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  1. #1

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    tail heavy wants to roll?

    This has got to be a easy basic question for most of you in this forum. I now have my latest plane (a foam acro-wot) that when rolling either way it tend to want to keep rolling. All four of my planes are foam ARF that do this. If banked maybe 45 degrees or less they're fine , but about 60 degrees or so then they tend to keep rolling over even when stick is centered.

    The only idea I have is that I have all of my CG (balance points) set too far back. They're not hard to control really , but I like to use very little elevator when inverted so I have my points set at the rear most point recommended in the manual usually. I remember first complaining about this on my cessna corvalis , yet it's balance point is inmop too far forward.

    Back to my latest acro-wot I only have 4 flights on it , and when inverted it takes "just a little bit" of elevator to maintain level flight at about half throttle. Any ideas why they tend to want to continue to roll over when the stick is centered. I know they have no dihedral in them , but wondered if it could really be that my planes are all "tail heavy"

    Maybe I should've just asked "Do tail heavy planes tend to want to roll over and be very unstable when banked?" I always heard that a tail heavy plane was nearly uncontrollable and I surely had one that was too. It was a real handfull to fly. It was nuts and made me look like a real goofball flying it. []
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  2. #2

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    RE: tail heavy wants to roll?

    I've a Spacewalker that suffers this and I contribute it to weight of motor, batteries and servos too high.

  3. #3

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    RE: tail heavy wants to roll?

    I don't think that the CG location is too far aft or affecting the roll stability. You shouldn't see a large cross-coupling between pitch and roll, in a conventional airplane. You may have a problem with the wings being out of balance. You could also be flying too slow and the wing tip on one side, is stalling. The CG location should not be causing a lateral(rolling) imbalance, if the the CG is exactly on the centerline of the fuselage.Also. check to assure that both ailerons are centered, when the aileron stick is in the neutral position.

    Check for a lateral imbalance by lifting the plane about an inch off of a flat surface, by holding only the prop, so that the plane is free to roll. Chances are, that you'll see the plane want to roll.

    Best of luck.
    President of RC Aeronauts, sharing knowledge and providing innovative techniques and software solutions to RC modelers

  4. #4
    David Bathe's Avatar
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    RE: tail heavy wants to roll?

    Having your CG set back because you like little forward presure on the elevator stick isn't the route to go.
    Even with the F3a pattern planes, folks think that because the roll/fly so neutral is that the CG is set back.
    Nothing could be further from the truth... the cg's are set quite considerably further forward than you'd believe.
    The trick is to add either opposite expo on down elevator making it more instead of less sensitive or offset your elevator push rod at the servos end giving you more down than up.
    Setting up either will still allow you to set a forward CG (which brings a mountain of benefits) whist still retaining the slight forward pressure feel on the stick you enjoy.

    Regarding your "roll over" problem".
    Must be something you're doing that has effected 4 different planes.
    The Acro Wots ( IC powered) I've flown never had that problem.
    Are you using slow servos? Are they centering correctly? just could tell.
    Do the above mentioned CG trick. Least you'll be able to cross that of the list of suspects.
    Best Regards: davidbathe.com
    Occasional Aircraft Illustrations.

  5. #5
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    RE: tail heavy wants to roll?

    Most aircraft with positive roll stability (wants to return to level on its own) will at some point (usually at about 60 degrees) become negatively stable in roll, or want to continue the roll. You will find that up to about 45 degrees you have to maintain a little pressure into the turn to stay banked and between 45-60 it maintains the angle well. After about 60 it will try to continue the roll with no input. Very normal for sport, trainer and some scale aircraft. If it is excessive in yours and not in others of the same design then I would say you have an issue, otherwise it is probably normal to that design.
    The three most useless things to a pilot, the sky above you, the runway behind you, and the fuel on the ground.

  6. #6
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    RE: tail heavy wants to roll?

    The Acro-wot is a low wing with little dihedral; it is naturally roll-unstable because the CG is higher that the CP (center of pressure).

    If rolled during a coordinated turn, it should be roll-stable at any angle due to the action of the centrifugal force "makes it think" that is flying upright.
    Lnewqban - "God will not look you over for medals, degrees or diplomas, but for scars. He has achieved success who has worked well, laughed often, and loved much." - Elbert Hubbard

  7. #7
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    RE: tail heavy wants to roll?

