Stall warning

[XPMorten]

Hi All,

I'm in the process of installing a stall warning on a 737 for the X-Plane flight simulator.

Does anyone know at what alpha the warning buzz comes on? Is there maybe other indications than alpha that play a role?

Or is it maybe simply taken from a lookup table with predefined minimum maneuvering speeds at different configurations?

Regards,

Morten

[Genghis the Engineer]

I don't know the 737, but from knowledge of other types, I'd estimate it to be based upon alpha, probably with a different trigger value depending upon flap/slat setting.

G

[John Farley]

Most stall warning systems for commercial and military aircraft include an alpha rate term so that if you are increasing alpha rapidly the warner realises this and blows off early in the hopes of preventing you charging through whatever angle really matters.

[XPMorten]

OK thanks :)

I guess getting any hard numbers on this is difficult.

Maybe try some reverse engineering then :D
e.g is it "normal" to get the alarm shortly after
rotate if you rotate too fast or pitch too much
on e.g a Boeing 737 takeoff?

Other situations on the edge of the "envelope" one can expect to get a stall warning?

Cheers,

Morten

[sycamore]

XPM,until you can get some hard info, try just the sim. in a clean config, with a speed reduction of about 1-2 kts/sec,to the stall, noting where you get any visual indications of buffet, roll/yaw etc, until it stalls. Then you can set a nominal warner at -10/15 kts, or -2-3 deg. Then work outwards with different configurations, checking where the warner operates in relation to the full stall.If you can tie in a` rate` as JF suggests, then increase your speed reduction rate to 2-3/3-4/4-5 kts/sec to fine -tune it. Should also work in steep turns ,60deg. bank @1.4 Vs.

[XPMorten]

Thanks sycamore :ok:

Will try that!

Cheers,

Morten

[blackmail]

hello every one,
stall is only function of angle of attack(alfa),eg, the wing of a parked aircraft in a 10kts headwind is not stalled.
i think that roughly 15° angle of attack will represent a stalled condition for a civil aircraft. less in a dynamic overshoot.
certification rules require a stall warning 7% above stallspeed, buffeting and/or stickshaker/pusher(for T-tails), assuming a speed reduction of 1kt/sec.

[Mad (Flt) Scientist]

Sorry, but stall is not just a function of angle of attack (alpha). It's also a function of the Mach number - even for 'low speed' stalls; it can be affected by the angle-of-attack rate (alpha-dot) and it can be affected by the weight and normal acceleration (total wing loading) due to wing bending changing the actual wing profile.

Civil aircraft with leading edge devices may considerably exceed that 15 degree value - stalling AoAs in the low 20s would not be unreasonable for a slatted aircraft, for example.

The 7% warning margin rule is not an absolute - there are aircraft out there with less "if the stall warning has enough clarity, duration, distinctiveness or similar properties". (The latter being a direct quote from FAR25.207(c) as of Amdt 25-42) 5% might be a typical number for the lower margin aircraft. Not sure which category the 737 falls into.

[John Farley]

Sorry, but stall is not just a function of angle of attack (alpha). It's also a function of the Mach number - even for 'low speed' stalls

Cor, I supect that will have a few people wondering....

I know what you mean but might it not be better to say something more on the lines of:

'while the stall is only a function of angle of attack, the precise function is affected by Mach number and aircraft configuration etc etc'

Sorry if you see that as a nitpick but I think the earlier poster was just trying to make the point that a wing does not stall because of the speed it is travelling at. And my word is that not the truth!

[XPMorten]

OK, heads up.

Found what I was looking for. Turns out this
information is available in Airworthiness Flight Test Schedules. You can find information on most
airliners HERE (http://www.caa.co.uk/default.aspx?categoryid=397&pagetype=68&groupid=786)

Here is the 737-300 stall warning table as
an example.

http://www.xplanefreeware.net/~morten/jpgs/stall.jpg

Cheers,

Morten

[Mad (Flt) Scientist]

That's interesting; I was always under the impression that CAA required a stall speed demo as part of that test.