Why is Yaw 2nd effect of Roll? (and explain Trim)

[blobber]

Why is Yaw 2nd effect of Roll?

and

explain how the trim works accurately and simply - why when we change airspeed does the plane need re-trimming and therefore why trim cannot be finalized until airspeed stabilizes?

I would like to know the most accurate and correct and understandable way of explaining these to a student.

Basically the most technically correct detailed explanation remastered into the most effective description

Thanks

[blobber]

the yaw thing is more tricky to explain/ understand than you think

[W.R.A.I.T.H]

Hello,

not an expert, and i'm already warming up for a party, so I really hope for someone wiser to come and offer a better explanation.

Re trim - the case boils down to the location of centre of lift, which is an acting point of the lift force, as integrated from lift distribution over a wing area. The point is also called the aerodynamic centre and as it is normally (reasonable regimes, conventional aircraft) located well behind the centre of grafity (of the aircraft), the resulting moment forces the plane nose down. That's why we've got the stabilizer back there. The point is that this aerodynamic centre, unlike the C/G, is nt constant, but linearly shifts backwards with growing angle of attack. The AoA, in turn, is directly related to amount of lift generated and hence to the airspeed. All summed up, an increase in speed results in a relocation of the aerodynamic centre, pitching moment changes and a retrim is desirable.
A cool web with animations here (http://selair.selkirk.bc.ca/aerodynamics1/Stability/Page7.html).


With the yaw-roll coupling, this is one of the more difficult bits in aerodynamics and further factors such as the dihedral and the sweep angle play a significant role here. Very much simplified, let's say an airplane yaws from a rudder input, to the right. Because of its dihedral, the underside area of the left wing is now exposed to the airflow in a relatively larger extent to that of the right wing, and also because of its sweep angle, more airflow now follows lengthwise along an airfoil section, than it does on the right wing. As a result of those two effects, the left wing now generates more lift than the right one and the plane banks to the right. The entire process is reversible and works the other way round just as well.

Anyway, if you guys manage to get even a glimpse of the concept from what i've just written, a great kudos to you. I would've never been able to understand the principle, even in the silliest version, from any amount of text. I did have, though, a set of pics drawn by a friend of mine (an Einstein reincarnation and a very hot babe :ok:), that helped a lot. I'll search them up and post, as soon as I recover from the hangover, which now appears inevitable.

[blobber]

Some good points well made there! just looking at the yaw thing. I was asking why we get yaw as a 2nd effect of roll, not roll as a 2nd effect of yaw. its different. the latter is partly and I think mainly because as you yaw the a/c the outer wing is travelling faster and thus more lift is created resulting in the roll. I seek an understanding of the first scenario

Cheers

[BEagle]

Roll=>sideslip=>yaw (fin effect).

Simplistic, but adequate at PPL level.

[ERIK C]

Power + Attitude (e.g. pitch) = Performance (ROC/ROD, speed)

Trim changes the neutral position of the elevator (pitch)

At constant power a pitch change will give you a certain ROC / ROD and the speed will decrease / increase

At constant pitch a power change will give you a certain ROC / ROD but at a constant speed

So you trim for a speed AND
Power makes the plane climb/descent

[blobber]

BEagle - its not good enough if you have a student who presses you for further explanation. Besides, if you roll gently to the left, the ball stays in the middle. If you roll with back pressure applied as you should, do you get yaw (without using rudder)?


18 Greens - Again, we are talking about Yaw as a 2nd effect of roll..Not the other way round.

Thanks

[DB6]

Roll/yaw as BEagle explains i.e. there is generally more side area (fin and fuselage) aft of the CoG, so if the aircraft is rolled the tail will tend to slip sideways slower than the nose hence yaw.
Blobber, you have to consider roll in isolation - if you introduce pitch (back pressure) then additional factors come into play and it all comes under 'Far Too Difficult'. In EoC 1, when demonstrating the secondary effect of roll, you cannot introduce back pressure or it all goes tits-up.
Trimming (in pitch) is necessary to offset any imbalance in the lift/weight and thrust/drag vectors: The thrust and drag vectors do not generally act on the same point in the vertical plane (e.g. Catalina, engines above fuselage in wings, or B737 below wings), thus when thrust is changed the thrust/drag coupling changes and retrimming is necessary. Changing airspeed changes the drag vector rather than thrust, but the principle is the same. Lift/weight similar but at right angles to thrust/drag and will generally change with airspeed and e.g. fuel burnoff.

[BEagle]

If you roll slowly, the sideslip velocity will be less and the resulting yaw will be less. The ball will remain almost central, but not precisely central.

KISS on EoC1!!

If the student wants to know more, research the full lateral stability quartic and go through each of the 16 elements in turn.......

[jamestkirk]

There is a book called 'flight without formulae'. i cannot remember who wrote it.

