I don't agree with this FAA ATPL Question
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The obvious addendum to this is the fact that there are various factors that affect the strength of the vortices, and the dominant factor may be changeable.
What I mean by this is that whilst heavy slow and clean is the standard answer, it is not quite so simple because a dirty aircraft can fly much slower.
A generic clean aircraft might have a min clean speed of 200kts.
This aircraft might have a fully configured speed of 130kts.
This means that the "clean" and "slow" aspects can be trading off against each other.
Yes, if you could fly clean at 130kts it would give greater vortices than dirty, but you can't, so on some aircraft dirty at 130kts may give a greater wake than clean at 200kts
Part of the reason that clean gives a large vortex is because the vortex comes only from the wingtip when clean, but once flaps are deployed, there is another vortex at the flap tip giving more than one vortex per wing which shrinks each vortex plus they can destructively combine reducing it further.
Boeing has even got plans to deliberately help this along:
Aero 14 - Airplane Trailing Vortices
Gear down has also been shown to have an effect on wake turbulence in some aircraft, but this is due to interference with the vortex formation through mechanical turbulence and as such will differ between aircraft.
NASA - NASA Dryden Technology Facts - Wake Vortex Research
Another aspect that must be considered is full length flaps. these will obviously cause entirely different effects.
What I mean by this is that whilst heavy slow and clean is the standard answer, it is not quite so simple because a dirty aircraft can fly much slower.
A generic clean aircraft might have a min clean speed of 200kts.
This aircraft might have a fully configured speed of 130kts.
This means that the "clean" and "slow" aspects can be trading off against each other.
Yes, if you could fly clean at 130kts it would give greater vortices than dirty, but you can't, so on some aircraft dirty at 130kts may give a greater wake than clean at 200kts
Part of the reason that clean gives a large vortex is because the vortex comes only from the wingtip when clean, but once flaps are deployed, there is another vortex at the flap tip giving more than one vortex per wing which shrinks each vortex plus they can destructively combine reducing it further.
Boeing has even got plans to deliberately help this along:
Aero 14 - Airplane Trailing Vortices
Gear down has also been shown to have an effect on wake turbulence in some aircraft, but this is due to interference with the vortex formation through mechanical turbulence and as such will differ between aircraft.
NASA - NASA Dryden Technology Facts - Wake Vortex Research
Another aspect that must be considered is full length flaps. these will obviously cause entirely different effects.
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However, I disagree.
The correct answer is what the teacher says it is- if you want to pass.
As to wingtip vortices- keep in mind that the various winglets work by taking a lot of the vortex " energy " and redirecting it in such a manner that a few percent better lift is generated. Thus the improved takeoff performance. And in some cases, by ' reducing' the trailing vortex during landing, closer separation **could** be allowed for trailing aircraft.
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In level flight total lift is constant irrespective of speed.
Vortices are proportional to the lift generated near the wing tip. With flaps down lift is increased at the root and reduced at the tip. So vortices are worse with flaps up.
Answer A.
Vortices are proportional to the lift generated near the wing tip. With flaps down lift is increased at the root and reduced at the tip. So vortices are worse with flaps up.
Answer A.
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Heavy: because the heavier the weight, the higher the lift requirement
Slow: because the slower the speed the greater the AOA
Flaps up: because the smaller the surface wing, again the greater the AOA
As for the gear down; yes it creates indisputably a disturbance, but nothing to compare with the magnitude involved in the wake turbulence itself.
Slow: because the slower the speed the greater the AOA
Flaps up: because the smaller the surface wing, again the greater the AOA
As for the gear down; yes it creates indisputably a disturbance, but nothing to compare with the magnitude involved in the wake turbulence itself.
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But below a certain speed, level flight cannot be maintained clean.
The question might be rephrased "Is the wingtip vortex worse at min speed clean or at min speed fully configured" because if the aircraft was to be flown at min speed fully configured without configuring, then the vortices will be very small indeed.....
There are two variables, ie speed and configuration which work against each other.
You cannot be slow and clean.
The dominant variable will be different for different aircraft wings.
The question might be rephrased "Is the wingtip vortex worse at min speed clean or at min speed fully configured" because if the aircraft was to be flown at min speed fully configured without configuring, then the vortices will be very small indeed.....
There are two variables, ie speed and configuration which work against each other.
You cannot be slow and clean.
The dominant variable will be different for different aircraft wings.
