AOA sensors
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AOA sensors
Ok, this might be a really daft question but its gone from being a simple "by the way what does this mean" query to a sad obsession!
On the 757 DFDAU, one of parameters is AOA1 and AOA2, I'm assuming these give readings of the AOA sensers on either side of the aircrafts nose.
In straight and level cruise, with a normal 2.5 (ish) degree upward nose attitude on the EADI the readings on the DFDAU are around -7.21 or -7.38 degrees.
Reading the maintenance manual, the sensors pivot at the front and a +ve reading indicates the vanes are pointing down (ie there is a downward AOA), but in the cruise the indications show they point up and therefore give an upward angle of attack.
However, as we slow down the reading increases (ie it goes from -7. something to -6. something), meaning the AOA is decreasing.
To maintain a level attitude with decreasing speed the AOA must increase - surely. Why do the sensors indicate a decreasing AOA with decrease in speed?
Can someone answer what may be a really obvious question?!
On the 757 DFDAU, one of parameters is AOA1 and AOA2, I'm assuming these give readings of the AOA sensers on either side of the aircrafts nose.
In straight and level cruise, with a normal 2.5 (ish) degree upward nose attitude on the EADI the readings on the DFDAU are around -7.21 or -7.38 degrees.
Reading the maintenance manual, the sensors pivot at the front and a +ve reading indicates the vanes are pointing down (ie there is a downward AOA), but in the cruise the indications show they point up and therefore give an upward angle of attack.
However, as we slow down the reading increases (ie it goes from -7. something to -6. something), meaning the AOA is decreasing.
To maintain a level attitude with decreasing speed the AOA must increase - surely. Why do the sensors indicate a decreasing AOA with decrease in speed?
Can someone answer what may be a really obvious question?!
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when it gioes from -7.something to -6.something the AOA is INCREASING - it's becoming more positive. Exactly as you expect. You can't just look at the magnitude.
Basically, there will be a conversion factor from vane AoA to 'body' AoA (AoA measured relative to the aircraft reference). By the sounds of it, at +2.5 body AoA the vanes read -7.5 Vane AoA. If I had to guess, I'd say that the vane reading probably changes by more then the body AoA for any change - let's assume a 2:1 ratio.
Then we have:
Body AoA=(Vane AoA+12.5)/2
So, for vane = -7.5, body = 2.5
and, for vane=-6.5, body= 3.0
and, so on.
(Incidentally, while the 2:1 is a guess, our aircraft have 3:2 ratios so its possible. What I don't understand is why they're bothering to display a confusing vane angle to the crew - it'd be easier to display the converted body value...but as you all know, I don't like displaying AoAs, and this is yet another example of why.)
Basically, there will be a conversion factor from vane AoA to 'body' AoA (AoA measured relative to the aircraft reference). By the sounds of it, at +2.5 body AoA the vanes read -7.5 Vane AoA. If I had to guess, I'd say that the vane reading probably changes by more then the body AoA for any change - let's assume a 2:1 ratio.
Then we have:
Body AoA=(Vane AoA+12.5)/2
So, for vane = -7.5, body = 2.5
and, for vane=-6.5, body= 3.0
and, so on.
(Incidentally, while the 2:1 is a guess, our aircraft have 3:2 ratios so its possible. What I don't understand is why they're bothering to display a confusing vane angle to the crew - it'd be easier to display the converted body value...but as you all know, I don't like displaying AoAs, and this is yet another example of why.)
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Thanks for that, I partly see where you are coming from with the factoring, but if the vane reads -7 degrees (indicating it is below the horizontal) and increases to -6 degrees its getting closer to the horizontal, giving less AOA at a slower speed. Shouldn't the reading decrease even further, beyond -7, to go further below the horizontal giving a greater AOA?!
As an aside I just re-read the info I was given and it pivots in the middle with the trailing edge being +ve when above the horizontal - this doesnt change anything just thought I'd correct myself!
As an aside I just re-read the info I was given and it pivots in the middle with the trailing edge being +ve when above the horizontal - this doesnt change anything just thought I'd correct myself!
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The factors don't matter.
-6 is MORE POSITIVE than -7, therefore the AoA of -6 is GREATER THAN the AoA of -7.
It's just that the system has a significant offset, and is reading negative numbers. But it's behaving in the correct sense.
-6 is MORE POSITIVE than -7, therefore the AoA of -6 is GREATER THAN the AoA of -7.
It's just that the system has a significant offset, and is reading negative numbers. But it's behaving in the correct sense.
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tailwheel, it seems you're thinking of the AOA as an angle from the horizon. Properly speaking, AOA is between relative airflow and chord line. However, if your relative airflow is horizontal, it easy to catch yourself thinking of it with reference to the horizon.
