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OmniAtlas

I can't stall the Airbus

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My question is how do the systems *know* you are about to enter a stall.

 

The systems, at least on the Airbus series, calculate VS1G from angle of attack, speed/Mach, altitude, thrust and CG. Using equations for lift, you can then deduce the VS1G (stall) speed.


Andrew Wilson

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When the systems know, then they activate the stick shaker.

 

In order to comply with the requirements aircraft designers may install a system that will constantly monitor the critical parameters and will automatically activate to reduce the angle of attack when necessary to avoid a stall. The critical parameters include the angle of attack, airspeed, wing flap setting and load factor. Action by the pilot is not required to recognise the problem or react to it.

 

 

Yes, this is what I'm looking for. So there is an equation depending on these values.

 

Found the equation -- V = sqrt[ 2W / (rho*S*Cl,max)]

 

http://www.ehow.com/how_5029873_calculate-stall-speed.html (dunno if ehow is accurate!)

 

The systems, at least on the Airbus series, calculate VS1G from angle of attack, speed/Mach, altitude, thrust and CG. Using equations for lift, you can then deduce the VS1G (stall) speed.

 

How about in GA aircraft that don't have computers to calculate values? Do they just assume certain values such as weight? CG of course will change depending on TRIM.

 

I might be talking out of my &@($*, I don't know anything about flying and FSX is just a hobby. :)


Soarbywire - Avionics Engineering

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You really don't need weight to determine a stall point.

 

All wings have a finite lift capability and it's directly tied to the wing's angle of attack. A stall vane is usually found on larger aircraft and a stall horn (if you look at it, it's a tube that has air pass through it) on most small GA aircraft that have stall warning systems.

 

These systems are what indicate a stall is about to happen. It's a mechanical device that's being used, not a computer that's dumber than a box of rocks.


Ed Wilson

Mindstar Aviation
My Playland - I69

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Yes, this is what I'm looking for. So there is an equation depending on these values.

 

Found the equation -- V = sqrt[ 2W / (rho*S*Cl,max)]

 

http://www.ehow.com/how_5029873_calculate-stall-speed.html (dunno if ehow is accurate!)

 

How about in GA aircraft that don't have computers to calculate values? Do they just assume certain values such as weight? CG of course will change depending on TRIM.

 

I might be talking out of my &@($*, I don't know anything about flying and FSX is just a hobby. :)

 

A wing stalls when it exceeds its critical angle of attack, period. It has nothing to do with speed or weight. (At least in the realm of GA flight, at high altitude and high Mach numbers the compressiblity of air starts playing a role).

 

The thing with stall speed is the following:

To keep an aircraft flying level you need the lift to equal the weight.

Lift depends on speed and angle of attack (and other things, but that's not important for this argument).

If you reduce speed you will lose lift, which you can compensate by increasing the angle of attack.

The maximum lift a wing can produce is at the critical angle of attack, just before it stalls.

The slower you fly, the closer your angle of attack needs to be to the critical angle in order to keep your lift equal to the weight.

At a certain minimum speed your angle of attack will equal the critical angle.

If you reduce speed further, one of two things can happen: (1) you keep the AoA constant, your lift is no longer equal to your weight and you start to descend.

(2) you increase the AoA further and stall the wing (at which point your lift is still not equal to your weight and you start to descend).

Stall speed is the slowest speed at which you can fly and still have a lift equal to your weight, which is achieved by maintaining your wing at the critical angle of attack. Unlike the critical AoA, the stall speed does depend on the plane's weight, as shown in your equation.

 

GA planes have a system that measures the angle of attack and acts (alarm, stick pusher, whatever) if the AoA gets too close to the critical angle.

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GA planes have a system that measures the angle of attack and acts (alarm, stick pusher, whatever) if the AoA gets too close to the critical angle.

 

Hi, thanks for the explanation. Is the system the stall vane warpd mentioned above? Found this picture --

 

Stall+Vane.JPG

 

So air has to be entering into this vane system to determine if you're at the stall warning point?

Couldn't you be at a high speed but still be at the critical angle, and therefore you are not at a risk of stalling?

 

Or is this where speed does play a role in addition to the AoA?


Soarbywire - Avionics Engineering

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Hi any idea about engine stall and is is simulated ??


