Balance your plane 25–30% of the mean aerodynamic chord back from the leading edge, with the battery in. A conservative first-flight start is 27–28%: measure the chord (leading edge to trailing edge), multiply by 0.27, and mark that distance behind the leading edge. An 8-inch-chord trainer balances about 2.2 inches back. (Straight, constant-chord wing: the MAC is just the chord, so measure anywhere. Tapered or swept: use the mean aerodynamic chord.) Enter your chord below and I'll give you the exact number — level or slightly nose-down on that mark is right, tail-heavy is the dangerous side.
Balance a conventional RC airplane at 25–30% of the mean aerodynamic chord, measured back from the leading edge, with 27–28% as the conservative first-flight starting point. Nose-heavy flies poorly; tail-heavy flies once. Slide the battery to trim before adding any ballast.
Straight (constant-chord) wings only — one number works at every station. Tapered wing? See the note under the result.
Tapered wing? The balance point moves with the mean aerodynamic chord, not the root. The Airplane Designer does that math and draws the CG to scale on your plane.
The plane balances like a seesaw on its CG — and the fulcrum isn't in the middle. Balance is a moment-arm problem: weight × distance from the CG = balancing moment. On the plane drawn here the tail sits about three times farther from the CG than the nose position, so 1 gram at the tail takes about 3 grams there to counter — but that ratio comes from this plane's distances, not a universal 3:1 rule. That's why builders keep the tail light, and why the fix for tail-heavy is sliding the battery forward — moving weight you already carry — not gluing lead to the nose.
This gives you the number. The Airplane Designer gives you the whole design — CG marked on a to-scale plan view, plus wing loading, thrust, servo placement, and a parts list, all from your wingspan and style.
My first-plane build has an 8-inch chord and flies like a trainer. The math: 8 × 0.27 = 2.16 inches behind the leading edge, with a safe range of 8 × 0.25 = 2.0″ to 8 × 0.30 = 2.4″. The cut guide for that plane says 2¼–2½ inches — same window. Mark it under the wing on both sides of the fuselage, put the flight battery in, and lift the plane on two fingertips at the marks. Level or slightly nose-down: fly it. Tail sinking: move the battery forward and check again.
The battery is the heaviest single thing in a foam-board plane, and it sits ahead of the wing. Balance the plane empty, call it perfect, then strap a pack in for the maiden — and the plane you launch is a different aircraft from the one you checked. Every balance check happens flight-ready: battery in, strapped where it will actually fly. It's also your free trim weight — on my 5-foot cargo plane, the fix that tamed a scary maiden wasn't a rebuild, it was moving the battery until the plane balanced on the mark.
The wing's lift acts near the front third of the chord, and the tail shifts the whole airplane's neutral point a little further aft. A conventional plane is stable when the balance point sits ahead of that neutral point — that margin is what makes it weathervane back to level instead of swapping ends. The 25–30% convention exists because it reliably puts the CG ahead of the neutral point on normal proportions. That's why the window is a percentage of chord and not a fixed number of inches — a 6-inch-chord slow flyer and a 12-inch-chord hauler both balance at the same fraction. New to all of this? The beginner path covers CG, throws, and the rest of the first-flight checklist in order.
Balance point sorted. Design the whole airframe around it — or work out why the last one wouldn’t fly right.
