negative AOA in cruise
 

http://i.imgur.com/u2vcsmw.jpg

737NG DFDAU in cruise will show negative AOA in cruise, I guess this is normal for transsonic speeds and supercritical wing?

I think that it's probably nothing more or less than a reflection on the zero reference used for the aircraft's AoA instrument. I don't think that it's telling you anything meaningful about the aerodynamics of the aircraft on its own.

Quotes from Genghis:
"I think that it's probably nothing more or less than a reflection on the zero reference used for the aircraft's AoA instrument."

Agreed! Unless the a/c is in a bunt manoeuvre...

"I don't think that it's telling you anything meaningful about the aerodynamics of the aircraft on its own."

Yes, and wouldn't it be nice if it was telling the crew something meaningful? Isn't it possible - even with a supercritical wing - to establish a reference that approximates a mean AoA for the whole wing, that would approximate at any speed or AoA?

I don't remember having a similar problem with the AIDS AoA readout on the A320, but perhaps someone current on Airbuses may comment.

To be fair, it can tell you something meaningful, as it's dead easy to determine correct / optimal values for IAoA for each of the major phases of flight, and from that use the AoAI as an alternative / reserve intrument in the event of uncertainty over correctness of readings on either ASI or the AI - or for that matter simply because the AI can be a bit over coarse.

But, unless you have calibration curves compared to the in-flight condition, which may not exist at-all, you can't tell anything about the basic aerodynamic behaviour of the aeroplane.

At the risk of stating the obvious, most non-symmetric aerofoils still produce lift at a small negative AoA.

True, Dave, but maybe you and Genghis will agree that in this particular case - assuming 1G cruise flight at F370 and IAS 255 (about M 0.75 - 0.80) - the AoA is likely to be something between +2 and +3. In fact it may well approximate the Pitch attitude shown on the PFD/ADI and the flight-control display in question (+2.7).

Genghis, you will recall that we've had much discussion on the possible efficacy of providing airline pilots with an AoA gauge: initially on the serial AF447 threads, but more recently on a specific thread, the title of which escapes me at the moment...

That depends entirely on what you choose for your datum.

A common choice is the so-called "geometric AoA" which uses a line drawn through the TE and the centre of the nose radius (with flaps and slats retracted). This will produce arbitrary numbers because the lift coefficient at "zero" will vary between airfoils and also vary with flap/slat deployment.

Another choice is the so-called "aerodynamic AoA" which is simply a datum where zero degrees AoA is the zero-lift coefficient angle. This could tell you useful things, but it would feel strange because for many airfoils zero aerodynamic AoA will occur at as much as minus 7 or 8 degrees geometric AoA. It would also lose its definition as soon as flaps/slats etc were deployed (unless you recalibrated the datum for each stage of flap, which would be weird).

I suspect the datum is chosen as much as anything to be something which allows for easy rigging and testing of the instrument.

Impossible to say Chris, for the simple reason that the zero AoA datum is arbitrary, particularly where you're talking about an aircraft indicator system, not wind tunnel data.

I do recall the AF447 aftermath and discussions. My opinion's not changed since then - an AoAI is potentially a valuable, even lifesaving instrument: but only if it has been developed, and pilots trained, with that use in mind.

I believe I am speaking for most jet drivers that very little training is given to fly on the AoA gauge. We did some on the last recurrent sim but it was limited to a few minutes only, VMC condition and I even had a defective indication on my PFD which the instructor didn't seem to bothered about. Wasn't the greatest learning experience.

Pitch + Power is king when you're faced with an unreliable airspeed indicator, imho. It's also something that we subtly get trained at on every flight as we keep our scan going observing a range of reasonable pitch attitudes and power settings for various stages.

"We keep our scan going" Since the flight director is (rightly or wrongly) the dominant instrument in current jet transports, most pilots invariably find to their concern they lose the skill needed to scan effectively. Their gaze is 100% on FD indications from lift-off to short final. Plus they are on full automation for hours at a time, then how does subtle training occur?:sad:

"Have the opportunity to practice on every flight" is a better description. I you pick up on that offer is up to the PF ;)

It's thread drift but I couldn't help myself from commenting on the need for an AoA gauge. From my own view point, I think it's of fairly limited value. Nice little thingy to look at and study, but when it's really needed the pitch + power is a more effective way to fly the plane.