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"slow" is a variable, not fixed though. It invalidates the question to declare speed to be one of the available variables and then declare 2 different speeds to be "slow"
This means that the answer is not as simple as "A"
Different wings will have different characteristics, and speed may dominate under some circumstances.
A full length flap will cause a different effect.
Most of the trials that feed the received wisdom seem to have been performed a long time ago on a limited number of aircraft with old fashioned wings.
I have seen no experimental data from trials of the 787 or A380 wake turbulence.
This means that the answer is not as simple as "A"
Different wings will have different characteristics, and speed may dominate under some circumstances.
A full length flap will cause a different effect.
Most of the trials that feed the received wisdom seem to have been performed a long time ago on a limited number of aircraft with old fashioned wings.
I have seen no experimental data from trials of the 787 or A380 wake turbulence.
Last edited by Tourist; 22nd Nov 2015 at 06:49.
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There is of course another variable not mentioned which is g loading. an aircraft in a turn will obviously have more wake all other things being equal, and that goes for the flare too......
Plus, on many aircraft, an increase in flap requires greater downforce from the tailplane due to the shift in the centre of pressure, which acts as an increase in weight thus affecting the third variable.
Plus, on many aircraft, an increase in flap requires greater downforce from the tailplane due to the shift in the centre of pressure, which acts as an increase in weight thus affecting the third variable.
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I Will Not Debate The Question Itself
I refuse to to debate the question, but you are missing a vitally important point: for better - and often for worse, in the U.S. of A., the FAA makes the rules and we have to live with them. Have they imposed bad rules or asked wrong questions before? Of course they have. It will happen again. I promise. The FAA is an arm of Government and G'ment makes LOTS of very stupid mistakes. even in 'merika, where most G'ment is is a committee function.
Why are you wetting your pants over One Single Question, or becoming frustrated after only two weeks? Responses from G'ment often take months. Don't know where you are from, but a two week turn around is a
dream for a response from the U.S.'s FAA. If you still do not have a response by July, 2016, please post your question afresh. While I do not wish to sound snarky here, two weeks for a response to a non-emergent question posed to the FAA hardly unusual. Heavens, even in emergent case, you are not likely to hear back that soon. As note, please write to us again in July, 2016 if this is still bothering you. If it helps you sleep better some engineers would favor two of the three responses offered, for different reasons. IMO, all three responses have some merit, but in Amerika, our testing procedures require the best of of the best. Sometimes. When G'ment gets involved, logic and reason tend to 'fly' out the window. Good luck.
Why are you wetting your pants over One Single Question, or becoming frustrated after only two weeks? Responses from G'ment often take months. Don't know where you are from, but a two week turn around is a
dream for a response from the U.S.'s FAA. If you still do not have a response by July, 2016, please post your question afresh. While I do not wish to sound snarky here, two weeks for a response to a non-emergent question posed to the FAA hardly unusual. Heavens, even in emergent case, you are not likely to hear back that soon. As note, please write to us again in July, 2016 if this is still bothering you. If it helps you sleep better some engineers would favor two of the three responses offered, for different reasons. IMO, all three responses have some merit, but in Amerika, our testing procedures require the best of of the best. Sometimes. When G'ment gets involved, logic and reason tend to 'fly' out the window. Good luck.
What happened to them??
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High speed==>less angle of attack==> less wingtip vortex.
wingtip vortex is a result of air pressure differential at wing tip.
With flaps down another wake vortices created at the tip of trailing edge flaps. I believe this will reduce the span wash which consiquently weakens wingtip vortices.
wingtip vortex is a result of air pressure differential at wing tip.
With flaps down another wake vortices created at the tip of trailing edge flaps. I believe this will reduce the span wash which consiquently weakens wingtip vortices.
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Well, I wanted B but you know what - what`s happening when you have no flaps and heavy and slow. Well, apart from you just got a good deal from your insurance man - you `aint got the lift with the flaps, I know, I know, but it is all generated by the lift from the wings. So - no flaps needs a greater angle of attack which mean more lift - more vortex - taking vortex to be the secondary effect of lift (and by passing drag altogether - just to make a point. Drag it on if you want. Swimmer seems to have got it in one! but my call is that with less "help" from the flaps you are generating more lift from the greater angle of attack -of the wings- and therefore more spanwise flow as that nice person just mentioned. . . . and so more Vortex. Trust me am a test pilot. (was)
Last edited by Natstrackalpha; 26th Nov 2015 at 14:34. Reason: (was)
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Natstrackalpha
I honestly hope you are joking about being a test pilot.