    I think the buzz word is negative static stability or negative static lateral stability. Most planes with little or no dihedral have a degree of it as opposed to plane with more dihedral having positive static stability. As explained above , the plane will want to keep moving a little bit but the controls back to =neutral should hold it there. It's not quite like the pendulum effect as you don't experience that until you have passed 180 degrees.
    For anyone who has flown larger float planes knows that when you want to stop the roll back at zero the weight of the floats wants to keep rolling the plane back up to and sometimes through 180 degrees.
    Brian Ray

  8. #8
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    RE: tail heavy wants to roll?

    A tail heavy plane can drag the wing into a stall condition and the result looks like the plane is rolling on it's own free will.
    WHO GUNNA FEED MAW KEEEIDS..???

  9. #9
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    RE: tail heavy wants to roll?

    ORIGINAL: Lnewqban

    The Acro-wot is a low wing with little dihedral; it is naturally roll-unstable because the CG is higher that the CP (center of pressure).

    If rolled during a coordinated turn, it should be roll-stable at any angle due to the action of the centrifugal force ''makes it think'' that is flying upright.
    This lay out can be like trying to balance a balloon on your nose...if the CG is too high.
    OTOH, a high wing trainer flown inverted has a very high CG and they fly solidly. I suspect that an aft CG contributes to the OP's troubles.
    Besides, low wing planes that have no dihedral just look kind of "stupid"...
    WHO GUNNA FEED MAW KEEEIDS..???

  10. #10

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    RE: tail heavy wants to roll?

    Thanks for all the responses guys. I'm trying to digest it all so far.

    Here's what I've done so far with the acro-wot. Moved the balance point back even further by removing 50 grams from the very front of the nose (on a 1000 gram plane) This sets the CG at the rear most point recommended in the manual. It is none worse than before and inverted requires still a bit of "down" In a dive test it nearly goes straight , but wants to recover and pitch up very slowly. It's kinda the way I like it.

    I never check the lateral balance on any of my planes because I've just never had a good way or tool to do it. The manuals speak about picking up by the prop and tail or rudder and I just can't seem to get repeatable results that way. The motor shafts or props are not always in the center and some of my tail wheels are rather sloppy side to side throwing off the results. I did , however do my best to check the lateral balance without battery and in doing it a number of times I couldn't guess which side was heavy so I think it's "good enough"

    I'm going to check again with battery installed in center. I've only got 6 flights so far and it does track well in loops and it also seems fine when inverted. The left aileron is visibly higher than the wing though. Lots of my planes are that way. Foam planes ya know. This one here had a bent wing and I remember straightening it before I installed it. My worst case plane is one that had/has a twisted fuse. This is not the first plane I've asked about that does this very same thing.

    The corvalis is so bad that I only fly it on calm days , it isn;t enjoyable and the balance point is quite forward http://www.rcuniverse.com/forum/m_10...m.htm#10983436
    and airplanes were in

  11. #11
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    RE: tail heavy wants to roll?

    I've noticed that on one of the pictures shown on the thread link that the foamy in the shoy has a quite considerably positive incidence on the wing.
    This will couse excessive nose down during invered regardless of CG position and require a decent amount of down elevator to maintain level inverted flight.
    Best Regards: davidbathe.com
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  12. #12

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    RE: tail heavy wants to roll?

    That has to be the one in the middle with the twisted fuse (tail). I'm going to look up wing incidence and study things a bit. Still digesting everything that has been posted here.
    and airplanes were in

  13. #13

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    RE: tail heavy wants to roll?

    so I'm thinking about this... but the only thing I can think of that would cause that is anhedral (The opposite of dihedral). dihedral causes an aircraft to have positive roll stability. another effect of dihedral is rudder to roll coupling (left rudder will bank left... vice versa) if you think about it, once and aircraft with anhedral gets past a certain point (probably about knife edge), itll almost behave like a plane with dihedral (kinda... in a sense that itll want to remain inverted. not the best attitude ever) just something to consider. another thing is called keel effect. if the majority of the mass of the aircraft is on top of the center of gravity of the plane, itll want to hang below the plane. move your battery or other stuff towards the bottom of the fuse. thats all I got! good luck.


    p.s. by the way... your center of gravity will never affect roll stability. unless you have a hardcore lateral balance problem (easy fix: pennies or dimes on the wings).

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    RE: tail heavy wants to roll?

    I have never had a model that wanted to keep rolling when the stick is centred.
    You could have a TX stick problem, maybe a sticky potentiometer that doesn't return to neutral.
    Some servos don't always centre very well. Check that.
    Seems strange that you have 4 models with the same thing.
    Does it do the same in a left and right roll.
    Dihedral or CG should not make this happen.
    You are correct when you say the model needs just a little "down" when inverted.
    If it is the right way up but wants to climb during a shallow dive when the speed is building up then the CG is too far forwards. You can move it back a little bit at a time but cut the elevator movement down as well or you could end up over controlling it as it becomes more responsive.
    You will need to retrim the elevator down a little as the cg is moved back.
    I don't understand David Bathe saying forward CG is the way to go, sorry David.
    Jim

  15. #15
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    RE: tail heavy wants to roll?

