more on the cause of "adverse roll couple"

[George Kennie]

Yeah, and then there's my Pro's Acq that has the wing ,stab on the datum line
with everything at zero and it pitches to the belly at 20%. Go figure.

Bob Richards wrote:

> 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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