If you are interested in principles of flight without all the boring eqaution stuff, it's good.

[TopBunk]

'Flight Without Formulae' by A C Kermode (1970) ISBN 0-582-01377-1, or that is what it says in the front of mine.

[Rarely Dble Amber]

Im no Instructor and definatley not one for the theory, but I thought this one was simple.. or am I missing something. You roll.. the wing that goes up produces more lift, lift = induced drag. Thus induced drag on the wing going up "pulls" the wing back. The yaw is the result of the increased drag on the climbing wing.
The Trimming thing.. faster = more lift .. slower = less lift. speed stabilised therefore = good time to trim. :ok:
Just tell em.. "Dont bother trimming too much untill you are neither speedin up or slowing down, 'cos you'll just hafta do it all again onced you are doin neither"... If that doesn't draw a blank look, I dont know what would:hmm:

[DB6]

RDA, lovely description of adverse aileron yaw there mate :ok: . Not unfortunately the same as the secondary effect of roll - in fact it couldn't be more opposite!

[BEagle]

Agreed!











.

[homeguard]

The question is a good one and needs settling. Pretty easy to describe Yaw resulting from slip but as already mentioned it won't happen if height is maintained.
The student will and should demand an explanation as to why rudder is used in the same direction as Roll, if they are only shown Yaw resulting from slip. Adverse Aileron Yaw is one explanation but differential ailerons largely eliminate that on modern aircraft. The other effect is Adverse Yaw resulting from Skid. The aircraft is not on rails and will not turn as if it was. Therefore there is a degree of skid and adverse yaw as the lift force develops until the turning force is fully established. The sideways impact on the fuselarge and fin causes adverse yaw.

The trim tab is aerodynamic and sensitive to speed and therefore maintains speed. Until the chosen speed is stable it follows that you cannot trim to the speed. With single engine propellor aircraft propellor slipstream will also produces an effect from the trim tab and therefore to be considered. RPM must also be as required and stable

[ERIK C]

When you roll right, the plane will yaw left for a brief moment because the upgoing wing produces more lift AND drag. About a second later the nose will yaw inside the turn due to the 'fin' effect as explained above.

[bookworm]

BEagle - its not good enough if you have a student who presses you for further explanation.

So peel the onion by another layer. BEagle's explanation is perfectly robust to that:

Explain why the angle of bank causes the C of G of the aircraft to start moving in a direction at an angle to its longitudinal axis: roll -> bank -> slip

Now explain why moving in a direction at an angle to its longitudinal axis causes yaw: slip -> yaw

It really is that simple.

If you roll with back pressure applied as you should, do you get yaw (without using rudder)?

Yes, of course you do.

[jamestkirk]

well done for having a go

[Whopity]

In a perfectly balanced turn, is Yaw present?

[BillieBob]

Without yaw, the aircraft will not turn (except at 90 deg bank!) - do I get a credit for the next seminar, Whopity?

[John Farley]

Why is Yaw 2nd effect of Roll?


Because by definition yaw is merely the horizontal movement of the nose round the horizon which happens in any turn (which follows a roll).

(Some people seem to think you cannot yaw with the ball in the middle)

[BEagle]

Yes - isn't the further effect of roll really sideslip?

Something to be avoided in the bona-jet, I understand, JF!

[bookworm]

Yes - isn't the further effect of roll really sideslip?

It all depends on what we mean by further or secondary. One could argue that the consequences of the application of aileron for period of time is some combination of a roll, a dutch roll, a spiral divergence, a phugoid and a short-period AoA oscillation. The only issue is what proportion they come in. But that's rarely a help to the student who's still working out which limb to move which way to make the aeroplane do what he wants... ;)

[hugh flung_dung]

Now I always thought that yaw was defined as rotation about the yaw axis, and that the yaw axis was perpendicular to the lateral and longitudinal axes. So all this talk about the nose yawing around the horizon (except with wings level) and needing 90 degrees of bank to turn without yaw is all a little strange ... or have I got my definitions wrong?

HFD

[homeguard]

Yes Bookworm, thats the point. What do we do with our plates of meat?
Too often it would seem that as part of Ex4 Effects of Controls; Roll-slip-yaw-more roll-more yaw etc is demonstrated. However, adverse yaw is dealt with without development only during ex6 and taken for granted it would seem; do as I say and don't ask why, for you do not need to know. Hence no doubt the question of this thread.
It does need explaining/demonstrating but simply I agree. Lets have some ideas on which we can all agree.

As for yaw around a point in space - OK Yuh!. We are talking here about the movement around the Normal/Longitudinal/Lateral axis of the aeroplane and the further/secondary effects.

