Why is Yaw 2nd effect of Roll? (and explain Trim)
Join Date: Jan 2007
Location: West Midlands
Age: 58
Posts: 33
Likes: 0
Received 0 Likes
on
0 Posts
s I said earlier, yaw (not surprisingly) occurs about the yaw axis, which is perpendicular to the other 2 axes and not related to where the horizon happens to be.
No such thing as a yaw axis its the normal or vertical axis
Yaw is not required in order for the aircraft to change heading EXCEPT when the wings are level.
A confusing statement which is technically incorrect, at slow speed adverse aileron yaw can momentarily produce a heading change in the opposite intended direction
A normal turn results from the horizontal component of the lift vector.
Yaw results from BANK when the vertical component of lift is insufficient to balance the weight - the aircraft sideslips and directional stability results in yaw.
You need to differentiate between into turn yaw and out of turn yaw. Into turn yaw will not occur if the height is maintained so again the above statement is not correct. The into turn yaw you are talking about occurs after the aircraft rolls and no corrective action is taken by the pilot.
You have also missed out;
The nose pitches down and eventually the result is spiral descent which is the most important point in the teaching of the further effects of aileron or rudder
Yaw results from ROLL because of differential drag caused by aileron deflection.
INCORRECT-- only out of turn adverse yaw is the product of aileron deflection. A non induced roll such as in turbulence will not produce adverse out of turn yaw if the ailerons remain neutral
Differential and frise ailerons reduce this but will only work optimally at a certain speed or angle of attack and (I believe) are typically optimised for cruise speed.
It is not possible to totally remove adverse aileron yaw hence to be correct an opposite rudder input is always required to offset this out of turn yaw
ASK CAPTAIN JON
No such thing as a yaw axis its the normal or vertical axis
Yaw is not required in order for the aircraft to change heading EXCEPT when the wings are level.
A confusing statement which is technically incorrect, at slow speed adverse aileron yaw can momentarily produce a heading change in the opposite intended direction
A normal turn results from the horizontal component of the lift vector.
Yaw results from BANK when the vertical component of lift is insufficient to balance the weight - the aircraft sideslips and directional stability results in yaw.
You need to differentiate between into turn yaw and out of turn yaw. Into turn yaw will not occur if the height is maintained so again the above statement is not correct. The into turn yaw you are talking about occurs after the aircraft rolls and no corrective action is taken by the pilot.
You have also missed out;
The nose pitches down and eventually the result is spiral descent which is the most important point in the teaching of the further effects of aileron or rudder
Yaw results from ROLL because of differential drag caused by aileron deflection.
INCORRECT-- only out of turn adverse yaw is the product of aileron deflection. A non induced roll such as in turbulence will not produce adverse out of turn yaw if the ailerons remain neutral
Differential and frise ailerons reduce this but will only work optimally at a certain speed or angle of attack and (I believe) are typically optimised for cruise speed.
It is not possible to totally remove adverse aileron yaw hence to be correct an opposite rudder input is always required to offset this out of turn yaw
ASK CAPTAIN JON
Join Date: Jan 2007
Location: West Midlands
Age: 58
Posts: 33
Likes: 0
Received 0 Likes
on
0 Posts
ISLANDER
Yaw is not required to turn an aircraft, otherwise the accurate pilot who keeps the ball exactly in the middle during turns, commonly known as an autopilot, would not function.
For the purpose of PPL training I will stick to the NASA explanation, which is
The aircraft is turned through the action of the side component of the lift force.
One simple statement thats all thats needed, Just because something is written in a book dosnt mean its correct or do you really believe people can walk on water too!
Yaw is not required to turn an aircraft, otherwise the accurate pilot who keeps the ball exactly in the middle during turns, commonly known as an autopilot, would not function.
For the purpose of PPL training I will stick to the NASA explanation, which is
The aircraft is turned through the action of the side component of the lift force.
One simple statement thats all thats needed, Just because something is written in a book dosnt mean its correct or do you really believe people can walk on water too!
Join Date: Jan 2006
Location: An island somewhere
Posts: 423
Likes: 0
Received 0 Likes
on
0 Posts
You need to differentiate between into turn yaw and out of turn yaw. Into turn yaw will not occur if the height is maintained so again the above statement is not correct. The into turn yaw you are talking about occurs after the aircraft rolls and no corrective action is taken by the pilot.
I give up! Try reading around the subject.
Also, might I suggest your footer slogan is a touch inappropriate?
Join Date: Jan 2006
Location: An island somewhere
Posts: 423
Likes: 0
Received 0 Likes
on
0 Posts
Yaw is not required to turn an aircraft, otherwise the accurate pilot who keeps the ball exactly in the middle during turns, commonly known as an autopilot, would not function.
