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Rudder and unexpected yaw behavior

#1
Hi dears,

I enjoy building my own airplanes and recently I have built 2 of them that the rudder is causing an unexpected yaw behavior. Please, help me to find out what is going on.

Before start I would like to point out that the controls are not reversed, and, besides the rudder, all other controls work very well and the airplanes fly really nice as long as I don't move the rudder stick.

So, what we expect when we move the rudder stick to the left is the rudder to move to the left too ( / ) which will increase the tail pressure to the right then the airplane nose will go to the left and also banking a bit to the left. But even though the rudder of my airplane also moves to the left ( / ) my airplane somehow nose dive to the right and I don’t see any sign of the normal yaw behavior. I have no clue what is wrong with it.

Here you can find the picture of 2 different airplane with the same rudder style that are behaving in the same way.

The airplanes are about 900 mm wingspan, tip chord 120 mm, root chord 220 mm, 700 mm fuselage length, 65 mm fuselage width and 700 grams when it is ready to fly.

Thanks for your help!
 

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PsyBorg

Wake up! Time to fly!
#2
The tail on the blue and white one looks all wonky and leaning... But that could be illusion from the picture angle. get some pictures from behind the plane looking down the fuselage so we can see how it all lines up with the main wing.

The other thing I am thinking is somehow you got a mix with the ailerons going on. Almost like you did a mix for turning but got the ailerons set the wrong direction.
 

Merv

Legendary member
#3
I agree with @PsyBorg, it could be a mix & it looks like the rudder is not vertical, it’s leaning to one side. If it is leaning, it acts like a V tail, left rudder will also act as down elevator.

If a mix, the ailerons could be going the wrong way or they could be going the right way and causing adverse yaw. Adverse yaw happens when the down aileron goes down more than the up aileron goes up. This causes more drag on that side, yawning the plane in the opposite side.

Make sure the up aileron goes up more than the down aileron goes down.
 

quorneng

Master member
#5
Assuming no aileron mix (electronic or otherwise) my own guess is a small fin and rudder with a substantial tapered wing with no dihedral. The rudder and fin are above the wing so the side pressure say to the left will tend to make the plane roll right.
Turn by banking with the ailerons (it is what they are for;)) and use the rudder just to counter any adverse aileron yaw.
Those planes are not 'rudder turn' types.
 
#6
Assuming no aileron mix (electronic or otherwise) my own guess is a small fin and rudder with a substantial tapered wing with no dihedral. The rudder and fin are above the wing so the side pressure say to the left will tend to make the plane roll right.
Turn by banking with the ailerons (it is what they are for;)) and use the rudder just to counter any adverse aileron yaw.
Those planes are not 'rudder turn' types.
Liking that. (y)
 

Tench745

Master member
#7
Assuming no aileron mix (electronic or otherwise) my own guess is a small fin and rudder with a substantial tapered wing with no dihedral. The rudder and fin are above the wing so the side pressure say to the left will tend to make the plane roll right.
I second this idea. If you extended the rudder to the full height of fuselage and fin, and maybe made the lower portion a little larger it would even out those rolling tendencies.
Another less likely possibility is that your v-stab is not stiff enough torsionally and when you give left rudder the whole fin twists instead, effectively giving right rudder. To explain it another way; the rudder may be acting like a trim-tab and the vertical fin acting like rudder.