The reason your nose gets closer to the horizon and yet your AOA is increasing is because you have to measure the angle of the chord line above the relative airflow (or above horizon if rel. airflow is horizontal). So if AOA goes from -7 to -6 the nose is getting closer to the horizon, but the angle of the chord line above the relative airflow is increasing.
The reason your nose gets closer to the horizon and yet your AOA is increasing is because you have to measure the angle of the chord line above the relative airflow (or above horizon if rel. airflow is horizontal). So if AOA goes from -7 to -6 the nose is getting closer to the horizon, but the angle of the chord line above the relative airflow is increasing.
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MP - tailwheel saidf level flight, so that explanation isn't going to help much I suspect - the flightpath (and hence the airflow) is horizontal - in free stream of course.
It's the negative numbers thing that's got him confused.
It's the negative numbers thing that's got him confused.
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Thanks for the responses. I know AOA is relative to airflow NOT the horizon, it is the -ve aspect I cannot get my head around. Thanks M(F)S for the answers so far but its just not gelling.
The maintenance manual says if the reading is +ve then the vane is pointing downwards, ie a downward AOA.
If you assume the AOA is 0 degrees for level flight. If the speed decreases the AOA should increase so shouldnt the trailing edge of the vane go below the horizontal and become negative, to indicate its pointing upwards and visa versa?
The maintenance manual says if the reading is +ve then the vane is pointing downwards, ie a downward AOA.
If you assume the AOA is 0 degrees for level flight. If the speed decreases the AOA should increase so shouldnt the trailing edge of the vane go below the horizontal and become negative, to indicate its pointing upwards and visa versa?
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Originally Posted by tailwheel76
The maintenance manual says if the reading is +ve then the vane is pointing downwards, ie a downward AOA.
If you assume the AOA is 0 degrees for level flight. If the speed decreases the AOA should increase so shouldnt the trailing edge of the vane go below the horizontal and become negative, to indicate its pointing upwards and visa versa?
Indeed, if the speed decreases, the 'real' AoA will increase - to 3, or 4, or whatever, depending how much you slow down.
But, and this is important, it actually increases from +2.5 to +3.0.
If you instead increased speed, the AoA would decrease - to +2.0, then to +1.0, then to +0.5, then to 0.0. Now just keep on accelerating. AoA HAS TO KEEP GOING THE SAME WAY - and the trend takes it to -0.5, then to -1.0, and so on. And for those last two points, as we got faster our AoA became a LARGER NEGATIVE number.
Now, lets stop at the speed that gave us -1.0 AoA. And start to slow down again. Well, we'll soon be back at -0.5. And there you see it - the speed decreased, the AoA became a smaller 'size' but because it changed between two negative numbers, becoming a smaller 'size' (or magnitude, to use the correct term) is the same as becoming a LARGER number.
What I suggest is draw yourself a graph with speed along the bottom, and AoA vertically. Make 200 knots 5 AoA and make 250knots 2 AoA. Draw a straight line, and extend it all the way to, say, 400 knots, where the AoA is negative. Look at what happens to the 'size' of the AoA above 300 knots as you speed up or slow down.
basically, it'll look like this, but with numbers on it. Or maybe this is good enough - look at the part on the right, where AoA is negative...
The key is, you can't just look at magnitude - you need to account for the sign.
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If you like, think of it in terms of money.
When you have $500 in the bank, if it goes to $250 suddenly, you have less money; your wealth - which is what the balance is a measure of - has DECREASED.
The wife (or SO) keeps spending cash, and soon you have $0 in the bank; your wealth decreased again.
Now she keeps on buying shoes, and soon you have even less wealth - -$250 is the balance.
And if she keeps buying trinkets, your balance will become -$500 - the raw number got bigger, but because of the "-" you didn't get more wealth - you got less.
thats all thats going on - AoA happens to be a negative number (due to the way the vanes are calibrated) and the 'normal' way of looking at increase/decrease by looking just at magnitude doesn't work.
When you have $500 in the bank, if it goes to $250 suddenly, you have less money; your wealth - which is what the balance is a measure of - has DECREASED.
The wife (or SO) keeps spending cash, and soon you have $0 in the bank; your wealth decreased again.
Now she keeps on buying shoes, and soon you have even less wealth - -$250 is the balance.
And if she keeps buying trinkets, your balance will become -$500 - the raw number got bigger, but because of the "-" you didn't get more wealth - you got less.
thats all thats going on - AoA happens to be a negative number (due to the way the vanes are calibrated) and the 'normal' way of looking at increase/decrease by looking just at magnitude doesn't work.