Join Date: Jan 2007
Location: West Midlands
Age: 58
Posts: 33
Likes: 0
Received 0 Likes
on
0 Posts
One of the fundemantal mistakes you are making is to offer a complicated pointless explanation which has little to do with the original post which asked for simplicity.
For the purpose of PPL, CPL & ATPL training using the ball as an indicator of yaw is perfectly adequate
In regard to the footer slogan your snide comment does little to strengthen your argument but much to highlight you character.
ASK CAPTAIN JON
PS I accept the Americans call it the yaw axis
For the purpose of PPL, CPL & ATPL training using the ball as an indicator of yaw is perfectly adequate
In regard to the footer slogan your snide comment does little to strengthen your argument but much to highlight you character.
ASK CAPTAIN JON
PS I accept the Americans call it the yaw axis
Last edited by rondon9897; 29th Mar 2007 at 13:13.
Islander2:
I'm always hungry to learn so please explain, but it'll need to be convincing to overturn a "certainty" that I've held for the last 30 years.
rondon1987:
Sigh. I was going to reply but it's probably better if I just suggest you re-read my append in its entirity, and that you consider the significance of the words "required", "roll" and "bank" (and yes, it's the vertical axis, AKA the yaw axis.)
As for the advice on how to teach EoC: thanks but I can probably manage that by now.
BTW, what is the significance of "ASK CAPTAIN JON"?
HFD
You will need to understand why that's completely wrong before any comprehension of the aerodynamics of turning is possible.
rondon1987:
Sigh. I was going to reply but it's probably better if I just suggest you re-read my append in its entirity, and that you consider the significance of the words "required", "roll" and "bank" (and yes, it's the vertical axis, AKA the yaw axis.)
As for the advice on how to teach EoC: thanks but I can probably manage that by now.
BTW, what is the significance of "ASK CAPTAIN JON"?
HFD
Join Date: Jan 2006
Location: An island somewhere
Posts: 423
Likes: 0
Received 0 Likes
on
0 Posts
For the purpose of PPL, CPL & ATPL training using the ball as an indicator of yaw is perfectly adequate
You will need to understand why that's completely wrong before any comprehension of the aerodynamics of turning is possible.
We will need to agree on the meaning of the words we use before we can possibly agree on the physics.
Wikipedia does a reasonable job of explaining the conventional meanings of the terms roll, pitch and yaw but even there the word "vertical" in the definition of yaw is unhelpful. The key idea is that the axis and therefore the angle is defined with respect to the airframe ("vehicle") not to the earth, nor to the instantaneous direction of motion.
An aeroplane in a balanced level turn (angle of bank between 0 and 90 degrees exclusive) has an angular velocity about a vertical (earth-vertical) axis. That angular velocity can be resolved, just like any other vector, into a different coordinate system, the vehicle axes, resulting in component about the yaw axis and a component about the pitch axis. Thus it can be said to both yaw and pitch.
If you choose to make the word "yaw" synonymous with "sideslip angle" or "change in sideslip angle", you may be able to reconcile some of the statements on this thread with the real world. But please be aware that this use of the word is not the conventional definition in flight dynamics.
Bookworm:
I agree, but this an instructing forum and we are debating how best to explain the secondary EoC to a pilot - pilots are taught from day 1 to use the aircraft axes as their control reference frame. Within this frame a high rate of yaw has the aircraft rotating around its vertical/yaw axis, regardless of the angle of bank or pitch attitudes.
A fascinating factoid from the wikipaedia flight dynamics pages (referenced from the page you linked): roll rate leads to fin lift causing yaw in the same direction. This had not occurred to me before but is blindingly obvious once its been pointed out.
(edited to fix a typo)
HFD
I agree, but this an instructing forum and we are debating how best to explain the secondary EoC to a pilot - pilots are taught from day 1 to use the aircraft axes as their control reference frame. Within this frame a high rate of yaw has the aircraft rotating around its vertical/yaw axis, regardless of the angle of bank or pitch attitudes.
A fascinating factoid from the wikipaedia flight dynamics pages (referenced from the page you linked): roll rate leads to fin lift causing yaw in the same direction. This had not occurred to me before but is blindingly obvious once its been pointed out.
(edited to fix a typo)
HFD
Last edited by hugh flung_dung; 29th Mar 2007 at 17:41.
I agree, but this an instructing forum and we are debating how best to explain the secondary EoC to a pilot - pilots are taught from day 1 to use the aircraft axes as their control reference frame.
Bookworm:
... but when you demo the primary EoC you show them with the aircraft in the reference attitude (approx cruise) and then demo in a climbing turn that the elevator still affects pitch and that the rudder still yaws the aircraft left/right from the pilot's perspective and irrespective of the aircraft's attitude.
