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I know that F=fighter, A=attack, F/A=fighter/attack, B=bomber. I can not find any information on what YF stands for. This type of aircraft is usually an experimental aircraft, but why do modern aircraft use X instead of YF?

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  • $\begingroup$ Modern aircraft? $\endgroup$ – egid Oct 23 '15 at 16:07
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    $\begingroup$ @egid guess the YF-22 and YF-23 weren't modern aircraft :) $\endgroup$ – jwenting Oct 23 '15 at 16:31
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    $\begingroup$ Well I suppose the most modern bits will always be in aircraft that have an "X" (Experimental) designation until they get incorporated into a prototype that will eventually become a line aircraft. (Though somehow I don't think anyone would call the X-1 "modern" anymore…) $\endgroup$ – voretaq7 Oct 23 '15 at 16:42
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The 'Y' in YF stands for prototype according to the Tri-Service aircraft designation system. The 'F' stands for fighter, so YF stands for prototype-Fighter. These aircraft are operated by the US Department of Defense (USAF or USN).

For example, YF-22 is the prototype (technology demonstrator) version of the F-22 Raptor.

The 'X' series is the name given for strictly experimental aircraft; i.e. they are not prototypes.

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    $\begingroup$ Why X-32 and X-35, then, when developing the JSF? $\endgroup$ – egid Oct 23 '15 at 16:08
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    $\begingroup$ @egid they were strictly experimentals, technology demonstrators, not actual prototypes. Mind I've not been able to find a source for the actual percentage of parts commonality, but that should be out there somewhere. $\endgroup$ – jwenting Oct 23 '15 at 16:29
  • $\begingroup$ According to "X-15 Extending the Frontiers of Flight" by D. R. Jenkins the X-series was created by NACA as they had no dedicated experimental aircraft program in the 1940s. All research, experiments and tests had to piggyback on other aircraft often unsuited for the tests being performed. The X-series was started in order to have a designation for aircraft which were never intended to progress into the prototype stage. While some may resemble the subsequent prototypes, they don't represent/contain the full technology stack or a complete set of systems as far as I know. $\endgroup$ – bjelleklang Dec 6 '17 at 20:07

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