There is an old, true adage in aeromodelling:
“A nose-heavy airplane flies poorly, but a tail-heavy airplane flies once.”
If your Center of Gravity (CG) is too far rearward behind the aerodynamic center, the aircraft becomes dynamically unstable. The moment it pitches up, aerodynamic lift exacerbates the pitch, resulting in an immediate unrecoverable stall and crash.
What is the Target CG Point?
For standard straight or gently tapered wings (like the VT-Simple Trainer and VT-Super Cub), the ideal maiden flight CG is located between 25% and 30% of the Mean Aerodynamic Chord (MAC) measured back from the leading edge of the wing.
For delta wings and flying wings (like the VT-F22 Raptor and VT-Mig29), the balance point is calculated via root chord, tip chord, and leading edge sweep angle (typically 15% to 20% MAC for stable beginner control).
How to Test and Balance Your Aircraft
- Fully Assemble the Aircraft: Install all wings, landing gear, servos, motor, ESC, receiver, and your flight LiPo battery.
- Mark the CG Point: Using a soft marker or two small pieces of masking tape, mark the designated balance distance on the underside of both wings next to the fuselage.
- Fingertip Balance Test:
- Place your index fingertips directly on the marks underneath the wings.
- Lift the aircraft smoothly off the table.
- Evaluate Aircraft Attitude:
- Level or 3° Nose-Down: Perfect! This represents optimal stability and gentle stall recovery.
- Tail-Heavy (Nose rises up): DANGEROUS! Do not attempt flight. Slide your battery pack forward in the fuselage nose cavity.
- Excessively Nose-Heavy: Slide battery slightly rearward until attitude levels out.
Fine-Tuning in the Air: The 45° Dive Test
Once your maiden flight is in progress, trim the plane for straight and level flight at half throttle. Then:
- Climb to safe altitude (at least 30 meters).
- Push into a 45° hands-off descent dive.
- Release the elevator stick and observe:
- Gentle shallow climb back to level: CG is optimal.
- Sharp pull-up: Plane is nose-heavy (excessive upward trim was compensating for heavy nose).
- Tucks under or steepens dive: Plane is tail-heavy! Land immediately and move battery forward.