    It will also want to stall.Put a few degrees of Dihedral Wings tilted up for stability.This will help the aircraft return to level flight .And help it stay level
    Charlie111 Looking for two HIGH PERFORMANCE motors with single channel speed control.BRUSH OR BRUSHLESS

  16. #16
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    RE: tail heavy wants to roll?


    ORIGINAL: Lnewqban

    The Acro-wot is a low wing with little dihedral; it is naturally roll-unstable because the CG is higher that the CP (center of pressure).

    If rolled during a coordinated turn, it should be roll-stable at any angle due to the action of the centrifugal force ''makes it think'' that is flying upright.
    Years ago I scratch built a 3D biplane without paying any attention to how the "vertical" CG would effect flight, especially with the plane flying inverted. As soon as you roll it inverted this plane will flip itself back to being right side up and continue to fly very nicely until you try to roll it on it's back again. I challenge anyone to take the sticks and hold this plane on it's back for 5 seconds..it can't be done. Needless to say, this plane sits in the attic...waiting for it's Viking Funeral someday.
    WHO GUNNA FEED MAW KEEEIDS..???

  17. #17

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    RE: tail heavy wants to roll?

    The original problem is that the model does not stop rolling after the sticks are centred?? Is that correct?
    I just can't see that adding dihedral or moving the CG around will make the slightest difference.
    This has to be a radio or servo problem because it happens with 4 different models.
    Jim

  18. #18

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    RE: tail heavy wants to roll?

    Yep, that is the original problem. I've since fixed a motor problem and was able to fly again. It's been poor visibility every flight so far and I can't wait to get a bright day so I can get a bit more aggressive with it.

    Of the 4 models I have that have this tendency , this particular TX does fly 3 of them. It also flies 3 other models real nicely though and one (GP e-stik) I even have to "hold the turn in" so it is rather the opposite. Different design plane though (high wing with no dihedral I think)

    The 4th model that has this tendency believe it or not is a 4' piper cub , but it flies with a different TX. That other TX also flies about 4-5 other models ok.

    I have looked at the display on the TX and watched the ailerons and servos to see if they didn't center well. I didn't notice anything funny. I also gave just a tick of opposite stick last time I flew to make sure it wasn't the servos/ailerons centering problem. This didn't help any.
    and airplanes were in

  19. #19

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    RE: tail heavy wants to roll?

    ORIGINAL: cfircav8r

    Most aircraft with positive roll stability (wants to return to level on its own) will at some point (usually at about 60 degrees) become negatively stable in roll, or want to continue the roll. You will find that up to about 45 degrees you have to maintain a little pressure into the turn to stay banked and between 45-60 it maintains the angle well. After about 60 it will try to continue the roll with no input. Very normal for sport, trainer and some scale aircraft. If it is excessive in yours and not in others of the same design then I would say you have an issue, otherwise it is probably normal to that design.
    This is the best description of how it acts. Maybe I am just flying too slow in the turns? I do seem to bleed off a LOT of speed when doing a 180 degree turn-around at the end of my field. I also notice the problem is not quite as bad or noticeable when doing a higher speed turn-around.

    If banked 50-60 deg , by the time the plane gets thru a 180 degree turn around it will be nearly 90 if the stick is centered immediately after banking. I'm not too great at using rudder yet either guys. I have to constantly remind myself to use rudder and this plane I use VERY little rudder.
    and airplanes were in

  20. #20
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    RE: tail heavy wants to roll?

    How hard would it be to..

    Test the control system for centering after you release the stick......

    Add dihedral to the wing, enough dihedral to resemble a head on view of a WWII fighter plane..?

    Keep your speed up in turns. Turning burns up power. Feel what it is like to fly a full flight at full throttle and then glide in dead. See for yourself what the plane should feel like with adequate power applied at all times.
    Never forget that gravity also adds power, so trim your plane with a slight amount of "down" when you release the stick. This will help keep your plane flying with adequate speed while you are having any moments of indecision.

    WHO GUNNA FEED MAW KEEEIDS..???

  21. #21

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    RE: tail heavy wants to roll?

