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 Post subject: Stall warning horn
PostPosted: Sat Apr 10, 2004 10:24 am 
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Hey all. Having trouble picturing how a stall horn actually works on a simple trainer ie. C150/172. What causes the horn to sound?

Is there a difference for something more complex, maybe a navajo?



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PostPosted: Sat Apr 10, 2004 12:01 pm 
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Wish i could draw pictures on this thing.

Normally the opening in the leading edge where the stall warning horn is is outside the low pressure area created on top of the wing. But when you get close to the stalling AOA, the low pressure area has now expanded to the point that the opening now has a suction on it from the low pressure. The horn is just a reed that vibrates when you suck on it.

On larger aircraft like a ho or king air there is a vane and electrical switch which work on the same principle. In normal flight the vane is pushed down but at high angles of attack the vane is pushed up. The switch activates a speaker in the cockpit. Due to one of my takeoffs last month i can attest that the one on my king air works. :oops:

When you get to jets and transport category size aircraft the stall warning is done by a computer.



Last edited by ahramin on Sat Apr 10, 2004 6:33 pm, edited 1 time in total.

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PostPosted: Sat Apr 10, 2004 12:15 pm 
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:shock:


Last edited by dont_snag_it on Tue Jun 29, 2004 11:30 am, edited 1 time in total.

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PostPosted: Sat Apr 10, 2004 6:04 pm 
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TOO LOW, GEAR
TOO LOW, GEAR
TOO LOW, FLAPS
TOO LOW, FLAPS
GLIDESLOPE
GLIDESLOPE

I'm getting kind of tired of that. And why is it a male voice? The US Air Force did a study and found that fighter pilots responded better to a female voice. So why didn't the rest follow suit?



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PostPosted: Sat Apr 10, 2004 6:15 pm 
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Hey Abramin :

I had a few girl friends that vibrated when I su.....no forget it, that is not aeronautical.

Cat


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 Post subject:
PostPosted: Sat Apr 10, 2004 10:01 pm 
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The stall warning device in a Cessna 152 / 172 is actually mounted in the wing root just beside the cabin vent on the pilot side. The opening in the wing leading edge is connected by a tube to the device, which is a small metal reed similar to what you might find in a New Years Eve noise maker (but much more expensive).

As the angle of attack of the wing increases air begins to flow over the opening causing a low pressure area in the opening. This draws air over the reed causing it to vibrate. If you look close at the opening inside the cabin you may notice that it is a small megaphone to amplify the sound.

The air movement is similar to blowing across the top of a pop bottle, where the noise results from air passing in and out of the bottle.


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PostPosted: Sun Apr 11, 2004 10:36 am 
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To get even more techy, when the AOA of the wing increases, the stagnation point moves closer to the underside of the wing. When it reaches a pre-determined point (stall warning indicator position) the dynamic pressure of the air now forces the metal tab up to complete the electric circuit, thus the stall horn. That's the basic concept. Also the lack of pressure above the tab, as stated above, also contributes to the movement of the tab.


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PostPosted: Sun Apr 11, 2004 10:48 am 
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I guess that would depend on whether you had an electric stall warning device or the kind usually found on the C152?C172"s most comonly used for flight training. I've never seen the electric device on a C152 or C172, but then I've never seen Nicole Kidman naked in my room either.

It could happen! :shock:


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PostPosted: Sun Apr 11, 2004 1:32 pm 
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Ah..yes yes. :oops:
Time takes it's toll on the memory and I'm trying to think back to the good ol'172 days.
Ah yes, now I remember the training, listen for a faint horn sound amongst the noise ;) but mainly watch for the onset of the buffet for a solid indicator.


Thx for the heads up OW.



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PostPosted: Sun Apr 11, 2004 1:33 pm 
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OW, the reason we are talking about electric stall warning devices is because he asked
Joey wrote:
Is there a difference for something more complex, maybe a navajo?

Not because we think they are on Cessna 150s and 172s. :)



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PostPosted: Sun Apr 11, 2004 2:55 pm 
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The stall warning devices on an airplane all depend on the stall characteristics of the airplane they are installed on. Real simple airplanes like a Cub did not have any because of very benign stall characteristics. The little hole in the wing and reed in a 172 is there so no electricity is required, Again simplicity. Most airplanes that have gentle stall characteristics have a simple vane on the wing. Same idea. The Navajo and Kingair have a vane on the wing but they also have very benign stall characteristics. In the King Air 350, when it stalls, just go to max power with the power levers and it will fly right out just on brute power alone. 2200 horsepower helps a lot. Eventually you get into airplanes like a Metro which can get downright ugly in a stall and the manufacurer has a choice, design bad stall characteristics out or prevent them from stalling. Most manufactures will design a system that prevents the airplane from stalling, usually with a stick shaker, an electric motor that spins an out of balance weight attached to the column, and a stick pusher, a device that will physically push the control column forward with up to 60 lbs force. In the Metro they prevent it from stalling with a stick pusher. There is an angle of attack vane, either on a post on the right wing or on the nose, that senses angle of attack and also compares stall angle with different flap settings and as the airplane slows down, we get an aural warning and if that is ignored, the stick pusher will push the control column forward. This system has to be tested daily and if it fails, it is a no-go item. There are emegency procedures if it quits in flight. In the Gulfstream G1. there is a stick shaker that vibrates the control pole approaching the stalll. In a Lear 25, there is both a stick shaker and if that is ignored, a stick pusher will push the control column forward. And if you get going too fast, a stick puller will pull the control column rearward to prevent overspeed. The DC 10 has a stall avoidance system for each wing. Sometimes these systems can get quite complicated. What ahramin was shouting about with the TOO LOW FLAPS etc is the ground proximity warning system in larger airplanes. A completely different system with a different set of protocols. Works off the radar altimeter, BARC (barometric altitude rate controller) and flap and gear settings.

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 Post subject:
PostPosted: Sun Apr 11, 2004 4:32 pm 
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Yup and the Bus has some computers that will not allow the pilot to stall it.

Cat


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 Post subject:
PostPosted: Sun Apr 11, 2004 4:54 pm 
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Or fly it.


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PostPosted: Sun Apr 11, 2004 5:17 pm 
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Well, actually you can sort of fly it but if you get to agressive the computers will over ride your input so I guess you can't really fly it. :D :D

But what wonderful colours on those screens............

Cat


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After over a half a century of flying no one ever died because of my decision not to fly.


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 Post subject:
PostPosted: Sun Apr 11, 2004 7:51 pm 
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Me sorry, only read the first line about the simple ones.

I would stand corrected, but I'm too tired, so I will sit corrected.

:roll:


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