Controls

Basic Aircraft Components

A typical light aircraft has the following parts to it:

• Spinner (the ‘cone’ of the prop)
• Landing gear / undercarriage
• (Wing/lift) strut
• Wing
• Ailerons
• Flaps
• Fuselage
• Horizontal Stabiliser
• Fin and dorsal (a part perpendicular to the horizontal stabiliser)
• Rudder
• Elevator
• Doors
• Seats
• Windscreen
• Engine cowl
• Propeller

Aircraft axes and control surfaces

The lateral axis (Y-axis) runs across the wing of the aircraft. The longitudinal axis (X-axis) runs perpendicular to the Y-axis (duh) and runs down the length of the aircraft’s fuselage and straight through its nose. The normal axis (Z-axis) runs perpendicular to the (geometric) plane created by the X and Y axes. All these axes intersect at the aircraft’s centre of gravity.

The aircraft’s control surfaces manipulate the deflection of aerodynamic forces on the aircraft such that it changes in a desired way. Normal straight and level flight requires no deflection on any control surfaces (minus the elevator trim) which means that air flows smoothly over all the control surfaces. A deflection in any of these surfaces will mean that the relative wind will create a force against that control surface, thus influencing the aircraft’s movement. For example, pulling back on the control column deflects the elevator upwards. The airflow over this part of the aircraft is now disrupted – it strikes the now-raised elevator and creates and downwards force on the tailplane, thus lifting the nose and vice versa.

In the case of the ailerons, turning the control column to the right causes a deflection upwards on the left aileron and the a downwards deflection on the right aileron. Similar to the elevator example above, the creates a downward force on the left aileron and an upwards force on the right aileron. This lowers the left wing and raises the right wing and sets the aircraft into a banked attitude – the differences in drag on these wings create the turn.

As for rudder, pressing on the left pedal causes the rudder to deflect to the left. The relative airflow will apply a ‘right-wards’ force on the tailplane. The tailplane, as such, will move towards the right. However, as the aircraft revolves around the normal axis, the pilot who is on the other side of the ‘circle’ observes the aircraft yawing to the left.

Design Features

The horn balance is a protuding extension of the control surfaces that assists with manipulation of the control surface by creating the opposite ‘effect’ on the other ’side’, similar to how turning to the left causes the ailerons to act in opposite directions. The following example highlights how it works with the elevator:

The horn balance

Mass balances are designed to prevent control surface flutter. This is a flight state where external disturbances have caused a control surface to deflect in the opposite direction. The deflection is ‘reflected’, for want of a better word, back towards the neutral position due to the airflow but, due to the surface’s interia, overshoots the neutral position. This cycle repeats and repeats, and can cause vibrations and structural damage. The mass balance is designed to bring the centre of gravity of the control surface back towards the hinge to make it more difficult for a force on the leading edge of the control surface to affect the control surface as a whole. These mass balances are often hidden within the control surface itself and are not visible from the exterior.

Trims serve to allow the pilot to ‘fine tune’ the aircraft’s control surfaces. This makes it easier for the pilot when certain attitudes are required for extended amounts of time. Furthermore, an aircraft’s four forces (lift, weight, thrust and drag) do not operate on the same centres – the weight and drag work from the centre of gravity where as the thrust and lift work from the centre of pressure. As the centre of gravity is typically located in front of the centre of pressure, the weight force will have a tendency of pitching the nose downards (note its balancing force, lift, is at the centre of pressure – is the word ‘resolves?’). The tailplane, and the elevator trim, helps the aircraft balance out this tendency among other things.

The trim is a tab attached to the end of the control surface. Depending on how it’s set, it will influence the resultant force acting on that control surface. For example, a control surface movement on the aileron to pitch up (i.e., elevator up) with a trim set to nose down means that the force pitching up is somewhat reduced by the force that is trimmed down. This diagram shows the forces in action and their resulting force:

Trim Forces

Trims are also available for ailerons and rudders on some aircraft. The theory behind their operation is the same. In practice, rudder and elevator trims are operated using some form of trim wheel inside the cockpit. Rudder trims are mostly found in twin engine aircraft in order to balance out any differences in thrust generated by either engine. I’m not too sure, and the book doesn’t explain, if there are wheels for ailerons, but there are ground-adjustable aileron and rudder trims available on some aircraft.