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Old Jun 12, 2007, 02:02 PM
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djklein21's Avatar
San Diego, CA
Joined Aug 2004
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Aileron to Rudder coupling and Aileron Diff

I have noticed that one of the most misunderstood and most overlooked set up in both slope and thermal gliders is roll-yaw coupling. Most pilots don't notice or understand how to set up some effective mixes to make your glider roll axially and turn efficiently.

I am hoping this thread can answer some basic questions about glider set up for combating roll-> yaw coupling

For starters letís define some key terms

Roll- a planes movement about its fuselage (rolling axially about its fuselage, usually controlled through its ailerons)

Yaw- a planes movement about its vertical axis (usually controlled through rudder movement)

Pitch- a planes movement about its horizontal axis (a planes movement about the axis projected through its wings, usually controlled by its elevator)

All deflection measurements are taken at the actual flight surfaces, not the settings in your transmitter. All settings can be a coupling of mechanics and electronic programming, so all meaningful discussion about deflection % is a measurement taken at the contol surface.

As it has been stated in the thread discussing snap flap, lift always comes with a drag penalty. When an aircraft gives an aileron deflection for a turn (lets assume a left turn) the outside aileron moves down to increase the lift on that wing, this increases the drag on that outside wing. The inside wing's aileron moves upward, decreasing the lift on that wing and also decreasing that wings drag. The plane has now banked into a left hand turn. The nose of the plane has however been pulled right, away from the intended left turn. This is called a skid. The increased drag on the outer wing tip and decreased drag on the inner wing tip has adversely coupled the roll and yaw movement of the plane. This is most dramatic in high aspect ratio planes, as the drag created on the wings has a larger moment arm to act on. Full scale pilots have an indicator to tell them if they are skidding, or opposite slipping). On gliders this is usually a piece of yarn taped to the windshield. This lets the pilot know if he or she is flying directly into the moving air. This is what we want to do.

Since most of us begin on small 60" gliders, we usually either don't worry about this or we combat this through aileron differential. Aileron differential refers to an aileron setting that makes the aileron travel further up than it does down. On most small planes we don't have a rudder, or if we do it isn't that effective. This makes us focus on aileron differential as the only cure to roll-yaw coupling issues. To keep our plane from yawing with a rudder command we usually add about 50% aileron differential, but all planes are different. If we combat roll-yaw coupling with aileron diff only we will get a plane that falls as it rolls back and forth. We have simply decreased the lift on the inside wing more than we increased the lift on the outside wing. So to effectively trim a plane to roll axially and efficiently we will need to add some rudder mix to the ailerons.

It is important to note that all of these trims are for a plane flying a single speed and that these mixes change with speed, C.G. and weight. Airspeed changes these trims the most, so we should concentrate on that for now. Low speed affects this mix more than high speed. Or in other words this adverse yaw is greatest at low speed, so your rudder mix and differential should be higher for slower flight modes. Tom Copp has shared with us all that he sets his slope racing planes up for two different conditions completely. Or to restate that for clarity he has separate programs for light air and heavy air with multiple flight modes for each. He has a 50 second air program, and a high 3X.XX second air setting.

I start by tuning my rudder mix first. I put about 40% differential in my ailerons and guess at a rudder mix. I program a switch for higher rudder mix and one for lower. Than I fly the plane and see which is closer. I adjust and try again. Once I get as close as I can, I switch to adjusting my aileron diff.

After my rudder setting is close, I look for a plane that doesn't drop its wings as it rolls. To restate this, the planes cg doesn't fall or rise when given a roll command. This is hard to see if you haven't got your rudder mix close. I fly the plane at the speed I am interested in and tune appropriately. When it is the best I can, I switch back to rudder tuning. It can take a few cycles to get all of this stuff right, but the plane will fly much better when you are done.

Remember these settings are flight speed dependent so use a three position switch to have a plane that flies well at all speeds.

This is an early write up, based on my experiences only. I am by no means an authority on this subject, so take my advice with a grain of salt. Enjoy, and please feel free to add to this discussion.

For more information look to Kevin Newton's site
http://www.knewt.com/planes/setup.htm

Mike Smith has made a great write up for tuning a HKM Sharon for TD flight at the USA F3B team site, under his bio
http://www.teamusaf3b.com/
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Old Jun 12, 2007, 06:01 PM
Sherpa
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La Quinta, Ca.
Joined Sep 2003
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DJ,

I have read your article with great interest. I am a full scale pilot and fly mostly PSS. I actually understand what you said which is surprising with my brains. However I do have one question. "WHY" I understand the need for snap flap on F3F but I am lacking the knowledge as to why I need axial roles. I can only think of "it looks neat". I am sorry I dont get the "need" for axial rolls.

Sherpa
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Old Jun 12, 2007, 06:33 PM
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Chino Hills, CA
Joined Feb 2003
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Quote:
Originally Posted by Schwemmer
DJ,

I have read your article with great interest. I am a full scale pilot and fly mostly PSS. I actually understand what you said which is surprising with my brains. However I do have one question. "WHY" I understand the need for snap flap on F3F but I am lacking the knowledge as to why I need axial roles. I can only think of "it looks neat". I am sorry I dont get the "need" for axial rolls.