    Your description is not unusual and does not indicate a balance problem. Many aircraft will do as you describe and it is best described as a design quirk than anything else. Full size pilots will often use a little 'opposite aileron' in a turn to maintain bank angle, as the aiplane will try to tighten the turn with a bit of elevator added to maintain height. So your observation is accurate for most airplanes. The fact that modellers do not often comment on it is more likely due to the small amount of time actually spent turning, a second or so compared with the much longer time a full size takes. To add to your observation, most of my scale models suffer the exact thing, and if sufficient time is allowed, then a little opposite aileron is needed to maintain a fixed bank angle once in a steady state turn. Left alone, as elevator is added to maintain height/speed, the models will start to roll into the turn. In fact, the only models I have that don't illustrate the effect are a couple of '70's aerobatic models, and they are specifically designed to have as little secondary effects as possible, and are pretty quick so you don't have much time to see it anyway...
    Evan, WB #12.

  22. #22

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    RE: tail heavy wants to roll?

    I finally had a chance to fly this today in some decent visibility. It's actually a very nice flier and it does tend to do this when flying slow. It also does it more to the left. I guess I was being too careful and cautious before because it was a new plane (to me) and visibility was terrible. I flew it 2-4 times out as far as before and it's easy to see and actually quite fast too once the battery gets warmed up.

    When flying fast I almost won't even notice a problem at all. I had such a good time with it I flew it back to back flights 5 flights in 1.5 hrs. (not much time to rest) and even a few inverted laps (which is pretty good for me)

    After I got it in I was eyeballing it up and the wing is twisted. I remembered that when I got this used it did have a bent wing and I straightened it. It's warped in such a way that explains all the left aileron trim it required too. I may try to twist the wing straighter or even replace it because I'm not much into repairing stuff.

    So for now I'm going to enjoy it still , try to fly inverted a lot with it and practice knife edge and rudder. The rudder sure acts funny , I don't even know how to describe it yet. Maybe pitches the nose under or down? I'll have to practise some stall turns and some flat turns too see what's going on with it.


    Here's a bad story about yesterday's flight: I noticed the neighbor's wood burner was smoking a lot and the wind was toward the field. The visibility looked about like it had been on my first 4 flights. I gave it a toss and realised that smoke was a whole lot worse than fog is. I flew it around like an idiot trying to fly an obstacle course where there was clear sky. About 1 minute later I realised I better land this thing so a very haywire landing commenced. It was rather dumb , but I learned a lesson. Don't fly in smoke , it's much worse than fog or clouds.
    and airplanes were in

  23. #23

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    RE: tail heavy wants to roll?


    ORIGINAL: combatpigg

    How hard would it be to..

    Test the control system for centering after you release the stick......

    Add dihedral to the wing, enough dihedral to resemble a head on view of a WWII fighter plane..?

    Keep your speed up in turns. Turning burns up power. Feel what it is like to fly a full flight at full throttle and then glide in dead. See for yourself what the plane should feel like with adequate power applied at all times.
    Never forget that gravity also adds power, so trim your plane with a slight amount of ''down'' when you release the stick. This will help keep your plane flying with adequate speed while you are having any moments of indecision.

    Testing the control system visually indicates that all is working well. Today I flew 2 other planes with the same TX (one that behaves like this and one that's the opposite) My e-stik require me to "hold the turn in" a lot. My sensei trainer will roll just about like this acro-wot does, and it would be the last plane I would expect to do it. I don;t know how I'd put in some dihedral with the one piece foam wing. Maybe when I straighten it I may try to bend it a bit? The acro-wot when inverted flies pretty nicely and requires just a little bit of down.
    and airplanes were in

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    RE: tail heavy wants to roll?

    Many models are not designed for doing perfect aerobatics but are made to look pretty and sell. I would say some manufacturers have little or no idea about airframe design and the models sell on looks alone.
    If the model dives during a knife edge then it is usually the tailplane height that is wrong.
    Using the rudder makes the model roll with the rudder if the dihedral is too much and against the rudder if too little. Perfect dihedral is when the rudder is used and you only get yaw with no secondary effects.
    If it is a high wing trainer then this is a bit different as you probably won't be trying to knife edge.
    Scale is also a bit of a problem as you can't alter it too much without changing the scale outline so you just have to learn how to fly around the problems.
    I have had models that want to roll over at around 45 degrees, a Fly Baby was one.
    Jim

  25. #25
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    RE: tail heavy wants to roll?

    Best remedies: build straight, lateral balance your aircraft, setup proper C/G, and build as light as possible, keep the wing loading down! Most all these types of problems go away with these 4 simple rules.

    Bob
    Fly It Like You Stole It!!!


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