3MilesToWRO
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How Does Stick Shaker Sensor Work?

Tue Dec 04, 2007 11:03 am

How is stall detected for stick shaker? Wikipedia says it's an angle of attack sensor, but this is, well, Wikipedia  Wink Is it indeed only AoA measuring plus wing configuration (flaps, slats etc.) and predefined table "when to shake" or are there any more direct ways of sensing?
 
boeing767mech
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RE: How Does Stick Shaker Sensor Work?

Tue Dec 04, 2007 11:19 am

On the MD-80 the stick shaker gets input from the Stall Warning Computer, which takes input from the AOA vane, the flap/Slat postion(from the Proxs box) and input from the air data computer. There maybe more inputs but without the LAM or maintainence manual infront of me these are the ones I can name of the top of my head

David
Never under-estimate the predictably of stupidty
 
fadecfault
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RE: How Does Stick Shaker Sensor Work?

Tue Dec 04, 2007 6:42 pm

It's going to vary for a/c to a/c but like Boeing767mech said the ADC/ADIRU is going to have some input like current airspeed. I'm sure there are more inputs and maybe some inhibit inputs as well but the AOA sensors are not in full command as the wikipedia makes it seem.
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A10WARTHOG
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RE: How Does Stick Shaker Sensor Work?

Tue Dec 04, 2007 8:23 pm

If I recall correctly a signal from the ice detectors also come into play.
 
pilotpip
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RE: How Does Stick Shaker Sensor Work?

Tue Dec 04, 2007 10:32 pm



Quoting A10WARTHOG (Reply 3):
If I recall correctly a signal from the ice detectors also come into play.

On the EMB 145 when the ice detectors detect ice we'll get an "SPS ICE SPDS" message on the EICAS for the remainder of the flight. It's a blue (advisory) message to let us know that the speeds for the Pitch limit indicator (PLI aka "Bart Simpson") will come down earlier than normal because the ADC now thinks there is ice on the wing. The only way to clear this message and return the speeds to normal is to test the system on the ground in between flights. I usually try to do this right after everybody is off and before the next flight is boarded but sometimes you'll hear us test it. We pull the yoke back, push the test button get the shaker, then pusher, while the aural warnings are going off as well.

It's loud, and doesn't sound good so I try to avoid letting pax hear it.
DMI
 
bobbidooley
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RE: How Does Stick Shaker Sensor Work?

Tue Dec 04, 2007 11:03 pm

I would like to know, not only, how it derives its input - but how does it physically shake? I would imagine it is somthing akin to a cell phone vibrator, only on a much larger scale. Also, on non FBW AC do the actual control surfaces vibrate to some degree?
Thanks,
Bobbi



edited - clarity

[Edited 2007-12-04 15:05:51]
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fadecfault
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RE: How Does Stick Shaker Sensor Work?

Tue Dec 04, 2007 11:16 pm

On the a/c I've worked it's just a motor with probably an unbalanced weight attached to the control column. I can't see it moving any flt surfaces to any degree.
You can see it in the middle of the fo's control column.


[Edited 2007-12-04 15:16:56]

[Edited 2007-12-04 15:17:12]
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boeing767mech
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RE: How Does Stick Shaker Sensor Work?

Wed Dec 05, 2007 12:02 am

There is a stick shaker on the column that warns of approaching a stall. When you stall you get a stick pusher and depending on the airplane the stall will auto extend. Interesting thing on the DC-9/MD-80 which do not have hydraulic powered flight controls, The down side of the elevator is powered, to push the nose over in a stall. You can do it on the ground if you slam the yoke forward.

David
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pilotpip
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RE: How Does Stick Shaker Sensor Work?

Wed Dec 05, 2007 1:09 am



Quoting Bobbidooley (Reply 5):
I would like to know, not only, how it derives its input - but how does it physically shake? I would imagine it is somthing akin to a cell phone vibrator, only on a much larger scale. Also, on non FBW AC do the actual control surfaces vibrate to some degree?
Thanks,
Bobbi
[Edited 2007-12-04 15:05:51]

The shaker is an unbalanced flywheel on the end of a small electric motor. When the motor turns on, the yoke shakes. It's a setup just like what makes your cell phone vibrate but it's on a much larger scale.

I doubt the controls are vibrating as a result of a stick shaker. The whole point of the stick shaker is to mimic an aerodynamic buffetting that occurs prior to stall. Most hydraulic systems are equipped with an artificial feel unit that allows the pilots to "feel" the aircraft's handling charateristics through it's range of speeds. Even on aicraft that use cables and pulleys you have a lot of cable to absorb any vibration.
DMI
 
bobbidooley
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RE: How Does Stick Shaker Sensor Work?

Wed Dec 05, 2007 1:54 am

That's kinda what I thought. Thanks Pilotpip.
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Jetlagged
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RE: How Does Stick Shaker Sensor Work?

Wed Dec 05, 2007 3:04 am



Quoting FADECFAULT (Reply 2):
It's going to vary for a/c to a/c but like Boeing767mech said the ADC/ADIRU is going to have some input like current airspeed. I'm sure there are more inputs and maybe some inhibit inputs as well but the AOA sensors are not in full command as the wikipedia makes it seem.

The basic inputs are AOA (measured by a vane) and LE/TE flap position. Airspeed is not needed because the key parameter is AOA. It may be that some sophisticated stall warning systems use other inputs to correct vane AOA to a more precise correlation with wing AOA. Pitch rate, for example, is important for converting vane AOA to wing AOA, especially if the vane is near the nose on an aircraft with rear mounted engines. However on Airbus the stall warning is basically very simple, one stall AOA value for flaps up, another for LE flaps extended. Boeings usually have a different stall AOA value for every flap position.

On complex modern aircraft the position of individual LE flap/slat sections may be taken into account to allow for asymmetry conditions. On older aircraft designs only one or two sections may have an input.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
fadecfault
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RE: How Does Stick Shaker Sensor Work?

Thu Dec 06, 2007 1:02 pm

Quoting Jetlagged (Reply 10):
The basic inputs are AOA (measured by a vane) and LE/TE flap position. Airspeed is not needed because the key parameter is AOA. It may be that some sophisticated stall warning systems use other inputs to correct vane AOA to a more precise correlation with wing AOA. Pitch rate, for example, is important for converting vane AOA to wing AOA, especially if the vane is near the nose on an aircraft with rear mounted engines. However on Airbus the stall warning is basically very simple, one stall AOA value for flaps up, another for LE flaps extended. Boeings usually have a different stall AOA value for every flap position.

From the 737NG AMM (SDS)



Notice the last sentence.

737NG SSM:


[Edited 2007-12-06 05:03:01]
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