Which aircraft have turned out to deliver the least in terms of their cost and performance? This is a tough question, particularly since there are a number of reasons why aircraft have never really fulfilled their design potential. One reason is that the plane may have never had to carry out its designed role.
One example of this is the B-36 heavy bomber. This was a large and expensive bomber controversial since there had been an effort to cut back on aircraft carrier production to pay for it. This caused a huge debate and the famous Revolt of the Admirals during which it was labeled a billion dollar blunder. However, things never got to the point where the B-36 would be put to the test of combat a fortunate thing, since the B-36 was strictly designed as a nuclear bomber (to cross the Atlantic to bomb Nazi Germany in case Britain fell under German control). To a lesser extent, this has been true of every strategic bomber built and entering service since the Cold War started (the B-52, the B-1B, and B-2A all fall into this category to one degree or another). The nuclear bombers have turned into superb conventional assets all the while managing to keep the peace in the nuclear balance.
Another reason could be that the aircraft was oversold to begin with. A classic example of this is the F-111. The original billing of the F-111 was to be a low-altitude interdiction aircraft for the Air Force, an interceptor for the Navy, and a close-air support aircraft for the Marine Corps. In essence, it was a late-1950s and early-1960s version of the Joint Strike Fighter. The aircraft only really succeeded in the first role, never got past the prototype stage in the second, and the third version never got off the drawing board. In essence, one airframe was being tasked to fulfill three widely disparate roles roles that needed three separate aircraft. Ultimately, the Navy pulled out of the F-111 program and instead built the F-14, which became a superb interceptor and air superiority plane. The Marines ultimately went for the AV-8B Harrier for close-air support. The Air Force, however, got a superb tactical bomber once the F-111s teething problems had been worked through.
Some aircraft also get the label, because they are still being proven, or have just entered service. Several American aircraft fit this category, most notably the V-22, F-22, and to a lesser extent the F-35. The press has had their field days reporting on perceived problems with these aircraft, but these are aircraft that have been going through the same process of working out the glitches that many now-proven planes like the F-14, F-15, F-16, and F/A-18 have. The Su-27/Su-30 family of aircraft also fall into this category. There have been very few combat tests of the Su-27/Su-30 to date (the only one known of is the Ethiopia/Eritrea conflict in 1999-2000).
These are some planes that DO deserve some criticism. These are planes that have gone to combat and have proven they cant hack it (at least to date). One of these planes is the MiG-29 Fulcrum, which got its first combat test in Desert Storm where it was on the wrong end of a 29-0 kill ratio. As many as four Yugoslavian MiG-29s were downed during the war in Kosovo, and as many as five were reportedly shot down in conflicts between Ethiopia and Eritrea. The only kill attributed to the MiG-29 was an unarmed Cessna on a humanitarian mission shot down by a Cuban MiG-29 over international waters. This has resulted in the MiG design bureau being eclipsed to a large degree by Sukhoi, whose Flanker has picked up the bulk of export orders from Russia. It never hurts to take claims that a system is junk with a few grains of salt. It sometimes might be the aircraft still has to work out some teething problems or hasnt had the chance to prove what it can do. Harold C. Hutchison (email@example.com)