twin vertical tails

[frankenfoamy]

A scale project I am working on has twin vertical stabilizers. (like a P38)
In the wind it seems to "hunt" on the yaw access.
Zagis seem to do this as their yaw stabilizers are on the wing tips.

In my model I have three ways to correct this.

Lengthen the boom without changing the fin area

Make the fin taller

Make the fin wider chord

What is the most effective method?

http://rcguy.tripod.com/

[Sparky Paul]

Add area.

[frankenfoamy]

Ok that is 1/2 way there.

vertical or width is best?

[vintage1]

Mm. Shall I be controversial and say reduce dihedral?

[Peter Mar]

Added vert stab area is most effective when out of the turbulence of the fuse.

All else being equal, go taller.

[Sparky Paul]

Ok that is 1/2 way there.

vertical or width is best?
.
Vertical... easier to scab on top than on the trailing edge..
Although..
Flying my Peck Polymer "Genesis" flying wing last week, during a close pass the plane hit the transmitter antenna.. and lost the horn balance on the rudder, a substantial part of the area.
I'd found the plane needs rudder assist when turning. Very reluctant to turn, so I had coupled the rudder to the ailerons.
With the loss of area, it seemed to respond -better- than it had with all the area there. Possibly due to less drag with the horn balance missing.

[frankenfoamy]

very little dihedral as it is.

I added a little area with a subfin (taller) to test.

The vertical fins are canted in a bit and I did not take that inconsideration when I laid out the plan. So the model had a smaller fin than the full scale.
This is a big no no.

[frankenfoamy]

That genesis had alot of rudder up high. The reason a rudder is on the top is for protection. It actually works against the best turning forces on top. Notice the UAV Blackwidow and Predator place the rudders on the bottom. The Blackwidow does not even use ailerons.

[Salto]

I sort of agree with V1. The symptoms sound to me like dutch roll, so fiddling the dihedral and/or the vertical tail volume co-efficient is the way to go.

Graham.

[steve wenban]

I sort of agree with V1. The symptoms sound to me like dutch roll, so fiddling the dihedral and/or the vertical tail volume co-efficient is the way to go.

Graham.
yep I concur

[ChrisP]

My buddy Harald has a Graupner electric sailplane that had a noticeable dutch roll tendency on its maiden flight.

The first thing we did was to reduce dihedral as it was easy to do at the field on this model in our club workshop. We just gave the wire wing joiners a decent whack with a hammer. Dutch rolling was reduced, but not eliminated.
So we then moved the centre of gravity forward as much as we could, about 10mm, to increase the stability margin. Dutch rolling was ALMOST totally eliminated after this.

When Harald has the time he will increase fin area which should knock this one on the head.

Amazing you can buy an expensive kit model that has basic aerodynamic bugs like this.