Week 2: Buying agility with stability

How an aircraft came to be built deliberately unstable, what that bought, and what it cost the person flying it

1 March 2027

A lecture about a trade that was made once, in the 1970s, and that every period of this course is downstream of.

Stability in an aircraft is a restoring tendency: disturb it, and it returns. That tendency also resists being pointed somewhere else, and by the late 1960s that resistance was the limiting factor on how fast a fighter could change what it was doing.

The trade that follows:

• move the centre of gravity aft, past the aerodynamic centre
• the restoring tendency inverts, so disturbances grow instead of damping
• the aircraft points very quickly, but a human can no longer fly it — the corrections required arrive faster than a person can produce them

That last point is acceptable only if a computer flies it instead.

The lecture covers what that computer does — sampling, voting, redundancy, and what the architecture assumes about its own failure. It ends on the question the rest of the course returns to: when the thing flying the aircraft is not you, what exactly is your job.

Reading#

The flight control system chapter, and one contemporary account of the type’s introduction to service. Bring a note of anything in the second that the first does not prepare you for.