adverse coupling

[bobplay33]

Hi,
Adverse roll coupling, such as knife edge flying often entails, is mysterious enough to me; but rolling opposite to rudder input, i.e., right rudder producing left roll, seems cruelly incomprehensible. Doubtless, someone will delight in explaining how obvious it is. Please do so in layman's terms.
Thanks,
bob

New: i'm told that dihedral can fix the rolling opposite to rudder input. If so, why?

[bobplay33]

check that one on adverse coupling.

[Ollie]

Off

[BMatthews]

...New: i'm told that dihedral can fix the rolling opposite to rudder input. If so, why?

Because dihedral responds to a yaw angle by producing a rolling action. It's what lets rudder/elevator gliders turn. It's not the rudder that turns the model but rather the rolling of the wing as it responds to the yawing force of the rudder.

Models in knife edge are using a LOT of yaw thanks to the top rudder. A tiny dihedral angle will make a lot of difference in that case. How much? Depends on how much rolling action you have. It needs to be experimented with to determine how much angle is needed for a specific design. There's no way to know without a LOT of calculating of a very fancy kind.

I think most pattern fliers just couple in a bit of aileron mix with the rudder to get rid of the adverse rolling action.

[Mat]

Is it bad if your plane tends to slide sideways in a normal turn?

[portablevcb]

It depends on what you mean by 'bad' :)

It you have an R/E/T plane it will always slide sideways (skid) in a turn before the dihedral of the wing will induce a bank. If you have ailerons and don't use rudder in a turn it will also 'skid', with the tail 'dragging' through the turn. It is not as noticable from the ground, but, it is if you are in the cockpit. A true coordinated turn requires both aileron and rudder, with or without dihedral.

Knife edges are very tricky if you have any sort of lateral stability built into the plane, such as you get with dihedral. The airplane will try to go back to level flight. You will have to hold aileron to keep it on edge while holding the rudder over to maintain altitude.

charlie

[portablevcb]

PS Adverse coupling is also caused by the increased drag of the downward aileron. That drag will make the plane want to yaw to the direction opposite of the aileron position. If it is severe enough, a left roll input to the ailerons will actually cause a right roll of the aircraft. Not usually a problem if you are flying a well designed plane.