more on the cause of "adverse roll couple"

[George Kennie]

Patrick,
Don't want to shake you up too badly here, but books on theory I
ain't got! I only know what happens when I design my own airplanes.
There are so many variables that enter into the equation that it can
make your head spin.I have found that some of the strongest
contributors
to the roll scenario are dihedral and the center of pressure of the
rudder. I know that if you have a ship that has proverse couple you
can correct
the problem, or at least affect it in a very substantial way, by
making the rudder taller and if that doesn't fix it, stick a balance
tab on the front of
the rudder ahead of the hinge line overhanging the top of the fin,
and if you go too far you'll end up with adverse couple.However, I
have found that the inverse of this
procedure, i.e., adding area to the bottom of the rudder is less
effective than the stuff you do at the top of the rudder and my
guess is that it's harder
to get the same leverage because the bottom is closer to the datum
line. So, knowing what effect the rudder produces, while it is
helpful knowledge
is still skinning the cat with a rubber spatula. The proper
aerodynamic fix is to saw the wing in half and add the correct
amount of dihedral to each
panel and if you tell me how far below the datum line the center of
the wing is, I'll tell you how much dihedral you need to add
(however, this figure
could be affected be the current shape of the rudder and may end up
being erroneous). Of course, as I mentioned before, the Edge 540 has
already
dealt with all these areas of frustration and flies appropriately.
It's getting close to auction time.I think you may have some
desirable equipment.
Good Luck,
Georgie

PPandelaers wrote:

>
> 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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