AoA
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AoA
G'day
Can Anyone give me a detailed explanation about aoa???
I hope to generate some what a detailed discussion, as I am
trying to better understand it.
Cheers
DNTB
Can Anyone give me a detailed explanation about aoa???
I hope to generate some what a detailed discussion, as I am
trying to better understand it.
Cheers
DNTB
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To be honest, I'm not sure its reasonable to ask for a detailed explanation about something relatively basic - there are plenty of sources available. Essentially you're asking the denizens of this place to write you a substantial part of a text book.
I don't wish to appear unhelpful (and a search of my posting history should show you I quite often provide lengthy technical answers here).
May I suggest you ask one or more questions thats a bit more targeted, perhaps about something specific you don't understand? At that point I'm sure people will be able to give you some helpful input.
Regards
pb
I don't wish to appear unhelpful (and a search of my posting history should show you I quite often provide lengthy technical answers here).
May I suggest you ask one or more questions thats a bit more targeted, perhaps about something specific you don't understand? At that point I'm sure people will be able to give you some helpful input.
Regards
pb
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Fair enough, I know my q's was vague I'm sorry about that.
I have been reading through stick and rudder and his exp is good
but I still feel my level of understanding is still vague. So I guess
i don't have any specific qs yet.
Once again I do apologize in adv for being vague.
I have been reading through stick and rudder and his exp is good
but I still feel my level of understanding is still vague. So I guess
i don't have any specific qs yet.
Once again I do apologize in adv for being vague.
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Perhaps suggesting the context, or operating situation for which you would like to further your undertanding of AoA would be useful to those who might attempt to help you...
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Well, I've found if someone is getting hung up on something, usually a good place to start with the basic terminology associated with it. Often a basic misunderstanding leaves the rest of someone's understanding built on a shaky foundation.
One technique I recommend for anything PofF related is to be willing to swap your frame of reference around. In many instructional texts a frame of reference of the aircraft is used. There are reasons for this (a) practicality of diagrams (stationary aerofoil with the airflow drawn as streamlines) and (b) relationship to a wind tunnel. However, there are many places in the study of PofF where it is very useful to learn to visualise the air as being stationary and the aircraft moving through it.
e.g. AoA changes during aircraft attitude changes are much easier to visualise.
In this contrext, its important to get your head around the idea that AoA is related to the difference between where the aircraft is pointing and where its going. (the other component being sideslip).
Try sketching out some diagrams to demonstrate this, and I recommend avoiding straight and level. Straight and level is Satans gift to the the PofF student! Why? Because a number of important definitions are made unclear because SandL is a special case where several important vectors just happen to be lined up with the horizontal or vertical axes, which then gives the impression they are defined in that manner.
e.g. people think the lift is defined in the vertical, drag in the horizontal etc. In this specific case, people can get pitch angle and AoA confused. Make sure you are rock solid on the relationship with flight path angle.
Make sure you appreciate that you may sometimes be considering an angle of attack for one section of the wing, and at other times you may be considering the whole wing, so a reference line that is not necessarily the chord line may be used.
I hope thats something to get you started.
pb
(p.s. Mods maybe this belongs in Tech Log)
One technique I recommend for anything PofF related is to be willing to swap your frame of reference around. In many instructional texts a frame of reference of the aircraft is used. There are reasons for this (a) practicality of diagrams (stationary aerofoil with the airflow drawn as streamlines) and (b) relationship to a wind tunnel. However, there are many places in the study of PofF where it is very useful to learn to visualise the air as being stationary and the aircraft moving through it.
e.g. AoA changes during aircraft attitude changes are much easier to visualise.
In this contrext, its important to get your head around the idea that AoA is related to the difference between where the aircraft is pointing and where its going. (the other component being sideslip).
