Key point: It could be useful, but a true stealth jet will always be superior.
A recent article concerning Boeing’s pitch to the U.S. Air Force of a cheap, modestly upgraded F-15X fighter elicited lamentations from some commenters that the Air Force was not enticed by a more ambitious proposal for the F-15SE Silent Eagle—an F-15 with a reduced radar cross section.
Reduced radar cross-sections (RCS) are a common feature in the cutting-edge o 4.5 generation fighters. Before reduced-RCS engineering was widely understood, manufacturers designed fighter like the F-15 or F-16 that had an RCS of around 3 to 5 m2 or greater.
By comparison, the U.S. military’s newest fourth-generation fighter, the the FA-18E/F Super Hornet, might as well be called the Silent Hornet with an RCS ranging between .1 and 1 m2. The French Rafale has a 1m2 RCS, and the Swedish Gripen and Eurofighter Typhoon around half that at .5m2. Relatively small, RCS-optimized fighters also include the F-16C (1.2 m2) and Chinese J-10 (1.5 m2) Even Russia boasts that its longtime counterpart to the F-15, the Flanker, has been improved with an RCS between 1 to 3 m2 in the most advanced Su-35S model.
However, RCS is not uniform across all aspects of a plane, so these designs are doubtlessly more visible from certain angles. Furthermore, RCS is often calculated from frontal radar visibility of a ‘clean’ airplane not carrying weapons. Of course, fighters will be carrying weapons, so the observability for fourth-generation fighters will usually be inferior in practice to these optimal figures.