more on the cause of "adverse roll couple"

[Bill Glaze]

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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