How these are worked out — the equations behind this sheet, their sources, and what each one assumes.
Where people go wrong
The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.
Balancing at the neutral point. The neutral point is where the aeroplane stops caring which way it is pointing. The centre of gravity belongs ahead of it by the static margin — that gap is the stability, and at zero there is none.
Using the static margin as the tail's moment arm. This sheet did exactly that until Rev B, and it overstated the tail download by roughly a factor of two. The arm about the centre of gravity is the distance to the wing's aerodynamic centre, not the margin.
Balancing a flying wing like an aeroplane with a tail. Its static margin band is 3 to 8% of MAC, not the 12 to 15% of a sport model, and it needs a pitching moment at zero lift that is not negative — which means reflex or washout, deliberately designed in.
Believing the fuselage allowance. Three per cent of MAC is a placeholder that suits a typical model, not a computation. Once the fuselage geometry exists, work its pitching-moment slope out and replace it.
Balancing the model with the battery out. On an electric model the pack is the heaviest single thing you can slide, and where it ends up decides the centre of gravity more than the tail does.
Balancing the aircraft for real
How to find the point on the wing
- Measure out from the centreline to the
ystation given above — that is where the mean aerodynamic chord sits. - At that station, measure back from the leading edge by the distance given.
- Mark both wings. Balance on fingertips or a stand at those two marks, wings level, model complete and ready to fly.
Battery, receiver and fuel all count. A model balanced without its flight battery is not balanced.
Start forward, then work aft
The forward limit is the safe end. A nose-heavy model flies sluggishly, needs speed on the approach and floats a long way — it survives. A tail-heavy model is unrecoverable in the hands of most pilots, and the first sign is usually a departure right after launch.
Fly the first flights at the forward figure. Move the balance back a few millimetres at a time, testing each step, only once it is flying well.
The dive test
Climb to a safe height, trim for level flight, then dive at about 30° with the power off and let go of the elevator.
Gentle pull back to level: balance is about right. Sharp pull-up: nose-heavy, move back. Steepens into the dive: tail-heavy — land, move the balance forward before flying again.