more on the cause of "adverse roll couple"

[Dean Pappas]

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
 

Dean Pappas 
Sr. Design Engineer 
Kodeos Communications 
111 Corporate Blvd. 
South Plainfield, N.J. 07080 
(908) 222-7817 phone 
(908) 222-2392 fax 
d.pappas at kodeos.com 

-----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On Behalf Of Nat Penton
Sent: Wednesday, August 10, 2005 5:00 PM
To: discussion at nsrca.org
Subject: Re: more on the cause of "adverse roll couple"


Patrick, the roll axis must be parallel to the wing zero lift line and pass thru or near the vert cg for the aerobatic airplane. All moments about the roll axis must total zero when in yaw.
Moment generators can be:
         1. Dihedral 
         2. CLA vs vert cg
         3. Rud/fin area distribution
and probably others I am overlooking.           Nat

----- Original Message ----- 
From: PPandelaers <mailto:ppandelaers at pandora.be>  
To: geobet at gis.net 
Cc: Discussion at Nsrca.Org 
Sent: Wednesday, August 10, 2005 12:29 AM
Subject: more on the cause of "adverse roll couple"


Hi Georgie, 

 

Would you mind giving your view on the source of roll effect in the first place (without dihedral applied to fix it)

>From the theory books  I read, here's what I understand:

I think what you call datum line is the following:

-  if on the fuselage. the center of lateral area (CLA) and the vertical CG are connected, you get the line which should be the wind-direction during flight as the plane faces it. This is in fact the theoretical datum line, in most construction drawings this line will be shifted up, but parallel with what I describe. So during the design of a plane, you assume a desired flight attitude of the fuselage, in this attidude youmake sure the CLA is on the height of the vertical CG.

- If the CLA is on this theoretical datum line during knife edge ( and I tend to set wing incidence, hence fuselage attidude,  to fly knife edge as neutral as possible), assuming no dihedral, than the only possible contributors to roll are:

*	Motor-side thrust. 

*	Center of lateral area of rudder, which should also be on this theoretical datum line 

Don't forget. The wing isn't flying.

 

Clearly, I must be overlooking something serious, such that we need to apply dihedral to fix it. Can you help?

 

Thanks!!

 

Patrick

 

 

 

What you're experiencing is known as "adverse roll couple"! You can prove this just by flying right side up and applying full rudder deflection in either direction and you will see that the airplane will roll in the opposite direction to whatever rudder you are applying.The cause of the phenomenon is that the wing is located too far below the Datum line without sufficient dihedral to compensate for the offset

 

 



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