How do you propose to use your flight dynamics definition of yaw reference in that situation and in a way that will add value to the poor old stude? One way may be to simply teach that the rudder cause the aircraft to sideslip left/right but that would be counter to (AFAIK) all current teaching - it would also make terminology a bit tricky when teaching aeros.
IMHO (not so humble, actually
) this debate has moved from the practical to the esoteric. KISS for the stude's sake. All control effects are taught referenced to the aircraft's axes, piloting discussions should use the same reference frames.
HFD
... but when you demo the primary EoC you show them with the aircraft in the reference attitude (approx cruise) and then demo in a climbing turn that the elevator still affects pitch and that the rudder still yaws the aircraft left/right from the pilot's perspective and irrespective of the aircraft's attitude.
How do you propose to use your flight dynamics definition of yaw reference in that situation and in a way that will add value to the poor old stude? One way may be to simply teach that the rudder cause the aircraft to sideslip left/right but that would be counter to (AFAIK) all current teaching - it would also make terminology a bit tricky when teaching aeros.
IMHO (not so humble, actually
) this debate has moved from the practical to the esoteric. KISS for the stude's sake. All control effects are taught referenced to the aircraft's axes, piloting discussions should use the same reference frames.HFD
Join Date: Jan 2006
Location: An island somewhere
Posts: 423
Likes: 0
Received 0 Likes
on
0 Posts
HFD
I believe you've misconstrued bookworm's post (either that, or I have, and he'll be along in a minute to correct me!).
The axes of reference that I believe bookworm is referring to (and certainly I'm referring to) when talking of pitch and yaw are the aircraft's axes. Any turn (whether level, climbing or descending) comprises a) a pitching motion around the aircraft's lateral axis together with b) a yawing motion around the aircraft's normal axis; the relative magnitudes of these two motions depend on the bank angle. Whilst rolling into a bank initiates the process, a key question to be answered (and understood) is what causes the pitching motion and what causes the yawing motion. These two motions are the secondary effects of roll. In other words, having created a sideways force by inclining the lift vector, why does the aeroplane take up a circular arc rather than merely 'lift' sideways? Or, to put it another way, if the sideways force (earth reference) is automatically a centripetal force, how come the vertical component (earth reference) doesn't have the same effect (and continually loop the aeroplane)?
Bookworm was, I believe, suggesting that some of the confusion in this thread arises from some posters interchanging yaw and sideslip as if they are the same thing. They are not! Edited to add that, even in the case of a 60deg bank turn for larger pitch influence, or 15deg bank turn for larger yaw influence, the rates of yaw and pitch are low compared to some other flight regimes and hence more difficult to perceive ... maybe this is part of the confusion.
I believe you've misconstrued bookworm's post (either that, or I have, and he'll be along in a minute to correct me!).
The axes of reference that I believe bookworm is referring to (and certainly I'm referring to) when talking of pitch and yaw are the aircraft's axes. Any turn (whether level, climbing or descending) comprises a) a pitching motion around the aircraft's lateral axis together with b) a yawing motion around the aircraft's normal axis; the relative magnitudes of these two motions depend on the bank angle. Whilst rolling into a bank initiates the process, a key question to be answered (and understood) is what causes the pitching motion and what causes the yawing motion. These two motions are the secondary effects of roll. In other words, having created a sideways force by inclining the lift vector, why does the aeroplane take up a circular arc rather than merely 'lift' sideways? Or, to put it another way, if the sideways force (earth reference) is automatically a centripetal force, how come the vertical component (earth reference) doesn't have the same effect (and continually loop the aeroplane)?
Bookworm was, I believe, suggesting that some of the confusion in this thread arises from some posters interchanging yaw and sideslip as if they are the same thing. They are not! Edited to add that, even in the case of a 60deg bank turn for larger pitch influence, or 15deg bank turn for larger yaw influence, the rates of yaw and pitch are low compared to some other flight regimes and hence more difficult to perceive ... maybe this is part of the confusion.
Last edited by Islander2; 29th Mar 2007 at 18:49.
Thanks Islander2, I agree.
HFD, I'm not trying to change any of that, I'm just asking you to recognise that the aircraft does yaw when it changes heading (except in the extreme case of 90 degrees bank when it's all pitch).
Pick a point on the horizon. Wings level, kick the left rudder. We agree that the point moves left to right across the windscreen, as the aircraft yaws to the left, right? That's how the pilot sees yaw, as a movement left <-> right across the windscreen of that distant point.
Now roll into a balanced level left turn at 15 degrees of bank. Does the point move left to right across the windscreen in the turn? You bet it does. It moves a little bit downwards as well, but it certainly moves left to right, so the aircraft is yawing. That is the "flight dynamics definition of yaw".
... but when you demo the primary EoC you show them with the aircraft in the reference attitude (approx cruise) and then demo in a climbing turn that the elevator still affects pitch and that the rudder still yaws the aircraft left/right from the pilot's perspective and irrespective of the aircraft's attitude.