[BillieBob]

Let us assume, for the purposes of the question, that by 'turn' we mean a change in heading or direction. In that case, if the aircraft's wings are level with the horizon a turn can be achieved only by yawing the aircraft around its normal axis (which is at 90° to the horizon). On the other hand, if the aircraft is at 90° angle of bank a turn can be achieved only by pitching the aircraft around its lateral axis (which is at 90° to the horizon). Consequently, if the aircraft is in a banked attitude between 0° and 90° any turn must be a combination of pitch and yaw.

[John Farley]

Well done BillieBob for combining piloting and aerodynamics!

[rondon9897]

Why is Yaw 2nd effect of Roll? (and explain Trim)
Why is Yaw 2nd effect of Roll?

You need to qualify that statement, yaw is the secondary effect of UNCORECTED roll

When the aircraft is banked the resultant of lift is inclined away from the vertical producing a decrease in the lift required to maintain level flight. The loss of lift causes the nose to fall towards the downgoing wing which can be seen as yaw. Corrected roll, whereby level flight is maintained by an increase in CC back pressure does not produce the secondary effect.(needs supporting diagram or model)

explain how the trim works accurately and simply - why when we change airspeed does the plane need re-trimming and therefore why trim cannot be finalized until airspeed stabilizes?

Speed is directly proportional to the production of lift so when airspeed is changed this produces a change in lift which either produces a nose up moment or nose down moment, (normally speed up, nose up and vice versa) If the pilot attempts to maintain the original attitude this trim change is felt through the CC by the pilot as a loading. This loading can be removed by aerodynamically producing an equal and opposite force by moving a small aerofoil surface known as a trim tab.(needs supporting diagram or model). As the speed changes so generally will the trim loading , when the speed stablises the trim loading change will remain constant.

I would also mention that the Tiger Moth has a trim wheel which uses a mechanical opposite force EG a big spring! This helps to explain force.

The above are very simple explanations and I am amazed there have been so many posts without adequate simple replies.

ASK CAPTAIN JON

[bookworm]

The loss of lift causes the nose to fall towards the downgoing wing which can be seen as yaw.

Why does a loss of lift, presumably at the centre of gravity of the aeroplane, cause the nose to drop? Why doesn't the aeroplane simply "drop" in the same orientation as it started?

[jamestkirk]

As gravity acts vertically downwards the aircraft decends with the resulting airflow hitting (for want of a better word) the fin area, with the fuselage acting as a lever arm, which in turn yaws the nose into the turn.

There is coupled with this the loss of lift in a turn due to lift acting at 90 degrees to the relative airflow. Actually, to explain that better than i ever could here. If you have a flight training manual 1 (PPL air pilots), go to medium level turns and look at the forces in a turn section.

[bookworm]

As gravity acts vertically downwards the aircraft decends with the resulting airflow hitting (for want of a better word) the fin area, with the fuselage acting as a lever arm, which in turn yaws the nose into the turn.

Exactly. In other words, it slips, which in turn causes the yaw -- exactly as BEagle described at the beginning of the thread. And what's more that still happens if you apply back-pressure. You can only avoid the slip by applying rudder, whether it's a level turn or not!

[rondon9897]

So then Bookworm you always use right rudder when going into a left level turn.




ASK CAPTAIN JON

[bookworm]

So then Bookworm you always use right rudder when going into a left level turn.

No. Left turn, left rudder.

[rondon9897]

Quote:
As gravity acts vertically downwards the aircraft decends with the resulting airflow hitting (for want of a better word) the fin area, with the fuselage acting as a lever arm, which in turn yaws the nose into the turn.
Exactly. In other words, it slips, which in turn causes the yaw -- exa.cnctly as BEagle described at the beginning of the thread. And what's more that still happens if you apply back-pressure. You can only avoid the slip by applying rudder, whether it's a level turn or not!

The quoted example above which you, Boookworm, say you agree with is talking about into turn yaw--you cannot prevent into turn yaw by pressing the rudder on the same side of the turn that just increases the yaw. You are confusing adverse aileron yaw with the further effects,. It dosn't happen if you apply back pressure and keep the aircraft level and what you will be demonstrating then is the effect of adverse aileron yaw.

An aircraft that is disturbed about it horizontal axis (without any aileron movement) will yaw towards the downgoing wing and produce no adverse aileron yaw.

[Islander2]

You are confusing adverse aileron yaw with the further effects,. It dosn't happen if you apply back pressure and keep the aircraft level and what you will be demonstrating then is the effect of adverse aileron yaw.I think you will find it is you that are confused, rondon, not bookworm who almost certainly is not talking about the use of rudder to avoid adverse aileron yaw!

To turn the aeroplane, it has to yaw (other than, as billiebob said earlier, in a level turn at 90deg bank angle - but since that's rather hard to sustain it can be ignored!). To yaw, it either has to have some small amount of sideslip, even in level flight, for the fin to provide a yawing force ... or a small amount of in-turn or bottom rudder is necessary to achieve the same effect.

