Center of Gravity & Balance
~10 min read · Keep CG inside limits and explain what a tail-heavy or nose-heavy sUAS does.
A drone with a bad center of gravity is a fight the flight controller slowly loses. The exam tests the concept: where the CG must sit, what happens fore, aft, or sideways of limits, and why mounting position matters as much as payload weight.
CG in a multirotor
The center of gravity is the point where the aircraft's mass balances. Multirotors want it centered among the rotors and within the vertical band the manufacturer designed for. Offset CG forces some motors to carry more thrust continuously: the flight controller trims it invisibly — until the loaded motors saturate. Symptoms of off-CG loading: persistent drift or lean in hover, one pair of motors running hot, asymmetric battery drain, sluggish response in one direction, and shortened endurance.
- Target: CG centered between rotors, at design height
- Off-center = some motors permanently overworked
- Watch for hover lean, hot motors, one-sided sluggishness
Fore-aft and lateral effects
Forward CG: nose-heavy — on fixed-wing sUAS this adds stability but costs efficiency and flare authority; on multirotors the rear motors work harder and forward flight consumes extra power. Aft CG: the dangerous direction on fixed-wing (reduced pitch stability, stall recovery gets worse); multirotors lose braking authority when stopping from forward flight. Lateral offset (a side-mounted accessory) makes one side's motors work harder and couples roll into other maneuvers. The battery — often the heaviest single component — is the usual CG adjuster: mounting it a centimeter off spec moves the CG meaningfully.
- Fixed-wing: aft CG is the stability killer
- Multirotor: any offset burns power and control margin
- Battery position is the biggest single CG lever
Loading practice
Mount payloads on the manufacturer's intended points, balance dual accessories symmetrically, re-check CG after every configuration change (new camera, bigger battery), and hover-test after reconfiguration: a clean hover with level attitude and even motor temperatures is the field check that balance is right.
Worked example
A crew straps a spotlight to the right landing gear leg for a night inspection and notices the aircraft drifts right in hover, right-side motors run hotter, and battery endurance drops from 22 to 17 minutes. What is happening and what fixes it?
The side-mounted light shifts the CG right of center. The right motors must produce extra thrust every second of the flight to hold level — that is the heat and the endurance loss; the flight controller's constant trim against the offset is the drift-and-sluggishness. Fix: move the light to a centerline mount, or add a balancing mass on the left leg (accepting the total-weight penalty), then hover-test: level attitude, symmetric motor temperatures, endurance recovers most of the loss. Exam framing: lateral CG displacement → asymmetric thrust demand → reduced controllability and endurance.
Common exam pitfalls
Thinking the flight controller 'handles' bad CG.
It masks the imbalance by overworking motors — consuming control margin you will want in gusts and emergencies.
Balancing weight but not position.
CG is about WHERE mass sits. The same payload centered vs offset produces entirely different aircraft.
Skipping a hover check after reconfiguring.
Every configuration change moves CG. A one-minute level-hover check catches it before the mission does.
Center the mass or the motors pay the tax.
Recap
- CG must stay within manufacturer limits in all axes
- Offset CG = constant asymmetric thrust = heat, drift, shorter endurance
- Aft CG is critical on fixed-wing sUAS (stability/stall recovery)
- Battery placement is the primary CG adjustment
- Mount payloads centered; balance symmetric accessories
- Hover-test after every configuration change
Prove it: 10 questions on this topic
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