Low wing aerobatic plane dives using only rudder??

[hoppy]

My ROTO, a semi symetrical low wing aerobatic plane has rudder/elevator/aileron control surfaces. All work freely and do not interfere with each other. No mixing has been added. Problem: When given full right rudder, which is about a 1" throw, the plane does not turn but noses sharply down. Anybody know whats happening?
hoppy

[steve lewin]

How much dihedral do you have on it, if any ? How much is 1" in degrees ? It depends on the size of the plane whether 1" is a little or lot.

Rudder will only cause the plane to yaw not to turn or bank unless it is helped out by dihedral. Any pitch (up/down) coupling will depend on all sorts of possible factors including the relative positions of all the surfaces, thrust angles etc. A picture of the plane might help.

Steve

[hoppy]

Thanks Steve,
It was a little lighter today with the sun out and I was able to see what the plane was doing a little better than yesterday.
Rudder throw is about 28deg. Power is just enough to do a loop from level flight. It defiinitely is on the mild powered side. Speed 300, 7:1 box, 10x7 prop, 8x800NiMH pack or 2s1200 Lipo. AUW 14oz. Plane has about 4deg dihedral (total angle between both wings).

What the plane does on full rudder application is a fast yaw of about 22 deg and than the nose drops.
My guess is that after the yaw, the plane stalls and the nose drops. Does that sound reasonable?

Does this mean the plane won't be able to do a knife edge? hoppy

[RSands]

Not at all, pitch to the belly is common with rudder application. The slipstream off the prop can sometimes aggrivate the effect, so if possible, try a different prop. Some radio mixing (given you have a computer radio) of up elevator with rudder application can make this easily controllable, if not eliminate the problem completely. CG can also have an effect, which in your case, a more forward CG may help.

[Sparky Paul]

I was flying a scale Ultimate today... serious nose-down pitch with rudder... enough to snap from level flight, but mild activity in KE.
Another bipe I have is dangerous due to pitch with rudder when landing.
Why?
I dunno!

[Ollie]

Here is a theory:

The full rudder deflection and yawed fuselage create enough drag to slow the model appreciably. The decreased airspeed reduces lift. The nose drops to increase airspeed and lift. The yawed fuselage wake slows the air flow over one side of the wing root further reducing lift on that side and, all that yaw, together with the modest dihedral, initiates a roll. The bank reduces the vertical component of lift and even more airspeed is needed. The plane noses down even more to regain the needed airspeed.

You could hold the wing up with opposite aileron to turn the maneuver into a side slip. If your airspeed is high enough to provide generous stall margin, a little up elevator could keep the nose up some without stalling.

[hoppy]

Thanks guys for the inputs. Todd of Todd's models suggested I tilt the axis of the rudder hinge line back 3/8" at the top. I tried this and it eliminated the yaw/dive problem. It just turns slowly now while losing a little altitude like a rudder plane would do.
hoppy