more on the cause of "adverse roll couple"

[PPandelaers]

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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