Cautrell05

I am trying to get a plane of mine in the air for more that 10 seconds at a time. The small version is a 4 ounce 18 inch wingspan pusher canard. Its stable in pitch with good atourity, and stable in yaw. but just after takeoff before its up to speed it rocks or wobbles back in forth(roll). Right now it has two verticals angled out at the top. Im wondering if it has something to do with the shape of the main wing and the placement of the verts. Any ideas? thanks
Nick

Rotaryphile

Your airplane has a lot of lateral area far ahead of the CG, which can destroy yaw stability. I would try taping on some temporary cardboard extensions to the vertical tail fins as a quick check. The CG may be a bit too far back as well, causing the nose to pitch up, and turbulent air from the wing and horizontal canard tail to blanket the vertical tail. Increasing the height of the vertical tail as well as its area might be a good idea, to get the vertical tail into cleaner air when the airplane is at high angles of attack.

Cautrell05

The CG is 1 inch ahead of the leading edge right now. I can move it back and forth a bit with battery placement if I need to but thats where the glide tests are best with the canards just above level. Like I said its stable in pitch and yaw. What I was wondering was if in high AOA if it rolled to one side if the outside fin wasnt getting any airflow and the inside one was. Air pressure on the inside fin pushes it back to the other side and so on and so on. ould a different angle help? Maybe straight up, as well as being wider when viewed from the side?
Nick

Tall Paul

It needs more vertical area aft of the cg.
Add some ventral fins to handle the high angle of attack wobble.

BMatthews

Try the ventral fins but from what you're describing I think it's due more to a sort of cyclic wing stalling that you'll be getting at higher angles of attack. It's probably due to the flat foamie wing section combined with what appears to be just angle cuts to a very sharp leading edge. If the wing's leading edge was rounded instead it may help. The LEX's of the rear wing are there to generate vortices that help delay a stall. But yours may not be big enough or stick out far enough. On stuff like the F18 they curve out for a larger size up near the front and then run back with an ourward taper. A lot of other LEX's share this. On the other hand yours are just simple triangles with a point at the front. Perhaps try making the LEX's larger?

A good way to tell if it's a yaw issue is if the nose is oscillating side to side along with the wing rock. If it's pure or near pure wing rock then it's not the fin area but just the wing's stalling or near stalling in a cyclic manner or responding to a cyclic formation of vortex flow that isn't stable.

WEDJ

I agree. It may be that the wide fuselage is blanketing the rudders, effectively making them useless at high angles of attack.

Tall Paul

There's a video on YouTube showing a model SR-71 flying quite well, despite the world's worst hand-launch, up until the model is slowed down for landing.
It then departs into an unrecoverable nose-high flat spin.
Interestingly enough, the real one had the same problem.
I've tried slopers of scale F-22s and F-117s which depart at high angles of attack.

otrcman

The other posts have pointed out the two most likely culprits: dutch roll or wing stall. Dutch roll is usually caused by a combination of low directional stability and high dihedral effect. Dihedral effect is the tendency to roll due to sideslip. The two sources of wing rock can be hard to tell apart visually because they both tend to occur at high angle of attack.

What you need to do is eliminate one possibility so you know what you are dealing with. The easiest tempory fix for dutch roll is to install much larger vertical tails, preferrably on the bottom of the fuselage. If you put the enlarged verticals on the top, you may be increasing the dihedral effect faster than you can increase the directional stability. If the rolling stops, then what you had was dutch roll. If the rolling does not stop with the larger verticals, then you have post stall gyrations.

Dick