SIG SUKHOI SU-31 ARF, Assembly Manual, Page: 19 / 26

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Set the model UPSIDE DOWN on the balancing fixture and shift it
back or forward until you find the exact spot where the model will
set perfectly level. Carefully measure the distance of that spot
from the leading edge of the wing.
IMPORTANT NOTE: Balanced means the airplane sets level
when supported at the desired balance point. Believe it or not,
we've had questions like; "When my model is balanced it should
sit slightly nose down, right?". NO, balanced means level - not
nose down or nose up - flat level!
BALANCE REFERENCE POINTS FOR THE SUKHOI SU-31
*
Percentage of Mean Aerodynamic Chord
** Distance aft of wing leading edge measured right along side
of fuselage
*** Distance aft of wing leading edge measured at the wing tips
% MAC* @ fuse side** @ wing tip***
25% 4.468" 2.874"
26% 4.619" 3.025"
27% 4.769" 3.175"
28% 4.920" 3.326"
29% 5.070" 3.477"
30% 5.221" 3.627"
31% 5.372" 3.778"
32% 5.522" 3.929"
33% 5.673" 4.079"
For initial test-flying and familiarization purposes, we suggest a
starting balance point of 27%, which is approximately 4-3/4",
behind the leading edge of the wing at the side of the fuselage
(this translates to about 3-3/16" aft of the leading edge at the wing
tip, for those of you who prefer to balance your airplanes that way).
As we all know, as the balance point is moved aft, an airplane
will become more responsive in all axis, but it will be especially
noticeable to a model pilot in "pitch" (up and down). We have flown
the SUKHOI at the 30% location and found it to be manageable,
with excellent aerobatic capabilities. However, we did notice an
increase in pitch sensitivity and had to decrease the elevator throw
to compensate. "Softening" elevator response with a reasonable
exponential percentage also works. In the end, the final balance
point and control throws you use will depend somewhat on how
you like to fly. Some pilots like their models extremely reactive,
others strive for more smoothness. The SUKHOI SU-31 is a very
capable aerobatic machine and can be tailored to fit your style.
We therefore suggest that you begin with the 27% CG location and
experiment from there.
you have had time to shift it around into exact position. Once you
have it in position, squeegee the excess soapy water out from
under the decal. Mop up the water with a dry cloth. Squeegee
repeatedly to get as much of the water out from under the decal as
possible. After setting overnight, the decal will be solidly adhered
to the surface.
BALANCE YOUR SUKHOI
In terms of the flight characteristics you will realize, this is
probably the single most important step in preparing your SUKHOI
for flight. The final placement of the longitudinal Center of Gravity,
or Balance Point, is extremely important and should be
approached with patience and care.
Completely assemble the model, including propeller, spinner, etc.
Do not leave anything off the airplane that will be on it in flight. DO
NOT fill the fuel tank for balancing purposes. Some people prefer
to balance their airplanes by lifting the model up by one finger at
each wingtip to find the spot where the model will sit perfectly level.
This has been done for years and is an acceptable way to balance
a model (we have included balance measurements for this method
below). However with an airplane as large as the SUKHOI, it is
virtually impossible to balance it by the wingtips by yourself. We
prefer to use a "balancing fixture" which checks the model's
balance point right along each side of the fuselage (balance
measurements are given below). It can be done alone and is
actually a little more precise than a fingertip balance.
You can make your own simple "balancing fixture" with a couple of
1/4" dia. dowels glued into a wood base at perpendicular 90
O
angles. The dowels need to be the same length and tall enough
to accommodate the height of the assembled airplane, as well as
the width of the fuselage, plus about 1/2" additional width. The
tops of the dowels need to be padded to avoid damage to the
wing. We used 1/8" thick rubber sheet, cut to 1/4" diameter, glued
in place, for the padding. There are commercial units available for
balancing but be sure they are large enough to support the weight
and size of the SUKHOI.