The Stack Overflow podcast is back! Listen to an interview with our new CEO.
2 Added more data
source | link

The 'Y' in YF stands for prototype according to the 1962 Tri-Service aircraft designation systemTri-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'X' series is the name given for the aircraft operated under the NASAstrictly experimental aircraft; i.e. they are not prototypes.

The 'Y' in YF stands for prototype according to the 1962 Tri-Service aircraft designation system. The 'F' stands for fighter, so YF stands for prototype-Fighter.

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

The 'X' series is given for the aircraft operated under the NASA.

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.

1
source | link

The 'Y' in YF stands for prototype according to the 1962 Tri-Service aircraft designation system. The 'F' stands for fighter, so YF stands for prototype-Fighter.

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

The 'X' series is given for the aircraft operated under the NASA.