[bookworm]

The quoted example above which you, Boookworm, say you agree with is talking about into turn yaw--you cannot prevent into turn yaw by pressing the rudder on the same side of the turn that just increases the yaw.

As Islander says, we need the aeroplane to yaw for it to turn (turn normally, at least). The object of using some into-turn rudder is not to prevent it, but to provide the yawing moment necessary for it rather than having that yawing moment provided by reaction to sideslip. The amount of rudder required (at least at high speeds and short wingspans) is typically small and shouldn't be confused with the use of the rudder to counter "adverse aileron yaw".

[jamestkirk]

Bookworm

It's now a long thread so I don't remember what BEagle wrote. Although, if BEagle and I agree, then great.

I was only trying to help.

[rondon9897]

NASA seem to share my confusion QUOTE

As long as the aircraft is banked, the side force is a constant, unopposed force on the aircraft. The resulting motion of the center of gravity (http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html) of the aircraft is a circular arc (http://www.grc.nasa.gov/WWW/K-12/airplane/objmotion.html). When the wings are brought level by an opposing motion of the ailerons, the side force is eliminated and the aircraft continues to fly in a straight line along a new heading. Notice that the rudder (http://www.grc.nasa.gov/WWW/K-12/airplane/rud.html) is not used to turn the aircraft. The aircraft is turned through the action of the side component of the lift force. The rudder is used during the turn to coordinate the turn, i.e. to keep the nose of the aircraft pointed along the flight path. If the rudder is not used, one can encounter an adverse yaw in which the drag on the outer wing pulls the aircraft nose away from the flight path.

http://www.grc.nasa.gov/WWW/K-12/airplane/turns.html

------------------------------------------------------------

AEROSPACE WEB share my confusion too

All this talk of anhedral and dihedral leads to the question of why one would want use either of these on an aircraft. The simple answer is they provide lateral (roll) stability. Let's consider an aircraft rolling to the right. As it does so, the right wing produces more lift than left wing, causing the rolling motion. At the same time, however, this increased lift creates an increased drag, which causes the aircraft to yaw to the left, an effect known as adverse yaw (http://www.aerospaceweb.org/question/dynamics/q0045.shtml). This is why pilots need to apply rudder in the direction of the turn.
http://www.aerospaceweb.org/question/dynamics/q0055.shtml
----------------------------------------------------------------------------------------------------------------------

Gliding New Zealand share my confusion too

http://www.gliding.co.nz/Operations/Training/notes/acert/009%20Effects%20of%20Control%20_Secondary_.pdf

Your right I am a very confused person but what confuses me most is how some people on here can be flying instructors

[hugh flung_dung]

To turn the aeroplane, it has to yaw (other than, as billiebob said earlier, in a level turn at 90deg bank angle - but since that's rather hard to sustain it can be ignored!). To yaw, it either has to have some small amount of sideslip, even in level flight, for the fin to provide a yawing force ... or a small amount of in-turn or bottom rudder is necessary to achieve the same effect.
As 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.
Yaw is not required in order for the aircraft to change heading EXCEPT when the wings are level.
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.
Yaw results from ROLL because of differential drag caused by aileron deflection. 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.

HFD

[Islander2]

Yaw is not required in order for the aircraft to change heading EXCEPT when the wings are level.You will need to understand why that's completely wrong before any comprehension of the aerodynamics of turning is possible.

See the earlier posts from John Farley and billiebob. Also, find a good book on the principles of flight that gives a vector diagram for a turn showing the relative contributions of pitching moment and yawing moment. All normal turns are a combination of pitching and yawing ... a bank angle is merely what enables these to occur (and if they didn't occur, the aeroplane would just slide sideways without turning).

[rondon9897]

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

[rondon9897]

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!

[Islander2]

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.Oh well, you can take a horse to water, but ........

I give up! Try reading around the subject.

Also, might I suggest your footer slogan is a touch inappropriate?

[Islander2]

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.One of the fundamental mistakes you are making is to equate ball position with yaw.

[rondon9897]

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

[hugh flung_dung]

Islander2:
You will need to understand why that's completely wrong before any comprehension of the aerodynamics of turning is possible. 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

[Islander2]

For the purpose of PPL, CPL & ATPL training using the ball as an indicator of yaw is perfectly adequateNot only isn't it perfectly adequate, it is completely wrong! The ball is an indicator of slip or skid, not yaw. A balanced turn with a high rate of yaw and the ball in the middle is readily demonstrable.

[bookworm]

You will need to understand why that's completely wrong before any comprehension of the aerodynamics of turning is possible.

If I could put that slightly less confrontationally...

