Hi¡¡
This last week-end I made the maiden flight of my scrap build RCM Advance Trainer (BTW, kudos to Mr. Bridi and Mr. Dewey, the designers). It flew great, but with a significant tendency to pro verse roll couple when using rudder. In fact, it seems you can fly the model without the ailerons at all. Strangely, knife-edge were very good and somehow easy.
I thought the reason for the roll-couple was too much dihedral, but the most experienced guy in our field told me that the reason was the tail moment, specifically the rudder. For those who doesn't remember the model, it has the rudder behind the elevator, and I think it could "deflect" some differential airflow to the elevator, once rudder is applied, but honestly, I don't know. As an additional information, the plane was almost impossible to enter a spin, but it was a little nose heavy...

So, has roll coupling something to do with tail moments? Is there any guide to the consequences of using different moments on a given plane I can use to somehow forecast model's behaviors in advance?

Thanks

Tail moment would not provide the proverse roll couple. It's the dihedral plus a high-wing configuration. That roll couple would be intended in the design to help make it more stable for at training situation. You can get the same effect in a low-wing model if you have enough dihedral.

If you look at full-size aircraft, you can see significant differences in the amount of dihedral in low-wing versus high-wing aircraft. Both a Cessna high-wing, and a Piper low-wing have proverse roll couple with rudder application, but the amount of dihedral in their wings is considerable and very apparent visually.

Yes, study aeronautical engineering. I'm not trying to be a wise a55. Using other than trial-and error (copy a known design and make small changes), the math required to predict with accuracy the roll couple characteristics of an original design, is not trivial.

Well written, Bax.

Thank you both...

Bax, dihedral was my first option, I was confused by our expert guy at the field. Make sense. I fly both Cessna's and Piper's, and the effect is not as dramatic as in my model, though...

Mesae, perhaps I miss constructed the question. What I meant was: Is there some (empirical) information about what would happen if I reduce tail moments, generally speaking?
I have seen this kind of information in other subjects, such props, engines, areas, etc. , so I wonder something like this already exists in moments... I don't think I would have enough time to become an aeronautical engineer, and I definitely will test in known designs, as I am doing today. I just wonder if someone else did the first steps.
Probably I should post a new topic...

Thanks again