Peter –
Probably no one has covered vertical CG in more detail than Al Rabe, because of his scale-looking, dihedral-winged stunters. Al has a simple, practical, test to find the correct leadout location in relation to the vertical CG:
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[photo not recovered: 27828.jpg]This pic is from topic 301170, the entirety of which is worth reading. (that is not the only thread in which Al has discussed vertical leadout location, dihedral, and vertical CG – do a search if interested) Vertical leadout location is not too complicated if you think about it a moment. Al has shown this picture of other planes; the wings and wheels are always “parallel” to the vertical wall. As you can easily picture, when the plane pictured flies around the circle, the plane can fly with level wings
both upright and inverted with the correct wing tip weight.
If you hang a stock U-Key against the wall as Al has done, the outboard tip will be further from the wall than the inboard tip. You can simulate the effect of additional wing tip weight (when flying upright) by pushing the outboard wing closer to the wall with your finger. You can simulate the effect of additional wing tip weight
while inverted by pushing the outboard tip
away from the wall! Whoa down! Clearly, you can’t get level flight both upright and inverted with additional wing tip weight – it helps to level the wings upright, and exacerbates the problem while inverted, and has a near zero effect overhead.
A trim tab alone will only sort of work. Since all the forces involved (aerodynamic pressure from the trim tab, centrifugal force, line tension, gravity) all vary with speed and orientation of the airplane in relation to gravity, you can get the forces in balance at one constant speed, line tension, and level flight both upright and inverted (a 1 g vector of vertical gravity either way) – that is,
if you don’t do any maneuvers, the wings will stay level at that one speed during level flight. The only
true solution is to locate the leadouts in line with the vertical CG. Al accomplishes this by raising the leadouts using wing dihedral. He could have more easily left out the dihedral and simply put the bellcrank and leadouts on top of the wing – hardly an acceptable solution when building a world-class stunt ship.
There is no inherent (CG) advantage to a leadout guide
in the wing tip,
above the wing tip, or
below the wing tip – it depends solely on where that leadout exit is placed in relation to the vertical CG. That is, Al’s stunters have a vertical CG location issue the exact opposite of the U-Key. So raising the guide is his solution. Lowering it is ours. If Al had a U-Key, I guarantee the leadout guide would be under the wing (or maybe he'd add anhedral
??)
Check this out by hanging
your U-Key against a wall from various locations near the wing tip, then locate
your leadout guide where it needs to go with 100% confidence the problem is solved.
Larry Fulwider
PS: My modified plane in post # 3 did in fact hang vertically in the "wall test" after re-locating the tip guide. As you can see in the center pic, the leadout guide is under the wing. It flew level in all orientations with the trim tab pretty straight after the final trimmming.
LF
Attachment
#1, (jpg file)