The lift is the same, ie equal to weight however you get it in un-accelerated flight at any speed. (except that due to downforce being greater on the tail, slightly more lift is required from the wing is required fully configured unless you can shift the CofG)
The difference is how much air the lift is being generated from/acted upon.
Lots of air a tiny bit at high speed, or not much air a lot at low speed.
I honestly hope you are joking about being a test pilot.
The lift is the same, ie equal to weight however you get it in un-accelerated flight at any speed. (except that due to downforce being greater on the tail, slightly more lift is required from the wing is required fully configured unless you can shift the CofG)
The difference is how much air the lift is being generated from/acted upon.
Lots of air a tiny bit at high speed, or not much air a lot at low speed.
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@Tourist
Wait on, wait on Mr. Tourist - I like what you say - but you are also suggesting, are you not that there is the same amount of vortices at fast speed level flight as there is at - look, there is more Vortex at slow and level and heavy than there is at fast - level and light - take it from there. Because if you are slow and fat and heavy then there is loads of lift generated by the wings (logically) and the angle of attack - You are suggesting that because the lift is ~ the same ~ then the vortices are the same at any speed - despite the fact that they are all fluff and all bunched up in a small amount of space in the sky during (in this case) a flapless approach in a heavy aircraft. Whereas screaming along at M0.82 level and clean (obviously clean). . . . . I hate you Tourist.
There is something missing here - How can the lift be the s, oh yes ok, . . ~?
because Lift <> weight . . . . ah, there it is - as you said if you are going slower you need more lift because there is less air . . . so to generate this lift you increase the angle of attack -
so if you are increasing the angle of attack that is producing more lift, then you must be chuffing out more vortices - THAN if you are flying along light and fast. As the lift is there but it is not being produced by a high angle of attack meaning there is less drag to overcome - howz that?
Also fully configured? WWe are clean in this scenario/question.
Ok anything - a train . . A train is going along at fantastic speed - it wants to maintain that speed so the driver opens the throttle a little.
Same train - At the station, ready to leave Drive uses loads of power to overcome inertia - clean big fat aeroplane is creating more inertia/drag due to increased angle of attack needs more lift - causes more spanwise flow - Seriously, what has the tail got to do with it. Looking at the Longitudinal Dihedral (I`d rather not) but that has no bearing on this. If there is not enough air pushing the Tail down then . . well, there is - I don`t know why you brought the tail into the equation . . ~~~? If you are saying the speed is too low for the tail so you use more power and greater angle of attack then fine - but you are still creating more vortexx as there is more to overcome Why? Because it `aint being generated by speed, or in your case lots of air. (on the wings)
Or rather than write all that - to simply quote you in your previous statement Quote: "Part of the reason that clean gives a large vortex is because the vortex comes only from the wingtip when clean" Unquote.
And, you also said: Quote "The lift is the same, ie equal to weight however you get it in unaccelerated flight at any speed. " Unquote - well, not really confusing - considering the Lift is equal to weight - yes, so, if yo increase the weight, you have to increase the Lift - if yo increase the Lift (especially Clean then you increase the Vortices. Or, you would get the same Vortices from a balsa wood model as you do from a 283 ton 744
Also, Oh retired one, A 747 cruising at max chat (forget VNE and any other obvious restrictions like ISA, mach crit etc) and level flight light as possible with minimum fuel except for this experimental flight +> then the lift required from the wing is less than that required (from the wing`s upper surface) than at heavy, Clean and slow. At fast speed, really light there is a comparative nose down attitude i.e., little or no angle of attack . The airflow from the Rel Air Flow (R.A.F.) is max under the wing - so that the wing is verily surfing across the air and is having a dramatic affect of the Lift whilst the airflow over the wing is also providing lift - the wing is at max efficiency in the the span wise flow and the subsequent vortices are flowing, twisting from the tips causing powerful yet small and concentrated vortices, as you know. Light, unloaded, fast 747. Level flight Clean.
Now take same aircraft - load to max payload and or fuel and make it real heavy and slow it down to as low as you dare without reaching going to near the stall-WARNER - the air under the wings is minimal compared to fast flight - there is a high nose attitude due to the increased angle of attack due to the wings having to grapple for height (think of it that way - (unless of course you are and engineer) The wing is using its design to stay airborne with little help from the Rel Air Flow - The airflow over the wings is no longer as laminar as it was at the fast speed run and some vortices and airflow break up is occurring perhaps, the airflow over the wing is being pushed up - not only close to the boundary layer but also continues up above the wing and flows down to the tailing edge (Bernoulli) - also the Centre of Pressure CP is starting to move forward more than before - animatedly, its hanging by its teeth, so all of the air is being actively utilised to provide all of the Lift - so, massive Vortices. Heavy, Slow, Clean.
