more on the cause of "adverse roll couple"

[Bob Richards]

Dean,

I agree with everything you said.

I have a couple of rather simplistic theories that
seem to work well when it comes to explaining
roll/pitch coupling. Take two airplanes with zero
dihedral, one a high wing and one a low wing. The high
wing will have proverse coupling, the low wing will
have adverse coupling. My theory is that the fuselage
will have high pressure on one side and low pressure
on the other side, when the rudder is deflected. Not
unlike a wing. :-) There will be a natural line of
separation close to the center of the fuselage (with
no wing or stab). If the wing is not centered in the
fuselage (close to this line of separation) there will
be a difference in pressure between the top and bottom
of the wing as a result, and will be opposite between
the left/right wing panels. This will result in a roll
coupling. Same goes for the stab location, a low stab
location will pitch to canopy, a high stab will pitch
away from the canopy. (And it will also have a small
affect on roll coupling). My Cap21 pitched horribly to
the belly in knife edge, took about 30% mix as I
recall. Not surprising since the stab was on top of
the fuse.

The real problem with using mix is that the required
mix is never linear.  A small rudder deflection might
not need much mix %, but large rudder deflections can
make the plane really stupid. :-)

In my opinion, the wing/stab position and dihedral
have a much larger effect on coupling than the
vertical CG. Also, it is much easier/practical to
affect a change in the dihedral and stab location than
it is to significantly change the vertical CG
location. Think about it, if you lower the wing, you
have lowered the vertical CG which you would think
would cause proverse roll, but it usually (always?)
causes the opposite.

Bob R.

--- Dean Pappas <d.pappas at kodeos.com> wrote:

> Hi Nat,
> Just a further complication, that if I remember the
> original E-mail, may be useful.
> If your plane pitches to the belly AND rolls adverse
> with rudder, or pitches to the canopy AND rolls
> proverse, then it is possible and likely that you
> have only one problem, and not two. If you fix the
> pitching, then the roll may be reduced, or if you
> stop the roll, the pitching may be reduced. In
> general, if a rudder to aileron couple fixes things,
> you will have less interesting behavior with rudder
> corrections in looping maneuvers. This is because
> most designs have an angle-of-attack sensitive
> yaw-to-roll couple. That knowledge can save your
> plane if you ever take off with the ailerons
> disconnected: slow down, get the nose up, and turn
> with the rudder. At high AOA, the plane will roll
> like a high wing trainer (well sorta!)
>  
> Regards,
>     Dean
>  
> 

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