I've never flown with an AoA. I've always been a pitch/power guy; probably because that was all I had, so learnt to make the best of it. I wonder at the clamour for these displays. I suspect the HUD guys might be in favour.
I do notice many cadets and line F/O's flying with the FPV On when the FD is also displayed. Now that seems daft to me. These same people don't understand the 'look thought the FD' concept and there they are with 2 displays. I ask them which they are looking at, and they claim both; but why.
There is no specific training given how to use the FPV, what its limitations are, what traps it has, when best to use it, when not. However, there are some who advocate its use on raw data ILS and on visual approaches in manual flight. IMHO it is so small and dynamic that it requires too much attention to follow it accurately, to the detriment of the whole scan; plus, on a visual approach I'd prefer to have more attention outside with a confirming glance inside at attitude, speed & VSI. On a low level base leg, after a circle, and with PAPI's, is the best place to focus your scan really on the FPV and not on the crash point? That teaching, by some SFI's, is personal opinion, but it is not taught as such.
FPV with no FD in a high bank level turn can be useful though.

My favourite is the wooden stick attached to the leading edge, with a bit of yarn tied to the front end. Worked for the Wright Brothers. http://cdn.pprune.org/images/smilies/embarass.gif

Genghis is right, it depends on the choice of reference for the aoa gauge.
PDR1 is right, the choice is biased towards something easy to set up and check. Fuselage centreline datum is often chosen
But to get to wing aoa you have to add the wing/body setting which is usually chosen to get a more or less level cabin floor in cruise.
On top of that, the actual average wing aoa will depend on the design twist as modified by aeroelastic twist under load.

Add to which that local flow at the location of the AoA vane is unlikely to be parallel to the free stream flow, due to flow modification by the fuselage.

Not to mention flow upwash ahead of the lifting wing

Oh, and fuselage bending!

The plot thickens...
 

This discussion is starting to beg the question of whether the concept of AoA is so meaningless as to have no relevance to flight whatsoever! :eek:

Quote (my emphasis):
"Genghis is right, it depends on the choice of reference for the aoa gauge.
PDR1 is right, the choice is biased towards something easy to set up and check. Fuselage centreline datum is often chosen"

Thanks, Owain. Presumably the same practicality may apply to the pitch attitude, as shown on the display above and the PFD/ADI? That assumption would at least be consistent with the fact that when most jets I've flown were parked or taxiing on a horizontal pavement the pitch attitude displayed on the PFD/ADI was roughly zero.

Unfortunately, however, that raises a dilemma that has been bugging my subconscious for years. Quote:
"the wing/body setting which is usually chosen to get a more or less level cabin floor in cruise."

So why is the indicated pitch typically about +2.5 in the cruise? That suggests a 4% gradient on the fuselage axis, which itself seems unlikely to be much different from the cabin floor(s).

Secondly, in the example pictured above by the OP, pitch is +2.7 (see also the PFD) and AoA -0.8. If they are both referenced to the fuselage axis, the incoming airflow at the relevant AoA sensor must be angled at -3.5 relative to the horizon. (In the conventional model, where the aircraft is stationary, as in a wind tunnel, that would represent a downwash of 3.5 degrees relative to the horizontal.) Is that feasible?

Both of the above points are predicated on the datum for pitch attitude being the fuselage axis. If it isn't, then what can it be?

@Chris Scott

I TOLD you I shouldn't get involved in this discussion!
Don't take my words literally - I will change them to
"Chosen to obtain an acceptable floor cabin angle in cruise"

Now to take an airplane with which I know you are familiar but using approximate numbers, the no lift angle in cruise is just under 2 deg, the lift at zero aoa is about 0.23, and the lift curve slope about 0.12 per degree. The wing body setting is a little over 3.5 deg.
The optimum cruise CL is about 0.53, giving a cruise aoa of 2.5 deg which is familiar to you. Without the wing body setting it would be 6 deg.

To take the other example (different airplane) if the pitch is 2.7 and the indicated aoa -0.8 then the incident flow would be 1.9 deg nosedown at whatever point the vane is located. This seems to me to be quite possible

Hi Owain,

I take your point about the deck angle. +3.5 degrees is a much higher "rigger's angle of incidence" on the A320 than I realised.

On the apparent, katabatic-like performance of the incident flow in the OP's example: if 1.9 degrees seems reasonable, what about 3.5 degrees? Combining the pitch of +2.7 and the AoA of -0.8, I make it the latter.

@ Chris Scott

Just noticed a typo - the zero lift aoa should read "is a bit more positive than minus 2deg"

Regarding your other point, I got the signs wrong. Yes it looks like 3.5 deg but whether or not that is sensible is impossible to say without detailed knowledge of the cesign