An intro to aviation - Control surfaces
We’ve looked at the ways that planes rotate - roll, pitch, and yaw. Now, we’ll look at how the plane’s control surfaces move around to achieve that rotation.
Control surfaces
Control surfaces are movable surfaces on the trailing (back) edge of each wing. Pilots control them with a set of pedals and either a yoke (which looks like a steering wheel) or a control stick. As the plane moves through the air, control surfaces deflect (change angle) and change the amount of lift that each wing generates.
The ailerons are the control surfaces on the back of the main wings and are used to roll the plane. Pilots control them by turning their yoke like a steering wheel or moving their control stick to the side. To roll left, the left aileron is moved up (to reduce the lift generated by the left wing) while the aileron on the right is moved down (to generate more lift).
The elevators are the smaller control surfaces on the horizontal stabilizers (the little wings at the back of the plane). They change the plane’s pitch and are controlled by the pilot moving the yoke or control stick forwards and backwards. Pushing the stick forward moves the elevators down and generates more lift at the back of the plane while the lift at the front stays the same, which pushes the nose down.
The last control surface is the rudder. It’s on the vertical stabilizer (the vertical wing that sticks out the top of the plane) and pilots use it to yaw the plane by pressing on their rudder pedals. The pedals can be a little confusing initially, since unlike a bicycle (where pushing forward on the right turns the bicycle left), pushing on the right pedal yaws the plane to the right.
More about rudder pedals
Pilots also need to be able to steer their planes on the ground, but without air flowing over the wings, a plane’s control surfaces don’t actually do anything. As a result, pilots steer by turning the front wheel of the plane1 like the front wheels of a car. It would be confusing if the pedals made the plane yaw in the air, but not on the ground, so that front wheel is also controlled by the rudder pedals - pilots steer on the ground with their feet!
Planes also have brakes on their wheels. In many aircraft, those brakes are also controlled by the pedals and are activated by pushing just the top edge of the pedals. Given that pilots push that top edge with their toes, those brakes are called toe brakes.
Coordinated turns
When a control surface deflects and causes a wing to generate more lift, that wing also generates more drag (which causes the wing to start slowing down). As a result, when the plane’s right aileron moves downwards to roll the plane to the left, the right wing also generates more drag and causes the plane to start yawing to the right. That’s called adverse yaw and the pilot counteracts it by using both the rudder and aileron inputs when rolling into a turn. Preventing adverse yaw is one of the uses for the little ball in the turn coordinator that I mentioned in An intro to aviation - The gyroscopic flight instruments.
The other wing bits
While we’re discussing the bits of the wing that move, we may as well talk about flaps, slats, and spoilers.
The flaps are on the main wings, inboard of the ailerons (closer to the body of the plane). Instead of being used to roll the plane, they can be extended out of the back of the wings to increase the lift that the wings generate 2. Doing so allows planes to fly more slowly, which is useful when taking off or landing. Slats also help increase lift at low airspeeds, but do so by extending out from the front of the wings
Finally, spoilers are the big panels that flip up from the top of the wings of some planes when they’re slowing down after landing. They both spoil (reduce) the lift generated by the wings to make the plane stay on the ground and create extra drag, which slows the plane down faster.
Next up
Next time, I’ll take a look at how planes are designed to stay stable in flight.
