The centre of gravity is the single point where the aeroplane's weight is assumed to act. Move it forward and the aircraft becomes very stable but pays a fuel and stall-speed penalty. Move it aft and it gets sprightly but trims out of stability. Every flight begins with a load-sheet calculation that puts the CG inside a published envelope — here is why that envelope exists.
The geometry of the problem
Lift acts at the wing's aerodynamic centre (AC), located at about 25% MAC for a conventional wing. Weight acts at the CG. The two are usually offset, so they make a moment about each other — a pitch-up or pitch-down twist that the tailplane has to cancel.
In normal cruising flight the centre of gravity is forward of the centre of pressure. The wing therefore produces a nose-down pitching moment which must be balanced by a download on the tailplane. — Oxford ATPL Principles of Flight (2020), Ch.10
In plain terms: with CG forward of AC, the wing wants to pitch the nose down. The tail must push down to stop it.
What changes when CG moves
Forward CG — safe but expensive
- Higher stall speed — The tail download adds to weight, so the wing must produce more lift → higher Vs.
- More stable longitudinally — The static margin (CG to neutral point) is large. Disturbances damp out fast.
- Heavier control forces — More elevator deflection is needed to flare or manoeuvre.
- More trim drag — Tail download means the wing carries more total lift → more induced drag.
- Worse range and endurance — A direct fuel-flow penalty — airlines prefer aft CG for cruise.
Aft CG — sprightly but unforgiving
- Lower stall speed — Tail download is small (or zero), so wing carries only weight.
- Lighter controls — A small elevator input produces a large pitch response.
- Less stable — Static margin shrinks. Disturbances damp out slowly; with CG aft of the neutral point the aircraft becomes statically unstable.
- Better cruise economy — Less induced drag from the wing, less trim drag from the tail.
- Higher cruise speed at same power — A few knots of TAS can come "free" by moving freight aft.
The envelope
Every aircraft has a published forward and aft CG limit, expressed as a percentage of MAC. The envelope is bracketed at:
- Forward limit — the aircraft must still rotate at VR at maximum take-off mass with the lightest practical elevator authority.
- Aft limit — the aircraft must remain positively stable in pitch, with adequate elevator authority to recover from a stall and re-trim.
The forward CG limit is set by stall recovery, take-off rotation, and landing flare; the aft CG limit is set by the requirement for positive longitudinal stability. — Oxford ATPL Mass and Balance (2020), Ch.3
A full load sheet then accounts for passengers (zone-based), cargo (forward and aft holds), fuel burn (CG drifts as fuel burns), and any seat reassignments — the resulting CG must stay inside the envelope for take-off, in flight (after fuel burn), and at landing.
Static stability in two sentences
If you push the nose down and let go, a stable aeroplane comes back. The further forward the CG is of the neutral point, the more aggressively it returns — this difference is the static margin. Beyond the aft limit the CG can sit at or behind the neutral point and the aeroplane no longer self-corrects.
A small worked example
Take a 60-tonne airliner cruising at FL350. Moving CG aft from 18% MAC to 32% MAC — a typical airline operations target — can:
- Reduce required tail download by ~ 4% of weight
- Cut trim drag by ~ 2–3%
- Save ~ 1.5–2.5% on fuel for the sector
Multiply by the airline's annual fuel bill and you can see why dispatchers fight for every centimetre of aft CG.
Quick check
- Forward CG → higher Vs, more stable, more drag.
- Aft CG → lower Vs, less stable, less drag.
- CG behind the neutral point → statically unstable — outside the envelope.
- Fuel burn typically moves CG forward (centre tank empties first) — planners check both take-off and landing CG.
- Strong nose-up trim setting on an empty aircraft is a clue the CG sits well forward.
Common mistakes
- Believing aft CG is always better. Only true within the envelope. Beyond the aft limit, controllability and stability collapse.
- Confusing CG with centre of pressure. CG is a property of mass, CP is a property of aerodynamics. They move independently with AOA.
- Forgetting that fuel burn moves CG. A loadsheet must check take-off, in-flight, and landing CGs — not just one.
- Thinking trim setting alone fixes a poor loadsheet. Trim cancels the force, not the underlying drag penalty.
Why it matters for exams and interviews
- DGCA Mass and Balance papers test "what happens to Vs if CG moves forward?" and "why is aft CG more economical?" almost every session.
- Tech interviews probe whether you understand that aft CG = better economics but tighter stability margin.
- Real airline dispatch teams watch CG trends across fuel burn — this is the conceptual model behind their loadsheet check.