more on the cause of "adverse roll couple"

[Bill Glaze]

Nat:
I agree with your postings.  It may be possible to cure the tucking 
problem by shifting the CG, but that also brings along it's owns set of 
illnesses.  I was just curious about Georgie's case, whether or not he 
had shifted the CG and found any effect.  I agree that, by curing the 
tucking (Positive or negative) with a CG shift, that it may make the 
airplane virtually unflyable in other, more frequently used, regimes.  
(Such as straight and level! ;-) )
Bill glaze

Nat Penton wrote:

> Bill, some years back a friend of mine was competently flying a west 
> coast design with wing and stab on the thrust line, O-O. He had to 
> move the CG to 50% MAC to cure the pitch up and the airplane only 
> required about 1/4 inch up/dn elevator. I notice he did not stay with 
> the design !!
>                                    Nat
>
>     ----- Original Message -----
>     From: Bill Glaze <mailto:billglaze at triad.rr.com>
>     To: discussion at nsrca.org <mailto:discussion at nsrca.org>
>     Sent: Saturday, August 13, 2005 10:20 AM
>     Subject: Re: more on the cause of "adverse roll couple"
>
>     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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>>>>>
>>>>
>>>>
>>>>
>>>>
>>>>=================================================
>>>>To access the email archives for this list, go to
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>>>>      
>>>>
>>       
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