OzLAME From Australia, joined Feb 2005, 338 posts, RR: 0 Reply 2, posted (7 years 11 months 1 day 14 hours ago) and read 10392 times:
The Stick Shaker is part of the stall warning system. On most small aircraft (some Cessna singles being one exception) the stall warning system consists of a switch in the leading edge of the wing which turns on an electric horn or bell behind the instrument panel.
On most larger a/c, (including bizjets, the Fairchild Metro and just about anything bigger) the system is an Angle of Attack Transmitter mounted on the nose (which looks like a blade or conical probe and is free to rotate) which sends a signal to a computer. The computer takes other inputs such as gear and flap position and when it senses an impending stall it will activate the shaker; the shaker is an electric motor with an eccentric or cam built in that causes it to vibrate rather coarsely (like an out-of-balance wheel on your car) and which is usually mounted on the control column so the column shakes. Many a/c will also have a Stick Pusher that consists of a servo that will actually forcefully move the control column forward (ie nose-down) when the computer detects the a/c is in a stalled condition.
I say 'mostly' and 'usually' because, as with all things, there is considerable variation between different types of a/c. For example the Mitsubishi MU-2 and Beech King Air and 1900 all have the same type of Lift Detector mounted under the leading edge of the wing which senses the angle of attack by how far forward or aft a vane is in a slot. The Beechcrafts have the computer send a signal to the crew Audio system, which generates an electonic tone for stall warning; while the MU-2 has a Stick Shaker. Piper Cheyennes and Fairchild Metros have a Stall Avoidance System and have Stick Pushers fitted as well as shakers; most Metro IIs have the AoA transmitter on a short probe forward of the wingtip while other IIs, and IIIs and 23s have the transmitter on the side of the nose. DC-3s have nothing except the behaviour of the a/c to warn of an approaching stall.
Monty Python's Flying Circus has nothing to do with aviation, except perhaps for Management personnel.
Kaddyuk From Wallis and Futuna, joined Nov 2001, 4125 posts, RR: 28 Reply 3, posted (7 years 11 months 1 day 2 hours ago) and read 10261 times:
Its a motor that turns an unbalanced flyweight. Inducing a vibration which transfers to the control column to indicate a stall or impending stall condition. You can activate this during a test and if you get a nice pilot, he can activate a test of the stick shaker via the CMC
Whoever said "laughter is the best medicine" never had Gonorrhea
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 4, posted (7 years 11 months 1 day 2 hours ago) and read 10254 times:
Part of the Stall Warning Circuit.An Unbalanced Flyweight attached to the Control Column receives current from an Angle of Attack sensor,at a high angle.The Unbalanced Weight causes the Control column to shake.The Warning comes on much before an actual stall occurs & gives the pilot time to recover.
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 7, posted (7 years 11 months 19 hours ago) and read 10168 times:
The L-19 Bird Dog had no stall warning. Just about at the stall, however, the prop would start to whistle. The next thing that happened would be falling off on one wing, the stick going kind of limp, and the altimeter would begin to unwind, regardless of the location of the ground.
Happiness is not seeing another trite Ste. Maarten photo all week long.
OzLAME From Australia, joined Feb 2005, 338 posts, RR: 0 Reply 8, posted (7 years 11 months 12 hours ago) and read 10098 times:
Quoting HAWK21M (Reply 4): An Unbalanced Flyweight attached to the Control Column receives current from an Angle of Attack sensor,at a high angle
That is not correct. As I stated in my earlier post, the AoA Transmitter looks like a blade or conical post on the side of the nose and is free to rotate, typically through an arc of about 30 degrees. What I should have added is that the Transmitter is positioned by the slipstream, hence it's angle relative to the fuselage is determined by the aircraft's wing's angle relative to the airflow (AoA). The blade or probe is connected to a potentiometer inside a case ,which also is often filled with a damping fluid so that the probe won't 'flop around', for want of a better term. There is also a heating element for ice protection.
Where HAWK21M is incorrect is in saying that the current goes from the AoA sensor to the flyweight. The potentiometer receives a fixed voltage from the Stall Computer and sends a different voltage back, which is proportional to the AoA. The computer determines whether the aircraft's wing is geting close to the stalling Angle of Attack (which is the same for all aircraft, regardless of size, shape, power or airfoil design), about 16 degrees for a wing in 'clean' configuration. The computer takes inputs for gear and flap position because the stalling AoA changes when the wing is 'dirty'. The computer will then send a voltage to turn the motor, which as I and other people have noted, has an eccentric or unbalanced flyweight attached to the end of it which makes it vibrate. The flyweight itself is just a bit of metal and receives no current.
In the LH photo below you can see two silver things above the crew seats on each control column. These are a pair of hose clamps such as you may find on the radiator hose on your car and they are holding the stick shaker motors on the front side of the control columns. On the very right of the RH photo, in the middle you can see part of a circle, this is the top of the shaker motor on the front of the RH control column. Unfortunately the crewmember's leg means that there is no contrast to see the black-painted motor in any detail. In the third photo you can see the AoA Transmitter blade above the Lufthansa logo.
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 9, posted (7 years 11 months 7 hours ago) and read 10065 times:
Quoting OzLAME (Reply 8): The potentiometer receives a fixed voltage from the Stall Computer and sends a different voltage back, which is proportional to the AoA
You are correct.I tried to explain it as brief as possible.
A little more Detailed Explaination is as follows:-
A stall warning system is provided to alert the pilots of an approaching stall condition.
The warning is accomplished by applying vibrations to the captain's control column.
Interconnection of the control columns enable the vibrations to be transferred to the first officer's column. The system consists of an angle of airflow sensor, flap position transmitter,stall warning module and a control column shaker.
The angle of airflow sensor measures theairplane angle of attack.
An electrical position signal from the airflow sensor is modified by the
flap position transmitter synchro and then fed into the stall warning module.
At predeterminedcombinations of flap position and airplane angle of attack, a stall-warning signal is emitted from the module to activate the control column shaker.
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 11, posted (7 years 10 months 3 weeks 3 days ago) and read 9863 times:
Quoting JDD1 (Reply 10): Am I right in saying that the stick-shaker, followed by a stick pusher, came about because of the deep stall crashes of the HSA Trident and BAC1-11 during flight test?
That's the way I heard it.
Stick pusher is an interesting experience in doing stall recovery in the sim. Flying the plane out of a stall requires some finesse. If you go so far out of steady flight as to get the stick pusher it requires a bit of brute force to overpower it. Brute force and finesse at the same time? That'll challenge you.
Happiness is not seeing another trite Ste. Maarten photo all week long.
Troubleshooter From Germany, joined Feb 2005, 423 posts, RR: 5 Reply 12, posted (7 years 10 months 3 weeks 2 days 22 hours ago) and read 9871 times:
The 737NG does not have a stick pusher. After reaching a certain AOA (and other valid inputs) the elevator feel shift module receives a signal from either of the two SMYD to send hydraulic system A pressure to increase the artificial feel on the elvator system. This shall prevent the crew from pulling the aircraft more up.