Originally Posted by
safetypee
MFS IIRC there is a clever method of using AOA to display (overlay) AoA related information such as stall margin or approach speed on EFIS airspeed scales.
I believe that the technique required knowledge of CL polars etc, and on some aircraft a trim input as an approximation of cg. As I understand this overcomes the problems of weight; are we discussing similar concepts?
I am confident that this was used on the Avro RJ – it used a Honeywell algorithm, which may have originated from the MD11, as both these aircraft had Honeywell EFIS speed computation. The Avro RJ did not provide any FMS derived aircraft weight input to the system.
It's the use of normalised AoA; we use it too. It depends on having a very good understanding of your lift-curve slope, as you note. It (invariably) assumes this to be linear (which it usually ISNT) which means its an approximation. Because the prime use of normalised AoA on the airspeed scale is to get the "low speed cue" aligned with shaker activation, it's "tuned" for that case; it can be CONSIDERABLY in error under other conditions.
(What you do is define an AoA for "zero lift" and an AoA for "CLmax" for each configuration, then you assume the relationship between CL and AoA is linear, and that the AoA for CLmax corresponds to stall speed.
So, "normalised" AoA is then defined as:
AoA(n) = [ AoA - AoA (zero lift) ] divided by [ AoA (CLmax) - AoA (zero lift) ]
With the assumption of linear CL variation, you can show that the square root of normalised AoA is proportional to the inverse of your stall speed ratio, that is, for
AoA (n) = 1, V/Vs = 1/sqrt(1) = 1.0 (naturally)
AoA (n) = 0.5, V/Vs = 1/sqrt(0.5)= 1.41 Vs
and so on.
If you know your shaker stall speed ratio (and hence AoA(n) for shaker) you can use this to display shaker info.
To put it onto an airspeed tape involves a bit more messing around, to account for current conditions, but not much.)
Normalised AoA would work exactly the same as flying to a dimensional AoA in terms of weight - you end up at the "right V/Vs" but the "wrong Vref" compared to what you work out from the 'book'. As I suggested, that's good for stall speed margin, bad for landing distance margin.