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  1. #1
    samtech's Avatar
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    Suhkoi rudder coupling

    Hi guys,
    Ok, I'm getting geared up to build a Sukhoi 26. I have read the designers notes and he mentioned it had some really bad rudder to elevator coupling. To the point he even tryed to lower the elevator to fix it. So me question for you guys is; What can I do about it?
    I'm thinking of a couple of different solutions. Solution one, Lower the elevator stab. Two, a slight anhidral in the elevator stab...if thats how you spell it... Three.... for some reason...just a gut feeling... extending the fuse a couple of inches on the tail moment. Or should I just leave it alone and build as is. The idea is to build a good acrobatic/ mild 3d plane.
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    Mahahaha (evil laugh)
    Revver Brotherhood #188

  2. #2

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    RE: Suhkoi rudder coupling

    Years ago, pattern fliers tried to refine their designs from version to version by doing just what that designer said he was going to do, "next version." Sometimes they actually cut and refit their latest version to see if they could fix it. They often didn't.

    My latest model is a Tiger60. Flew it for the first time yesterday. It's got a significant rudder couple. Two of them as a matter of fact. Rudder-pitch and rudder-roll. Since both of them affect how the airplane handles on takeoffs and landings when you need to steer the airplane with rudder both on approach and on departure, I've seen the need to trim out most of those couples. I did it in the first 3 flights with my transmitter. Setup a rudder-to-elevator mix and a rudder-to-aileron mix.

    If you wish to redesign the airplane to remove the couples, you're really going to need to do it by building a number of versions. And to remove the roll couple, you're probably going to need to change the wing dihedral. How much to remove, and how far down to move the horizontal tail? There are no formulas available to predict those two. But you probably won't have to build more than a couple of versions to decide if that option is working for you.
    Good flying wit ya today

  3. #3

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    RE: Suhkoi rudder coupling

    I am not sure just what is meant by rudder coupling. I know what it means as applied to the radio control ,but not from the design of the aircraft. Does it mean when you give rudder it also creates an elevator effect to and elevator to rudder effect as well? If that is the case.then it looks to me that you need to get rid of some of the rudder that is below the elevator and lower the stab some to. I am by no means an airplane designer ,would not such a large rudder effect the elevator and vice versa?
    Do unto others as you would be done unto,well most of the time anyway!

  4. #4
    samtech's Avatar
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    RE: Suhkoi rudder coupling

    Crasherboy, You got it. Give it some rudder and it not only starts turning...it starts diving. Makes landings difficult to say the least.
    Mahahaha (evil laugh)
    Revver Brotherhood #188

  5. #5

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    RE: Suhkoi rudder coupling

    There are two types of rudder couple.

    One is pitch or elevator. You guys just discussed it. Some planes pull up. Some planes tuck down. Some planes don't have rudder-pitch couple and simply rudder turn.
    The other is roll or aileron. When the rudder is moved, the plane rolls. It might roll into the turn the rudder is starting or it might roll away from the turn.

    My Cap had both. And bad. When flying along straight and level, if I gave it rudder, it would tuck and roll. When I tried to knife edge, it would turn away from me like I'd hit the elevators PLUS it would roll like a bandit. The roll was almost as fast as an aileron roll on high rates. Any airplane that does this is usually going to crash on takeoff or landing sooner or later when you need to steer with rudder and the rudder couples are the last straw for a wing that's about to stall as it is. Experienced pilots use rudder for takeoff and landing steering in order to NOT affect AOAs that are already critical.

    There is a trim condition that experienced flyers call "pure response". It happens when the deflection of a single surface causes the airplane to do just what that one surface is supposed to do and nothing else. Most flyers today trim their models to give "pure response" by using mixing in their computer radios. With my Cap, I setup a rudder-elevator mix and a rudder-aileron mix.
    Good flying wit ya today

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    RE: Suhkoi rudder coupling

    If it tends to pitch up, set your rudder hinge line canted forward a few degrees, if it pitches down tilt it rearward. How much would depend on an educated guess and trial and error.

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    RE: Suhkoi rudder coupling

    If it tends to pitch, set your rudder hinge line .............
    This often is extremely difficult to work out.