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 (http://en.wikipedia.org/wiki/Tait-Bryan_angles) 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.

[hugh flung_dung]

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

[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.

I also agree, but from an instructional point of view the worst thing you can do is teach the pilot a non-standard definition of the word "yaw".

[hugh flung_dung]

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

[Islander2]

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.

[bookworm]

Thanks Islander2, I agree.

... 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.

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".

[hugh flung_dung]

:ok: 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

[bookworm]

and that yaw is the angle between the longitudinal axis and the line of travel

Not the "line of travel", no. I'm not sure why that would ever be different from the direction of the "relative wind", unless you're trying to distinguish between velocity over the ground and through the air.

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.

[Islander2]

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!

[BEagle]

For F***'s Sake!!

Roll=>sideslip=>yaw (fin effect).

Simplistic, but adequate at PPL level.

BEagle BSc(Aero Eng)......just!

[DB6]

:ok: :ok: :ok: :ok: :ok: :ok: :ok: :ok: :ok: :ok: love it!
Definitely with BEagle on this one. I was beginning to lose the will to live.

[Islander2]

For F***'s Sake!!

Roll=>sideslip=>yaw (fin effect).

Simplistic, but adequate at PPL level.

BEagle BSc(Aero Eng)......just!

Thought that was what I was saying! Merely emphasising, for those that had expressed a different view, that the mechanism is identical in a level or climbing turn. Sorry to have bored.

Islander2 'O'-level physics......just!

[jamestkirk]

where does the sideslip come from?

[jamestkirk]

CSE - home economics, grade d

[hugh flung_dung]

BEagle, succinct and perceptive as always - couldn't agree more.

HFD (BSc, and a bit more)

[blobber]

I think its an interesting topic that isn't 100% clear cut which is why I asked the question in the first place. It's easy to explain on PPL level, but going into it further seems to cause alot of debate.
On a simplistic level, I say that when you roll (turn) the aircraft, the reletive air is now hitting the fuselage and fin which causes the yaw. It just seems that if you pitch up, this yaw doesn't occur but pitching up is not meant to prevent yaw is it?

Blobber 'C & G in Personal Development and Achievement' (Princes Trust)

[BillieBob]

I say that when you roll (turn) the aircraft...... Here we go again with the sloppy terminology. Do you mean roll or do you mean turn? The two are not synonymous - you can roll an aeroplane without turning it just as you can turn an aeroplane without rolling it.

To get back to basics, we shouldn't be referring to the further effect of roll in any case but the further effect of aileron - Effects of Controls, remember?

If, in straight and level unaccelerated flight, the aileron is displaced and none of the other controls is moved, the aircraft will roll with the result that the lift vector will be displaced from the vertical causing sideslip. The directional stability of the aircraft will cause yaw and, if uncorrected, the aircraft will enter a spiral descent.

Roll => Sideslip => Yaw. What else is there to explain?

BB - No degree, just CFS A1/A2(H)

[Islander2]

If, in straight and level unaccelerated flight, the aileron is displaced and none of the other controls is moved, the aircraft will roll with the result that the lift vector will be displaced from the vertical causing sideslip. The directional stability of the aircraft will cause yaw and, if uncorrected, the aircraft will enter a spiral descent.

Roll => Sideslip => Yaw. What else is there to explain?Oh dear, having already incurred the wrath of the invigilator, this is probably unwise.:*

However! :rolleyes: Judging from many of the contributions on this thread, what else seems to need explaining is the mechanism for 'correcting the yaw to prevent a spiral descent' (if indeed that is what one does ;))

Islander2......11+ (marginal pass)

[BigEndBob]

In my mind the secondary or further effect of aileron is adverse aileron yaw.

The effect of being 'banked' is sideslip leading to yaw towards the lower wing, as might occur in turbulence.

If i roll an aircraft from being banked to straight wing level flight does yaw occur?
Adverse aileron yaw does.

What happens on say a MU-2 with wing spoilers for roll contol.

[blobber]

Here we go again with the sloppy terminology. Do you mean roll or do you mean turn? The two are not synonymous - you can roll an aeroplane without turning it just as you can turn an aeroplane without rolling it.


It isn't sloppy terminology to say Roll (turn). We use roll to turn the aircraft, we use the rudder to purely maintain balance. When do you roll and not turn?

..the aircraft will roll with the result that the lift vector will be displaced from the vertical causing sideslip.

You haven't explained sideslip at all. all you have stated is that the lift vector is displaced from the vertical which causes sideslip. This results in a movement about the longitudinal axis and causes a change of direction (not the sideslip). Sideslip is caused by that subsequant change of direction in that the relative airflow is now causing the aircraft to turn about the normal axis due to its new path in relation to the fuselage and fin.

The directional stability of the aircraft will cause yaw and, if uncorrected, the aircraft will enter a spiral descent.


