Reducing Control Forces - ATPL 081 — Principles of Flight

Every deflected control surface pushes back. The air load on the surface, acting behind the hinge, is a hinge moment the pilot must hold through the control run (the linkage from cockpit to surface), and it grows with deflection and with the square of the airspeed. On a light aircraft at approach speed the forces are pleasant; scale the surface up and the speed up, and unaided human arms lose the fight long before the aircraft reaches transport size. This lesson covers the two answers, in the order history found them: aerodynamic balance (arranging the surface so the air itself cancels part of the hinge moment) and powered controls, with the artificial feel that fully powered systems must add back.

Aerodynamic balance: the purpose

The built-in balances (the horn and the internal balance below) share one idea: put some of the surface's area, or some of the airflow's pressure, ahead of the hinge line, where the air load generates a moment helping the deflection instead of fighting it. (The tabs, covered afterwards, reach the same goal by the opposite route: a small force on a long arm behind the hinge.) The purpose is exactly this: to reduce the hinge moment the pilot must hold (keeping control forces within human limits across the whole speed range) while leaving the surface's effectiveness essentially intact. The balance must stay a minority share: a surface balanced past its own hinge moment would drive itself to full deflection, which is the over-balance every design carefully avoids.

Horn balance

The horn balance is the visible version: at the tip of the surface, a portion of its area, the horn, extends forward of the hinge line, filling a notch cut into the fixed surface ahead of it. Deflect the surface and the horn (rigid with it, but on the other side of the hinge axis) moves the opposite way: trailing edge down, horn up. The air load on the horn, acting ahead of the hinge, produces a moment opposing the main surface's hinge moment, and the pilot holds only the difference. Horn balances are easy to spot on the elevators and rudders of light aircraft.

Internal balance

The internal balance hides the same trick inside the parent surface. The control surface's nose extends forward of the hinge into a bay within the wing or stabiliser, and a flexible seal connects that nose to the structure, dividing the bay into an upper and a lower chamber. Deflect the surface and the pressures on the upper and lower skins near the hinge gap change: the gaps between the parent surface's shrouds and the control's nose vent different pressures into the two chambers, and the pressure difference acts on the balance area ahead of the hinge, pushing it the way the pilot is already pushing. Same relief as a horn, but the balance area never meets the airstream, so it costs no extra drag.

The tab family

The subtler tools are tabs: small hinged surfaces at the trailing edge of the control surface itself. A tab is tiny, but it acts at the greatest possible distance from the control surface's hinge: a small force on a long arm, moving or resisting the big surface. Four members, distinguished entirely by what drives the tab and which way it goes:

Balance tab — geared to deflect opposite to the control surface: surface up, tab down. The tab's air load helps rotate the surface, so the pilot's force is lightened, at the price of a small loss of surface effectiveness, since the tab's own lift works against the surface's.
Anti-balance tab — geared the other way: it moves with the surface, deliberately increasing the hinge moment. Its home is the surface that is naturally too light: the all-flying tailplane above all, whose hinge moments near its pivot are so small that without an anti-balance tab the pilot would feel almost nothing and could over-control effortlessly.
Servo tab — the pilot's controls connect to the tab only; the main surface floats free on its hinge. Deflect the tab and its force drives the main surface to a new equilibrium: the pilot steers a large surface with tab-sized forces. The weakness is the mechanism's strength in reverse: at low IAS the tab commands little force, so control response near the stall and on the runway is poor.
Spring tab — a compromise: the pilot's linkage connects to the surface through a spring, with the tab driven by the linkage's relative motion. At low speed the hinge moments are small, the spring barely compresses, and the pilot moves the surface directly: full low-speed response. At high speed the moments are large, the spring gives, and the tab takes over progressively (servo-tab relief exactly where it is needed).

Power-assisted and fully powered controls

Past a certain size and speed, aerodynamics alone cannot keep forces humane, and hydraulics take over, in two distinct arrangements the syllabus wants clearly separated:

Power-assisted controls — the pilot's mechanical run still connects to the surface, and a hydraulic actuator adds most of the force, but only most. A share of the real hinge moment still reaches the pilot through the run, so the controls keep natural feel: forces that grow with speed and deflection, scaled down to human size.
Fully powered (irreversible) controls — the pilot's input goes to a valve, and the actuator alone moves the surface. The system is irreversible: air loads on the surface cannot drive the actuator backwards, so nothing of the real hinge moment reaches the pilot.

The differences to state: who carries the load (all of it, versus most of it), whether real feel survives (none, versus a proportional share), and what must be added (fully powered controls need artificial feel; power-assisted controls come with their feel built in).

Artificial feel

A fully powered aircraft with plain spring feel would be flyable, and dangerous. The advantages the syllabus asks for follow from what real feel used to do. Artificial feel (typically a spring gradient plus a q-feel unit fed with pitot–static pressure so that forces rise with dynamic pressure, exactly as natural forces would) restores three things:

protection — at high speed, heavy forces discourage the large, rapid deflections that would overstress the structure; the stick force per g minimum of the stability module, rebuilt in hydraulics.
information — force gradients tell the pilot's hands about speed and load factor without a glance at the instruments.
consistency — the aircraft handles like a conventional one across its whole envelope, so nothing new must be learned or unlearned in an emergency.

What the exam asks

The tab family's four members and the powered-control distinction are classic bank material. The overview, following the learning objectives:

Describe the purpose of aerodynamic balance

Describe the working principle of the horn balance

Describe the working principle of the internal balance

Describe the working principle and application of balance, anti-balance, spring and servo tabs

State the differences between fully powered and power-assisted controls, and describe power-assisted controls

Describe the advantages of artificial feel in fully powered controls