more on the cause of "adverse roll couple"

[Bill Glaze]

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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