E*nealio

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Almost all of my foamies have some roll coupling to the left... Any ideas?????

airmcn_3

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Most Foamies are developed in a garage. Most people unless they are flying Competition dint take the time to tweak their setups before releasing them to the public, You will find that there are a few out there that have been tweaked and designed without roll coupling, The Yak 54 is one of them. Reflection is another. Both of these birds had a extensive amount of testing and time in them before they were released to the public. In a nutshell its just lack of time in the design, You build something and go fly it and it flys really good, But has a little roll coupling, So then you make a decision. do I possibly mess up the way that it flys and try and eliminate R.C. or do I just leave it alone. The most common answer is Just leave it alone it flys too good now. I hope this answers your question, Try and design and build one and see if you can get the R.C. out of it, You may find it a bit of a challenge. Good luck Chris

E*nealio

My Feedback: (10)

No joke, i have built quite a few and still have had a lot of roll coupling in every one. I thought it might have to do with thrust angle but i have seriously measured it 4 or five times. I build em perfectly straight, but still just a little R.C to the left each time. Just thought it might mean something if they always rolled to the left.

exeter_acres

My Feedback: (2)

Which foamies?

My HL Yak 55 has very little.. and it also needs very little rudder for a HAKE
while others I have had that costed much more were awful

E*nealio

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3D foamy Katana, 3D batix Diablo......Dont know what to build at this point... I really dont want to spend 50 on a foamy but looks like i might have too.....

bob_nj

My Feedback: (62)

There are lots of planes that aren't foamies that have roll coupling to some degree.
Mix it out and have fun_bob

emnm79

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its all about rudder design... counterbalances suck for coupling if you don't make em right...

E*nealio

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that might be my problem? I have no idea how that might efffect the R.C.... Could you expand on that theory?

bdavison

My Feedback: (1)

Ive got one word...TORQUE. Any time you apply left rudder, the added torque of the motor will cause it to roll left. With right rudder, the torque counteracts the turn, so it doesnt dip the wing as much.

Dont think of a foamy as a airplane. Think of it as a thrust vectoring device. You basically got a high powered fan, and the control surfaces just direct its flow. Yeah...I know flat wings can be airfoils, etc...blah blah blah....but a majority of our snappy maneuvers are a result of vectoring the thrust. Thats why we use 60 degrees of throw on our foamy surfaces. You need that much thrust deflection to get it to move.

Think of a hover. There is no forward momentum to produce lift over the wings. So why does it fly? Its basically going from a forward flight airplane where lift is produced by the wings to a helicopter where lift is produced by the propellor. Its flying on thrust alone.

Dont believe me...Pull up into a hover and pull all the power off...
Point made.

To keep it stable in a hover your deflecting this thrust using the control surfaces. As anyone that has hovered knows, what do you normally have to hold to keep it in a hover....RIGHT aileron. Why? Torque.

emnm79

My Feedback: (5)

ok, well, in the case of say a tensor, where the rudder is centered over the thrust line, completely symmetrical, you shouldnt' need any right thrust, theoretically... so, when you have a normal rudder, that is larger at the bottom, it is shaped that way so the force it applies to the fuselage is closer to the thrust line/ vertical CG, follow me? if you have a huge counterbalance on a rudder that doesn't have much taper, and doesnt go very far below the thrust line/cg, when you give it rudder, its like you are pushing the back of the fuselage sideways at the top of the V stab, so because the center of the applied force is not centered around the center of gravity, it will apply a torque, and make it roll... giving it the effect of an aileron with some wierd differential... of course, all plane's vertical center of gravitys don't lie on the thrust line, so you have to consider that.. an easy way to think of it, is just imagine what would happen if your rudder was 2 feet long and way above the plane, don't you think it would spin the plane just like an aileron? take the same rudder and center it to the vertical cog, now what happens? perfect yaw

torque doesn't really apply when you have alot of air over the wings.. these little props don't have enough torque to force the wing through the air sideways (roll)... high alpha is a different story, it is alot easier to turn the wing..

lwatson

My Feedback: (2)

Perhaps all the readers know what Roll Coupling is. But, I certainly don't. Is it?

