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

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 24th Mar 2007, 18:48 #1 (permalink) Join Date: Jan 2007 Location: uk Age: 37 Posts: 42 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 Last edited by blobber; 24th Mar 2007 at 18:49. Reason: typo
 24th Mar 2007, 20:04 #2 (permalink) Join Date: Jan 2007 Location: uk Age: 37 Posts: 42 the yaw thing is more tricky to explain/ understand than you think
 24th Mar 2007, 20:22 #3 (permalink) Join Date: Aug 2006 Location: pub Age: 30 Posts: 69 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.
 24th Mar 2007, 21:37 #4 (permalink) Join Date: Jan 2007 Location: uk Age: 37 Posts: 42 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
 24th Mar 2007, 22:20 #5 (permalink) Join Date: May 1999 Location: Quite near 'An aerodrome somewhere in England' Posts: 20,216 Roll=>sideslip=>yaw (fin effect). Simplistic, but adequate at PPL level.
 25th Mar 2007, 00:53 #6 (permalink) Join Date: Nov 2006 Location: UK Age: 43 Posts: 33 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
 25th Mar 2007, 07:04 #7 (permalink) Join Date: Jan 2007 Location: uk Age: 37 Posts: 42 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 Last edited by blobber; 25th Mar 2007 at 07:05. Reason: typo
 25th Mar 2007, 07:18 #8 (permalink) Join Date: Apr 1999 Location: Dundee, Scotland Age: 50 Posts: 1,195 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. Last edited by DB6; 26th Mar 2007 at 15:32. Reason: to add a bit
 25th Mar 2007, 07:56 #9 (permalink) Join Date: May 1999 Location: Quite near 'An aerodrome somewhere in England' Posts: 20,216 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.......
 25th Mar 2007, 09:36 #10 (permalink) Join Date: Oct 2004 Location: UK Posts: 660 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.
 25th Mar 2007, 09:56 #11 (permalink) Join Date: Dec 2000 Location: on the golf course Posts: 1,931 'Flight Without Formulae' by A C Kermode (1970) ISBN 0-582-01377-1, or that is what it says in the front of mine.
 25th Mar 2007, 10:01 #12 (permalink) Join Date: Aug 2005 Location: Australia Posts: 27 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
 25th Mar 2007, 10:42 #13 (permalink) Join Date: Apr 1999 Location: Dundee, Scotland Age: 50 Posts: 1,195 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!
 25th Mar 2007, 13:10 #14 (permalink) Join Date: May 1999 Location: Quite near 'An aerodrome somewhere in England' Posts: 20,216 Agreed! .
 25th Mar 2007, 15:40 #15 (permalink) Join Date: Nov 2003 Location: notts Posts: 633 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
 25th Mar 2007, 20:28 #16 (permalink) Join Date: Nov 2006 Location: UK Age: 43 Posts: 33 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.
26th Mar 2007, 13:48   #17 (permalink)

Join Date: Aug 2000
Location: UK
Posts: 3,495
Quote:
 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.

Quote:
 If you roll with back pressure applied as you should, do you get yaw (without using rudder)?
Yes, of course you do.

 27th Mar 2007, 08:36 #18 (permalink) Join Date: Oct 2004 Location: UK Posts: 660 rarely dble amber well done for having a go
 27th Mar 2007, 14:06 #19 (permalink) Join Date: Oct 2004 Location: Europe Posts: 3,888 In a perfectly balanced turn, is Yaw present?
 27th Mar 2007, 15:18 #20 (permalink) Join Date: Aug 2002 Location: United Kingdom Posts: 1,963 Without yaw, the aircraft will not turn (except at 90 deg bank!) - do I get a credit for the next seminar, Whopity?

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