Really? So directional stability causes yaw in the direction of the roll? I always thought that directional stability creates yaw in the opposite direction due to there being more lift and therefore more drag on the upward moving wing causing yaw towards that wing (opposite to the turn)

[Islander2]

For F***'s Sake!!

Roll=>sideslip=>yaw (fin effect).

Simplistic, but adequate at PPL level.And so clearly understood by ALL pilots as a result! I rest my case.

[BillieBob]

Blobber - Your total lack of uderstanding of the most basic principles of flight is utterly astounding. Please, please tell me that you are not an instructor.

When do you roll and not turn? - In any properly executed slow roll

I always thought that directional stability creates yaw in the opposite direction [to the roll] I think you might be well advised to quit before you make yourself look an even bigger fool.

[blobber]

Mate, you're taking the piss. Go and crawl under your rock you arrogant ****. I believe I have made a mistake in what I have said about directional stability I was thinking of something else, but so what! People are entitled to make a mistake about something about a subject they dont use every day. As for for the roll issue, I think you are just being an arse. I doubt you would talk that that to anyones face, and if you did to me kick **** out of you :)

[rusponge]

I have not read this post, cause it bores me……..boot the rudder what happens????????????????????????????????its the basics!!!! the aircraft yaws about the normal axis then it rolls…..every aircraft I have tried does the SAME THING!!

[Islander2]

I have not read this post, cause it bores me……..boot the rudder what happens????????????????????????????????its the basics!!!! the aircraft yaws about the normal axis then it rolls…..every aircraft I have tried does the SAME THING!!Well, if you try reading this post, you'll find that wasn't the question!!

[FL8]

Now, before anyone bites my head off, I'm not challenging anything that's gone before!! And apologies for prolonging what looks to have been an agonising debate... :ugh: This is one I've always had a bit of difficulty with, and now I have to explain it to babiators.

Ex 4.1 is effects of controls, not effects of attitude.

We all (presumably?) agree that there is no secondary effect of pitching with elevator. Certainly that is what I have been taught to teach. Change in IAS and ROC is a function of the pitch attitude we find ourselves in. We use elevator to achieve a given pitch attitude, but we don't consider what happens as a result of that pitch attitude to be a secondary effect of the controls.

Is not yaw which results from sideslip which results from bank the same thing, ie a result of aircraft attitude rather than a secondary effect of the controls?

In which case, is not Wikipedia's answer (though I hesitate to admit it) mentioned above the most accurate, in that while changing our angle of bank by rolling with aileron, we create sideways lift on the vertical stabliser / fin?

I admit that it would be bloomin difficult to demonstrate. In my experience the yaw is only noticeable once established at an angle of bank with no elevator input.

Come to think of it, none of my babiators have ever shown much interest in why, so maybe it isn't that important. On the other hand none of them seem too interested in flaring at the mo, so that's possibly no guide!

[VFE]

We all (presumably?) agree that there is no secondary effect of pitching with elevator.

Airspeed decreases and houses change shape.

What more can one say?

Note to ones self: do stay in touch VFE!!!! Again, have I missed something apart from most of this thread? ;)

'K.I.S.S.' anyone?

VFE.

[bookworm]

We all (presumably?) agree that there is no secondary effect of pitching with elevator.

This all comes down to semantics: what do we mean by secondary effect? If you restrict "secondary effect" to the moments associated with the control application itself, then these are limited: yawing moment from aileron drag, roll moment from rudder application, yawing moment from power application in a single.

If by "secondary effect" you mean consequences, then... well, see above! I would suggest that looking at the consequences is more useful for the student.

[FL8]

All very interesting. And it does presumably (must stop presuming things!) suggest my FIC instructor was being inconsistent describing the yaw resulting from sideslip as a secondary effect of controls but categorically refusing to say the same about the shrinking sheep and plummetting ASI needle resulting from elev application.

Semantic, I agree, but thought provoking. Thanks for the (control) input.

[ghostwhowoks]

All simplifications loose detail and accuracy, how much you are willing to loose and simplify depends on how much detail the student needs to understand what they are doing. It is fine to simplify for ab initio students roll=>sideslip=>yaw (fin effect) as succinctly stated by BEagle. However for advanced manoeuvres (aeros ie vertical rolls, STOL landing in high tail aircraft, spinning, etc) a more accurate understanding of effects of controls and aerodynamics is required to understand what inputs are required when. As an instructor you must know when to simplify the explanation, but not confuse the simplified explanation with the facts. Instructors should have the detailed, correct understanding required to expand simplified explanations if the students asks for more or requires more for advanced sequences.

