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Help with coupling Ailerons and Rudder

Old 06-23-2003, 08:57 PM
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Default Help with coupling Ailerons and Rudder

Could use some help with the % to use with coupling ailerons and rudder together. Here are some specifics.
1. Radio used is an Airtronics, has this capability to percentage the amount of rudder to ailerons
2. Planes that I am considering using this on are: Four * 60 and Dynaflight Giant Chipmonk.

If you are currently using this method, please advise what you like or dislike about it, thanks.
Old 06-23-2003, 09:52 PM
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Default Help with coupling Ailerons and Rudder

You can program it, starting with a small amount, like 25% and work up from there, or do in manually and figure out how much you are using then program that in, or just do it manually when you want mixing. Start small and work up from there is the easiest way to do it in the radio though.
Old 06-24-2003, 12:02 AM
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Default Why Aileron to Rudder mix?

Rene If I may lets talk about why this mix is sometimes used. When we use the ailerons in an airplane the downward moving aileron produces more drag than the one that moves up and when we make a turn with just ailerons that outside aileron trys to drag the nose away from the direction we are banking and want to turn. you will see this very ungainly turn particularly with trainers all the time, not pretty.

Full scale pilots use coordinated rudder with the ailerons for most manuvers to combat this. Most RC pilots do not and yes its a good thing to practice but in honesty even those that do only do it for a portion of the flight. There is another way of minimizing this 'Adverse Yaw' either mechanically in the linkage geometry of the ailerons or electronically in some radios and that is 'Differential Ailerons' where the downward moving aileron does not go down as much as the other goes up. This reduces the effect of adverse yaw but not completely eliminate it. This is where using aileron to rudder mix can improve your flying considerably particularly when you are lazy and not use the rudder.

What kind of airplanes benefit from this? That an important question. The effect of Adverse yaw is most pronounced in high wing cabin or parasol wing types. And that is why you see so many trainers up there wallowing around like a sick whale. Shoulderwing, midwings and low wings experiance progressively less adverse yaw and generally barn door ailerons (such as a Cub) will produce a bit more than strip ailerons.

Generally low Wings do not really need it and in some cases a very neutral stability aircraft like a low wing pattern ship will become a bit more twitchy to fly. High wing airpanes particularly with barn door ailerons can and will benefit the most. I use it on all my highwing aircraft and almost never with low wings.

Indeed Flyboys suggestion for a starting point of 25% is a good one. If you wanted to try it on your low wings perhaps 10 or 15% would be better. My preferance on those is not to use it.

John, enjoy
Old 06-24-2003, 12:51 AM
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Default Help with coupling Ailerons and Rudder

I'm far from an expert, but I believe Aileron/rudder coupling works best on flat bottom airfoils to alleviate adverse yaw.

Not really needed on fully or semi-symmetrical airfoils.

But, if you want to do it anyway, just make sure that the rudder moves the same distance as the aileron. If the aileron deflects 1/2" up, make the rudder move 1/2".

You can do it with a computer radio or a Y-harness.

Sure helps trainers turn flat in windy conditions.
Old 06-24-2003, 12:59 AM
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Default Help with coupling Ailerons and Rudder

Good reply, John. I, too, use differential ailerons on most of my high-wings, especially those with flat-bottomed airfoils.

I was away from the hobby for awhile (new house) and when I came back, I couldn't find any of the nylon bellcranks specifically made for adding differential anymore. I believe they were called, "60 degree" bellcranks but think I can recall they also made "120 degree" versions. Do you know anyone that still makes them?

I've got some models that have them buried in the wings but probably still couldn't tell who made them even if I tore the wings open to see them.

On larger models, I'd build the wings with a servo for each aileron but I'm referring to smaller models where you might still use only one aileron servo and use the offset bellcranks to achieve the differential.

I have also built some where the differential was achieved right on the servo wheel, but I was curious what happened to the bellcranks that used to be available for this purpose.

Thanks,

Jon (linclogs)
Old 06-24-2003, 02:04 AM
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Default Help with coupling Ailerons and Rudder

Originally posted by Crash_N_Burn

But, if you want to do it anyway, just make sure that the rudder moves the same distance as the aileron. If the aileron deflects 1/2" up, make the rudder move 1/2".
You can do it with a computer radio or a Y-harness.
Dave that is a 100% mix that you just described and decidedly what you do not want, it will make even a trainer quite difficult to fly It is never neccessary to use that much mix to correct for adverse yaw in even the worst case type airframes i.e. cabin type with barndoor ailerons and yes I did neglect mentioning a high lift airfoil as another contributor. Its a big mistake to use a 'Y' cord for an aileron to rudder mix. For a Highwing airplane to use this mix start out with no more than 25% and much less for a low wing.


Jon

most Radio manufacturers make a simple round servo wheel that can be drilled easily that does the job. I think Futaba has a star shaped one that will do it. For those who would like to know what we are talking about:

To minimise the effects of Adverse yaw with a high wing airplane and a single aileron servo its possible to add differential ailerons (the downward moving one does not go as far as the up moving one) without a computer radio with just a slight change in aileron linkage. Obtain a star or round servo output wheel and place on the aileron servo now instead of having the pushrods attach ninety degrees to the right and left abeam of the servo output shaft you just make the rods a little longer and the attach to the wheel 45 to 60 degrees forward past the output shaft and thats it, you now have aileron differential without fancy electronics. This will give any conventional trainer an entirely different character and make the pilot look great. Don,t even get me going on Cubs. There are so many cub designs out there (worst case agine: High lift, High aspect ratio, barn door ailerons) that are so hard to fly smoothly and this one simple geometry change will make you look a pro.

Jon its easy on a two servo ailerons too just make the pushrods longer and rotate the output arms on both servos forward on the splines 45-50 degrees. And your right Jon I doubt you could buy differential bell cranks anymore but probably moot anyway since hardly anyone does articulated aileron linkages any more.


John

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