negative AOA in cruise

[vlkyplky]

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?

[Genghis the Engineer]

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.

[Chris Scott]

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.

[Genghis the Engineer]

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.

[DaveReidUK]

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

[Chris Scott]

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...

[PDR1]

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

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.

[Genghis the Engineer]

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.

[172_driver]

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.

[Centaurus]

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:

[172_driver]

"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.

[RAT 5]

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.

[DaveReidUK]

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).

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

[Owain Glyndwr]

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.

[Genghis the Engineer]

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.

[Owain Glyndwr]

Not to mention flow upwash ahead of the lifting wing

Oh, and fuselage bending!

[Chris Scott]

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?

[Owain Glyndwr]

@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

[Chris Scott]

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.

[Owain Glyndwr]

@ 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

[Mad (Flt) Scientist]

@PDR1
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).

Thse simplest option, and the one on every aircraft I've been involved in, is to define the "aircraft" angle of attack as being purely in reference to the same fuselage plane to with pitch is referenced. So an aircraft with zero pitch and zero AoA has zero gamma. Making tht simple equation work.

That avoids all the questions about what bits of the wing to use as refences moot - you don't use any of them. (Not only does any wing reference move around as surfaces are deployed, but with washout, which wing station would you use anyway). Since the wing based reference ends up being arbitrary anyway, may as well keep things simple at the aircraft level and use fuselage AOA.

That still leaves to issue of local AOA at any point you might sense it not being truly the same as the nominal AOA. In flight test you 'solve' that with a nice long boom. Otherwise, you calibrate your fuselage mounted sensors/vanes and hopefully get a decent relationship. In any case, as long as the use the AOA is put to is understood in the right context, it doesn't much matter if the relationships are a bit off. (If I define my shaker firing, say, as a function of vane angle and flap, any calibration changes with flap can be masked)

[Chris Scott]

Quote from OK465:
"I'm curious why the ND is in the PLAN MODE and the standby altitude indicates 36880 level in cruise, presumably at 370 (maybe that's within tolerance for the STBY, don't recall). Turbulence ? Hand flown ?"

Afraid I know so little about the B737NG that I had foolishly mistaken that S/B flight display for the PFD... :O But, apart from the ND being in PLAN mode, it looks like a routine cruise regime, including the engine parameters.

Apart from the AoA, the parameter that I don't understand on the DFDAU display is the STAB angle of 5.8, which taken on face value would probably be 5.8 degrees nose-up trim (i.e., minus 5.8 degrees aerodynamically). That's more than double what I would have expected at cruise speed (clean).

[underfire]

Looks like an indication of the vertical winds.

Pitch angle is good, and the AoA reacting to vertical influence?

Curious to know what the winds were...

[Concours77]

What kind of load plan would require so much trim? It is not terribly efficient, and I might venture to say "abnormal"?

[Smilin_Ed]

The US Navy uses AoA only for setting the optimum speed on landing approach. Why?
Because AoA is highly sensitive at approach speeds but varies hardly at all with speed at cruise. Thus you can get a very accurate speed indication where it is most critical, on landing. Further, since carrier landings are made by driving the plane directly to the deck with NO Flare you hold the correct AoA and drive it on. That also ensures that the end of the tail hook stays at the correct height in relation to the fuselage and main landing gear.

US Navy AoA gauges indicate "units" and not degrees. I guess this keeps down any speculation as to the actual number of degrees of angle of the wind in relation to the wing, which matters not to the pilot,

[Concours77]

Hello,
"That also ensures that the end of the tail hook stays at the correct height in relation to the fuselage and main landing gear."

Did you fly the F8U Crusader? Variable incidence wing? Elaborate?

Could you also say then that AoA is critical at speeds approaching Stall?

[Piper19]

I recall from back in the days when I was studying aircraft design that it is possible for an AOA in cruise to be negative. We studied some NACA profiles on which we got -1.5° for cruise lift, and much higher negative values for zero lift point. Full explanation has long left my brains, but reduction of induced drag was involved.
Whether that value is normal for the Airbus I cannot say.

