The F-84 and Derivatives
By Bruce Craig, Webmaster
The Republic P-84, later changed to F-84 when the US Army Air Force was replaced by the US Air Force and the "P" for Pursuit was changed to "F" for Fighter, was variously known as the Thunderjet (in its straight-wing or "plank wing" form), as the Thunderstreak (in its swept-wing with nose intake form), as the Thunderflash (in its swept-wing with wing-root intakes form), unofficially as the Thundershriek or Thunderscreech (the XF-84H turboprop test aircraft), and the Super Thunderstreak (the XF-84J "lip-intake" prototype). A related aircraft, the Republic XF-91, was known as the Thunderceptor.
The Bell P-59 and the Lockheed F-80 were the first and second jet fighters ordered by the USAAF; both of those aircraft used centrifugal flow turbojet engines. The P-84 was the third jet fighter ordered by the USAAF and the first to use an axial flow turbojet. The Thunderjet, as it came to be called, was a more slim and clean design than either of the other two. Because jet aircraft were relatively new, and their capability for achieving speeds much higher than propeller driven aircraft was accompanied by many problems associated with the unknowns of aerodynamics and materials, these early fighters were plagued with a myriad of problems. The Thunderjet was not imune, and suffered through many assorted problems and failures which lead to accidents, caused the fleet to be grounded on occasion, delayed manufacture and delivery at various times, and lead to many changes and improvements being implemented on the production line within production blocks.
Production of the Thunderjet got underway during a period when understanding of aerodynamics principles was advancing at a rapid pace. Data acquired from German WWII records about the benefits of swept wings was implemented by many aircraft manufacturers. Republic's iteration arrived as their swept-wing modification to an E-model Thunderjet, then designated YF-96A, the first prototype of the series that became the F-84F Thunderstreak. Then the Korean War began, and pressure to produce the newer-and-better F-84 intensified.
However, production of the F-84F was delayed for a number of reasons, including engine problems, lack of facility to produce the wing-spar forgings, aluminum production shortages, and flight instability. Meanwhile, as the Thunderstreak was not available to the USAF for the Korean War, the "plank-wing" Thunderjet was upgraded to the G-model and these aircraft, along with some of the earlier D- and E-model Thunderjets, served in Korea instead of the Thunderstreak.
The result was that the Thunderjet, although a stable gun platform, simply was not capable of the speeds needed for it to serve in an offensive or even defensive role against the faster MiG-15 which rapidly dominated the skys over Korea. The vacancy was filled by the North American F-86 Sabre while the Thunderjets were relegated to the Attack Bomber role, in which role they served admirably, albeit without the prestige and fanfare accorded the F-86.
And therein lies the irony of history; although the Sabre got the lion's share of the press and prestige while the Thundejet and its derivatives plugged away in the shadow of the F-86, there were more F-84s manufactured than F-86s, the F-84 served in a wide variety of test-bed roles while the F-86 got more press as a chase plane for tests, the F-84 was used longer in NATO service than was the F-86, and the F-84 in its various iterations served in a wider variety of service-duty roles than did the F-86. Effectively, the F-84 became the plow-horse while the F-86 went to the Kentucky Derby.
In parallel with production of the F-84, Republic also undertook testing and development of others in the F-84 family. Among these were the XF-91, the XF-84H, and the YF-84J. The XF-91 Thunderceptor was a test-bed for rocket engines, inverse taper wings, and variable incidence wings. The XF-84H "Thundershreik" was a test-bed for turboprop engine and supersonic propeller designs. The YF-84J was a development of the F-84F with a larger engine and "lip" style intake. Also, test programs included the YF-96A, YR-84F, YRF-84F, YF-84K, TipTow, FICON, ZELMAL, YF-84A with NACA Intake Ducts, and thrust reversers.
Generally then, a case could be made that the F-84 series was a better aircraft than was the F-86. But, it is all a matter of prespective; the fighter jocks and fighter aircraft always have occupied the "glamour" role in the press and the eyes of the public, while the rest of the aircraft in the inventory were the "grunts" and the "go-fers." Just ask any F-105 or A-10 pilot. And, that is perhaps the final irony; like the F-84, the F-105 and A-10 are products that came out of Repbulic.