    For example, the Tigers by Carl Goldberg all have strongly canted rudder hinge lines. Both my TigerII and Tiger60 have an obvious tuck when only rudder is used. I've just been tuning the 60 the last few days. I'm dialing in rudder-elevator mix and haven't gotten to "pure response" yet. If the rudder hinge line were angled any more on that 60, it'd look pretty awful.

    Also, when an airplane (like my Cap) exhibits both a strong pitch and a strong roll couple, the two suggested changes in design work differently when combined. What you think would work for one is now affected by the other change. So if you had an idea how much to move the stab vertically, that is probably not a good amount if you're also redesigning the angle of the rudder hinge line.

    Good flying wit ya today

  8. #8
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    RE: Suhkoi rudder coupling

    How much couple can you eliminate with mixing in your tx? Limitless?
    I have often seen reviews were mixing around 2-5% have been regarded huge. What about 20% or 40%? Will that have other negative effects you absolutely don’t want?

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    RE: Suhkoi rudder coupling


    ORIGINAL: KjellKo

    How much couple can you eliminate with mixing in your tx? Limitless?
    I have often seen reviews were mixing around 2-5% have been regarded huge. What about 20% or 40%? Will that have other negative effects you absolutely don’t want?

    How much can you eliminate with mixing?

    Everything you can see.

    As for advice about how much you should use, or how much does what........... It's really dangerous in our hobby to try and make soundbyte suggestions. Or to look for a one sentence answer that applies to all airplanes.

    It ought to be intuitive that different amounts do different things with different airplanes.

    I've got a model that needs a mild amount of rudder-aileron mix because it only shows a bit of rudder-roll couple in knife edge and no rudder-pitch.
    I had a Cap that almost broke in half when the rudder was moved for a knife edge. The sucker couldn't figure out which it ought to do first, break in half in a tuck, or rip the wings off in roll. It took a fair amount of rudder-elevator mix to cure the tuck, but only about 1/3 the amount of rudder-aileron mix.

    Our hobby is too broad to have "one answers covers all" sound byte wisdoms.
    Good flying wit ya today

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    RE: Suhkoi rudder coupling

    I spent a pretty wonderful couple of hours last evening at the field. Temperatures are coming down and it was pleasant. Gentle breeze, mostly along the runway. Blue sky and some brilliantly white puffies. Down on a river flat with a mountain in the background. ........ uh what was I going to talk about........

    I went out to check if I'd finally gotten enough elevator throw in the Tiger60 to get the sucker to snap honestly. And in about a minute had discovered that problem was solved. No way was I going home early, so I started dinking around with the airplane's rudder-roll couple. It almost wasn't worth messing with, but I mess with stuff for the fun of it. Rather mess than anything. And didn't have any props to test/mess with/compare.

    There was a guy there who didn't know what I was talking about when I mentioned that I was going to trim out the "couple". So I showed him what the airplane did. And he could see it right away.

    He was really interested then and wanted to see what I did and what that did to the airplane. It took a couple of shots. Set what I thought would work and showed him the effect on the plane. Took her up and he could see the improvement. Landed right away and tweaked the settings. Right back up. Could see improvement. Landed right away and tweaked. Took her up and turned out that she was cured of the couple AND guess what.......... she snapped even better. Why? think about it. It's actually pretty obvious. What does the elevator have to do to get the airplane into a snap. And a snap uses rudder. And if the rudder couples..... against what the elevator is trying to do............. Nope, the stuff we do isn't sound byte simple.
    Good flying wit ya today

  11. #11

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    RE: Suhkoi rudder coupling

    Want to see if your airplane has a rudder couple? Simple..............

    Full bore, level flight. Hit the rudder.

    Does she start to tuck? pitch up? roll?

    Anything other than simply yawing, and whenever you use the rudder you're going to get other "mess" you didn't ask the plane to do.

    Say you want to steer with the rudder on takeoff since it's ten times safer to rudder steer than it is to try to steer with the ailerons. So you are going to safely steer with the rudder and the rudder has a pitch up couple. That can't be good, especially since you probably don't expect the rudder to do the elevator's job. And you got an airplane with it's wing cranked up a couple of degrees under the stall AOA and your blasted rudder throws a couple more degrees of UP at the poor wing.

