AoA and ASI low speed awareness
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How does it realize that you don't weigh 100T, and that it needs to show speeds for 110T?
Much of the basics are captured in other posts but the following may make things a tad simpler. As a sideline point, some of the ideas in a few posts are a bit wide of the mark but that's aviation - ask ten pilots a question and get twenty different answers ...
The basics are comparatively straightforward even if there is a lot of wind tunnel work and computer code cutting to make it work in the cockpit.
The underlying computer models built into the magic black boxes will know the relevant parameters which determine lift from onboard measuring systems - incidence (AoA), density, speed, and a reference area to balance up the units. In addition (and most folks either forget or are not aware of these) Reynolds Number and Mach Number affect the shape of the lift curve slope (incidence against lift coefficient) as well as incidence.
If we take the simple case of cruise flight, for instance, we can put
lift = weight = some constant x lift coefficent x speed squared
so, if we know the weight and the speed, then we can figure out what lift coefficent we should have for a given set of conditions. The result is that we know at what incidence the aeroplane should be flying for that lift coefficient.
If the calculated incidence is higher or lower than the real world incidence on the day, then the presumed weight is wrong. It follows that we can reverse the sums to figure out what the weight is.
Stall is proportional to the square root of weight so the magic box can figure the correct stall speed and adjust the presentation made to the pilot accordingly.
Why can't PPRuNe have a QUOTE button like every other forum?
If you look along the top line (starting with the bold symbol) you will find the quote symbol.
Simpletons like me just highlight quoted material by italicising or similar.
Given that the critical AoA remains constant
A simplistic (if useful for basic pilot training) view. But be aware that Re and M come into the lift curve shape as well as incidence (AoA). To save my having to try and write a complicated equation, you might like to have a look at any of the web sources which cover this sort of stuff, such as here.
The flight is not affected if we do not enter the weights.
.. because the boxes can figure out what the weight is to a reasonable accuracy.
You will stall the wing above AoA max regardless of your airspeed when you get there.
.. but be careful of some sideline issues. Published stall data usually is based on certification testing at low pitch rates. Things are a bit different at high rates - accelerated or g stalls. Indeed, at very high pitch rates, the stall angle can be pushed much higher due to the formation of spanwise vortex flows above the wing.
Much of the basics are captured in other posts but the following may make things a tad simpler. As a sideline point, some of the ideas in a few posts are a bit wide of the mark but that's aviation - ask ten pilots a question and get twenty different answers ...
The basics are comparatively straightforward even if there is a lot of wind tunnel work and computer code cutting to make it work in the cockpit.
The underlying computer models built into the magic black boxes will know the relevant parameters which determine lift from onboard measuring systems - incidence (AoA), density, speed, and a reference area to balance up the units. In addition (and most folks either forget or are not aware of these) Reynolds Number and Mach Number affect the shape of the lift curve slope (incidence against lift coefficient) as well as incidence.
If we take the simple case of cruise flight, for instance, we can put
lift = weight = some constant x lift coefficent x speed squared
so, if we know the weight and the speed, then we can figure out what lift coefficent we should have for a given set of conditions. The result is that we know at what incidence the aeroplane should be flying for that lift coefficient.
If the calculated incidence is higher or lower than the real world incidence on the day, then the presumed weight is wrong. It follows that we can reverse the sums to figure out what the weight is.
Stall is proportional to the square root of weight so the magic box can figure the correct stall speed and adjust the presentation made to the pilot accordingly.
Why can't PPRuNe have a QUOTE button like every other forum?
If you look along the top line (starting with the bold symbol) you will find the quote symbol.
Simpletons like me just highlight quoted material by italicising or similar.
Given that the critical AoA remains constant
A simplistic (if useful for basic pilot training) view. But be aware that Re and M come into the lift curve shape as well as incidence (AoA). To save my having to try and write a complicated equation, you might like to have a look at any of the web sources which cover this sort of stuff, such as here.
The flight is not affected if we do not enter the weights.
.. because the boxes can figure out what the weight is to a reasonable accuracy.
You will stall the wing above AoA max regardless of your airspeed when you get there.
.. but be careful of some sideline issues. Published stall data usually is based on certification testing at low pitch rates. Things are a bit different at high rates - accelerated or g stalls. Indeed, at very high pitch rates, the stall angle can be pushed much higher due to the formation of spanwise vortex flows above the wing.