Put the plane in loop, and it has a tendency to roll one way or the other. So you can correct for this by mixing in some elevator to aileron.

Put the plane in a knife edge, and it has a tendency to roll out of it. So you can correct this by mixing in some rudder to aileron.

Put the plane in a hover, and have it torque roll. (I don't think this is it?)

Please let us know the behavior that you don't like. That way I can test all my planes and see if one doesn't have roll coupling.

Larry

exeter_acres

My Feedback: (2)

That one.... and it will not only roll out but fall to either the belly or canopy.....

My HL Yak 55 has very little coupling at all.....

DHG

My Feedback: (7)

Bdavidson is right (as usual) about the torque, but some airplanes are 'way better than others. I recently wore out my Lee Ulinger Yak-55 and replaced it with a Katana ... now I wish I had the Yak back! I could cut nice KE figure-8s with the Yak, but the Katana says "no way". As soon as I push in enough rudder to hold the nose up, it starts to claw its way out of KE. Lots of cross-aileron will stop it, but that seems too much like work.

Re the counterbalanced surfaces: Once upon a time I modified a Lou Andrews Trainermaster by reducing the fin height by an inch and adding a sub-fin and counterbalanced rudder, like on a Sig Spacewalker. I thought that would improve the rudder effectiveness for better KE. Well, the rudder was about as effective as before, but my modifications gave it roll coupling and also a pitch-down tendency just for good measure. There's no reason that should've happened, but it did. Reminds me of the Harvard Law of Animal Behavior: "Under carefully controlled laboratory conditions, a test animal will do whatever it darn well pleases."

MRMcFarlandM

Hi Guys,
Are U concerned about roll coupling or pitch coupling. Either way try the Ikurus shockflyers they have very little of either. Pitch coupling is some what depends or your cg. So if you have the CG way tail heavy chances are you chances are U will need to mix a little up in with the elevator
Mike,

Charley

My Feedback: (2)

I am concerned that, while there have bben many posts about how to deal (mix) with roll coupling, there have been no posts
about what causes it.

Don't any of you know any aerodynamics?

CR

S1Pitts

My Feedback: (4)

Hi All
I spent my early years of r/c in old school pattern flying. We spent many hrs flight trimming planes as the radios of the day had no mixing to correct RC, etc.

We solved RC by several methods eg: Rolls left with application of left rudder & rolls right with right rudder = add dihedral to main wing or anhedral to horizontal stab

Don Lowe wrote a column in RCM called Flying Lowe & explained many of the issues affecting the flight of pattern planes & how to correct them. I doubt the foamies are immune from the same forces that we battled back then too.
Jim Kirkland wrote an in depth article many years ago (30 yrs) on flight alignment of r/c aircraft. Maybe some of the answers lie in the too basic alignment that the foamies are being produced at.
Just a thought.
KR

exeter_acres

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Guess not

Eagerly waiting your description

Charley

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I wrote too soon. Somehow, I hadn't seen all of the posts in the thread. I'm confused now about what the term "foamie" means. I have a TM-400 that I thought was a foamie but there's at least one post that implies that a foamie is a profile model with only flat surfaces.

Anyhoo, I always thought of RC as the tendency of a plane to roll in the direction of applied rudder. The simple rule, AFAIK, is: the more dihedral, the more RC.

CR

exeter_acres

My Feedback: (2)

that is true,,,,, dehidral and KE don't get along
(byt the by, I was just playing with ya!)

and in this context, foamies are flat depron (or similar) foam airplanes with no airfoil.. (or very little on a couple)
things like the shock flyer, Hobby Lobby Yak, 3Dfoamy.com planes, etc. etc.

emnm79

My Feedback: (5)

on other thing I forgot to mention... side area on the fuselage... if the center of the area is not in line with the center of gravity, it will roll...

tf2psycho

alot or all of it is how you build the thing...
the rest is the plane itself ...

and i hated the recommended prop on my plane
so try different things
mod the hell out of her like you do your video games to make it better
....good luck to u....

