Rex du Pont wrote:
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? H. 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 point
should
be 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 otherpoint if any of the gear are touching the ground. During takeoffrotation and landing de-rotation, for example, the aircraft will rotateabout the main gear.
It seems to me there would otherwise be no need for a refferencepoint.
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 aftalong the mean aerodynamic chord. The reasoning for this is that forlow speed aircraft, it is a reasonable estimate of the wing's center ofpressure (i.e. where you'd place the lift vector both chord- andspan-wise). In truth, this location varies with (at least) the airfoilsection, wing planform, and Mach number, so when using it as a constantreference point we have to call it something different. I chose tocall it the aero reference point. And since someone might reducemeasured data using something other than the 1/4 chord of the MAC, thisnumber needs to be configurable.
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