Stall warning

[ben123]

Hello, Currently studying engineering at college. I really can't find out the answer to this question:-

A stall warning will be given for the aircraft’s particular configuration at a minimum percentage above stalling speed of?

I have tried searching the net + CAA legislation that I can find, but can't find a percentage that all aircraft must comply to for stall warning systems.
Any help would be appreciated.

Thankyou, Ben

[JUL]

Hey Ben

An aircraft can stall at any given speed. It only depends on the angle og attack. The angle of attack is the angle between the chord line and the relative airflow. Stall warning systems measures this angle of attack and gives a warning before the critical angle of attack is achived, in form of a buzzer, horn, lights or stick shaker.

A typical stalling angle is about 16 degrees, but this depends on the configuration on the aircraft. On large aircraft the stall warning system is feed with information about the configuration on the aircraft.

Hope this helped.

Simon

[silentbob]

"Part 23 requires that the stall warning begin at a speed exceeding five knots, but not more than the greater of 10 knots of 15 percent of the stalling speed and must continue until the stall occurs."

Had a quick search on google "Stall warning certification" if you want to try and clarify. Got this answer at

http://www.ntps.edu/thapdfs/07stalls.pdf

[Lucifer]

A stall warner for a smaller GA aircraft will operate in a less sophisticated way - where although obviously designed to go at a certain speed in a certain configuraiton, will warn when the airflow breaks up of the leading edge. It is often placed around the mid-length of the wing, and will therefore sound when about half the length of the wing along the leading edge from from wingtip has disrupted airflow - leaving half the wing with full airflow to be able to recover.

[silentbob]

P.S. I also read in my travels that not all aircraft are required to have a stall warning. Excemptions are given where aerodynamic warnings such as the obvious buffet occur early enough to warn the pilot.

[Mad (Flt) Scientist]

FAR Part 25 Aircraft

25.207(c)
The stall warning must begin at a speed exceeding the stalling speed (...) by seven percent or any lesser margin if the stall warning has enough clarity, distinction, duration or effectiveness.
Amdt 25-42

Note that this refers to warning, not to any particulkar form of warning source. Some aircraft will use natural stall warning, some artificial (e.g. stick shaker). There must be some form of warning; the only option is which kind.

[Genghis the Engineer]

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