Alaa A. Riad
Just love to fly...............

W11 64-bit, MSFS2020, Intel Core i7-8700 CPU @ 3.20 Ghz 6 Cores, 2 TR HD, 16.0 GB DDR4 RAM, NVIDIA GeForce GTX 1060 6 MB GDDR5
 

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Hi, thanks for the explanation. Is the system the stall vane warpd mentioned above? Found this picture --

 

Stall+Vane.JPG

 

So air has to be entering into this vane system to determine if you're at the stall warning point?

Couldn't you be at a high speed but still be at the critical angle, and therefore you are not at a risk of stalling?

 

Or is this where speed does play a role in addition to the AoA?

 

I'm not quite sure how that system works, but I would assume it needs to be in contact with the airflow somewhere to be able to measure the AoA.

 

You can indeed be at high speed at the critical angle of attack, and if you are you will still stall. You stall when you exceed the critical angle of attack, regardless of your speed. During normal flight it's unlikely to happen however, because if you are near the critical angle at high speed your lift will be much larger than your weight, so you will have a huge upwards acceleration. The most likely situation this would happen is if you're flying at high speed and suddenly yank the stick back. You definitely can stall the plane like that, as anyone who's flown in IL-2 Sturmovik on high realism settings will have found out.

 

At high speed (more than 100 m/s or 200 knots) you start noticing the effects of air being compressible, and if I remember correctly this can affect the critical AoA at which the wing stalls, but compressible aerodynamics was a while ago, I'm a structures and materials guy now.

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I'm not quite sure how that system works, but I would assume it needs to be in contact with the airflow somewhere to be able to measure the AoA.

 

You can indeed be at high speed at the critical angle of attack, and if you are you will still stall. You stall when you exceed the critical angle of attack, regardless of your speed.

 

Hi John, but isn't your critical angle of attack a variable depending on your speed? Its not a constant value?

 

I'll try some high speed angle of attacks tonight. Thanks.


Soarbywire - Avionics Engineering

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Stall speed is not a constant value.

 

As example, when you bank an aircraft... the speed you stall at climbs because you're translating some of the lift from the wings in the lateral direction. Since flight is a balance of forces, something has to 'give'. In a turn aircraft tend to have a nose-up attitude which reduces the amount of angle of attack left for the wing before reaching the critical angle of attack. Combine that with the reduction in lift to offset the weight... and *poof!* (<-- magic happens here!) the stall speed increases in value.

 

FYI, the stall speed equation referenced above is only valid for level flight.


Ed Wilson

Mindstar Aviation
My Playland - I69

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Hi John, but isn't your critical angle of attack a variable depending on your speed? Its not a constant value?

 

I'll try some high speed angle of attacks tonight. Thanks.

 

I'm not sure about the compressible range (speeds above 100 m/s), but in the incompressible speed range the critical angle of attack is always a fixed value that does not depend on speed, only on the shape of your wing. If it wasn't constant the equation you gave above wouldn't work, as C_L_max would not be constant either.

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When yo rise the nose the speed slows down and speed up when you lower the nose.

 

Stalls depend only on angle of attack, not airspeed. However, the slower an airplane goes, the more angle of attack it needs to produce lift equal to the aircraft's weight. As the speed slows further, at some point this angle will be equal to the critical (stall) angle of attack. This speed is called the "stall speed". An aircraft flying at its stall speed cannot climb, and an aircraft flying below its stall speed cannot stop descending. Any attempt to do so by increasing angle of attack, without first increasing airspeed, will result in a stall.

The actual stall speed will vary depending on the airplane's weight, altitude, configuration, and vertical and lateral acceleration. Guidelines for the case of zero acceleration are provided by the following V speeds:

  • VS: The computed stalling speed with flaps retracted at design speed. Often has the same value as VS1.
  • VS0: The stall speed in landing configuration (full flaps, landing gear down, spoilers retracted).
  • VS1: The stall speed in a "clean" configuration (flaps, landing gear and spoilers all retracted as far as possible).
  • VSR: Reference stall speed.[clarification needed]
  • VSR0: Reference stall speed in the landing configuration.
  • VSR1: Reference stall speed in the clean configuration.
  • VSW: Speed at which onset of natural or artificial stall warning occurs.

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