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Jimmy,
<br>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.
<br>I think I would try to check the motor thrust with some nice vertical
up-lines and see which way it's pulling.
<br>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.
<br>Georgie
<p>"Woodward James R Civ 412 TW/DRP (Test Ops)" wrote:
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<div class="MsoNormal"><span style='font-size:10.0pt;
font-family:Arial'><font face="Arial"><font size=-1>Hi
All,</font></font></span></div>
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<p class="MsoNormal"><span style='font-size:10.0pt;
font-family:Arial'><font face="Arial"><font size=-1>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. </font></font></span>
<p class="MsoNormal"><span style='font-size:10.0pt;
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<p class="MsoNormal"><span style='font-size:10.0pt;
font-family:Arial'><font face="Arial"><font size=-1>"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?</font></font></span>
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<p class="MsoNormal"><span style='font-size:10.0pt;
font-family:Arial'><font face="Arial"><font size=-1>Thanks,</font></font></span>
<p class="MsoNormal"><span style='font-size:10.0pt;
font-family:Arial'><font face="Arial"><font size=-1>Jim</font></font></span>
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12.0pt'></span></div>
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