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-   -   Yaw/roll - affect on heading (https://www.pprune.org/private-flying/528720-yaw-roll-affect-heading.html)

Maulkin 27th Nov 2013 23:04

Yaw/roll - affect on heading
 
Hi all,

I've recently started my PPL, and am slightly confused as to the use of yaw - I understand the basics, but am struggling to understand how way and roll interact.

I get the secondary effects of ailerons and rudder, but I'm not entirely sure how side slip can work - if inducing yaw, and maintaining straight and level flight, how does the aircraft not change heading? Similarly, when inducing roll, and correcting with the rudder, why does the plane bank?

Surely (well, obviously not!) the aircraft would remain in direction of flight due to the forward vector produced by the thrust of the engine, and similarly changing the direction of the thrust with yaw - the plane should change direction!

What am I missing?

Thanks,
Neil

Dash8driver1312 27th Nov 2013 23:35

Yaw/roll - affect on heading
 
For the side slip, try thinking of it like this...with left aileron and right rudder (for example) the lift vector is tilted to the left, so the aircraft is constantly turning to the right (you are after all producing more lift with the main plane than the tail to stop the nose dropping, right, so less lift at the tail...) and the right rudder is making the aircraft nose constantly go to the right. If you are putting in enough yaw to counteract the sideward a component of the lift vector, you are not changing heading...

The rest if the time you want to be balanced, using your rudder to counteract any aileron drag, slipping, etc.

What wide words have your instructors given you over tea and biccies on this topic yet? Or have they been 'too busy' and left you to the books?

Don't be afraid to ask questions any time! We never learn otherwise...

(Just to throw you a curveball, now go think about how tailless designs operate in yaw ;-) )

Maulkin 27th Nov 2013 23:43

Ah, yes, of course, thanks - that does make sense! But how does that work with just yaw? Keeping straight and level wings by correcting with ailerons, are you actually "over correcting" and producing a resultant thrust that goes straight forward?

To be fair, I've not brought this up with the instructors yet, I think they get a bit worried when I say angle of attack at this stage in my training, I'm not sure I'm "meant" to know that yet :) I'll mention it tomorrow morning, assuming weather holds!

Dash8driver1312 27th Nov 2013 23:43

Yaw/roll - affect on heading
 
Edit "turning to the left" for the aileron but above. Too much slipping.

Dash8driver1312 27th Nov 2013 23:48

Yaw/roll - affect on heading
 
If you keep the wings level, you will yaw and change heading, sideslipping by definition means you have a wing down...

The Brothers Wright inter alia used yaw for turning anyway. Roll control was just to stop the machine spiralling in. With increasing speed, and new thought, it was safer and more practical to alter heading with roll surfaces (ailerons, wing warping, spoilers, etc) than yaw surfaces (rudder) and the rudder became the stabilising device. It feels much nicer too to be in a careful coordinated balanced rolling turn.

john_tullamarine 28th Nov 2013 00:05

sideslipping by definition means you have a wing down

Usually the two are associated but not necessarily. Slip just requires the wind to be coming from other than straight ahead.

Dash8driver1312 28th Nov 2013 00:25

Yaw/roll - affect on heading
 
John, sure you're not now talking about drift?

Andy_P 28th Nov 2013 01:39

I think OP is possibly referring to straight and level flight? Ie, at cruising speed, you should need no rudder otherwise it will provide unwanted yaw.

Rudder is used to correct adverse yawing that results from banking (roll) or prop yaw. Ie roll to the left, plane yaws right.

3 Point 28th Nov 2013 09:23

Drift or sideslip???
Yaw or sideslip???
Roll or bank????

Such confusing use of terminology! How do we ever expect students to understand what we are talking about??

If we use precise terminology then things are much clearer.

Yaw is an angular motion, rotating around the vertical (or normal) axis of the aeroplane; it is controlled by rudder.

Sideslip is the angle between the aircraft's fore and aft (or longitudinal axis) and the relative airflow (not the wind!).

Roll is an angular motion, rotating around the fore and aft (or longitudinal) axis, it is controlled by ailerons.

Bank is the angular displacement of the aircraft's lateral axis from horizontal (ie are the wings level with the horizon).

Relative airflow is the direction of the air as it flows past the aeroplane.

wind is the direction of airflow across the earth.

Drift is the angle between the aircraft's heading (its longitudinal axis) and its track (its direction over the ground), drift is caused by wind.


So; to turn an aeroplane we need some sort of force acting towards the centre of the desired turn (a centripetal force). Normally we create this by rolling to an angle of bank which tilts the lift vector. The resulting horizontal component of the lift vector provides the centripetal force and the aeroplane turns.

