chrisp32

i understand that when you deflect the rudder to the right or left the corresponding wing will dip caused by the reduced lift from the wing being slowed. i am wondering then why does the plane also roll when it is in knife edge and why id doesnt when it is inverted? what is all goin on that i am not getting? thanks in advance for the education and insight


thank you
Chris

wellss

Sounds like the effect of sweep. At positive angles of attack, you get roll in the direction of yaw. At negative angles of attack, the opposite.

Dihedral is a factor. The plane will always roll in the direction of applied rudder, regardless of attitude. The effect of the fin/rudder acting like half a wing/aileron, is also a factor, depending on how far above the CG it is.

LouW

The tendency to roll with application of rudder is not due to any momentary speed difference in the wings. Instead, when rudder is applied, the tail swings to point the nose in the direction in which the rudder is applied. This swing causes the airplane to move sideways through the air (yaw). If the wings have dihedral, the wing in the direction of motion hits the air at a greater angle (angle of attack) which increases its lift while the other wing has reduced angle of attack (less lift) so the airplane rolls opposite the direction of yaw.

This tendency to roll opposite a sideslip is called “dihedral effect�. While actual dihedral of the wings is by far the most powerful influence, other things effect this tendency. Placement of the wing on top of the fuselage will result in some dihedral effect even though the wing is flat. Placement of the wing on the bottom of the fuselage will have the opposite effect. This is the reason many low wing aircraft have more physical dihedral in their wings than similar high wing planes.

Sweepback will produce dihedral effect. This is why airplanes with a lot of sweep are usually built with negative dihedral. A tall fin and rudder will produce some dihedral effect,

In knife edge flight the airplane is in a continuous sideslip. The tendency to roll opposite a sideslip (dihedral effect) will cause it to try to roll back level and aileron must be held to oppose the tendency. In this case it is sometimes called “roll coupling�.

Some dihedral effect is desirable in trainers and general sport aircraft because the tendency to maintain level flight makes the aircraft easier to fly. In aerobatic aircraft which involve knife edge flight and other sideslip maneuvers it is generally not.

mjfrederick

Your first assumption is not always true. It depends on the design. My pattern plane does not have roll or pitch coupling with rudder application (or so little it can't be noticed). I can do a 360 degree rudder-only turn without using ailerons or elevator to maintain altitude. And no, I don't use any mixes.

Lnewqban

Chris:

Check these links:

http://en.wikipedia.org/wiki/Dihedral

http://en.wikipedia.org/wiki/Dutch_roll

http://en.wikipedia.org/wiki/Spiral_divergence

Regards!

Tall Paul

I have a couple of planes where using rudder when landing is very dangerous! The nose drops and the plane rolls rapidly.. My Stearmann is a particular offender with this.

Lnewqban

Paul:

I have a similar problem to solve, but I don't know how.

I have researched about rudder-pitch coupling; however, I don't fully understand the forces that produce that reaction, or how to correct it.

Can anybody help?

Regards!

rmh

It's all case by case
here are a few things
the crabbed attitude from adding rudder can reduce the effectiveness of the horizontal stabilizer
this makes the model pitch down .
Some guys try to fix this with cg shifting -and that can help.
However in some cases the rudder will overpower any reasonable cg setup.
case by case
One a plane starts skidding, the entire airframe adds different forces.
but if you have a fairly common sporty plane such as a semi scale EXTRA- shift the cg a little - or add a little elevator up with either rudder application
and watch - and see how increasing rudder application changes things
On each type model setup, the application of rudder interacts differently
NO hard fast rules .
Some -the rudder makes a roll in th e direction of the rudder application
Some- the roll is opposite
Some- the model pitches up
some - it pitches down
Some- it simply yaws and does not change direction of flight
Some it turns evenly , level, in a circle
The text books cover a little of this but not all of it .
The reasons are because the speed ranges , relative fuselage area and location on models , is often far different than full sized passenger carrying craft.
It is a cut n try thing.
The guys who actually have experience in flying the stuff will be of best help .
ask around .

Tall Paul

I believe it's a "feature" of the orientation of the inertial axes of the plane, with the longitudinal axis is skewed, passing under the rudder, which gives the rudder a moment arm around the longitudinal axis. This rolls the plane with rudder deflection.

rmh

That may do it
proove it by adding an opposing rudder beneath the rudder with reversed throw.

Lnewqban

Thank you very much, Dick and Paul.

It is an interesting reaction.

Regards!

pimmnz

With the application of any control the model will slow slightly. With most normal cg positions and trim setups the airplane will attempt to regain speed by dropping the nose. It is not really an 'elevator' effect as you don't have to move the elevator for this to happen. Take a model, any model, out to the field, get it dong a bit of 'S&L' all by itself, then apply any control and hold it. Note that, if you hold it long enough, the model will impact the ground. You can do this with either aileron or rudder, without touching the elevator at all. All to do with the lift being angled off with the attitude of the model, and not being allowed to correct because the continued application of the control.
Evan.

MTK

Do I understand it right? Model will pitch down and roll in the direction opposite of applied rudder? That is, apply left rudder and the model violently pitches down and rolls right simultaneously.

A simple fix for such a state is to increase dihedral in the wing

MattK


edited to correct quote display