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

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

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

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.


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.

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

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

Simplistic, but adequate at PPL level.

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

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

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.

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.......

blobber
 

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.

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

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:

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!

Agreed!











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Adverse YAW
 

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

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.

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.

Yes, of course you do.

rarely dble amber
 

well done for having a go

In a perfectly balanced turn, is Yaw present?

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

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)

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

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

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... ;)

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

What is our roll?
 

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.

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.

Well done BillieBob for combining piloting and aerodynamics!

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

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

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?

bookworm-trying to be clear, so sorry of i am not
 

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.

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!

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




ASK CAPTAIN JON

No. Left turn, left rudder.

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.

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.

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".

what have i done!
 

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.

Confusion
 

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 of the aircraft is a circular arc. 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 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

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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. This is why pilots need to apply rudder in the direction of the turn.
http://www.aerospaceweb.org/question...cs/q0055.shtml
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Gliding New Zealand share my confusion too

http://www.gliding.co.nz/Operations/...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

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

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).