Are you now going to show the difference between IC and electric?
Static solutions are nice, but don't explain the difference.
I have no idea why the CG needs to be (or can be) different. I also only have weak arguments for the aft leadout position (although I think most people are not actually running a static yaw offset). But I am concerned with the rest of the theory and I have seen nothing since to change my mind, far from it. The original was in 2004 on SSW, as I recall.
The point I made before and still see is that there are a few things missing from Igor's illustrations once you try to turn a corner. Certainly they are correct as far as they go, but the kinematic and dynamic effects are missing.
Take the last drawing, where rudder offset is balanced with the leadouts. I think we all generally agree that you have to have that, regardless of what yaw angle you want to run. If you have rudder offset, you have to move the leadouts back to compensate or you end up with wild line whip reactions when you start to maneuver.
To turn without incurring any sort of disturbance, the airplane has to rotate around the vector Fc (the radial vector, from the pilot to the CG). The problem is, unless you do something to force it, *it can't and won't rotate around that axis* because they aren't the principle axes. Barring other inputs, it wants to rotate around the Y axis of the airplane, and that isn't aligned with the radial vector. If the airplane rotates around its own Y axis by 90 degrees, it winds up vertical with a large roll angle, effectively appearing to have inadequate tip weight. The usual solution is to add tip weight to keep it from hinging. But the effect of tip weight varies depending on the pitch acceleration, so you can't have it balance at all rotation rates (including zero). This leads to what I previously called "Twister Disease". No one appears to have been able to resolve this satisfactorily that I have seen over *many* examples.
A possible solution would be to skew the principle axes with respect to the airplane, so the momental Y axis lines up with the radial vector at whatever yaw angle you want to run. You can skew the principle axes around the yaw axis by adding mass at the inboard tip TE and the outboard tip LE, and it has certainly been done. The problem is that to get any consequential shift, you have to use a lot of mass and effectively that means you can only make small adjustments rather than wholesale changes. And, in any case, if you were to successfully get the principle axes shifted around, now you get huge aerodynamic torques because, for example, the lift from the tail is centered in the wrong place, there's a huge dihedral effect, and a host of other issues.
The alternative is to create some torques that oppose all this. I can easily believe that the moving rudder can compensate for a torque that appears as a function of the pitch rate as was intended. Precession is certainly that way, but the kinematic effect is completely independent of the rate and cannot be corrected by moving the rudder in the direction of the elevator. You might be able to move the leadouts off the natural "trail" position and use the dihedral coupling roll and yaw to induce a yaw angle that then rolls the airplane as required, but that also requires that you put force on the lines that almost guarantees that the line whip will get excited.
This was all hashed out long before in various SSW posts. The method I recommend for adjusting the rudder and leadouts will align the geometric axes of the airplane to that Y lines up with the radial vector and remove most of the roll.yaw dihedral effects. Building the airplane with matched parts from side to side will get the inertial Y axis lined up pretty close to the geometric Y axis. I think it's close enough that small dynamic balance effects and precession are both in the negligible range most of the time, or at least close enough that other effects like details of workmanship will swamp the effect. If you have enough passive yaw stability and don't use excessively large props, and don't build too light (which increases the line tension stabilizing forces without exacerbating the other issues), it's below the threshold of caring about it.
I have seen next to no one able to trim with a significant static yaw angle successfully. The few exceptions are those case (like the Firecracker) where there is very little passive yaw stability so that slewing around in yaw doesn't encounter any significant impediment - not that it doesn't yaw, it just doesn't care. Otherwise you get wild fishtailing in the maneuvers. The worst example I have ever seen in an otherwise competently constructed airplane was a 54-ounce, PA65 airplane with a 3-blade Bolly 13-4 or so and a very large fin with lots of offset.
big fin = lots of passive yaw stability/authority
offset fin = lots of aerodynamic yaw torque
huge very heavy prop = lots of precession and other aerodynamic destabilizing force
abnormally light weight = reduced stability provided by the lines and in conjunction with very heavy .018 lines, huge influence of line whip effects
This airplane was in an almost continual pretty high frequency roll/yaw oscillation that only damped out in the two laps between maneuvers in perfect air. Anything else, and it got upset, and stayed upset with very light damping. It would make about 3 swings in the space of one leg of a square loop, for example.
Topic: Triming Question....Wing Waggle (Read 13249 times)