Sherpa
Pure speculation... but I'm guessing you'd be wasting a lot of energy with non-axial rolls since the roll axis will be outside of the fuse center line thus moving the whole fuse when rolling. Also, axial rolls will be much faster too.

Adrian

Adrian
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Old Jun 12, 2007, 07:18 PM
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djklein21's Avatar
San Diego, CA
Joined Aug 2004
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When the fuselage slips or slides, it creates a large amount of drag do to flow separation around the fuselage. The tail the wags back and forth creating even more drag.

Think of it like wagging the rudder while flying down the straights. Every time you give an aileron input, the tail wags and scrubs off energy.
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Old Jun 12, 2007, 07:36 PM
F3X
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Costa Mesa, CA USA
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Quote:
Originally Posted by djklein21
<<SNIP>>
Tom Copp has shared with us all that he sets his slope racing planes up for two different conditions completely. Or to restate that for clarity he has separate programs for light air and heavy air with multiple flight modes for each. He has a 50 second air program, and a high 3X.XX second air setting.

Hi Dave,

Not so much really, I think I said you "Can" do this if you like but my set up is basically the same for all conditions. I am not sure if this is good or bad but it works for me. About the only change I have is a D/R about 5 points softer on low rate but I rarely switch it on.

Diff when set to provide a axial roll or really axial to 90-120deg is all we really need takes the workload of the pilot, if the model automatically rolls to say 120 deg and the fuse tracks perfectly the turn entry is 100% predictable and easy to execute. If you nail the turn entry the turn exit is almost guaranteed. Some guys fly with less diff and use rudder but I am not that good so I cheat and use what I consider the right amount of diff. I have some info on my "How To" page.....Read "Stop that Fllickin" and see if it makes sense.

Good post Dave!
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Old Jun 12, 2007, 10:56 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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I like to have rudder mixing on and off with a switch, and use it when the plane is in slower air, or light lift.
Using the rudder manually in light lift makes you realize that it has some serious benefits efficiency-wise.

I usually don't worry about using any rudder in fast air; if the aileron diff is good, that enough for me.

Target
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Old Jun 13, 2007, 11:24 AM
Daryl Perkins's Avatar
United States, VA, Falls Church
Joined Mar 2007
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I haven't raced for years, but... back in the day.... when I would set up a slope racer, the very first thing I would pay attention to was CG. After that, I would work on model tracking. In my racers, and my F3B models in the speed task, I tend to run a bit less differential than most, with a very slight amount of rudder coupling. For very fast runs, just enough to see it move... adverse yaw will allow the nose to come up on turn entry (this is bad), and just a hint of rudder coupling will help to keep this from happening. (Keep in mind, that the roll in on entry into the turn is usually more aggressive than the roll out on exit. )

The problem is, as mentioned above, is this is airspeed dependant. If I were racing F3F today, I would have probably two separate set ups also. For light air, very slightly more diff and very slightly more coupling. As a matter of fact, for 50+ second air, I would probably run in section (no reflex) to carry more weight. For big air, I would run more like a F3B speed run set up.

I have mentioned rudder coupling to hardcore F3F'ers, and they have told me I don't know what I'm talking about. I'm not talking extreme amounts - just hints.

Nice to see a F3 forum.

dp
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Old Jun 13, 2007, 01:02 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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Quote:
Originally Posted by Daryl Perkins
I have mentioned rudder coupling to hardcore F3F'ers, and they have told me I don't know what I'm talking about. dp

Me too! But I just use what works for me anyway...
And some never even use the rudder at all period.

Target
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Old Jun 25, 2007, 03:43 AM
isoaritfirst
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Not a Brummie/but close
Joined Sep 2005
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I've recently changed the settings on my Furio V to include a rudder to aileron mix, (note R/A not A/R) very, very small amount that is activated on the throttle stick when in thermal mode.
It is an analogue control, so the mix increases as the throttle advances, also when in thermal mode the throttle progressivly activates the camber, takes out the snap and takes out the aileron to flap mix, and reduces the aileron throw. So as the model flies slower more mix is added.
It has the effect of making the rudder slightly more powerful with less deflection. I tend to fly using mostly rudder and elevator during very light conditions,and use the ailerons as support when all else fails.

I mainly just rely on differential when flying fast, and have sufficient set to lift the tail slightly as it enters a corner. I am considering changing this (adding A/R) to try to achieve the same result with less throws.

Having reduced throws in the thermal mode has definatly made the model nicer to fly, previously it could bite occasionally as a surface stalled.

I enjoy the fiddling.
Mike
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Old Jun 25, 2007, 09:09 AM
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Southern California
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I use rudder mixing for F3F because I like the way it "sets up" my entry turn. It usually allows for a little "roll in" with the fuselage angle that helps me pull hard through the turn. It's only 10 to 20% on my transmitter depending on the plane and how effective the rudder mix is--like X tail vs V tail--but I've always flown with it. It seems to require less aileron throw to acheive the same roll in I'm looking for-- and puts the fuse on the line I want for turn entry. And like Tom says above-- if you get the entry--the exit is easier!
FWIW
Tim
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Old Jun 25, 2007, 09:24 AM
4 wheels move a body; 2 a soul
Saratoga, Ca
Joined Feb 2004
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Didnt Knewt say he doesnt use coupled rudder in F3F?
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