Test pilots try to see if it won`t break.
There is something missing here - How can the lift be the s, oh yes ok, . . ~?
because Lift <> weight . . . . ah, there it is - as you said if you are going slower you need more lift because there is less air . . . so to generate this lift you increase the angle of attack -
so if you are increasing the angle of attack that is producing more lift, then you must be chuffing out more vortices - THAN if you are flying along light and fast. As the lift is there but it is not being produced by a high angle of attack meaning there is less drag to overcome - howz that?
Also fully configured? WWe are clean in this scenario/question.
Ok anything - a train . . A train is going along at fantastic speed - it wants to maintain that speed so the driver opens the throttle a little.
Same train - At the station, ready to leave Drive uses loads of power to overcome inertia - clean big fat aeroplane is creating more inertia/drag due to increased angle of attack needs more lift - causes more spanwise flow - Seriously, what has the tail got to do with it. Looking at the Longitudinal Dihedral (I`d rather not) but that has no bearing on this. If there is not enough air pushing the Tail down then . . well, there is - I don`t know why you brought the tail into the equation . . ~~~? If you are saying the speed is too low for the tail so you use more power and greater angle of attack then fine - but you are still creating more vortexx as there is more to overcome Why? Because it `aint being generated by speed, or in your case lots of air. (on the wings)
Or rather than write all that - to simply quote you in your previous statement Quote: "Part of the reason that clean gives a large vortex is because the vortex comes only from the wingtip when clean" Unquote.
And, you also said: Quote "The lift is the same, ie equal to weight however you get it in unaccelerated flight at any speed. " Unquote - well, not really confusing - considering the Lift is equal to weight - yes, so, if yo increase the weight, you have to increase the Lift - if yo increase the Lift (especially Clean then you increase the Vortices. Or, you would get the same Vortices from a balsa wood model as you do from a 283 ton 744
Also, Oh retired one, A 747 cruising at max chat (forget VNE and any other obvious restrictions like ISA, mach crit etc) and level flight light as possible with minimum fuel except for this experimental flight +> then the lift required from the wing is less than that required (from the wing`s upper surface) than at heavy, Clean and slow. At fast speed, really light there is a comparative nose down attitude i.e., little or no angle of attack . The airflow from the Rel Air Flow (R.A.F.) is max under the wing - so that the wing is verily surfing across the air and is having a dramatic affect of the Lift whilst the airflow over the wing is also providing lift - the wing is at max efficiency in the the span wise flow and the subsequent vortices are flowing, twisting from the tips causing powerful yet small and concentrated vortices, as you know. Light, unloaded, fast 747. Level flight Clean.
Now take same aircraft - load to max payload and or fuel and make it real heavy and slow it down to as low as you dare without reaching going to near the stall-WARNER - the air under the wings is minimal compared to fast flight - there is a high nose attitude due to the increased angle of attack due to the wings having to grapple for height (think of it that way - (unless of course you are and engineer) The wing is using its design to stay airborne with little help from the Rel Air Flow - The airflow over the wings is no longer as laminar as it was at the fast speed run and some vortices and airflow break up is occurring perhaps, the airflow over the wing is being pushed up - not only close to the boundary layer but also continues up above the wing and flows down to the tailing edge (Bernoulli) - also the Centre of Pressure CP is starting to move forward more than before - animatedly, its hanging by its teeth, so all of the air is being actively utilised to provide all of the Lift - so, massive Vortices. Heavy, Slow, Clean.
Test pilots try to see if it won`t break.
Last edited by Natstrackalpha; 30th Nov 2015 at 11:03.
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Flaps can break up the vortices if they're partial span flaps vs. full span flaps.
The 757 produces larger than expected wake turbulence due to the continuous nature of it's flaps vs. the inboard/outboard flap design of many jets.
Just looking at the 787 flaps you'd think they'd be in the 'more' group like the 757 flap design.
The 757 produces larger than expected wake turbulence due to the continuous nature of it's flaps vs. the inboard/outboard flap design of many jets.
Just looking at the 787 flaps you'd think they'd be in the 'more' group like the 757 flap design.