Proper weight and balance calculation is required for safe UAS operations — particularly when carrying variable payloads or operating custom-built platforms. This guide covers the theory behind UAS center of gravity, how to calculate weight and balance for your specific platform, and how to plan payload configurations that stay within approved limits. Applicable to all platform types from small multirotors to Group 2 and 3 fixed-wing systems.
Try the Interactive Calculator - Calculate CG in real-time with visual feedback
Understanding and calculating weight and balance is critical for safe UAS operations, especially for custom-built or payload-equipped platforms.
Why Weight & Balance Matters
Safety
- Flight stability - Improper CG causes instability or loss of control
- Control authority - Out-of-range CG reduces control effectiveness
- Motor stress - Imbalanced load increases motor wear
- Battery performance - Excess weight reduces flight time
Performance
- Flight time - Lighter aircraft = longer endurance
- Maneuverability - Proper balance improves handling
- Maximum altitude - Weight affects climb performance
- Wind resistance - Heavy aircraft struggle in wind
Regulatory
- Part 107 weight limits - 0.55 to 55 lbs
- Registration requirements - Know your weight category
- Operations over people - Category limits by weight
- Insurance compliance - Operating within design limits
Center of Gravity (CG) Basics
What is CG?
The center of gravity is the point where all weight is balanced. Think of it as the balance point if you placed the aircraft on your finger.
Why CG Location Matters
CG Too Far Forward:
- Nose-heavy flight attitude
- Increased motor load
- Reduced control authority (pitch up difficult)
- Higher stall speed
CG Too Far Aft:
- Tail-heavy flight attitude
- Unstable, twitchy flight characteristics
- Risk of tip stall
- Difficult to recover from pitch disturbances
CG Just Right:
- Stable, predictable flight
- Optimal motor efficiency
- Full control authority
- Easiest to fly
How to Calculate CG
Step 1: Set Reference Datum
Choose a reference point (datum) on your aircraft - typically the nose, motor mount, or leading edge of wing.
Step 2: Weigh Components
Weigh each major component:
- Airframe
- Motors
- Battery
- Flight controller
- Payload (camera, sensors, etc.)
- Landing gear
Step 3: Measure Arm Distance
Measure distance from datum to center of each component (in inches or cm).
Step 4: Calculate Moments
Moment = Weight × Arm Distance
For each component, multiply its weight by its distance from datum.
Step 5: Find CG Location
CG Location = Total Moments ÷ Total Weight
Weight & Balance Calculator (Manual)
Example Calculation
Aircraft: Custom Fixed-Wing UAS Datum: Nose of fuselage
| Component | Weight (oz) | Arm (in) | Moment (oz-in) |
|---|---|---|---|
| Fuselage | 12.0 | 15.0 | 180.0 |
| Wings | 8.0 | 18.0 | 144.0 |
| Motors (2x) | 6.0 | 20.0 | 120.0 |
| Battery | 18.0 | 16.0 | 288.0 |
| Flight Controller | 2.0 | 17.0 | 34.0 |
| Camera Payload | 4.0 | 14.0 | 56.0 |
| Landing Gear | 3.0 | 12.0 | 36.0 |
| TOTAL | 53.0 oz | — | 858.0 oz-in |
CG Location = 858.0 ÷ 53.0 = 16.2 inches from nose
Total Weight = 53.0 oz = 3.3 lbs (under 55 lbs for Part 107)
CG Range Determination
Finding Acceptable CG Range
Most aircraft have a CG range specification:
- Forward limit - Typically 25-30% of wing chord
- Aft limit - Typically 30-35% of wing chord
For fixed-wing:
- Measure wing chord (distance from leading edge to trailing edge)
- CG should be at 28-32% of chord (typical range)
- Example: 12" chord → CG should be 3.4-3.8" from leading edge
For multi-rotor:
- CG should be at geometric center
- Typically coincides with flight controller location
- Small deviations compensated by motor mixing
Weight & Balance Worksheet
Use this template for your calculations:
WEIGHT & BALANCE WORKSHEET
Aircraft: ____________________ Date: __________
Datum Location: ____________________________
| Component | Weight | Arm | Moment |
|------------------|--------|-----|--------|
| 1. _____________ | ______ | ___ | ______ |
| 2. _____________ | ______ | ___ | ______ |
| 3. _____________ | ______ | ___ | ______ |
| 4. _____________ | ______ | ___ | ______ |
| 5. _____________ | ______ | ___ | ______ |
| 6. _____________ | ______ | ___ | ______ |
| 7. _____________ | ______ | ___ | ______ |
| 8. _____________ | ______ | ___ | ______ |
| 9. _____________ | ______ | ___ | ______ |
| 10. ____________ | ______ | ___ | ______ |
|------------------|--------|-----|--------|
| TOTAL | ______ | | ______ |
CG LOCATION = Total Moment ÷ Total Weight
CG = _______ ÷ _______ = _______ inches from datum
ACCEPTABLE CG RANGE
Forward Limit: _______ inches
Aft Limit: _______ inches