[john_tullamarine]

Pitch + Power is king when you're faced with an unreliable airspeed indicator, imho

Likewise, I incline to raw data and pitch/thrust.

However, it only goes so far .. one must be doing it throughout the flight so that one knows what's what in the bigger scheme of things.

Falls in a heap if the aeroplane is stalled and the pilot fails to recognise this .. cf AF447. Sure, one can recover on instruments without too much angst if one is practised and maintains I/F proficiency but, if the skill is not there, all bets are off.

I have no experience in the cockpit with a specific AA indication but hold the view that it gives one the extra data useful either for precise flying on approach or in extremis.

[LOMCEVAK]

With respect to flying indicated AOA or airspeed on the approach there are a few other factors to consider. First, indicated AOA may be affected by sideslip dependent upon where the sensor is mounted and, therefore, may have an error when flying OEI. Secondly, if you have a wing that has a steep Cl vs alpha curve then a small change in stabilised AOA will equate to a relatively large change in airspeed. Therefore, flying approaches using purely AOA works well for swept wing aircraft but for straight winged types can result in imprecise airspeeds. Also, any rapid pitch control inputs will result in AOA fluctuations whilst airspeed will not change short term. Overall, the optimum technique is to scan both parameters but to bias that scan to one or the other depending on the lift characteristics of the wing and the functionality of the AOA sensing system, with an SOP to calculate the airspeed required for the approach AOA as a function of AUW.

Some aircraft such as the Buccaneer and Phantom FG1 had an audio AOA system so you flew the approach on that rather than using the gauge.

[PDR1]

Speed is not relevant for stalling - wings do not stall at a speed; they stall at an angle. So if AoA is uses it is to give the driver access to the prime information, not as a substitute for airspeed.

If an aeroplane can be landing at a wide variety of weights the stalling *speed* will be variable, but the stalling AoA will always be the same. This is more of an issue for combat aircraft (which may routinely return with unexpended ordnance) than for most civil airliners (which tend to land at more predictable weights in normal circumstances).

[Concours77]

" wings do not stall at a speed; they stall at an angle. So if AoA is uses it is to give the driver access to the prime information, not as a substitute for airspeed."

An aircraft stalls at Vs. that is a velocity, which infers angle of attack. It also infers, for practical purposes, separation of airflow? Flying can be done easily without any reported data at all, but instruments make flight ever so much safer? AF447 had no artificial horizon, as I recall, and certainly no AoA. Stalling is an event, not an indication. I thought the moderator has said it all:

"Falls in a heap if the aero plane is stalled and the pilot fails to recognize this...."

Recognition is the key to flying, and must precede any problem, "SA"....?

Instruments are important, but if they become critical to survival, something has gone very wrong....and the day UAS becomes unrecoverable, it isn't a condemnation of equipment.

[PDR1]

" wings do not stall at a speed; they stall at an angle. So if AoA is used it is to give the driver access to the prime information, not as a substitute for airspeed."

An aircraft stalls at Vs. that is a velocity, which infers angle of attack.

The point is that the airspeed "Vs" varies with weight, altitude, normal acceleration and lots of other parameters. The stalling AoA remains essentially constant, changing only slightly with air density. So for any given wing configuration (flap/slat setting) you can rely on remaining clear of the stall using the AoA gauge directly, whereas to use the ASI you will need to refer to calculated or tabulated data to produce estimates based on the known and assumed state of the aeroplane.

[LOMCEVAK]

PDR1,

Firstly, stalling AOA varies quite markedly with both Reynolds number and Mach number, and for a propeller driven aircraft will vary with any propwash over the wings.

