Video Demo of Pilot-Induced Oscillation

Link to download the video from NASA Dryden's site and a nice description of the F-8 Digital Fly-By-Wire program.

Play the video using QuickTime Player.  Setting the A/V Controls to 1/2x Speed may be helpful.  Also, in the lower left corner of QuickTime Player, you can set the time display to "Frame Number".  You can then use the double-arrow buttons to step through each frame.

In this system, the controller is the pilot.  The plant is the aircraft.  The pilot uses his perception of the motion of the aircraft to determine inputs of the control-surface deflections.  You can see the input to the plant by watching the motion of the stabilator.  The unstable motion is related to the lag between the input from the stabilator and the output motion of the aircraft.

Sample Frame History:

Frames 348-350: pilot pulls up on the stabilator

Frames 354-365: aircraft starts climbing

Frames 366-373: pilot has pushed down on the stabilator, but the aircraft is still climbing

Frames 374-377: aircraft finally levels out

Frames 379-386: pilot pulls up on the stabilator again

Frames 388-398: aircraft starts climbing again

The feedback of the phase-lagged output drives the system unstable.