How Ailerons, Elevators and Rudder Steer an Aeroplane

A conventional aeroplane is steered with three main control surfaces: ailerons on the wings, an elevator on the tail and a rudder on the tail fin. Each one tilts the aircraft about a different axis. The main point is that every surface works by changing the airflow over a part of the aircraft, which creates a force away from the centre of gravity and so turns the plane.

Three axes of rotation

NASA's education material explains that an aircraft can rotate about three axes that pass through its centre of gravity and are at right angles to one another. Each rotation has its own name and its own control.

• Roll: the wingtips move up and down, about an axis running along the fuselage. The ailerons control it.
• Pitch: the nose moves up or down, about an axis lying along the wings. The elevator controls it.
• Yaw: the nose swings left or right, about an axis perpendicular to the wings. The rudder controls it.

NASA adds that each surface produces a force at its own centre of pressure, which sits away from the centre of gravity. That offset makes a twisting force, called a torque, and the aircraft rotates.

Ailerons: roll

Wikipedia's article on flight control surfaces describes ailerons as hinged parts on the rear edge of each wing near the tip. They move in opposite directions. Raising one reduces lift on that wing, and lowering the other increases lift on its wing. The aircraft rolls toward the wing with less lift and begins to turn.

Elevator: pitch

The elevator is the movable rear part of the horizontal stabiliser, the small wing at the tail. Both sides move together. Raising the elevator pushes the tail down and the nose up, which increases the angle at which the wing meets the air. Lowering it does the opposite. This is how a pilot climbs, descends and sets the nose position.

Rudder: yaw

The rudder is usually hinged to the rear of the vertical stabiliser, the tail fin, and is moved by foot pedals. Wikipedia calls it the primary means of controlling yaw. It points the nose left or right but does not, on its own, bank the aircraft.

Why a turn uses more than one control

Rolling with the ailerons starts a turn, but there is a side effect called adverse yaw. The wing that is producing more lift also produces more drag from that lift, so the nose tends to swing opposite to the direction of the roll. Wikipedia says this is most noticeable in low-speed aircraft with long wings, such as gliders.

Pilots counter adverse yaw with the rudder. Designers also help with differential ailerons, where the aileron that moves up travels further than the one that moves down, reducing the difference in drag.

Trim tabs

Holding a control against air pressure for a whole flight would be tiring. Trimming balances those forces so the pilot does not have to keep pressing on the controls. Wikipedia notes that elevator trim is the most common. Larger aircraft may also have rudder trim, to counter thrust that is stronger on one side, and aileron trim, to offset a heavy wing such as one with more fuel. On very large aircraft, the whole tailplane is often adjusted instead, because small tabs cannot supply enough force.

Combined surfaces on other designs

• Flaperons: ailerons that also droop when flaps are lowered, so they act as both aileron and flap
• Elevons: surfaces on the rear edge of delta wings that combine the jobs of elevators and ailerons

These show the same idea in other shapes. What changes is how many jobs one surface performs.

A bit of history

NASA states that the Wright brothers' 1902 glider was the first aircraft to achieve complete active control. Having control about all three axes is what separates an aeroplane that can be steered from one that merely glides where the air takes it.

Summary

Ailerons roll the aircraft, the elevator pitches it and the rudder yaws it. Each works by changing airflow and so creating a turning force about the centre of gravity. Turning properly uses all three, with the rudder dealing with adverse yaw and trim tabs taking the effort out of holding steady. Modern designs fold these roles into combined surfaces, but the three axes remain the same.

Sources

• Aircraft Rotations, NASA Glenn Research Center
• Flight control surfaces, Wikipedia