#yf-110

Century Series - McDonnell Douglas F-110A Spectre

For a brief period before the 1962 United States Tri-Service Aircraft Designation System, the McDonnell Douglas F-4 Phantom II was known as the F-110 Spectre in United States Air Force service and F4H by the United States Navy. Both services settled on F-4 Phantom II as the eventual designation for the aircraft.

Note: The United States Air Force also used the designation "YF-110" for MiG-21 and Chinese F-7 aircraft acquired and flown under the Constant Peg program.

YF-110, YF-112, YF-113, YF-114, YF-116, and YF-118 - Captured/Obtained Soviet Aircraft flown by the US Air Force "Red Eagles"

Captured (or otherwise obtained) foreign aircraft used for evaluation and aggressor training were given designations in sequence using the pre-1962 "F-series." These aircraft were flown by the 4477th Test and Evaluation Squadron under the program "Constant Peg" from 1980-1988.

The aircraft were designated the following:

YF-110 - MiG-21 

YF-112 - Sukhoi Su-22

YF-113 - MiG-23

YF-114 - MiG-17

YF-116 (rumored) - MiG-25

YF-118 (rumored) - MiG-29

via Wikipedia, Area 51 - Black Jets (Google Books), and the 4477th Red Eagles Alumni Association.