irrelevant? may be, but, it is hard to consider him as a 'ignorant' when he explains all his assessments with detailed literature, formulas and calculations.
As i said, i haven't seen anyone thoroughly debunking his assessment and conclusions point by point with detailed analysis, rather, all i came across was a bunch of naive people who started to label kopp as idiot, biased etc.
by your logic of the car,well, i will also go with the one who has years of experience in driving, but, in this case, where are those professionals? and how many of these experienced professionals have clearly debunked kopps assessment?.
neither kopp nor those who label kopp as a fool have any kind of experience in flying an aircraft, nor were involved in actual design of the aircraft, in such a case, both are unreliable source of information, but, kopp makes an attempt to explain his assessment through known standard fundamental physics and formulas, while others completely stay out of it and call him a 'fool'.
If you go through his recent article:
A Preliminary Assessment of Specular Radar Cross Section Performance in the Sukhoi T-50 Prototype
he has made a highly detailed analysis of the Russian T-50 fighter, and this is the only scientific assessment that is out there in the public.No one has come forward to debunk this nor has made a attempt to analyse the T-50 at such a revealing level.
When both of them have no experience in driving a car, i will go with the one who tries to explain the details with fundamentals and sound reasoning and not with one who never justifies his opinion.