If we use rudder to yaw the aeroplane but use ailerons to hold the wings level we will have an element of force acting horizontally across the aircraft (from one wing tip to the other), this is caused by the air loads on the side surfaces of the fuselage. There will also be an element of the engine thrust acting to the side because we are no longer pointing the aeroplane and hence the trust straight at the relative airflow; these two forces, in combination, will provide centripetal force to turn the aeroplane. That's how the Wrights did it but it's awfully uncomfortable and not very effective so we don't do it that way any more.

For a steady heading sideslip you can roll one way to a steady bank angle (side force is created by the horizontal component of lift) and then use rudder to sideslip in the opposite direction so the side forces from sideslip and bank are equal but in opposite directions and therefore cancel each other out. Presto, sideslip with no turning!

Maulkin, get your instructors to discuss this with you using a model to demonstrate; a picture paints a thousand words! And be careful of people explaining things using sloppy or incorrect terminology; many of the answers on here confused the h*ll out of me!

Happy landings and good luck in your training

3 Point

3 Point 28th Nov 2013 09:41

PS

Maulkin, you are actually pretty close to it when you say "over correcting".

In a steady heading sideslip you are using one control (aileron or rudder) to command a turn on one direction then using the other control (rudder or aileron) to command a turn in the opposite direction cancelling out the first command.

Effectively "over controlling" one command with the other to cancel them out.

Andy P; in straight and level flight you are not yawing by definition (yaw being an angular motion how can you be yawing in straight (ie constant heading) flight?). You may or may not be sideslipping (if the balance ball is not in the centre you are sideslipping) but will still be flying straight (ie constant heading).

Of course we don't want to fly like that (it's inefficient due to increased drag and it's uncomfortable) so we would use rudder to yaw the aeroplane (angular change) aligning it with the relative airflow and centring the slip ball. We may or may not then have to hold some rudder to maintain the ball centred; if that's the case we need to adjust the rudder trim so that the aeroplane maintains straight flight, in balance and with no requirement for the pilot to hold a constant rudder force.

Happy landings

3 Point

cockney steve 28th Nov 2013 14:11

Great post, 3-point! even I understood your precise definitions.:O

Am I right in thinking, -to maintain a track in a crosswind, it's necessary to bank and yaw into the crosswind component in order to not be blown downwind of track?

3 Point 28th Nov 2013 14:37

To maintain track in a crosswind you need to turn to a heading such that your heading plus the drift equals your track. You would then be flying in balance (ball in the middle) and travelling in the desired direction across the ground.

If you are talking about the specific case of tracking the runway centreline on finals with a crosswind you can either turn into wind and fly the approach with the aeroplane in balance tracking the runway centreline or you can use rudder to point the nose straight down the centreline (aircraft heading equals runway axis) and then apply bank into wind to make the aeroplane sideslip at a sufficient rate to match the crosswind and cancel its effect. The first option is normally known as the "crab technique" and the second is the "wing down method"; both are valid and have their merits but I'm not getting into that debate just now:)

Happy landings

3 Point

Maulkin 28th Nov 2013 16:43

Thanks to all, especially 3 Point and Dash8driver1312 :)

Unfortunately the weather was 4000 BR BKN006 BKN014 this morning, so no flying for me. Mind you, I've now been playing with my model plane, so a combination of the above seems to make much more sense, thanks again!

http://www.halon.org.uk/stuff/postit_plane.jpg

AirborneAgain 28th Nov 2013 17:25


If we use precise terminology then things are much clearer.
:D :D :D :D

3 Point 28th Nov 2013 17:35

Excellent training model! I love it!

3 Point

Heston 28th Nov 2013 20:44


Excellent training model! I love it!
And a damn sight cheaper than that wooden red thing...

john_tullamarine 29th Nov 2013 03:42

John, sure you're not now talking about drift?

Two different animals. 3 Point's definitions are adequate .. even if he does chide me for my relaxed terminology ...

Dash8driver1312 4th Dec 2013 18:39

Yaw/roll - affect on heading
 
Maulkin, I love that model! Anything that helps visualise can't be bad, and that beast can be made anywhere!

I'll be sure to mention the designer if I need one myself.

Maulkin 6th Dec 2013 18:44

Hah, thanks. Some may notice the upgraded double trim tabs. That's an upgrade. Nothing at all to do with me cutting too many times... :O

flarepilot 9th Dec 2013 13:34

the wright brothers used rudder to overcome ''warp drag'' (we would now call this adverse aileron yaw). initially wing warping ( in place of ailerons but doing the same thing) did not properly turn aircraft. (indeed ailerons were even included in the patent)

even curtiss couldn't turn using ailerons, it was after the secret of coordinating aileron / wing warping AND rudder to compensate for adverse aileron yaw/ warp drag that reliable turns could be made and it was this method of control that made the wrights the true inventors of the airplane/aeroplane.

as we are approaching the 110th anniversary of the Dec 17 first flight, it would be worthwhile to seek out a very fine film called, "The Winds of Kittyhawk". While it is a dramatization, the "AH HA" moment of warp drag and its final conquering is amazing.


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