I recently tried to establish the CG position for my Europa Classic and have
found the process a bit puzzling.
Per Section 6 of the Nov 1995 Owner's Manual, the reference point is the
front of the cowl. With the aircraft level in both directions, using a plumb
bob from the front of the cowl, the floor was marked and the positions of the
main and tail wheel were also marked as was the position of the aft cowl
joggle.
Newsletter 21 page 17 gives an updated method for locating the Europa's CG in
which the cowl joggle is assigned as F.S. 29.25 and the measurements are all
relative to this. The method given in this newsletter is independent of cowl
length which I took to mean that it should work for both the Classic and the
XS configurations.
My expectation was that my older model would show the cowl joggle at 29.25,
consistent with the new technique. However, with the aircraft leveled using
the door rebate as a reference, the cowl joggle is not vertical as
anticipated. Using the horizontal split line of the cowl as the reference,
the vertical line of the cowl is clearly canted such that the bottom is
further forward than the top. Measuring from points established by the plumb
bob from the intersection of the cowl joggle and the horizontal split of the
cowl, the cowling measures 29 7/8 from front to back while dropping a line
---From the joggle at the bottom center of the cowl shows 29 3/4 from the front
of the cowl.
Thus, the two methods (front of cowl vs rear) of establishing the CG differ
by 3/8 to 1/2 inch. While this is not a huge difference, it doesn't seem
trivial either. That is, if the CG moves forward by 2 inches from the
nominal position of 60 it is at the limit, and the difference due to
measurement technique is near 1/4 of this range. So, which method of
measurement is the one to trust? Is there a better way?
My understanding of aeronautical design is limited, but for what it is worth,
I thought that the underlying idea was that the CG must be in a range forward
of the wing's center of lift by some amount to ensure pitch stability. Since
the goal is to establish the CG position vs the wing, it would seem to this
uninformed builder that the position of the wing's leading edge would be a
better reference point (more directly related to the desired result) than the
aft edge of the cowl.
The point of the above is that a further difference between the wing's center
of lift and either cowl reference point is possible due to a difference in
the way the seat module is bonded into the fuselage, especially the exact
angle of the seat back as well as the amount the bushings protrude toward the
wing spar, etc. The sum of these little differences could add to the cowl
length imponderables noted above. I suspect the effect of all of these
little random variables would be eliminated if the position of the wing LE
rather than the cowl were given as a specific FS.
In support of the random variation of measured points on the fuselage,
consider the following measurements:
Main Tail
Nov 95 manual 47.3 175.1
Revision 45.2 175.5
A044 44.68 175.75
Range 2.6 .65
It is interesting to note that the nose to tail range is fairly small while
the location of the main wheel varies considerably. Doesn't this mean that
the location of the CG vs the main wheel varies by 2.6 inches between
aircraft? Wouldn't this affect ground handling?
Lots of questions from what seemed a straightforward task...
John A044
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