Here is an example of an aftermarket stability augmentation (stabaug) system for an existing aircraft...
Cool City Avionics Receives SAS STC
The new Stability Augmentation System (SAS) improves aircraft dynamic stability in response to turbulence upsets while hovering and in flight. A SAS will typically remove upsets requiring less than 10% cyclic motion and will reduce the amplitude of larger upsets, thus requiring smaller pilot cyclic corrective inputs. The results are easier, and more accurate, hovering in turbulence and a more comfortable ride in cruise with fewer control inputs required by the pilot. The SAS uses roll and pitch actuators in series with the pilot and is transparent in operation. The SAS uses integral solid state gyros for high reliability and typically weighs less that 10 lbs. installed.
Note the highlighted.
The philosophy behind stabaug is this: Whatever the command (desire) for the aircraft, the system is supposed to make the execution of that command as stable (smooth) as possible. In order to do that, the system must have as much data about the aircraft as possible, most importantly, aircraft responses
DURING that maneuver.
In level flight, and level flight is a maneuver, and it does not matter if the aircraft is fixed or rotary, the gyros and accels feeds the system those aircraft responses. For level flight, P-R-Y responses are effectively nulls. Those nulls are exactly what the aircraft commander want. Turbulence want to deviate the aircraft from what the commander want, so the system uses those nulls as a reference to command the flight control surfaces to counteract those unwanted deviations. The result is a largely 'hands-off' straight and level flight with the aircraft automatically compensating for any disturbances.
Maneuvers are where things gets confusing for the layman. Confusing but not technically complex.
For example...If the command is a pitch up maneuver, throughout the entire maneuver, from initiation to end, there are tiny moments of level flight. It can be difficult to understand, but the analogy is this: If an arc (of a circle) is examined microscopically, there are segments of straight lines. Just like on a sphere, there are microscopic flat surfaces. So throughout the entire pitch up maneuver, there are very briefs moments of level flight
REGARDLESS of where the nose of the aircraft is pointing. If the nose is .0000000005 deg pitch up, at that moment in time, that is level flight, which lead back to the previous paragraph about stability in level flight when there are unwanted deviations. Then when the nose is .0000000006 deg pitch up, at that moment in time, that is also level flight, and lead back to the previous paragraph again. And so on...
The meaning of the word 'level' is not confined to horizon fix, as in eyeballing directly at the horizon. It equate to attitude and stabaug is supposed to maintain attitude stability until the next change, from .0000000005 to .0000000006 deg pitch up. So throughout this pitch up maneuver, stabaug will detect and compensate for any unwanted deviations to ensure a stable transition from .0000000005 to .0000000006 deg.
What distinguishes one stabaug system from another, as in 'good enough' to 'excellent' to 'great', depends on the hardware that provides those important aircraft responses and the software to exploit them. The analogy is how many segments of an arc can be divided to find those straight lines. A 'good enough' system, like this under $20,000 aftermarket system, may not find any microscopic straight lines at all. Just a bunch of smaller arcs, or voltage values. If an unwanted deviation, or voltage value created by a gust of wind, is greater than the ones capable of monitoring by this aftermarket system, then the aircraft will feel that deviation and the pilot will have to take control. If less, then nothing needs to be done.
There is no need to be any better than this for a helo. A helo is not going to fly at Mach or execute high g loops.
If a stabaug system can be aftermarket, that mean stabaug is very much an accessory but not a necessity. But for a high performance aircraft designed to defend national interests, stabaug must or should be built in from when the aircraft is on paper. Avionics, Aerodynamics, and Propulsion cannot be isolated from each other. This under $20,000 aftermarket system apparently does not need air data inputs -- altitude and airspeed -- and there is nothing wrong with that. But a jet fighter intent on killing an adversary must be able to dissect that arc as fine grain as possible to find as many straight lines as possible in order to provide the pilot automated flight stability to allow him to concentrate on being a killer instead of a mere 'stick actuator'. Altitude have different air density and therefore affect aerodynamic forces on the flight control surfaces. Airspeed also affect those same forces on those same surfaces. Different altitude-airspeed combinations will produce extremely diverse levels of aerodynamic forces, so a high performance stabaug system need air data inputs to calculate appropriate amount of flight control surfaces deflections to give the pilot that automated stability.
A fly-by-wire FLCS cannot be turned off. A lot of popular but low technical value articles often make that claim. It is stability augmentation which is an accessory that can be deactivated.
