Explain yaw-roll coupling...

[Christo]

I am flying a PBF 20"x20" with elevons and rudder. The rudder is on top only (nothing below the wing). When I give right rudder, the plane also rolls right and the same with left rudder. Can someone explain why it rolls in the same direction? With the rudder being on top of the wing I would expect the plane to roll in the opposite direction - I'm trying to "visualize" the forces inlvolved and don't get it.

BTW, the plane has no dihedral either...

[lrsudog]

Depending on the geometry of the plane of course, the yaw actually can force the "Outside" wing to fly faster that the inside wing. More speed = more lift on that side, along with less lift on the inside wing.

There have been a few no-dihedral aircraft with no ailerons that would bank with just yaw input alone. The V-1 Buzz bomb being one off the top of my head.

[Robert May]

Draw an inverted flat V on a piece of paper , nice and big! This is your wing

now draw 6 short lines vertically , 3 on the left wing 3 on the right, these short lines represent the airflow.


rotate (yaw) your piece of paper as if you have applied rudder, about 15degrees. one of your wings is now closer to perdendicular to the airflow and presents a wider edge to the air and therefore has more lift, and therfore roll.

[RobinBennett]

If I can add to what the others have said: don't just think of the rudder applying a force - the rudder forces the plane to fly slightly sideways, the sideways airflow causes the roll.

[Christo]

Thanks for the explanations - I guess it all makes more sense now. I always thought that if I had just as much rudder below the fuselage as above it, it should eliminate most of the roll coulping but I guess not necessarily. I may try it sometime. The PBF flies great as is though. The roll induced by the rudder is actually useful once you get used to it. At times I would like it to have no couling :cool:

[headless]

Question: Would it thus make sense to program a mix for x rudder apply y aileron so using the rudder only resulting in yaw, and no roll? Would this help in sitations such a knife-edge to keep the plane from rolling out of it?

[Robert May]

Question: Would it thus make sense to program a mix for x rudder apply y aileron so using the rudder only resulting in yaw, and no roll? Would this help in sitations such a knife-edge to keep the plane from rolling out of it?


Yes, on the pattern ships before computer radios it was necessary to re-set the dihedral of the wing to tune out this yaw roll couple, now it is easy to do just that.

Once you have dialed out the roll you might find that the model now drops its nose quite significantly with a rudder input, you then end up having not only having the Rudder and Ailerons coupled but you will also need Rudder and Elevator coupled as well. Thats what all the free mixers are for on modern computer sets.

Once you start noticing these discreet tendancies in your planes you learn to love your Radio

Regards

Robert

[headless]

I've had my comp radio (Hitec Eclipse 7) for months and am using a grand total of zero mixes... it's hard for me to figure out what do mix in to counter certain effects (goes with the n00b to flight territory i suppose!). I'll start playing with mixing rudder to ail and elevator and see what happens... maybe i won't need to correct w/ roll and elev in knife edge as much as a result!

[Magnum9]

Mixes have their place, but I think for things like knife edge, you should just learn the right movements on the sticks. It will improve your overall skills and give you a better feeling for the plane in general.

Plus, before you can set up a mix, you need to know what to mix and how much. In addition to this, a knife edge mix is unlikely to work well in normal flight, so you will be flicking switches on entry and exit to KE.

However, I think once you understand all of this, it is obviously quite beneficial to be able to roll into KE, flick a switch, and hold a perfect pass with rudder only.

I suppose what I am trying to say, for newbies like myself, is that you can't rely on a computer radio to do your flying for you. You first need to learn the skills, then the computer can assist you to hone other skills.

[Threshold]

I don't understand why the plane rolls right with right and left rudder input though. Left rudder must cause a left yaw and left roll why do both inputs result in a right roll?

On mixes, I fly about 10 aerobatic planes, and mix out the coupling, each plane is different, some DIVE and roll RIGHT with LEFT aileron, some CLIMB and turn LEFT with LEFT aileron, I could not fly them accurately without mixing it out or spending a few flights getting used to each plane again. Mixes are there, I use them. So do the experts, thats good enough for me, if they need them then I need them more!

[djindivik]

I once had a Chris Foss, Acro Wot, that would do a complete 360degree bunt, on application of rudder, in either direction. After numerous posts on various NG's, i never did get a satisfactory answer as to why. Maybe someone here knows? Ray