Howard is absolutely correct...and as usual, this belongs on the engineering board. That anyone expects this question to have a convenient "home remedy" answer is unfortunate. I don't care whether one WANTS it to have such an answer; it DOES NOT.
Now that the majority have tuned out to beliebve what's comfortable, I'll @#$% uphill or into the wind with the rest.
FIRST, it is not reasonable nor more "understandable" to use technical terms wrong and then state that you know what they actually mean to the masses. Nose moment means the torque around the center of mass exerted by the sum of the moments (torques) ahead of it. NOTHING MORE! If you choose it to mean the "distance" from the "whatever" to the leading edge, it is still absolutely meaningless, because it does not describe accurately what actually happens when you place a weight there. You can kid yourself all you want and suggest that it's healthy for others to think this way, but it is inaccurate in any inferences you may choose to draw from such characterizations. Forces that act ahead of the c.g. exert torques on the plane that are proportional to their distance from the c.g., NOT the leading edge. There is significant and quite measurable difference, and they have resulted in numerous errors, for instance, in adding prop extensions on light weight engines for certain purposes. These have resulted in historically disadvantageous tail sizes and lengths.
Hinge to hinge measurements too are not proportional to what really happens when tail moments and volumes are changed. Such measurements are irrelevant to real-world happenings. The only reason they seem - that is SEEM - to work is that most stunters have settled on the same approximate configuration, and hodge-podge solutions in trim "cure" design mistakes.
These are the effects relevant to the question, and their analysis and use are not all simple and NOT always easily understood.
1) Wing taper determines MAC placement, which in turn determines how far in on the span the a.c. lies. The more the taper, the further inboard the a.c. lies, until the taper ratio is zero (pointed wing), in which case the theoretical a.c lies at 1/3 the half span. In reality, all a.c.'s lie inboard of their computed value, because of tip losses and resulting lift distributions. This is significant to wind gusts, which is why more highly tapered wings like the "Firecracker's" are preferred in Australia and why elliptical wings are considered better in the wind (gusts). Leading edge sweep also moves the a.c. further aft of what any "home remedies predict; the famous icon of stunt design mentioned above once made that mistake and apparently wrote off a viable design, built later successfully by others who then placed the c.g. appropriately (you do not place the c.g according to root chord with any significant leading-edge sweep). Taper allows wings to approximate more closely the proved ideal of elliptical lift distribution (although idealized root bending moment calls for a variation). The limit is dictated by Reynolds numbers near the tip - too short a tip chord leads to inefficiency, flow separation, and tip stalls.
2) Spanwise-straight or swept back hinge lines should be more stable in yaw on normally tapered wings, since the forward yawed wing will usually have less lift and drag than the aft one with the more perpendicular hinge line and more projected (usually) planview area.
3) in order to achieve the straight hinge line and still have reasonable taper, the leading edge must be swept back. If flaps are reasonably proportioned (talk to Ted, Howard, or Brett) then the amount of sweep is determined by preferences of efficiency and wind (gust) performance. Sweep back causes a dihedral effect while otherwise stabilizing a wing in respect to yaw, due to drag. The dihedral effect causes complications in roll with gusts, and there is always a yaw-roll couple, meaning simply that if you cause either yaw or roll, you almost always get the other too.
4) Control line leadout position relative to rake helps diminish the yaw and roll effects. A lot of control-line fliers don't care. Some do.
Ultimately, it is comfortable for people to claim that these things like hinge-to-hinge or "nose moment" (whatever that actually is supposed to mean) measurements are significant, but they are not meaningful past the point that in a "normal" stunter, a longer one of these measurements means that the proper and significant measurement is also longer. HOWEVER, they are NOT proportional, and such fictitious "accepted" measurements, when actually used quantitatively, give the wrong answers, which are then gleefully accepted as meaningful, after trim has been applied to mask their shortcomings.
Do we always have to denigrate people who just try to use perfectly understandable terms by bastardizing these terms and casting aspersions on the character of people who use them correctly and post over and over all that's needed to truly understand? She-e-e-e-sh!!!
SK
Topic: Leading edge sweep (Read 23839 times)
