Fabulous discussion...and worth reading twice
During my TP training we were given the opportunity to fly three sorties in a variable stability Lear Jet while we tried to wrap our heads around the concepts discussed above. We were given a number of exercises where the student was given a 'mystery' aircraft - i.e. the fly by wire system of the Lear was programmed to mimic a certain set of flight control laws - and he/she had to puzzle out the handling qualities of the aircraft, deliver a verdict on problems observed, and then suggest a fix.
One of the areas that came up frequently (and that I have also dealt with several times in my career as a TP to date) was that of relaxed or reduced longitudinal stability. We were given several examples of both neutral and negative apparent static longitudinal stability to fly, as well as several with negative longitudinal stability under manoevure. The description posted earlier of the technique required to fly these 'simulated' aircraft was spot on - sharp, direct, and continuous inputs in an effort to keep up with the aircraft. Needless to say, we were most often unsuccessful and the safety systems of the variable stability Lear disconnected our controls to prevent us from overstressing the aircraft. With a bit of practice it was certainly possible to fly these aircraft, but the workload was high and if one did not have a clear understanding of what was happening at the time the end result was inevitable.... Flight in cruise is one matter; flight in the slow or high speed regimes is quite another and it is often during an 'upset' that relaxed stability renders a crew's trained reactions ineffective.
Not to muddy the waters further, but it is important in these discussions to have an accurate appreciation of the control system of the aircraft. The C150, with its convention/reversible control system where one sets and commands an angle of attack (alpha stable) is a very different beast from the A340 with a fly-by-wire system and flight path stability, or the C17 which uses its FBW system to command a pitch attitude (theta stable). I don't throw this out to confuse the issue, but rather to point out that there can be a myriad of reasons why a 'simple'
subject like stability can mean so many different things in different situations.
Pilots expect their aircraft to behave in a safe and predictable manner - it is the purpose of the certification guidelines (such as minimum stick force gradients) to help ensure that they do! Force gradients are one way that an aircraft provides the pilot feedback - without him or her ever having to look at an instrument of crosscheck.