i have built a bi plane (foam) to run only rud and elev. the thing with it though is it is not self righting , if you were to just let it go straight it probably would go for a little bit fine but as soon as you touch the rudder it's rolling back and forth no correcting at all then of course hit the ground.

The only thing I can think of is maybe not enough di hedral or way to much throw on rudder. any help as to what would cause this left to right crazy roll

thxs

If it's rolling hard from the rudder input then you've got lots of dihedral. It sounds like you may have way too much rudder area or rudder travel though. Pictures would help.

What works on a glider flying at 15 mph is going to be WAAAAAYYYY to big on a power model flying at 35 mph. I'm guessing you thought that rudder control wasn't going to be effective enough compared to "all that area" normally found in ailerons?

I have a rudder/elev small model - which will fly inverted and the rudder does not "reverse" when flying inverted
imagine that
better get out the calculator and the airfoil charts It also climbs vertically FAST under full power to OSS in a veryfew seconds - power is a tiny CDrom motor on 11v 3 cell Lipo
rolls are rather barrel shaped- 275 sq in 7 ozs all up

the little bipe in question is apparantly setup incorrectly likely quite tail heavy as well as a mismatch on dihedral and effective rudder location and perhaps a rudder servo/linkage that is dicey

Jeez, Dick, that is a geogeous little model. Is that one of your designs? It's quite interesting that it appears to have quite a lot of tail for the wing area, and that tail has quite a moment. Interesting. What drove that choice of layout?

You've got a couple of performance results that usually don't happen to one airplane at the same time. Interesting bird you built. It'll be great fun developing it.

Self righting (if you mean on the roll axis) usually is a function of dihedral or dihedral effect. Sounds like there isn't any. Do you mean roll or pitch or yaw for "self righting"?

When a model rolls from rudder input, that's usually from too much dihedral or dihedral effect. Yeah, try reducing the rudder throw a lot and see what happens.

Got any pictures?

Let us know what you try and what that does. You've got a very interesting test subject.

The camera angle is skewing the relative wing/tail size - but actually the model is an ARF from CZECH Republic- about 8 years old.
a fibreglass pod n boom and balsa fram covered in heat shrink monofilimant
It's as good a rudder /elev setup as any I have ever flown
mainly due to extremely low wing loading. Ichanged out the brushed motor for the outrunner (CDrom) andthe battery/motor weight savings provided the ballistic climb-just like a old competition free flight 1/2 A but MUCH faster climb.and the vertical is amost hands off! It thermals easily- the radio - a 6100 DSM2 rx and servos weigh 6+6+4 grams
It is amazing what can now be done with small models -
I bought a VAPOR (Horizon Hobby) 16 grams all up and my son flies it for the kids to watch- fully areobatic - rudder / elev- and slow speed -likely 3-4 mph is all that is required for good flying- stunning performance
That thing is a result of "microfilm" design and a 3 gram 2.4 radio-
And to think -all the time I spent on sanding and doping and etc...

OK, you've got a plane that responds well to rudder input but doesn't self-right.
This means you have enough dihedral and you have too much vertical fin area.
The airplane in the photos has a long tailboom and generous tail area, while it has a relatively short span.
When your plane is in curved flight the outside wing has more drag than the inside wing due to it's higher airspeed. Theoretically, if the rudder is in neutral there will be some adverse yaw present, that is the inside wing should be leading the outside wing a bit. This adverse yaw means that the plane is in a slight side-slip, and the dihedral angle will cause the inside wing to have a higher angle of attack than the outside wing.
This will produce more lift on the inside wing, thereby causing the airplane to be roll-stable, or "self-righting".
But you have a powerful amount of directional stability, due to the ample amount of vertical fin area, and the long lever arm (tailboom).
This large amount of directional stability is preventing enough adverse yaw and slip angle to provide a differential angle of attack to roll the plane back to level. And so your plane is "spirally unstable".

If your airplane were to be fitted with a much longer span wing it would tend to be more roll stable.
The wingtips being much farther apart would have more leverage to produce the adverse yaw required to make for stability.
Or, if the vertical fin were reduced in size, or moved closer to the wing (and the cg) you would have less leverage, more adverse yaw, and therefor more roll stability.
After a while we manage to "calibrate our eyeballs", and just looking at a design will give us a good idea of it's stability and handling qualities.
When I first looked at the photo I saw the generous "Tail Volume Coefficient" created primarily by the long tailboom, in comparison to the short span of the wing. It looks funny, and apparently "flies funny", according to your description, anyway.

You can learn to live with the roll instability, but you'll want to tone down the rudder power by reducing the throw. Otherwise you'll be constantly making inputs to either increase or decrease the bank angle.
If it was mine: I'd try reducing fin area, as that would be easiest, unless I had a much longer wing to just strap on it.
But it is a pretty little plane. The proportions are like an indoor free-flight rubber-powered plane.