"I took envelope to mean the operating conditions under which the control system is stable." Yes, that's exactly what I meant. I kind of borrowed the term from

"I took envelope to mean the operating conditions under which the control system is stable." Yes, that's exactly what I meant. I kind of borrowed the term from aviation where this is used to describe conditions under which the behavior of the aircraft is known and predictable. Doesn't mean it is not possible to fly outside the envelope, you just don't get any guarantees.