I am trying to learn how to hover a fixed wing aircraft.
The rudder on my plane is triangular with the largest surface of the rudder below the horizontal stab.
While using the rudder to maintain yaw in the hover, won't this tend to make the plane roll?
If the rudder was rectangular with approximately the same surface area above and below the horizontal stab it seems this would neutralize this.
Am I thinking wrong?

The short answer is no. Most aircraft do not tend to roll badly with rudder input while in a hover, even if they have signifigant roll coupling in level flight. And no, I can't explain that one.

Second, it depends on your plane in particular whether the rudder shape will cause it to roll with rudder input. There are several other factors that influence that, and your rudder's shape may actually be designed to negate that. Some other factors that influence roll coupling are wing placement (high, low, mid) dihedral, horizontal stab placement, verticle stab and rudder placement in relation to the datum line, as well as the rudder's geometric shape. By the way, what plane is it?

What plane are you flying? Because the answer is the standard "it depends".

Aerobatic planes frequently have rudders larger at the bottom than at the top in order to move the the rudder forces closer to the center line of the aircraft. One of the reasons this is done is to reduce the tendency for rudder application to cause some yaw-roll cross coupling.

It's not the rudder's relationship to the stab that causes this phenomenon but the rudder's relationship to the aircraft center line.

If most of the rudder is on one side of the CL, deflection of the rudder creates the intended yawing moment but since the force is all on one side of the CL, it will also try to rotate the aircraft. Think of it as an aileron on only one side. No force on the opposite side of the fuse so it causes some roll.

If the rudder area were equal on both sides of the CL, the yaw forces would be equal above and below the center line so there would be no tendency for rudder to cross couple to roll. Think of it like an elevator, except it's vertical. The forces are balanced so there's no roll.

Since the rudder usually doesn't extend past the bottom of the fuse, the way to move the rudder force towards the center line (and thereby reduce yaw-roll coupling) is to reduce the area at the tip of the rudder and increase the rudder area near the center line of the fuselage. Thus the generally triangular rudder shape you describe.

Dave

The plane is a Taco.- Sorry, I should have included that in my original post.

Ahh- a Paul Swanson design. I don't think you'll need to worry about fixing his design before you fly it. He does a lot of testing before he releases anything. I've not built the Taco, but if it's anything like the Mojo, you'll love it.

The wing is not loaded in hover. It's the only reason why the model or full scale plane will allow the engine torque to roll the crate

In my experience the rudder shape (of reasonable size and configuration) plays practically no role. Other model parameters as you noted (namely wing design and placement and dihedral angle) play the biggest role. Stab location vertically on the fuse also plays a very, very minor role.... vertical movement would need to be extreme to really show any effect on most low-mid wing planes

MattK

We've got two different situations being described in this thread. And they're worlds apart.

When our models go into almost zero airspeed flight almost all aerodynamic rules are turned off. The airplane is being dragged around by the prop and any movements come from prop blast on the large surfaces and their large deflection. And ideas like roll couple really don't apply like they do when the airplane if flying. So basically all the things LDuck said about things like roll couple are true in flight. And relative location of surfaces do matter in flight as stated.

Flight is a different world than 3D. And aerodynamic things are well known for flight.

And we need to be clear in these discussions if we're talking flight or 3D.

Yes indeed we do....I answered both of the man's questions in order. Did you read what the man asked?

edit.......................
Yes, I certainly did and since I wasn't answering his post, but trying to give other readers some help sorting what had been posted as answers, I don't see the point in your question.
Since our discussion with each other is off topic it's going to end in this thread right here. ..................... da Rock


edit.......................
And if you read from the top, his question was answered in the first sentences of the 2nd post. da Rock