stab placement

[george kennie]

Jimmy,
The fact that you reference "adverse roll couple" indicates that the
wing does not have enough dihedral, however, the fact that it only
requires 4% correction means that it's not very far off. The fact that
the wing is 2.5" below the datum line increases the pitching moment of
the engine thrust, which is the same as increasing downthrust which is
pulling to the belly big-time with the additional 2 degrees of built in
down thrust. Also right thrust contributes to the downthrust equation by
virtue of precession so  no matter what you do with the stab on this
airplane you will probably not achieve your objective.You've really got
an airplane that  needs a major make-over. On my designs I try to place
the center line of the stab on the same plane as the center line of the
wing with 50% dihedral compensation. In other words if the wing has 1/4"
dihedral I raise the stab 1/8" above the centerline of the wing.
I think I would try to check the motor thrust with some nice vertical
up-lines and see which way it's pulling.
All the above being said, I should add that no matter how carefully you
try to design and no matter how thoughtfully the design process is
approached, your best efforts can still be easily thwarted by many
gremlins that just seem to be waiting to attack the best aerodynamic
concepts.
Georgie

"Woodward James R Civ 412 TW/DRP (Test Ops)" wrote:

> Hi All,
>
> I was having a conversation about stab placements with someone and
> wanted to gather some opinions.  Imagine the following setup:  Main
> wing is 2.5" below thrust line and at 0.25 degree positive incidence.
> Stab is placed 0.5 - 0.75" or so above main wing, and at zero degrees
> incidence.  Engine down and side thrust are 2 and 2.5 degrees
> respectively.  Knife edge flight requires 10% up-elevator mix to
> rudder.  CG is set so that when flying inverted, it requires just a
> little touch of down elevator (not tail heavy, Inverted 45 degree
> uplines do fall to earth ever so gently).  Plane has 4% aileron to
> rudder mix, so that adverse roll couple is removed when flying
> knife-edge.
>
> "In - General" - if the stab was lowered to being 0.25-0.5" above the
> wing CL, would the mix require for K.E. flight be reduced?  If the
> stab is lowered, would this impact the adverse roll-couple at all?
> Or, would raising the wing effectively reduce the pitch and adverse
> roll couple?
>
> Thanks,
>
> Jim
>
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