negative AOA in cruise
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@PDR1
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)
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).
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).
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)
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...
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).
"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...
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).
When To Use AoA
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,
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,
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?
"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?
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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.
Whether that value is normal for the Airbus I cannot say.
Last edited by Piper19; 30th Dec 2016 at 02:22.
Moderator
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.
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.
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.
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.
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).
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).
" 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.
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.
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.
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!
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!
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.
More on AoA
"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?
Did you fly the F8U Crusader? Variable incidence wing? Elaborate?
Could you also say then that AoA is critical at speeds approaching Stall?
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,
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.
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!
Last edited by PDR1; 31st Dec 2016 at 11:30.
