more on the cause of "adverse roll couple"

[George Kennie]

Yeah Bill,
I remember Hanson trimming this way. He shifts the C.G. 'til he gets
zero pitch couple on K.E., but on a plane that pitches to the belly
strongly you end up with a plane that pulls to the canopy on down
lines and is elevator pitch sensitive to speed changes because you
end up carrying too much up trim. It can really go round and round.
G.

Bill Glaze wrote:

>  Would a change in CG have any effect on the tucking?  r did yu
> ever try it?  Just curious.  Bill
>
> George Kennie wrote:
>
>> Really can't remember, Bill, but I think it was around 35%.
>>
>> Bill Glaze wrote:
>>
>> >  Georgie:  CG?  Bill Glaze
>> >
>> > George Kennie wrote:
>> >
>> >>  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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>> >>
>> >>
>> >>
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>

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