Try sketching out some diagrams to demonstrate this, and I recommend avoiding straight and level. Straight and level is Satans gift to the the PofF student! Why? Because a number of important definitions are made unclear because SandL is a special case where several important vectors just happen to be lined up with the horizontal or vertical axes, which then gives the impression they are defined in that manner.
e.g. people think the lift is defined in the vertical, drag in the horizontal etc. In this specific case, people can get pitch angle and AoA confused. Make sure you are rock solid on the relationship with flight path angle.
Make sure you appreciate that you may sometimes be considering an angle of attack for one section of the wing, and at other times you may be considering the whole wing, so a reference line that is not necessarily the chord line may be used.
I hope thats something to get you started.
pb
(p.s. Mods maybe this belongs in Tech Log)
More accurately, AoA is the difference between the chord line and the relative airflow. The Chord Line is a straight line from the frontmost part of the wing ie the bit that first comes in contact with the airflow - the leading edge - to the rearmost part (erm...the bit that last sees the airflow, the trailing edge).
Imagine a thin plank moving through the air or, better yet, find thin plank and hold it horizontally while you spin around (or even hold it out the window while driving but best get someone to help). If the plank was held so that the leading edge was at the same height as the trailing edge as it moved horizontally through the airflow the plank would have 0 deg AoA. If it was rotated 90 deg so that the front edge was directly above the rear edge then the plank would have a 90 deg AoA. Each degree that the plank is rotated to raise the leading edge above the trailing edge increases the AoA likewise.
Angle of Attack is always measured against where the airflow is coming from, not the Earth's surface. That same plank could be moving vertically and still have zero, 90 or some value in between for its AoA, depending on how you oriented it.
For a flate surface (ie not cambered or wing shaped) zero lift is produced at 0 deg AoA, increasing as the AoA is increased to some amount and thereafter reducing until at 90 deg there is no lift being produced.
Imagine a thin plank moving through the air or, better yet, find thin plank and hold it horizontally while you spin around (or even hold it out the window while driving but best get someone to help). If the plank was held so that the leading edge was at the same height as the trailing edge as it moved horizontally through the airflow the plank would have 0 deg AoA. If it was rotated 90 deg so that the front edge was directly above the rear edge then the plank would have a 90 deg AoA. Each degree that the plank is rotated to raise the leading edge above the trailing edge increases the AoA likewise.
Angle of Attack is always measured against where the airflow is coming from, not the Earth's surface. That same plank could be moving vertically and still have zero, 90 or some value in between for its AoA, depending on how you oriented it.
For a flate surface (ie not cambered or wing shaped) zero lift is produced at 0 deg AoA, increasing as the AoA is increased to some amount and thereafter reducing until at 90 deg there is no lift being produced.
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So...trying to keep this thread going, a question I've heard different answers to;
If an aircraft maintains it's pitch angle by using the yoke, (eg 2 degrees up on the AI) while deploying slats, does the "aoa" change? Seeing as there is now a different line drawn when connecting the LE to the TE.
If an aircraft maintains it's pitch angle by using the yoke, (eg 2 degrees up on the AI) while deploying slats, does the "aoa" change? Seeing as there is now a different line drawn when connecting the LE to the TE.
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If the wing is reconfigured while maintaining a particular pitch attitude, the AoA may or may not change, because the question doesn't consider the path of the relative wind. All else being equal, with no change in airspeed or vertical motion (aircraft doesn't climb or descend), then yes, reconfiguring produces a change in AoA relative to the aerodynamic chord of the airplane (as camber increases, one can't necessarily draw a line between the trailing edge of the wing and the leading edge and use that as the actual reference chord line for determining the aerodynamic chord).
Another consideration is the upwash created ahead of the airfoil. It's easiest visualized with the aircraft traveling parallel to a given frame of reference, such as flat, level ground. If the aircraft travels level, moving forward, one might presuppose the relative wind to be opposite to the direction of travel. In still air, for example, if the aircraft is traveling due west, level, the relative wind might be assumed to be from the west, at the same velocity as the aircraft. This doesn't work in reality, however, as the airflow is altered ahead of the wing, and altered by other aerodynamic factors such as the development of a shock wave; airflow approaches the wing from a different angle than "head-on."
