RC Airplane CG Ballast Calculator
Enter the airplane weight, measured CG, target CG, and the station where ballast can be mounted. The calculator solves the aircraft moment equation backward and shows the added weight, final flying weight, and corrected CG check.
Required ballast (g)250
- Ballast direction
- Nose or forward ballast
- Corrective moment (g-mm)
- 20,000
- Final flying weight (g)
- 2,250
- Ballast share of final weight
- 11.11%
- Corrected CG check (mm)
- 80
Stations are interpreted as mm from one datum, with larger station values aft.
Required ballast is more than 10% of final flying weight; moving existing equipment may be better than adding dead weight.
How to use this calculator
- Choose metric or imperial units and use that same unit system for every entry.
- Enter the current ready-to-fly weight before adding ballast.
- Enter the measured current CG and the target CG from the same datum.
- Enter the station where the ballast center will sit, then check whether the result calls for nose or tail ballast.
How the ballast amount is calculated
This calculator treats the airplane and the ballast as point weights along one fore-aft reference line. Station values must use one datum and one sign convention. The entries here assume station numbers increase toward the tail, so a smaller station is forward and a larger station is aft.
Let W be the current ready-to-fly weight, xc the measured current CG station, xt the target CG station, xb the ballast station, and B the added ballast. The moment equation is:
xt = (W * xc + B * xb) / (W + B)
Solving that equation for the unknown ballast gives:
B = W * (xc - xt) / (xt - xb)
If the current CG is aft of the target, the numerator is positive and the ballast station must be forward of the target. If the current CG is forward of the target, the numerator is negative and the ballast station must be aft of the target. A negative result means the selected mounting location is on the wrong side of the target CG, so adding weight there would move the balance farther away instead of correcting it.
What changes the answer most
The distance between the ballast station and target CG is the main lever. Moving ballast farther from the target gives the same corrective moment with less weight. That is why a small amount secured in the nose can correct an aft-heavy model more efficiently than a larger amount placed near the wing saddle.
The corrective moment is B * |xt - xb|, which is the same as W * |xc - xt| apart from rounding. The final flying weight is W + B, and the ballast share is 100 * B / (W + B). The corrected CG check recomputes the combined moment after adding ballast so you can confirm it lands on the target.
What this calculator leaves out
The tool does not decide whether the target CG is safe for a particular airframe. Use the manufacturer stated CG range for initial flights, and do not treat this point-mass calculation as a substitute for range checks, control checks, structural judgment, or flight testing.
It also leaves out lateral balance, loose equipment, fuel burn, payload changes, flexible structures, and whether the attachment point can carry the flight and landing loads. Added ballast increases wing loading, so a large result is a sign to consider moving the battery, receiver pack, motor, or other existing equipment before adding dead weight.
Worked example
Suppose a 2,000 g model balances at 90 mm aft of the datum, but the target is 80 mm. A nose ballast pocket is available at 0 mm. The calculation is B = 2000 * (90 - 80) / (80 - 0), so the required ballast is 250 g.
The corrective moment is 250 * |80 - 0| = 20,000 g-mm. The final flying weight is 2,250 g, the ballast is 11.11% of final weight, and the corrected CG check is (2000 * 90 + 250 * 0) / 2250 = 80 mm.
Common questions
How much nose weight does an RC airplane need?
It depends on the airplane weight, how far the measured CG is aft of the target, and how far forward the ballast can be mounted. With aft-positive stations, nose weight is needed when the current CG number is larger than the target CG number and the ballast station is forward of the target.
Where should ballast be installed in an RC plane?
Install ballast securely on the side of the target CG that corrects the imbalance. For an aft-heavy model, that usually means as far forward as practical; for a nose-heavy model, it means aft of the target. The attachment point must be strong enough to hold the weight through flight loads and landings.
Why does moving ballast farther from the CG require less weight?
CG correction is a moment problem. Moment equals weight times arm, so doubling the distance from the target CG roughly halves the ballast needed for the same correction. This is why moving an existing battery can be more efficient than adding lead near the wing.
Can I move the battery instead of adding lead?
Yes, and it is often better because it changes the CG without increasing flying weight. Treat the battery move as a weight moved from its old station to a new station, then reweigh and rebalance the model. Use added ballast only when component placement cannot reach the target safely.
Why did the calculator return a negative ballast amount?
A negative result means the ballast station is on the wrong side of the target for the correction you need. Adding weight there would move the CG farther from the target. Choose a station forward of the target for an aft-heavy model or aft of the target for a nose-heavy model.
Does adding ballast change stall behavior?
It can. Added ballast increases final flying weight and wing loading, which can raise stall speed, shorten flight time, and change handling. After a significant ballast change, recheck controls, launch conservatively, and keep the first flight within the manufacturer recommended CG range.