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I think this Boeing document on AoA might answer a bunch of questions in this thread.
http://www.boeing.com/commercial/aer...ero_12/aoa.pdf
http://www.boeing.com/commercial/aer...ero_12/aoa.pdf
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Now I THINK I see what you are asking. You understand that the AoA gauge does not need to know the aircraft weight to tell you your margin from the stall, but you are asking how the computer transposes that information into a forecast speed which it paints onto the speed tape?
Is that your question?
Is that your question?
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The underlying computer models built into the magic black boxes will know the relevant parameters which determine lift from onboard measuring systems - incidence (AoA), density, speed, and a reference area to balance up the units. In addition (and most folks either forget or are not aware of these) Reynolds Number and Mach Number affect the shape of the lift curve slope (incidence against lift coefficient) as well as incidence.
If we take the simple case of cruise flight, for instance, we can put
lift = weight = some constant x lift coefficent x speed squared
so, if we know the weight and the speed, then we can figure out what lift coefficent we should have for a given set of conditions. The result is that we know at what incidence the aeroplance should be flying for that lift coefficient.
If the calculated incidence is higher or lower than the real world incidence on the day, then the presumed weight is wrong. It follows that we can reverse the sums to figure out what the weight is.
So it seems that the the weight is calculated to display speed tape information. I think I'll go re-examine the report on the Emirates A340 incident in Melbourne. I wonder what the speed tape looked like in that instance.
If we take the simple case of cruise flight, for instance, we can put
lift = weight = some constant x lift coefficent x speed squared
so, if we know the weight and the speed, then we can figure out what lift coefficent we should have for a given set of conditions. The result is that we know at what incidence the aeroplance should be flying for that lift coefficient.
If the calculated incidence is higher or lower than the real world incidence on the day, then the presumed weight is wrong. It follows that we can reverse the sums to figure out what the weight is.
So it seems that the the weight is calculated to display speed tape information. I think I'll go re-examine the report on the Emirates A340 incident in Melbourne. I wonder what the speed tape looked like in that instance.
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I think this Boeing document on AoA might answer a bunch of questions in this thread.
http://www.boeing.com/commercial/aer...ero_12/aoa.pdf
A very useful document. Thanks. I'll have to have a read.
http://www.boeing.com/commercial/aer...ero_12/aoa.pdf
A very useful document. Thanks. I'll have to have a read.
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That's exactly my question.
Lift = 0.5 rho V^2 S Cl
You can disregard the 0.5 for this. rho (density) can be calculated using static pressure and total air temp probes, you'd need a humidity measurement to get precise figures of density. V^2 is your TAS squared. S is the surface area of the lifting surface which can be stored in the computer. Cl would be related to your angle of attack in a certain configuration. That graph would change quite a bit and there are calculations/transformations for all changes. If you read the article by Boeing I just posted here it'll explain quite a bit. The Cl vs AoA graph would change depending on a number of variables that is explained more in the Boeing article.
The total lift at 1G will = the total weight. By calculating what the total lift is (including the lift on the horizontal stabilizer), they can know what the total weight of the aircraft is. They could even calculate the Center of Gravity! Knowing all this for 1G, they could apply transformations that would calculate the weight of the plane when it's not at 1G.
All of this stuff is easy to calculate. But does your airplane have the capability? - I don't know. I'm not intimately familiar with Boeing or Airbus systems on each aircraft so I don't know what their capability is.
Last edited by italia458; 6th May 2012 at 19:40. Reason: Correction.
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Perhaps it is even simpler. The system interprets the difference between the current AoA and the stall AoA as a margin between the current speed and the stall speed. So in the example shown below, if the current speed is 120 kts at an AoA of 9 degrees, it would place the stall speed bug at 100 kts.
P.S. The curve shown as an example to illustrate the principle is defined in the text box on the graph, with V/Vs=SQRT(CLmax/CL). It is typical of a large jet transport in clean configuration at low Mach numbers, and will be different for each airplane type. For any given airplane type, it is subject to the variations described in the Boeing article linked in post #22.
P.S. The curve shown as an example to illustrate the principle is defined in the text box on the graph, with V/Vs=SQRT(CLmax/CL). It is typical of a large jet transport in clean configuration at low Mach numbers, and will be different for each airplane type. For any given airplane type, it is subject to the variations described in the Boeing article linked in post #22.
Last edited by HazelNuts39; 7th May 2012 at 09:36. Reason: P.S. (in response to comments)
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Cl would be essentially a graph of your AoA vs TAS
Probably not.
Generally presented as a curve relating incidence to lift coefficient. If a bit more generalised, it will be a parametric presentation involving M and Re.