Airflow will be drawn somewhat from beneath the wing and gain an "upwash" or upward moment as it approaches the aircraft wing, in most cases, and this upwash moment, or upward vector increases as the camber increases when the aircraft is configured for takeoff or landing. This angle will be an upward vector to some degree relative to the free airstream, such that angle of attack at the wing isn't the same as the chordline of the wing relative to the free airstream (the airstream not adjacent to or near the wing).
Another consideration is the upwash created ahead of the airfoil. It's easiest visualized with the aircraft traveling parallel to a given frame of reference, such as flat, level ground. If the aircraft travels level, moving forward, one might presuppose the relative wind to be opposite to the direction of travel. In still air, for example, if the aircraft is traveling due west, level, the relative wind might be assumed to be from the west, at the same velocity as the aircraft. This doesn't work in reality, however, as the airflow is altered ahead of the wing, and altered by other aerodynamic factors such as the development of a shock wave; airflow approaches the wing from a different angle than "head-on."
Airflow will be drawn somewhat from beneath the wing and gain an "upwash" or upward moment as it approaches the aircraft wing, in most cases, and this upwash moment, or upward vector increases as the camber increases when the aircraft is configured for takeoff or landing. This angle will be an upward vector to some degree relative to the free airstream, such that angle of attack at the wing isn't the same as the chordline of the wing relative to the free airstream (the airstream not adjacent to or near the wing).
Another consideration is the upwash created ahead of the airfoil.
To suggest otherwise would be to say that different parts of the aircraft have different AoAs. They may have different flow-paths, but AoA as presented on lift and drag curves is the simple angle presented above.
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Its actually common (Id say normal) when dealing at aircraft level to ignore the local chord line and simply define AOA by reference to aircraft level geometry.
Angle of attack for the aircraft then becomes the angle between the aircraft reference line and the free stream (far field) air flow, much as pitch attitude is the angle between the same aircraft reference and the ground plane.
Only when this is done can the 'normal' relation ship between angle of attack alpha, pitch attitude theta and climb angle (flight path angle) gamma hold in still air:
theta=alpha + gamma
if you start accounting for wing geometry changes and the like it's extremely difficult to simply model aircraft dynamic for changing configurations. it also becomes a mess trying to analyse aircraft aerodynamic data consistently, or to correlate things like AOA vanes on the fuselage with aircraft behaviour.
the 'chord line' concept is fine when comparing aerofoils, but it's just an impediment when dealing with a real aircraft - what about washout? what about aeroelasticity? and so on.
so to answer hawk37, if the gamma does not change due to the slat deployment, then neither does alpha.
Angle of attack for the aircraft then becomes the angle between the aircraft reference line and the free stream (far field) air flow, much as pitch attitude is the angle between the same aircraft reference and the ground plane.
Only when this is done can the 'normal' relation ship between angle of attack alpha, pitch attitude theta and climb angle (flight path angle) gamma hold in still air:
theta=alpha + gamma
if you start accounting for wing geometry changes and the like it's extremely difficult to simply model aircraft dynamic for changing configurations. it also becomes a mess trying to analyse aircraft aerodynamic data consistently, or to correlate things like AOA vanes on the fuselage with aircraft behaviour.
the 'chord line' concept is fine when comparing aerofoils, but it's just an impediment when dealing with a real aircraft - what about washout? what about aeroelasticity? and so on.
so to answer hawk37, if the gamma does not change due to the slat deployment, then neither does alpha.
Thanks MFS,
So we are talking about the angle between an arbitory reference line (chord line being one possible reference) and the general airflow, NOT the angle at which the local airflow meets the Aerofoil.
So we are talking about the angle between an arbitory reference line (chord line being one possible reference) and the general airflow, NOT the angle at which the local airflow meets the Aerofoil.
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That's the usual meaning when AOA is being used.
