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.


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 ), 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]

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.

[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.
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

[DB6]

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

[BEagle]

Agreed!











.

[homeguard]

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

[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]

rarely dble amber

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?