Flying Instructors & ExaminersA place for instructors to communicate with one another because some of them get a bit tired of the attitude that instructing is the lowest form of aviation, as seems to prevail on some of the other forums!
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.
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
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.
Last edited by blobber; 25th Mar 2007 at 07:05.
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
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
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.