However you have to be EXTREMELY careful about context; if I'm, say, discussing aerodynamic loads on an aircraft component I may well talk in terms of local angle of attack in which case I am changing both the local geometric reference and the free stream reference possibly. The tailplane is an excellent example where this occurs. Usually when discussing the aerodynamic characteristics of the tailplane one will use TAIL AOA which is measured between the local freestream (which includes the effect of wing downwash) and the local tail reference (which moves as the tail moves on an all-moving tail, but doesn't move if the elevator alone moves). But if I were analysing the tail effect at an aircraft level, I'd be going back to using aircraft level AOA as described before.
It all depends on the context the person is using and thats why a generic "what is AOA?" question can result in many different answers, which may all be correct in the right circumstances
However you have to be EXTREMELY careful about context; if I'm, say, discussing aerodynamic loads on an aircraft component I may well talk in terms of local angle of attack in which case I am changing both the local geometric reference and the free stream reference possibly. The tailplane is an excellent example where this occurs. Usually when discussing the aerodynamic characteristics of the tailplane one will use TAIL AOA which is measured between the local freestream (which includes the effect of wing downwash) and the local tail reference (which moves as the tail moves on an all-moving tail, but doesn't move if the elevator alone moves). But if I were analysing the tail effect at an aircraft level, I'd be going back to using aircraft level AOA as described before.
It all depends on the context the person is using and thats why a generic "what is AOA?" question can result in many different answers, which may all be correct in the right circumstances
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MFS, this may well be an unanswerable question, but do the major authors of texts on the subject of aerodynamics, or aircraft performance, generally follow the definition of aoa as you have given it?
Hawk,
Perhaps a clue is that most diagramatic explanations show the chord line as the Datum. I would suggest that most Lift and drag curves use the same, though as it really is an arbitrary reference, it wouldn't matter if it was or not.
Most Aircraft that actually measure AOA do so with a vane attached to the fuselage, so what they are measuring is entirely independant of the wing.
Perhaps a clue is that most diagramatic explanations show the chord line as the Datum. I would suggest that most Lift and drag curves use the same, though as it really is an arbitrary reference, it wouldn't matter if it was or not.
Most Aircraft that actually measure AOA do so with a vane attached to the fuselage, so what they are measuring is entirely independant of the wing.
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Wiz, you said "Most Aircraft that actually measure AOA do so with a vane attached to the fuselage, so what they are measuring is entirely independant of the wing."
True, I think it "measures" aoa, but does the pilot see this ever? What may be presented to the pilot is not aoa, for example if one uses the common teledyne (sp?) system. The aoa probe provides info to the system, what the pilot sees is the indicator on the gauge, which would generally show buffet area above .8, with the true stall supposedly at 1.0
This was irrespective of configuration. The "stall" (ie 1.0 on the gauge) was the same for clean, slats, and slats+flaps. despite the aircraft being at a different pitch angle, and different aoa.
And so I think there was an input to the gauge so that it would know the configuration, and make the correct adjustment.
I haven't used other systems to know if they actually show aoa, and not percent of stall
True, I think it "measures" aoa, but does the pilot see this ever? What may be presented to the pilot is not aoa, for example if one uses the common teledyne (sp?) system. The aoa probe provides info to the system, what the pilot sees is the indicator on the gauge, which would generally show buffet area above .8, with the true stall supposedly at 1.0
This was irrespective of configuration. The "stall" (ie 1.0 on the gauge) was the same for clean, slats, and slats+flaps. despite the aircraft being at a different pitch angle, and different aoa.
And so I think there was an input to the gauge so that it would know the configuration, and make the correct adjustment.
I haven't used other systems to know if they actually show aoa, and not percent of stall
I've certainly worked with military aircraft whose AoA gauges showed a nominal value in degrees; Jaguar springs to mind, where the safety limits were set pre-flight but varied with external stores configuration (12deg with centreline stores, 17 without I think, but it's been years so I'm probably wrong).
G
G
