If an aircraft stalls, there are stall warning systems called as Stall computers. Anyone has idea how they predict that its a stall and how they raise the alarm/warning?
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$\begingroup$ possible duplicate of What is the immediate cause of stall? $\endgroup$– rbpMay 14, 2015 at 12:23
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20$\begingroup$ @rbp I don't think this is a dupe: this question is asking how stall warning systems work, the other question is asking what causes a stall $\endgroup$– PondlifeMay 14, 2015 at 12:24
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2$\begingroup$ Agree with @Pondlife , this is more asking for a technological question trying to understand how previous question is reflected in a warning electronic system. $\endgroup$– Trebia Project.May 14, 2015 at 12:36
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2 Answers
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Audible Warning
On light aircraft there is a reed (much like used on a musical wind instrument) mounted on one wing root, which is angled such that at the Angle of Attack which would cause a stall, the reed "plays" which can be heard in the cockpit.
Here is a view of where this system is mounted on a Cessna:
(source: weekendcfii.com)
On some aircraft, it is a similar principle; however instead of a reed, it uses a fin which at critical AoA pushes a micro-switch which activates a buzzer/horn inside the cockpit.
Here is the assembly on a Beech 18:
(source: eaa1000.av.org)
So, if you ever see anyone doing this:
They are not kissing their aircraft for good luck, merely testing the stall warn as part of pre-flight checks!
Tactile Warning
In addition to an audible reed/buzz/horn, most aircraft also provide tactile feedback to the pilot of an impending stall. In purely mechanical aircraft the aerodynamic forces on the control surfaces are transmitted through the mechanisms and are felt directly by the pilot when approaching a stall. In hydraulic or fly-by-wire aircraft, this is provided by a mechanical "stick shaker" linked to the same system as sounding the audible alarm.
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16$\begingroup$ @FreeMan You can stall at any airspeed (see this question and this one) and in any attitude, only the AoA is important because that's what causes the airflow to separate from the wing $\endgroup$– PondlifeMay 14, 2015 at 13:23
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1$\begingroup$ Recently, my kids were playing with "blowout" party favors. The sound reminded me of the reed-type stall warning in a Cessna. $\endgroup$ May 14, 2015 at 17:23
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1$\begingroup$ This would be improved by adding information for typical jet aircraft (see other answer). $\endgroup$ May 14, 2015 at 17:49
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1$\begingroup$ @Lnafziger do you really think I should update this answer in favour of leaving the other answer to its own merits? I've written about the sort of aircraft I have experience of, in honesty until I read the other answer I never knew that is how airliner-sized aircraft monitor stalls. $\endgroup$– Jamiec ♦May 15, 2015 at 8:09
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1$\begingroup$ One of the goals of the site is to revise answers to make them better, so that ultimately we have that "one perfect answer" the the question. It's the wiki part of the site, and the way that it is now, neither answer completely answers the question. For what it's worth, I would not have added a second answer (since it is not a complete answer) but would have edited yours had I been @porcupine11. $\endgroup$ May 15, 2015 at 15:17
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Large commercial aircraft typically rely on either Angle of Attack (AoA) Vanes or Differential Pitot Tubes (Smart Probes (PDF)) to supply input to flight computers for the purpose of calculating AoA.
AOA Vane
When the computer(s) calculate an AoA nearing the critical angle, an impending stall is communicated to the pilots. There are visual indications, such as those given on the speed tape and pitch limit indicators (PLI), and tactile indications such as the stick shaker and pusher. Here is a Boeing Article on AoA measurement (PDF). Figure 11 shows the indications on the primary flight display.
