Rants from RTF Thumper - RC Groups
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Dec 11, 2011, 12:35 PM
Big Jim
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Rants from RTF Thumper

More of what I know, to help others


I have read a few post her that lead me to believe that some of us here are not sure what a CG is or how to find it. Some here write as if the CG is a movable point on the plane that can be solved with trim. I hope this bit of info clears it up. Now I am no pro but have now built 3 scratch aircraft one with no plans and of my design.

Correctly balancing rc airplanes is so important for safe flying, because any deviation from the model's Centre of Gravity (CG) can potentially result in the model being quite uncontrollable.

Every rc airplane (and all other aircraft) has a specific CG position, it's the mean point where all gravitational forces act upon the plane and hence the point where the model balances fore-aft correctly. You can liken a plane's Centre of Gravity to the fulcrum of a see-saw, for example. The CG point is determined during the design stage of the airplane or aircraft and is typically shown on a plan as a disc split in to four quadrants,

If you've built from a kit & plan the CG should be clearly marked on the plan but if you've bought an ARF or RTF plane then the instruction manual will likely give the CG position in terms of distance back from either the leading edge of the wing or from the nose.
Incidentally some model aircraft manufacturers specify a range that the Centre of Gravity can fall in to, rather than a single point. If you're unlucky the manual might not even mention anything about balancing rc airplanes and the CG!

Balancing an rc airplane correctly about its Centre of Gravity is so important because a very badly balanced rc airplane will, at best, be hard to control, this is especially true for tail-heavy planes. At worst, the plane will crash within seconds of getting airborne.

How to find it if you made it

One theory is that all aircraft should balance at a point defined as 25% of the Mean Aerodynamic Chord, with no consideration given to tail area or moment arm. For the scale model RC airplane, I prefer to take these facts into consideration.

When designing a scale model we do not have the ability to change dimensions or ratios-we are restricted by our wish to build an accurate reproduction of the full size.

The formula I have used, came from a book written a few years ago by Gordon Whitehead titled "Radio Control Scale Aircraft Models for Everyday Flying" ( A great book that every scale modeler should own)

Here is the formula- CG POSITION = MAC/6+(3 X TAIL AREA X TAIL MOMENT ARM ) / 8 X WING AREA. Note that the Moment Arm is defined as the distance from the 25% MAC of the wing to the 25% MAC of the tailplane.

I hope this helps not my thoughts just the Info I used to find sucsess
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