The tail assembly, a horizontal stabilizer with elevator and a vertical fin with rudder, keeps the aircraft pointing into the airflow and balances the wing’s pitching moment, and it works only if the aircraft’s centre of gravity lies ahead of its neutral point. A centre of gravity too far back makes the aircraft pitch-unstable and often unrecoverable, and the permitted range is specific to each aircraft, so it must be found by calculation or taken from a proven design, not from a general percentage. Weigh the aircraft and find the centre of gravity before every configuration change.
Part of Aerodynamics and Flight
The stabilizing surfaces at the rear of an aircraft, why centre-of-gravity position governs stability, and how the tail is sized and set.
The Tail Is A Small Wing With A Long Lever
If the nose rises, the horizontal stabilizer’s angle of attack rises too and it makes more lift behind the centre of gravity, which pushes the nose back down. That restoring moment is stability. The fin does the same sideways. A tail with a longer arm and a larger area gives a stronger restoring moment.
The Centre Of Gravity Is The Master Variable
The wing’s lift acts at its centre of pressure, but the stability of the whole aircraft is set by the neutral point, the position of the centre of gravity at which the aircraft is neither stable nor unstable. With the centre of gravity forward of the neutral point the aircraft is stable; at the neutral point it is neutral; behind it, pitch diverges and a pilot cannot hold it. The gap is the static margin.
The carrier states a centre-of-gravity range of 15-30% of wing chord from the leading edge. A range 15% of the chord wide is a general claim, and the safe range for one aircraft depends on its tail size and arm, its wing and its fuselage, and is usually narrower. Do not adopt that figure as your limit; find the neutral point for your own aircraft, or copy a proven design’s proportions exactly, and work from there.
Method: Setting And Checking The Tail
Step 1: Fix the proportions from a proven design. Tail area and arm are design choices that interact with the wing; the corpus gives no formula.
Step 2: Set the stabilizer incidence so that the elevator is near neutral in cruise; adjust by trim after test.
Step 3: Build in rigidity. A flexible tail loses effectiveness and can flutter (Control Surfaces). Fabric tension can warp light tail frames (Fabric Covering).
Step 4: Check the fin and stabilizer are square to the wing and to each other, and to the centreline.
Step 5: Weigh and compute the centre of gravity (Ground Testing), and mark the limits on the aircraft where the pilot can see them.
Common Mistakes
- Adding weight at the tail — a heavy tail wheel, battery or ballast moves the centre of gravity aft.
- Refitting the engine, tank or pilot seat without reweighing.
- Cutting the stabilizer short to save weight.
- Confusing a stall with a tail problem — a tail that cannot lower the nose at a stall (too small, or in the wing’s wake) is a design flaw, found the worst way.
Safety
An aft centre of gravity cannot be flown out of
If the centre of gravity is behind the neutral point, a nose-up disturbance grows by itself, the stall comes with no way to push the nose down, and recovery may be impossible. Never fly outside the centre-of-gravity limits, and never fly a configuration you have not weighed.
What’s Next
- Learn next: Propulsion Systems, the front end of the aircraft
- Combine with: Four Forces of Flight, the moment balance