Example Data Table
| Model Type |
Root Chord |
Tip Chord |
Span |
Target CG |
Suggested Check |
| Trainer |
12 in |
9 in |
52 in |
28% MAC |
Start slightly forward |
| Sport Low Wing |
11 in |
7.5 in |
46 in |
30% MAC |
Check inverted feel slowly |
| Scale Warbird |
13 in |
6.5 in |
58 in |
25% MAC |
Use conservative first flights |
Formula Used
Moment: Moment = Weight × Arm.
Total weight: Total Weight = Sum of all part weights.
Total moment: Total Moment = Sum of all part moments.
Current CG: CG = Total Moment ÷ Total Weight.
MAC for tapered wing: MAC = 2 ÷ 3 × Root Chord × (1 + Taper + Taper²) ÷ (1 + Taper).
CG percent of MAC: CG% = ((CG From Datum - LEMAC) ÷ MAC) × 100.
Target CG from datum: Target CG = LEMAC + (Target Percent ÷ 100 × MAC).
Battery movement: Battery Move = Required Moment Change ÷ Battery Weight.
Ballast weight: Ballast = Required Moment Change ÷ (Ballast Arm - Target CG).
How To Use This Calculator
- Choose one datum point. The nose is often easiest.
- Measure every arm from that same datum.
- Enter wing root chord, tip chord, span, and tip sweep.
- Enter the target and safe CG percentages from your plan.
- Add component weights and arms in the table.
- Enter battery weight and battery arm separately.
- Press the calculate button.
- Move the battery as suggested, then recheck the model physically.
- Download CSV or PDF records for your setup log.
Why RC Airplane CG Matters
Center of gravity controls how an RC airplane feels in pitch. A forward balance point usually gives calmer tracking. It can also need more elevator and landing speed. An aft balance point can reduce stability. It may make stalls sharp and recovery slow. This calculator helps you compare the real balance point with a chosen safe range.
Advanced Setup Checks
The tool uses component weights, component arms, battery position, and wing geometry. Each arm is measured from one datum, usually the nose or spinner back plate. The same datum must be used for every part. You can enter motor, receiver, servos, landing gear, payload, and spare items. The battery is separate because moving it is the easiest trim change.
Wing Geometry Results
For tapered wings, the mean aerodynamic chord is more useful than the root chord alone. The calculator estimates MAC, the leading edge of MAC, and the current CG as a percent of MAC. It also reports wing area and wing loading. These values help compare different models, packs, and payloads.
Battery And Ballast Planning
After calculation, the tool shows the moment difference needed to reach the target point. It then estimates how far the battery should move. Positive movement means move the pack aft, when larger arm values point rearward. Negative movement means move it forward. If the battery cannot move enough, use the ballast estimate. Place ballast only in secure locations. Recheck every model after repairs, covering changes, new packs, or hardware changes.
Practical Field Advice
Use the designer range first when available. Many sport models start near 25 to 33 percent of MAC. Trainer models often prefer the forward side. Aerobatic models may use a rearward setup after careful testing. Mark the final balance point inside the fuselage. Record the pack, prop, payload, and throw settings. Export the results before the first flight. Small notes prevent repeated measuring and make future changes safer. Keep the aircraft level during measurement. Support it at matching marks on both wing panels. Remove loose tools from the bench. Check the model twice, with the hatch installed. A small measuring error can hide a large balance change. Always confirm control direction before every careful test glide.
FAQs
What is CG on an RC airplane?
CG means center of gravity. It is the balance point where total aircraft weight acts. It strongly affects pitch stability, takeoff behavior, stall recovery, and landing feel.
Where should I measure arms from?
Use one fixed datum for all measurements. Many builders use the nose, spinner back plate, or firewall. The exact point is less important than using it consistently.
What does positive battery movement mean?
Positive movement means the battery should move aft when larger arm values go rearward. Negative movement means it should move forward toward the datum.
Is 30 percent MAC always safe?
No. Thirty percent MAC is only a common starting point for many models. Always follow the designer range first, especially for scale, swept, canard, or unusual aircraft.
Can I use grams and centimeters?
Yes. Choose metric units and keep every weight in grams. Keep every length and arm in centimeters. Do not mix unit systems in one calculation.
Why does ballast show a negative value?
A negative ballast value means that the selected ballast location cannot correct the model toward the chosen target. Try an opposite location or move the battery instead.
Does this replace a physical balance check?
No. This calculator helps planning and record keeping. Always balance the finished aircraft on stands or fingertips before flying, with the battery installed.
Should I check CG after repairs?
Yes. Repairs, paint, covering, new wheels, larger batteries, and camera mounts can shift balance. Recalculate and physically check the aircraft before the next flight.