Sendary effect of rudder

[BOAC]

Originally Posted by Andy-P

why does an anhedral wing roll opposite to yaw

- it doesn't - it rolls due to sideslip. There is a danger of getting confused here. We must be careful how we define 'yaw' - it is simply a rotation around the vertical axis. In pure aerodynamic terms, an/dihedral have no effect on rolling moments due to yaw. They DO have an effect on rolling moments due to sideslip (motion along the lateral axis) CAUSED BY YAW which is where your 'conundrum' occurs. There, the roll due to sideslip is the greater force. (If we are not careful we will start Dutch rolling....)

The ultimate test would be to rotate two models with an- and di- hedral around the vertical axis - with no forward motion - and see that there is indeed a roll couple due to wing 'speed' difference and this would not differ between the two models.

Conversely, presenting both models at a fixed yawed angle to an airsteam will produce a rolling couple purely due to wing setting with zero 'yaw' input.

Never realised flying were so complukated.....

[Andy_P]

Ahh see I thought yaw is rotation about the normal axis in motion, so perhaps my terminology is wrong . However, you are correct. Perhaps the easiest way to describe it is by saying the aircraft is yawed relative to the airflow.

Either way, the myth that the outside wing is travelling faster and generating more lift is plain wrong and is not the reason for the secondary effect of the rudder.

That link I provided earlier is an excellent read. The guy dispels a bunch of common myths that are incorrectly taught to students like myself. I have only just started to read it, but I have already learned a heap from it. Useful given I am about to sit my BAK exam.

[BOAC]

Either way, the myth that the outside wing is travelling faster and generating more lift is plain wrong and is not the reason for the secondary effect of the rudder.

- no it is not plain wrong. It is part of the reason. It is that the roll due to sideslip and wing blanking is normally predominant.

You are in practical terms correct about 'yaw' since unless you are in VSTOL mode, there will always be translational motion of some sort and the moment you stat 'yawing' you start sideslipping.

While we are being 'technical', this discussion is about the effect of yaw, not the 'secondary' effect of rudder! The secondary effect of 'rudder' is indeed roll, but caused by the sideforce generated by the rudder being applied above (or below) the roll axis and generally insignificant. The roll we are talking about is generated by both the yaw AND the subsequent sidelsip in differing amounts.

If you flew on a constant heading with crossed control (ie with sideslip) there will be a roll couple of either sign due to an-/di-hedral and any other 'vertical' surfaces and obviously there is no speed difference between the wings.