    Nah, our stuff isn't sound byte simple.
    Good flying wit ya today

  12. #12
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    RE: Suhkoi rudder coupling

    The model you have selected has a nearly scale outline. It will have to be completely re-designed to become a decent so-called 3D plane. Not that it wouldn't be a thoroughly enjoyable plane to fly; you just will not have something equivalent to a QQ Yak or the likes. You'll have a (nearly) scale model. If you want something really neutral, pick a different plan to build.
    \"The purpose of time is to prevent everything from happening all at once.\"

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    RE: Suhkoi rudder coupling

    BTW, I just looked again at the plans. And noticed just how swept the wing's LE is.

    A swept LE can cause an airplane to fly as if it had dihedral. If it already has dihedral, that is emphasized in flight. When dihedral airplanes are yawed, they roll. It's how dihedral gliders with no ailerons turn. It's that strong an effect. Just yaw the glider and the dihedral rolls the glider into the turn.

    Darned if that airplane is going to look like what it is after all it's flaws are reshaped out of it.

    Setting two mixes in your radio works quite well. My Cap flew very cleanly after I spent a day working out the mixes.

    BTW, I talked recently with a very serious IMAC dude. I knew he and the other IMAC flyers encounter rudder couples quite a bit, so asked him what all the IMAC guys do. He said that a few tried moving horizontal tails but gave it up. And asked why would I not want to mix since it works so well.
    Good flying wit ya today

  14. #14
    MTK's Avatar
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    RE: Suhkoi rudder coupling


    ORIGINAL: da Rock

    BTW, I just looked again at the plans. And noticed just how swept the wing's LE is.

    A swept LE can cause an airplane to fly as if it had dihedral. If it already has dihedral, that is emphasized in flight. When dihedral airplanes are yawed, they roll. It's how dihedral gliders with no ailerons turn. It's that strong an effect. Just yaw the glider and the dihedral rolls the glider into the turn.

    Darned if that airplane is going to look like what it is after all it's flaws are reshaped out of it.

    Setting two mixes in your radio works quite well. My Cap flew very cleanly after I spent a day working out the mixes.

    BTW, I talked recently with a very serious IMAC dude. I knew he and the other IMAC flyers encounter rudder couples quite a bit, so asked him what all the IMAC guys do. He said that a few tried moving horizontal tails but gave it up. And asked why would I not want to mix since it works so well.
    Darock,
    Did you change the amount of dihedral the wing was originally designed to have? If you flattened it even a small amount, then the plane will adverse roll upon rudder application. That is....left rudder right roll and probably pitch to the belly. On the other hand, if the wing has too much dihedral, then left rudder application will roll the model left and pitch is up also (very slight). If pure response is what you want out of your Tiger, I'd recommend fixing the dihedral first then messing with slight amount of electronic mix for very slight adjustments.

    Unfortunately electronic mixing has become the first approach for so many folks with mix problems. It really should be the last fix and only for small trim type improvements not gross changes attempting to "fix" aerodynamic inadequacy.

    MattK
    Regards,
    MattK
    (Rcmaster199@aol.com)

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    RE: Suhkoi rudder coupling


    ORIGINAL: MTK

    Darock,
    Did you change the amount of dihedral the wing was originally designed to have? If you flattened it even a small amount, then the plane will adverse roll upon rudder application. That is....left rudder right roll and probably pitch to the belly. On the other hand, if the wing has too much dihedral, then left rudder application will roll the model left and pitch is up also (very slight). If pure response is what you want out of your Tiger, I'd recommend fixing the dihedral first then messing with slight amount of electronic mix for very slight adjustments.

    Unfortunately electronic mixing has become the first approach for so many folks with mix problems. It really should be the last fix and only for small trim type improvements not gross changes attempting to "fix" aerodynamic inadequacy.

    MattK
    Actually, there are two (at least two) opinions of the right way to fix things. And they're both correct. Which happens all the time in modeling. Heck, sometimes there are more than two best ways to fix something.

    No, I didn't change the dihedral of my Cap, or the one I built for a friend immediatly after finishing mine. And I didn't lower the horizontal tail. Nor raise the wing. Or flatten it. And since tapered wings stall at the tips first, while non-tapered stall at the root first, I could have changed the wing planform to rectangular and gotten less stall, but I didn't do that either. And since tricycle gear planes handle better than tail draggers I could have changed that, but didn't either.