Probably not.
Generally presented as a curve relating incidence to lift coefficient. If a bit more generalised, it will be a parametric presentation involving M and Re.
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Perhaps it is even simpler. The system interprets the difference between the current AoA and the stall AoA as a margin between the current speed and the stall speed. So in the example shown below, if the current speed is 120 kts at an AoA of 9 degrees, it would place the stall speed bug at 100 kts.
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It seems elegantly simple.
Just depends on which way you apply parallax to your contemplation.
If the machine knows the incidence (plus the other stuff), it knows the (approximate) weight which gives the Vs and V/Vs. So there is no problem representing things by a simple relationship between incidence and V/Vs. If that explanation floats your boat comfortably then run with it.
One needs to be a little careful extolling the virtues of incidence measurements - useful at high alpha but still subject to errors like everything else we deal with and a fairly coarse measurement - which is one of the reasons why the certification fraternity tends to stick with the weights and speeds side of things.
Just how the OEMs implement this stuff in the black boxes I can't say, but the underlying basics philosophically are fairly straightforward.
Just depends on which way you apply parallax to your contemplation.
If the machine knows the incidence (plus the other stuff), it knows the (approximate) weight which gives the Vs and V/Vs. So there is no problem representing things by a simple relationship between incidence and V/Vs. If that explanation floats your boat comfortably then run with it.
One needs to be a little careful extolling the virtues of incidence measurements - useful at high alpha but still subject to errors like everything else we deal with and a fairly coarse measurement - which is one of the reasons why the certification fraternity tends to stick with the weights and speeds side of things.
Just how the OEMs implement this stuff in the black boxes I can't say, but the underlying basics philosophically are fairly straightforward.
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That suggestion seems the easiest for me to comprehend. Can anyone verify if it's correct? It seems elegantly simple.
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Thanks guys. I don't expect it to be a nice smooth curve like the one shown, but I just wanted to know that the computer is relating AoA to V/Vstall. That's easy enough for my little brain to comprehend.
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I think pretty much everything has been covered on this thread, but I thought another anecdote might provide a different view of the same concept.
I just re-qualified on the MD-80, thus gaining an opportunity to refresh my original instrument scan from years gone by, and relieving me of the need to remember what all those colored little bugs and notches and hash marks are supposed to mean.
However, the check airman demonstrated a simple technique for checking your actual weight before beginning the approach. The 80, of course, has an alpha protection mode incorporated in the autothrottle speed mode, so that it will theoretically prevent you from decelerating into the protected stall margin by adding whatever power it might be able to scrounge up. We manually bug up the min maneuvering speeds prior to approach based on what we think we weigh, which is of course paperwork plus known fuel. If, upon dialing the speed back for the first speed reduction, you get an ALPHA annunciation on the FMA at a speed somewhat higher than the min maneuvering bug, then you know you weigh more than you think you do. The computer is telling you that you are already at a higher angle of attack than you think you are, and thus the protected stall margin is displaced to a higher speed range.
What you hope to see is the ALPHA annunciation illuminate just as the speed bug is dialed down past the manually set min maneuver bug; that says your weight is correct.
John:
Just depends on which way you apply parallax to your contemplation.
What a marvelous metaphor...must keep that one squirreled away for future use, with appropriate acknowledgement, of course.
I just re-qualified on the MD-80, thus gaining an opportunity to refresh my original instrument scan from years gone by, and relieving me of the need to remember what all those colored little bugs and notches and hash marks are supposed to mean.
However, the check airman demonstrated a simple technique for checking your actual weight before beginning the approach. The 80, of course, has an alpha protection mode incorporated in the autothrottle speed mode, so that it will theoretically prevent you from decelerating into the protected stall margin by adding whatever power it might be able to scrounge up. We manually bug up the min maneuvering speeds prior to approach based on what we think we weigh, which is of course paperwork plus known fuel. If, upon dialing the speed back for the first speed reduction, you get an ALPHA annunciation on the FMA at a speed somewhat higher than the min maneuvering bug, then you know you weigh more than you think you do. The computer is telling you that you are already at a higher angle of attack than you think you are, and thus the protected stall margin is displaced to a higher speed range.
What you hope to see is the ALPHA annunciation illuminate just as the speed bug is dialed down past the manually set min maneuver bug; that says your weight is correct.
John:
Just depends on which way you apply parallax to your contemplation.
What a marvelous metaphor...must keep that one squirreled away for future use, with appropriate acknowledgement, of course.