While not important for basic student to understand that during a banked turn the aircraft is both yawing, pitching and rolling, instructors certainly should know it, and they should not deny the fact if queried. Otherwise you will finish up with very confused students and perpetuate some of the mis-information evident in this thread. :=

The original question was "Why is Yaw 2nd effect of Roll?". Answers for basic students are:

Adverse yaw is secondary effect of aileron roll due to increase in drag on wing going up.
Into turn yaw is secondary effect of bank due to sideslip and directional stability.

Expanded answer for inquisitive students to the second "secondary effect" goes something like this:

In a bank the lift vector is tilted sideways. Resolving it into vertical and horizontal components means there is a small vertical component of weight remaining, and a horizontal component in the direction of the bank. When combined these result in a sideslip vector toward the low wing. Directional stability caused by change in AOA on the tail fin results in into turn yaw. Note that increasing the wing loading with back stick to maintain the vertical component of lift equal to weight (and therefore mantain level), still results in a sideslip component, and therefore into bank yaw (and also gives us a pitch rate). The combination of the roll, yaw and pitch result in a turn. In a conventional design (dihedral and/or sweep back for roll stability) in a steady turn (constant rate of roll, pitch and yaw), there is a requirement to hold some into turn aeileron to maintain the AOB. This causes a (slight) adverse yaw, requiring into turn rudder to remain balanced.

More detail is required to explain variations caused by delta wing, high tails, spoilers, canards, high/low wing, etc, but there isn't room here.

Is that clear? :confused:

Edit: left out the constant pitch rate in steady turn.

[ghostwhowoks]

Because the rate of roll in a steady level turn is very small, the adverse yaw associated with it is much less than when applying a large control input to demonstrate adverse yaw. But it is still there, and without designs such as differential ailerons and horns, some slight rudder will be required to remain balanced. Don't confuse this with there is NO rudder required in a steady turn. := If rudder is not used, there will be a slight out of bank yaw, however this will be masked by releasing the back stick very slightly to remain level (but the aircraft will not be exactly balanced, and the rate of turn will be slightly lower than for a balanced turn).
:E

[apollo85]

DONT EVERY FORGET:

POWER + ATTITUDE = PERFORMANCE

IF POWER REMAINS CONSTANT AND YOUR AIRSPEED IS CHANGING:

FOR 90KTS YOU WILL NEED A LOWER NOSE ATTITUDE TO REMAIN LEVEL, AND 80KTS YOU WILL NEED A SLIGHTLY HIGHER NOSE ATTITUDE AND AT 70KTS YOU WILL NEED A SLIGHTLY HIGHER NIOSE ATTITUDE AGAIN TO REMAIN LEVEL.

THE REASON WE TRIM IS TO NOT ONLY RELIEVE LOADS ON THE CONTROL COLUMN BUT TO SET THE REQUIRED AOA(I.E WE SEE ATTITUDE) WE NEED.

SO SIMPLY: AS SPEED CHANGES WE NEED A CHANGE IN ATTITUDE >> WE NEED TO RE TRIM.

HOPE THAT HELPSq!

[ghostwhowoks]

Sorry Apollo, but I beg to differ. You should never trim to set attitude. You should set the attitude, then trim to remove control forces. Is this just being pedantic? No.

If you use trim to set the 70 kt attitude while doing 90 kts, the aircraft pitching moment due to moving centre of pressure will require you to continue to trim to maintain the attitude as the speed decreases. If you don't the attitude will drop and the airspeed will finish up stabilised well above 70 kts. It is also a slow and inaccurate way to set an attitude, making it inefficient except for very small speed changes.

Remember: change, check, hold then trim!

Cheers

Edit: added old flying dictum

[apollo85]

ummm yeah!!! realise that..
if you read the orginal thread the guy is asking why we dont! trim while airspeed is changing!

try to read between the lines a bit....

: "AS SPEED CHANGES WE NEED A CHANGE IN ATTITUDE >> WE NEED TO RE TRIM."

that is >>> re-trim (once you have set desired attitude as a result of airspeed change).... not trim why our attitude changes!

[ghostwhowoks]

Your original post:

THE REASON WE TRIM IS TO NOT ONLY RELIEVE LOADS ON THE CONTROL COLUMN BUT TO SET THE REQUIRED AOA(I.E WE SEE ATTITUDE) WE NEED.

Trim is not to set either AOA nor attitude (which are not the same). The reason we trim is to relieve loads on the control column. That was my point.

Except of course if you lose all your flight controls and all you have left is trim and engine power! Have a look at this incident http://www.strategypage.com/respect/articles/military_20041031.asp. These guys know why secondary effects of controls are importance!

Cheers

Edit: added link to incident report.

[apollo85]

think about why u want to fly "hand off"....!?!?!?!?!??!?!

yes the effect it has in the cockpit is to relieve the pressure on the control column.... but this is because it makes it easier for us to maintain the desired attitude!!!

we set our pitch attitude through the elevator - - - - therefore if we balance the lift and drag through this control surface (with the trim tab) we will maintain our desired ATTITUDE by maintain our desired ANGLE OF ATTACK!!!

