How does a plane steer?

The principle

Step 1: Three ways to turn. A plane in the air can rotate three ways around its centre. Pitch tips the nose up or down. Roll tips one wing up and the other down. Yaw swings the nose left or right, like shaking your head.

Step 2: A flap changes the lift. A wing makes lift by bending the air that flows past it downwards; the air pushes back up on the wing. Tilt a hinged flap at the back of a wing down and that part gets more curve (more angle to the air), so it bends the air more and makes more lift. Tilt it up and that part makes less lift, or even pushes down. These hinged flaps are called control surfaces.

Step 3: The elevator, for pitch. The small wing on the tail has a hinged flap called the elevator. When the pilot pulls back, the elevator tilts up, the tail is pushed down, and the plane pivots so the nose rises.

Step 4: The ailerons, for roll. Near each wingtip is a flap called an aileron. They move in opposite directions: one goes down (that wing gets more lift and rises), the other goes up (that wing gets less lift and drops). The plane rolls.

Step 5: The rudder, for yaw. The upright tail fin has a flap called the rudder. Swing it to the right and the air pushes the tail to the left, so the nose swings right.

Step 6: Lean to turn. Here is the surprise: the rudder is not what turns a plane. First, where does lift push? Always straight out of the top of the wing, at a right angle to the wings. With the wings level it points straight up and just holds the plane up. Now the pilot uses the ailerons to roll the plane, so it leans over to one side. The lift arrow leans over with the wings. Think of that leaning arrow as two parts added together: an up part, which still holds the plane up, and a sideways part, which points to the side the plane is leaning towards. The up part is now a little smaller than before, so the plane would slowly sink; the pilot pulls back a bit, the wings make a bit more lift, and the height is held. Pilots call this leaning “banking”.

Step 7: The sideways part turns it. Seen from above, the sideways part keeps pulling the plane towards the inside of the turn, so its path bends into a curve, the same way a bike leans into a corner and its lean carries it round. The rudder only keeps the nose lined up with the curve, so the tail follows the nose instead of skidding sideways. The steeper the lean, the more lift the wings need: in a steep 60 degree lean they have to carry about twice the plane’s weight. A gentle “standard rate” turn is 3 degrees a second, a full circle in 2 minutes.

Step 8: Plane or helicopter?

• Plane: Upside: fast and efficient, the wings give lift for very little fuel. Downside: it must keep moving to stay up, and it needs a runway.
• Helicopter: Upside: it can hover and land almost anywhere; to move or steer it tilts its whole spinning rotor that way. Downside: it is slower and burns more fuel for the same trip.

Try it yourself: next time you fly, look out at the wing as the plane starts a turn. You will see the aileron tip up on one side and the whole plane lean before the view starts to swing.