Dealing with our aircraft’s center of gravity on our RC models is taken for granted. I want to make it simple and useable to get the flying qualities we want. I
have been in aviation for all of my life, and for full-scale
aircraft the CG is one of the most important things to
check.
I have flown aircraft with forward CG and aft CG. In
the 1960s I flew a Cessna 182, similar to the
172, but with a larger engine, resulting in a
forward CG. Pilots had to make sure to get the
nose up before running out of elevator during
the flare to keep from planting the nose gear.
The elevator feel was heavy. If you did hit
hard on the nose gear—we called that a $1,500
landing—which was the cost to repair the
wrinkled firewall. I flew my original-design
pusher with an aft CG at roughly 3% forward
of the neutral point. The neutral point definition is that
CG location for which there is no stability. The stick had
practically no elevator feel.
In an RC airplane, we have only the feel of the
transmitter and the flying quality. We can change the
CG to get the quality we want without using a lot of
exponential.
Most airfoils have an aerodynamic center at roughly
25% of the mean aerodynamic chord (MAC), so that is
our reference point. The MAC of a rectangular wing is
the chord length, and calculated for a tapered wing, but
the average chord length is good enough for us. So from
this 25% chord, the CG going forward is good and makes
flying easier and stable.
Going aft with the CG makes the aircraft unstable
and harder to fly without exponential; however, this is
generally preferred for more aerobatic or 3-D flight. If you
go back far enough to hit the neutral point, good luck.
You’ve probably lost your aircraft! Fighters use computers
to fly aft of the neutral point; humans can’t do it.
The neutral point is calculated and proven in flight or in
By Stan Burak
CG1 Model Aviation FEBRUARY 2014 www.ModelAviation.com