E*nealio

My Feedback: (10)

Well this is great. I finished building the new 3Dbatix Extra 330s. This has some great qualities that i really really love. For instance, a rudder turn is nearly horizontal, just like those very "unrealistic" simulator planes that perform everything flawlessly. Next, there is LITTLE roll coupling!! Yahoo!!! The thing takes a little to keep it in knife edge but not as much as the katana or Diablo! I swear by this airframe, i was doing tight knife edge circles around me today at the park. I'll post the video very soon, as soon as i edit "the tree" out of it hahaha.
Anyways, continuing on with this forum....am i just going to use one of my pmixes and dial this thing out, secondly do you guys fly with the mix on all the time or do you assign it to a switch? Im working with a 9C....so i can accomidate both ways.
Thanks for all the input,
Nealio

bdavison

My Feedback: (1)

Good luck on finding any information about the aerodynamics of "roll coupling" or "pitch coupling".

There are three things you have to understand first.

1. Positive stability - if the plane is displaced a little ways from its equilibrium position, it will generate a force tending to push it back towards equilibrium. Like a High-wing trainer with dihedral...roll it, and it wants to roll back to level.
2. Neutral stability - if the plane was in equilibrium and you displace it slightly, it remains in equilibrium. No force is generated. Like some pattern planes, designed to go exactly where you point it, no matter what attitude its in.
3. Negative stability - if the plane is displaced a little ways from its equilibrium position, it will generate a force that tends to push it farther from equilibrium. Like trying to fly a flat wing trainer inverted. You have to fight it.

Ive heard roll coupling explained multiple different ways.
Perhaps the most common is "Artifact of induced drag"explanation. Basically, when you roll the aircraft up to knife-edge, the induced drag drags the higher wing aft. When combined with sideslip (as in knife-edge flight) it produces lateral-directional instability. Then the plane just "falls" in whatever direction other influences push or pull it. Like torque as I mentioned above.

The problem with this is that a "fix" for one problem usually causes a problem in another area. There are a couple of fixes.

1. Adding thrust angles to the motor mount. Usually Down and Right.
This adversely affects the planes linear rolling charectoristics during regular flight though. Some foamies will tend to snap or barrel roll instead of rolling on axis.
2. Increasing tail fin area.
Increased tail weight, requires a movement of the CG, and can make elevator and rudder more sensitive.
3. Decreasing Dihedral
Not really an option on the Tensor or most other foamies.
4. Increasing or Decreasing Sweep on the wings.
A good fix, but also not an option on most "ARF" foamies.
5. Making sure that the rudder does not extend above or below the wings on a bi-plane.

Im going to try to be as nice as possible about this, but its going to tick some people off.

The Tensor4D has some serious issues, and I think it started with its design. Here's what happens.
A designer goes and designs a plane. He finds that it has a small amount of instability in knife-edge. Why doesnt he change the design to fix it. Because you pretty much have to start over from scratch. If you start tweaking the original design, you risk getting an even worse problem. The problem with the Tensor is that its so close to the edge in instability....that it makes the plane really difficult to get to fly right. It was designed to ride the fine line between stability and instability, to take advantage of aerodynamic capabilities that other planes cant do....the only problem is that while its walking the fine line, its leaning towards the instability side.

So how do you fix the Tensor?

Chunk it, and buy a Reflection....just kidding.

hmmmm. First, dont use the rudder so much during knife-edge. If the nose is dropping during knife-edge flight, use throttle to keep it up, or move the CG farther back so that it maintains level flight without needing so much rudder. The more rudder you use in KE, the more instablility and roll coupling its going to get.

emnm79

My Feedback: (5)

agreed on the tensor, but when set up right... oh boy

Flypaper 2

I think my turkey tensor is on that fine edge Cg is set so it sits at the same attitude whether upright or inverted with el. trim. With the throttle set right, does knife edge with no rudder input and fuse is at about a 30 degree attitude. If I move the ailerons over about 1/8 in. from neutral it will do rolling circles with no other input. Fun plane when the wind ain't blowin, A handfull if it is.