I've never been one to doubt Russian Engineering. Cut corners here and there they may but they do get the job done and more often than not, they turn good stuff out. What they have accomplished with designing and building the T-50 is significant even if it does borrow on our designs somewhat. As one of our members here correctly pointed out, there's only so many ways you can achieve stealth and certain performance parameters within a given design. What Sukhoi is running into now is that they now realize the elevated challenges of building such an aircraft and the current limitations of their engineering and available materials and technology. The final engine is still a ways off as continuing technical issues are slowing progress of that. There are still numerous other issues currently and ahead that could push the production start and service entry nearly 10 years or more away. It's not unlike the challenges and delays we faced with the F-22 and now F-35 but I am confident that eventually they will get it right. Some sources say if they rush the T-50 into production, it will end up more a Gen 4.5 than a Gen 5 Fighter. One piece of advice I've read, ditch thrust vectoring on the plane and focus on stealth, radar/sensor systems and weapons that have an engagement and maneuver envelope far beyond what any manned or unmanned aircraft can evade.