Originally Posted by tailwheel76
The maintenance manual says if the reading is +ve then the vane is pointing downwards, ie a downward AOA.
Sounds right. Its the way our vanes work. But AOA sensed by the vanes ISNT the 'real' AOA. It's related to it, by the kind of equation I suggested (or something even funkier, but linear with an offset would be pretty common I think)
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?
You know that your AoA isn't zero for straight, level flight. You said your pitch attitude was 2.5deg nose up, which means in level flight your 'real' AoA is also +2.5 degrees.
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.