more on the cause of "adverse roll couple"

[George Kennie]

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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