CG & performance

031 Mass & BalanceChapter 5

CG position affects longitudinal stability, control forces, stall speed, range, and VMC. Forward and aft extremes produce opposite trends — exam questions love comparing them.

CG neutral point

Definition — CG neutral

CG located at the aerodynamic centre (CP) — no inherent pitch stability or instability from CG position alone.

Forward CG effects

Key fact

Forward CG increases: longitudinal stability, stall speed, stick force, fuel consumption (longer moment arm to tail), elevator/tailplane loading. Decreases: range/endurance, absolute ceiling, rate of climb, mileage.

Definition — Too far forward

Difficulty or inability to flare — nose-wheel touchdown risk. Requires nose-up trim → higher fuel burn. Insufficient aft pitch control authority at landing.

Exam trap

Forward CG has the same performance effects as a heavier aircraft — higher stall speed, reduced range, reduced climb. Both increase tailplane download requirement.

Aft CG effects

Key fact

Aft CG decreases: longitudinal static stability, stall speed, drag, fuel consumption, power required for a given speed. Increases: cruise range/endurance, light pitch stick forces (risk of overstressing).

Definition — Too far aft

Difficulty recovering from a spin. Degraded nose-wheel steering — less weight on nose wheel. VMCG (minimum control speed on ground) is determined by aft CG limit.

Exam trap

Aft CG = less stable but more range — the classic trade-off. Do not pick “increased stability” for aft CG.

Elevator & tailplane

Key fact

Elevator and tailplane effectiveness in all flight conditions is most likely to affect range and CG position. Forward CG demands more tail-down force; aft CG reduces it.

Mass increase vs CG shift

Effect	Higher TOM	Forward CG	Aft CG
Stall speed	↑	↑	↓
Stability	—	↑	↓
Range	↓	↓	↑
Stick force	↑	↑	↓ (light)
Rate of climb	↓	↓	— (lighter feel)
Landing distance	↑	↑ (nose wheel)	—

Formula — Stall speed vs mass

Vs₁ = Vs₀ × √(W₁ ÷ W₀)

An increase in weight increases stall speed proportionally to the square root of the mass ratio.

Key fact

Higher TOM also increases: Vmu, gliding speed (glide range unchanged), fuel flow, AOA, drag, tyre wear. Decreases manoeuvrability and safety margin.

Landing gear & CG

Key fact

Gear extending forward (typical main gear) → CG moves forward. Gear retracting rearwards → CG moves aft. Always account for gear position in CG calculations during transition phases.

Exam trap

Glide range is unaffected by mass (in ideal conditions) — only glide angle/speed change. Mass increase does not shorten glide range in the same way it shortens powered range.

Exam trap

VMCG is tied to aft CG limit — loss of an engine at aft CG is the critical case for directional control on the ground.