Static test airframes, or more commonly called, “iron birds” are partially built, non-flying airframes or old formerly flying airframes that are used by agencies and manufacterers to test either the strength of than airframe, various design components or aircraft subsystems (avionics, flight control, engines, etc).
The iron birds used for strength testing are typically full scale representations of the aircraft that are rigged to gaint gantry cranes with weights and strain gauges attached. See the pic:
Lockheed’s F-35 test airframe installed on gantry cranes with strain gauges.
Once installed on the cranes the airframe is literally pulled and pushed to properly simulate all the aerodynamic forces that the aircraft will encounter throughout it’s flying career. Often the iron birds are tested till destruction.
This is a VC-10 undergoing wing fatigue testing. Note the bending wing.
Some iron birds are formerly flying airframes that have accumulated too many flying hours and are no longer consider safe to fly. These aircraft are typically stripped of most equipment (engines mostly) and used to test various aircraft subsystems in support of other programs.
This NASA’s F-8 Crusader iron bird that was used to test software for NASA’s Digital Fly-By-Wire program in the 1960s,
As the latest example of NASA’s iron bird, this is an F/A-18 Hornet used by NASA to support many of the F/A-18 test programs.
Iron birds aren’t limited to NASA. The US military also used them for the same purposes.
This B-2 at the National Museum of the USAF was never an actual flying airframe. This “aircraft” appropriately named “Fire and Ice”was used for fatgiue and climatic testing.
A close up of “Fire and Ice’s” nose gear door.
You can learn more about that particular aircraft here.
As an aside, old airframes are also typically used as maintaince trainers in the military. These are called ground instructional airframes: