Balancing an RC plane

If there's one thing that decides whether a plane flies well or crashes on its first flight, it's the centre of gravity. A plane can survive a lot of setup errors, but not a badly wrong balance point.

What the CG is

The centre of gravity is the point where the plane balances. For the plane to be stable, it must sit slightly ahead of the wing's centre of lift. Most trainers balance around 25 to 33% of the way back from the leading edge of the wing; 25% is a safe starting point for a first flight (Flite Test guide to CG). Always use the figure in your plane's manual.

Why it matters so much

• Correct CG: stable, predictable, glides well.
• Nose-heavy: stable but sluggish, needs more up elevator, lands faster. Safe.
• Tail-heavy: unstable, twitchy and prone to stalling. Dangerous.

The old saying holds: a nose-heavy plane flies poorly, a tail-heavy plane flies once.

How to check the CG

1. Set the plane up ready to fly: battery fitted in its usual position, canopy and hatches on.
2. Find the marks. Many foam trainers have CG marks or bumps under the wing. Otherwise, measure back from the leading edge at the wing root using the manual's figure.
3. Lift the plane on two fingertips (or a balancing stand) at those points, one under each wing near the fuselage.
4. Watch the nose. Level or very slightly nose-down is right. Nose-up means tail-heavy.

How to fix it

• Move the battery. The easiest adjustment: forward to cure tail-heaviness, back to cure nose-heaviness. Mark the right position on the tray.
• Use a different battery of the recommended size; packs vary in weight.
• Add weight to the nose or tail as a last resort, securely fixed.

After repairs or changes

Recheck the CG whenever you:

• Change battery brand or size.
• Repair the tail with glue, tape or reinforcement (tail repairs add weight in the worst place).
• Add lights, a camera or other equipment.

Crash repairs are covered in repairing a crashed RC plane.

Lateral balance

Check side-to-side balance too: lift the plane by the propeller hub and the tail. If one wing drops consistently, add a little weight to the light wingtip.

The CG check is part of every RC plane pre-flight checklist, and battery sizes are explained in LiPo batteries explained. Then it's time for your first flights. Back to the RC planes hub.