..... (it isnt just so you can have nice floppy wrist and fly without getting a wrist ache!!!!)

[apollo85]

MAYBE ITS TIME TO GET THE NOGGIN BACK INTO THE OLD BASIC AEROKNAUTICAL KNOWLEDGE CHAMP!!!

THEY ARE ALL RELATED>>BE IT THEY ARENT THE SAME

YOU MOVE YOUR ELEVATOR>> YOU CHANGE YOUR ATTITUDE>> YOUR WINGS WILL HAVE A DIFFERENT ANGLE OF ATTACK!!!>

:ugh:

[ghostwhowoks]

You say "therefore if we balance the lift and drag through this control surface"
Please enlighten me how we balance lift and drag? :confused:

Why is attitude not the same as AOA? Well if you select a higher attitude and increase power to maintain airspeed is your AOA more or less?

BTW, do you have a problem with your caps lock key? :(

[bothok_teri]

1. If the air craft is banking, your rudder will produce lift, so your plane will yaw
2. flight principles, just demonstrate to your student when straight and level flight add power so in a moment the speed will increase and your plane will climb, after that your speed will decrease same as straight and level.... thats called trimmed speed, :=

[ghostwhowoks]

bothok_teri wrote:
1. If the air craft is banking, your rudder will produce lift, so your plane will yaw
Rudder will only produce lift if either a rudder input from neutral is made or the aircraft is sideslipping.

roll->bank->sideslip or rudder->yaw

[bookworm]

In a conventional design (dihedral and/or sweep back for roll stability) in a steady turn (constant rate of roll, pitch and yaw), there is a requirement to hold some into turn aeileron to maintain the AOB. This causes a (slight) adverse yaw, requiring into turn rudder to remain balanced.

In a balanced turn, the rolling tendency is into-turn, not out-of-turn. Hence out-of-turn aileron would be required. The need for rudder in a steady balanced turn comes from the desire to eliminate the sideslip (which you have correctly identified as contributing to the into-turn yaw).

If the turn is not properly balanced and the sideslip is allowed to persist, the slip-roll coupling (dihedral, sweepback or fuselage position effects) will indeed cause an out-of-turn roll. But in most conventional aircraft it won't exceed the into-turn rolling tendency, otherwise the spiral mode would be stable.

[jamestkirk]

Someone please close this thread before we need therapy, a hug, the samaritans or a painless way to take our own lives.

Or all the above in any order:O

[bookworm]

Someone please close this thread before we need therapy, a hug, the samaritans or a painless way to take our own lives.
One to beam up Scotty, easy on the yaw... ;)

[ghostwhowoks]

the rolling tendency is into-turn, not out-of-turn. Hence into-turn aileron would be required
Isn't that bar$e ackwards?

But you are correct, the sideslip is eliminated by rudder in the balanced turn. After blowing some cobwebs off I remembered it is the increased AOA on the down going wing (constant roll rate) that causes the out of turn roll, and therefore requires into turn aileron.

To those voting closure, you don't have to keep coming back to read it, you know. Some of us weird types find aerody interesting. :} Trying to explain it makes you remember the bits you forgot (if that makes sense). :O

[jamestkirk]

Khaaaaaaaaan! Khaaaaaaaan!

[bookworm]

Isn't that bar$e ackwards?.

Yes :blush: I've corrected it above. I simply didn't write what I was thinking and stand by my point.

After blowing some cobwebs off I remembered it is the increased AOA on the down going wing (constant roll rate) that causes the out of turn roll, and therefore requires into turn aileron.

If the aircraft were rolling, yes, that's roll damping. But in a level balanced turn it is not rolling.

[casccadeair]

When the yoke is moved to roll the a/c, the downward moving aileron generates more lift, and consequentlhy more drag. The upward moveing aileron generates less drag, because producing lift produces drag, and vice-versa. Thus as the a/c rolls, the ascending wing lags behind the axis and yaws the aircraft. Very Simple.

[BEagle]

A plausible description of adverse aileron yaw, but nothing whatsoever to do with the further effect of roll.....

ROLL>>SIDESLIP>>YAW

Is teaching and learning of such utterly basic aerodynamics really so pi$$ poor in today's so-called training world?

[BillieBob]

If the secondary effect of aileron was yaw in the same direction as the intended roll, when you roll wings level, the aircraft should yaw to the left. It wont thoughOh, yes it will! If, from a level balanced 30° AOB turn to the right, left aileron is applied (to roll the aircraft towards wings level) without any movement of the rudder, the aircraft will also yaw to the left as a consequence of the applied aileron.