CG WITHIN LIMITS? YES NO
TOTAL WEIGHT CHECK
Aircraft Weight: _______ lbs
Max Takeoff Weight: _______ lbs
Weight Within Limits? YES NO
Part 107 Compliance: YES (0.55-55 lbs)
CALCULATED BY: _______________________
DATE: ____________ SIGNATURE: ___________
Adjusting CG
If CG is Too Far Forward
Add weight aft:
- Move battery rearward
- Add ballast weight to tail
- Relocate heavy components aft
Remove weight forward:
- Use lighter nose components
- Remove unnecessary forward equipment
- Shorten nose section (if possible)
If CG is Too Far Aft
Add weight forward:
- Move battery forward
- Add ballast weight to nose
- Relocate heavy components forward
Remove weight aft:
- Use lighter tail components
- Remove unnecessary aft equipment
- Reduce tail moment arm
Practical CG Testing
The Finger Test (Multi-Rotor)
- Balance aircraft on finger at geometric center
- Aircraft should sit level (no tilt)
- If it tilts, move battery opposite direction
- Retest until level
The String Test (Fixed-Wing)
- Tie string at estimated CG location (25-30% chord)
- Suspend aircraft by string
- Aircraft should hang nose-slightly-down (~5° angle)
- Adjust battery position if wrong attitude
- Mark final CG location
Flight Test
Warning: Only if confident in calculations
- First flight in calm conditions
- Short duration (2-3 minutes max)
- Test control authority at altitude
- Note any trim requirements
- Adjust CG if excessive trim needed
Weight Reduction Tips
Airframe
- Use lightweight materials (carbon fiber, foam)
- Minimize excess structure
- Remove non-essential components
- Use hollow fasteners where possible
Electronics
- Select lightweight flight controller
- Use efficient ESCs (less weight + better performance)
- Minimize wiring (use shorter, lighter gauge)
- Consolidate components (fewer boards = less weight)
Battery
- Right-size battery for mission
- Don’t over-capacity for required flight time
- Consider weight vs. endurance tradeoff
- Use high-discharge batteries (lighter for same power)
Payload
- Mount cameras/sensors efficiently
- Use minimal mounting hardware
- Integrate payload into structure
- Consider weight in sensor selection
Weight Categories for Part 107
Registration Requirements
Under 0.55 lbs (250 grams):
- No FAA registration required
- Still subject to Part 107 if commercial
0.55 lbs to 55 lbs:
- FAA registration required ($5 for 3 years)
- Standard Part 107 operations
Over 55 lbs:
- Not covered by Part 107
- Requires Section 333 exemption or different authorization
- Stricter requirements
Operations Over People Categories
Category 1: <0.55 lbs Category 2: <55 lbs, meets injury criteria Category 3: <55 lbs, over closed or restricted access sites Category 4: Small UAS with FAA-accepted Declaration of Compliance
Weight affects which category you can operate under.
Common UAS Weights
Consumer Drones
- DJI Mini 3 Pro: 0.55 lbs (249g)
- DJI Mavic 3: 2.0 lbs (899g)
- DJI Phantom 4: 3.0 lbs (1380g)
- DJI Inspire 2: 7.6 lbs (3440g)
Professional Platforms
- DJI Matrice 300: 14.8 lbs (6.7 kg)
- Freefly Alta X: 26 lbs (11.8 kg)
- Custom Group 2: 15-55 lbs
FPV Racing Drones
- Typical 5" quad: 1.1-1.5 lbs (500-700g)
- Cinewhoop: 0.7-1.0 lbs (300-450g)
Tools & Equipment
Essential Tools
- Digital scale - 0.01 oz / 0.1g resolution
- Measuring tape - Inches and centimeters
- String or thread - For suspension test
- Calculator - For moment calculations
- Notebook - Record configurations
Advanced Tools
- Laser measurement tool - Precise arm distances
- CG balancer - Specialized tool for fixed-wing
- Component database - Track weights of common parts
- Spreadsheet - Automate calculations
Interactive Web Calculator
We’ve developed an interactive web-based calculator that:
- Automatically calculates CG from your inputs
- Stores common component weights for quick selection
- Provides visual CG range diagram with real-time feedback
- Saves and loads multiple aircraft configurations
- Shows Part 107 weight compliance
- Displays weight as percentage of MTOW
Launch Interactive Calculator →
The calculator includes presets for common configurations:
- Custom Fixed-Wing platforms
- Quad Multi-Rotor systems
- Quick-add library of common components (flight controllers, batteries, payloads)
All calculations are performed locally in your browser - no data sent to servers.
Professional Training
Learn proper weight and balance procedures, along with hands-on platform development.
Our Practical UAS Development program includes comprehensive weight and balance instruction for custom-built platforms.
View Training Programs | Contact Us
Always verify weight and balance calculations before first flight. When in doubt, consult an experienced builder or instructor.