I've been watching this discussion for about a week. I am using the JSBSim model on a
real airplane development project, to give you some background.
Seems to me that the question is about how to make a 3D model look right from outside,
probably from a distance of several wingspans away. In that case, you do not have to be precise
about the congruence of the point of rotation and the cg, so long as you are not using the
rotation point for any of the physics. I would guess that the cg is near the 25% chord point, and
within a foot or so of the center of the wing vertically, so that if you know where that is relative to
the nose, or other origin point, you can make it look right in flight. I'd vote for 25% chord point
on the wing center line for rotation. If the model uses the nose as a reference, you would just
have to know the correct offsets, or estimate them from a side view.
You could have a problem at or near the ground for the reasons discussed before. On the ground you
rotate about the gear, not the cg. You could either accept having the wheels sink into the runway
(or float above it) or incorporate an "gear offset" when there is any WOW. I suppose this could jerk the
plane onto the ground if the point of rotation is set low, but, on the whole it would give you a pretty realistic flight. I suppose you could resolve this by empirically setting the vertical CG position so that the
gear looks right at rest, from outside, by cut and try.
The FDM model must know where the CG is. The 3D view must know where the wings are in relation
to the nose, etc. Where the MAC is on a swept wing is a matter of educated guesswork, unless someone
tells you, but you could come pretty close visually. I would suggest that you try something like rotating in the air around something simple like the 25% chord guesstimate, on the centerline, and then adjust vertically to have the gear just touch the ground at rest. How does the 3D deal with gear extension? Hmmmm. Try what looks right on the ground, then watch it take off and see what further adjustments you need.
Rex du Pont
Tony Peden wrote:
--- Erik Hofman <[EMAIL PROTECTED]> wrote:Jon Berndt wrote:Well, to rotate the aircraft realistically the refference pointshouldbe known by the 3D modellers, but that aside.
The rigid body rotates about the CG, not the aero ref. pt.Even when in motion?
In the FDM's (all of them, AFAIK), yes. Always.
In reality, the aircraft will rotate about the cg in air and some other
point if any of the gear are touching the ground. During takeoff
rotation and landing de-rotation, for example, the aircraft will rotate
about the main gear.It seems to me there would otherwise be no need for a refference
point.
It's still needed.
In order to calculate the moment coefficients from measured moments,
one needs to set a point on the aircraft to take the moment arms from.
This has traditionally been chosen as the point 25% of the way aft
along the mean aerodynamic chord. The reasoning for this is that for
low speed aircraft, it is a reasonable estimate of the wing's center of
pressure (i.e. where you'd place the lift vector both chord- and
span-wise). In truth, this location varies with (at least) the airfoil
section, wing planform, and Mach number, so when using it as a constant
reference point we have to call it something different. I chose to
call it the aero reference point. And since someone might reduce
measured data using something other than the 1/4 chord of the MAC, this
number needs to be configurable.Erik
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