    What I did was build a couple of Caps. We both liked the looks of the airplane. So I built the airplane.

    As for wanting pure response out of my Tiger, I got it. The same way fullscale pilots get their airplanes to fly better. There are trim tabs all over most full scale airplanes. So the pilot can trim the plane to do what he wants it to do. Our trim just happens to work from the TX. And the airplane still looks like a Cap.

    I agree that it's unfortunate that some modelers' first response to fixing stuff is to twiddle the settings in the TX, but that's not what is always done. However, it shouldn't be the last thing to try when you have the desire to keep the airplane looking like it did when it caught your eye in the advertisement and you decided you like that airplane. Too many guys discover their elevator is too sensitive on the new model and instantly hit the TX to change the endpoint adjustment. And lots of them have the pushrod rigged backward. But that's quite a bit different than sawing an airplane that exists into pieces to fix stuff.

    More than two ways to do things right.
    Good flying wit ya today

  16. #16
    samtech's Avatar
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    RE: Suhkoi rudder coupling

    Well,
    I've studied the plans to death and decided to add two inches in the tail moment and set the tail incidence to positive 1.5 degrees. Also, I'm going to make the gear bit more narrow and stream lined. The designer mentioned that he though is was the landing gear causing most of the fuss and that he had to put a good amount of down elevator trim to keep it level. I'll post the results once I get it in the air.... Give it two...two and a half months.
    Mahahaha (evil laugh)
    Revver Brotherhood #188

  17. #17
    MTK's Avatar
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    RE: Suhkoi rudder coupling


    ORIGINAL: samtech

    Well,
    I've studied the plans to death and decided to add two inches in the tail moment and set the tail incidence to positive 1.5 degrees. Also, I'm going to make the gear bit more narrow and stream lined. The designer mentioned that he though is was the landing gear causing most of the fuss and that he had to put a good amount of down elevator trim to keep it level. I'll post the results once I get it in the air.... Give it two...two and a half months.
    Mahahaha,

    Increasing the stab incidence will do quite a bit of good to the down pitching with rudder. It will make the tail ride high which means the rudder's center of pressure as deployed will be higher relative to the fuselage and stab. This increases the effective down moment at the tail when rudder is applied, which results in up pitch of the model. Some noted designers of pattern models (where pure response is paramount) have learned this approach eons ago.

    Narrow and stream lined landing gear is fine and will not hurt anything..... I seriously doubt your gear is any issue though. Who is the model's "designer" who says the landing gear is causing all this fuss?? I'd question that statement

    MattK
    Regards,
    MattK
    (Rcmaster199@aol.com)

  18. #18

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    RE: Suhkoi rudder coupling

    Stab incidence changes how much drag the airplane winds up with after the wing decides where it's AOA is going to be. The weight of the airplane, it's speed, wing area, and the airfoil decide and doesn't care much at all what the tail wants to do. And once the wing decides, it does it, and the fuselage goes where it's told. And takes the stab with it. And then the stab incidence either causes more drag or it causes less drag. It also happens to balance out any pitching moments the wing has decided it wants to create, but it won't be doing that on a Sukhoi with it's symmetrical wing.

    The wing establishes the AOA of the whole airplane. All the parts. And then the parts like the tail try to do whatever they're permitted to do at the AOA they find themselves. Most times when the stab is sensibly set, the elevator won't change it's neutral. And the drag back there isn't increased over best case. But move the incidence of the stab so it's not right at the AOA the wing gives it and what the tail does is make more drag and need the elevator neutral changed. And the airplane winds up flying a couple mph less than it could. And the wing needs a tad bit more AOA because of that and TAKES IT.
    Good flying wit ya today

  19. #19
    samtech's Avatar
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    RE: Suhkoi rudder coupling

    MTK,
    Here is the designers web page. Real nice guy, send him a mail and I'm sure he'll tell you what he thinks. I kinda dought it's the gear aswell but it's food for thought none the less.[link]http://www.geocities.com/rcsu26mx/su_intro.html[/link]
    Mahahaha (evil laugh)
    Revver Brotherhood #188


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