and this includes re-designing the pylons as well. The enclosure must be RCS efficient. If they are to be retained instead of discard then it must be understood that they will affect aerodynamics and fuel, but the discard option must still be available for the pilot in the event his life depends on its ejection from the aircraft.
- Re-design the canopy. The cockpit is a well and it will create high EM resonance.
- Smooth out surface flow as much as possible. This is about small surface discontinuities that the closer they are to each other -- real estate wise -- the greater the odds of their diffracted/resonance cumulative effects be detected and focused upon by a seeking radar. This effect was easy to incorporated into all radar systems.
- Re-design the engine exhaust.
Those 'saw tooth' patterns -- geometric absorber -- on the F-35's engine exhaust is actually more effective at scattering diffracted signals every which way but back to source direction than people realize. It is rare to look straight into an engine from the 'six' position so this technique is more effective than the simple straight edge to deny the seeking radar some energy. Not all, but enough. We have done plenty of model/predict and measurement on this.
So how much money will it take to do all of the above? No one really know. But if it approaches the point where it will cost to design 'stealth' from paper...Then is it worth it?
