Well done Ben, you've spotted that stalling is the second worst taught subject in university academics (the worst is spinning) and gone outside for advice - you've obviously sussed the academic environment!
Anyhow, first point - the stall as defined in certification codes (para 201, virtually any of them) is not necessarily the wind-tunnel / graph defined aerodynamic stall, the classical definition is the point at which the pilot ceases to have absolute control over the aeroplane, this is often defined simply by full back-stick meaning that the aircraft can't be pitched up any more. Make sure you know what definition is in use for a particular aeroplane.
Secondly, the different standards vary in their requirements for warning - in civil codes it'll be (if there's anything) somewhere between paras 201 and 220. Worth also looking at Def-Stan 00-970, the UK military standard (look for the link in tech-log's directory). Ultimately it's got to be considered adequate by the assessing test pilots - that's absolute, the wording of the standard is to some extent negotiable. Also some standards allow leeway on warning depending upon the severity of the stall characteristics. In one or two types (for example the Jetstream) it's marked by operation of the stick-pusher.
Stall warning can be furnished in a variety of ways, most of which are natural - an artificial stall warning is a last resort and not normally used unless absolutely necessary. The following are stall warnings I've seen, with a few examples of types to which they applied...
- Klaxon (PA28, Jaguar in operational alpha limits)
- Airframe Buffet (PA38, C150, L1011, DC3)
- Stick shake, artificial (some airliners)
- Stick buffet, natural (memory shakey here, Bulldog?)
- Pitch attitude (most light aircraft)
- Stick position (most microlights, both 3-axis and flexwing)
- Wing rock (Jaguar in flight test alpha limits)
Do bear in mind that most artificial stall or stall-warning systems use alpha gauges (usually a vane such as on the A10, Jag, a yawmeter (Tucano? - not sure), or a simple flip-vane such as the PA28, CH601, Aviasud Mistral). Artificial stall or stall warning systems are NEVER based on airspeed since this will change with loading both weight and in manoeuvre.
Finally, for the various standards, the minimum warning is..
Section S - at test pilots discretioon
Section K - 5kn, less at TP's discretion
VLA - 5-10kn
JAR-22 - 5-10%, less or none if totally controllable down to stall.
JAR-23 - 5kn
JAR-25, greater of 5kn / 5%
FAR-23 - 5kn
FAR-25 - 7%, less if sufficient to prevent inadvertent stall (as MFS' post).
This is obviously a summary, you do need to look carefully at the exact wording in each standard as if it were a legal contract !
G