Much of the earlier discussion relates to flying an approach, and speed is very important for landing distance considerations. As I said before, using just AOA may not be accurate enough in some aircraft for achieving the required landing speed accuracy criteria, especially with the potentially dynamic nature of AOA indications in turbulence and attempts to fly the required flightpath under such conditions. I have had an AOA probe stick on an approach in an aircraft whereby a single source AOA indication was the main method of speed control. It was only by cross-checking the standby ASI that I identified this failure.

I do wonder from your posts whether you have ever actually flown an aircraft where AOA indications are used on the approach; the practicalities are not as simple as straight aerodynamic theory may imply!

[Concours77]

I think I got a little wordy. The point I make should flow from the fact that AF 447 had no AoA indicator. Pitch and Power is important, no, critical, but if an airliner is anywhere near stall in cruise, something has gone wrong despite instruments. And instruments should not be responsible for nearing the Stall? With AF, the Stall Warn was active at the very beginning of the failure, there was much discussion about whether it was a valid warning or no.

[Smilin_Ed]

"That also ensures that the end of the tail hook stays at the correct height in relation to the fuselage and main landing gear."
Did you fly the F8U Crusader? Variable incidence wing? Elaborate?
Could you also say then that AoA is critical at speeds approaching Stall?

No, I did not fly the F8U. My primary planes were the A-4 and the EA-6B.

At any given weight, fuselage angle is determined by AoA. If you are too cocked up, the hook will be too low and you can have a "inflight engagement" of the arresting wire. The main gear need to be on the deck at or before engagement or there will be serious damage. Essentially that means you will be pulled down instead of landing normally and then engaging the wire. Thus, you need to be on AoA or you risk an inflight engagement.

AoA is your most sensitive parameter near stall so it allows you to get closer to the optimum speed for your weight.

For carrier operations, the captain tries to keep the wind directly down the angled deck so that there is no sideslip. Landing with a sideslip is an invitation to collapsed landing gear.

[PDR1]

PDR1,

Firstly, stalling AOA varies quite markedly with both Reynolds number and Mach number, and for a propeller driven aircraft will vary with any propwash over the wings.

Much of the earlier discussion relates to flying an approach, and speed is very important for landing distance considerations.


And it was in the context of those discussions that I made my remarks. And for aeroplanes "on approach" the Re, Mach and blown effect of propwash will be pretty similar (minor variations due to runway density altitude) so the stalling AoA will be within a tightly confined range, whereas stalling speed will vary by much larger amounts due to differences is weight. I therefore suggest that where an accurate AoA instrument is available it is a better reference. Airspeed is used as a parametric for AoA, but it has no absolute meaning. If you don't believe me try stalling an airfoil in a wind tunnel by reducing the airspeed!


As I said before, using just AOA may not be accurate enough in some aircraft for achieving the required landing speed accuracy criteria,


As I said - there are no fundamental landing speed criteria. There are approach and landing *AoA* criteria whose achievement is measured by monitoring airspeed, but many pilots have lost track of the detail that the airspeed itself is not the fundamentally important parameter - it is a parametric for AoA.

...especially with the potentially dynamic nature of AOA indications in turbulence and attempts to fly the required flightpath under such conditions. I have had an AOA probe stick on an approach in an aircraft whereby a single source AOA indication was the main method of speed control. It was only by cross-checking the standby ASI that I identified this failure.


These are complaints about specific implementations of AoA instruments. (not really relevant, but never mind). If AoA instruments were provided with a standby and with gauging that had similar damping to that we are used to in ASIs we would find them just as usable. You seem to suggest ASI is always available in a reliable form, which is (of course) far from the truth. The history of accidents and incidents due to pitot-static errors, icing, covered ports etc etc is a pretty large book.


I do wonder from your posts whether you have ever actually flown an aircraft where AOA indications are used on the approach; the practicalities are not as simple as straight aerodynamic theory may imply!

Oh right - attack the poster rather than discuss the points, that's gonna work.

[john_tullamarine]

Ten deep breaths whenever required to regain composure, chaps ....