Pick a point on the horizon. Wings level, kick the left rudder. We agree that the point moves left to right across the windscreen, as the aircraft yaws to the left, right? That's how the pilot sees yaw, as a movement left <-> right across the windscreen of that distant point.
Now roll into a balanced level left turn at 15 degrees of bank. Does the point move left to right across the windscreen in the turn? You bet it does. It moves a little bit downwards as well, but it certainly moves left to right, so the aircraft is yawing. That is the "flight dynamics definition of yaw".
so we're in sync about the frame of reference; that's a relief.Presumably we agree that sideslip is the angle between the longitudinal axis and the relative wind, and that yaw is the angle between the longitudinal axis and the line of travel. With these definitions I agree that a balanced turn has yaw but I don't agree that this has any relevance to teaching someone EoC.
For instructional and pilotage purposes it seems perfectly adequate to teach that aileron deflection leads to roll which leads to bank which by itself leads to sideslip which leads to a yaw rate (because of directional stability) which all leads to ... a spiral dive. Later we introduce adverse yaw and explain that an aircraft turns because of the horizontal component of the lift vector. AFAIK this is the way its been taught since day one and the average stude understands easily and manages to use the information to fly an aircraft.
HFD
and that yaw is the angle between the longitudinal axis and the line of travel
The problem with using the "line of travel" is that it changes if the aircraft is going in anything other than a straight line. In physics terms, it's not an inertial frame.
Does it all matter from the point of view of explaining EoC to a student? Probably not, but then as I said at the start of the thread, I think the best and simplest explanation is roll => bank => slip => yaw to explain why you end up with a heading change after some aileron input.
Join Date: Jan 2006
Location: An island somewhere
Posts: 423
Likes: 0
Received 0 Likes
on
0 Posts
HFD
As bookworm said, yaw isn't the angle between the longitudinal axis and the direction of travel - in the yawing plane this angle reflects slip or skid (angle of attack in the pitching plane - well alright, that's chord line and direction of travel, but near enough!). It isn't really relevant to talk about yaw as being any angle ... it describes a motion about the normal axis, or a moment that may give rise to such a motion. Think about roll as an analogy - roll describes the motion about the longitudinal axis, bank angle describes the primary result (using an earth reference).
Your instructional 'brief' seems fine, as far as it goes. Unfortunately, it only covers the specific case where the angle of attack isn't increased sufficiently to enable a level turn to take place and which therefore results in a spiral dive. Quite a lot of this thread, however, has been concerned with what is happening in the level banked turn ... and that's where most of the misunderstanding has arisen. Furthermore, what about a climbing turn? In all cases, the aeroplane is yawing (and pitching) in the turn, and the ball may well be centred. How? Doesn't the student need to comprehend these other cases too?
As you said, your brief is the one that been used since time immemorial. Whilst not statistically significant, the number of incorrect descriptions that have been put forward here suggests that it's somewhat inadequate for a more-general understanding of the principles of flight as applied to turning!
As bookworm said, yaw isn't the angle between the longitudinal axis and the direction of travel - in the yawing plane this angle reflects slip or skid (angle of attack in the pitching plane - well alright, that's chord line and direction of travel, but near enough!). It isn't really relevant to talk about yaw as being any angle ... it describes a motion about the normal axis, or a moment that may give rise to such a motion. Think about roll as an analogy - roll describes the motion about the longitudinal axis, bank angle describes the primary result (using an earth reference).
Your instructional 'brief' seems fine, as far as it goes. Unfortunately, it only covers the specific case where the angle of attack isn't increased sufficiently to enable a level turn to take place and which therefore results in a spiral dive. Quite a lot of this thread, however, has been concerned with what is happening in the level banked turn ... and that's where most of the misunderstanding has arisen. Furthermore, what about a climbing turn? In all cases, the aeroplane is yawing (and pitching) in the turn, and the ball may well be centred. How? Doesn't the student need to comprehend these other cases too?
As you said, your brief is the one that been used since time immemorial. Whilst not statistically significant, the number of incorrect descriptions that have been put forward here suggests that it's somewhat inadequate for a more-general understanding of the principles of flight as applied to turning!
Last edited by Islander2; 30th Mar 2007 at 15:03.
Join Date: Jan 2006
Location: An island somewhere
Posts: 423
Likes: 0
Received 0 Likes
on
0 Posts
For F***'s Sake!!
Roll=>sideslip=>yaw (fin effect).
Simplistic, but adequate at PPL level.
BEagle BSc(Aero Eng)......just!
Roll=>sideslip=>yaw (fin effect).
Simplistic, but adequate at PPL level.
BEagle BSc(Aero Eng)......just!
Islander2 'O'-level physics......just!