How do Airplanes Fly...

where we want them to?

We talked about how airplanes fly. But how do airplanes fly in the specific direction the pilot dictates?

Just as you turn the wheels in your car to change direction, a pilot changes the deflection of the control surfaces to change direction.

Let's take a look at how this works by examining the fundamentals of airplane aerodynamics.

Axes of Rotation

The center of gravity is the point at which the airplane balances both laterally and longitudinally. When the airplane is in flight it rotates around this point. To fly an airplane we must control this rotation in all three dimensions.

We can define three axes that go through the center of gravity. Each axis is perpendicular (90 degrees) to each other. This will allow us to define the orientation of the airplane by the amount of rotation it has around each of these axes.

The three principal axes are pitch, yaw, and roll.

The pitch axis is perpendicular (90 degrees) to the yaw axis and is parallel to the wings. This axis starts at the center of gravity and points towards the right wing tip. A pitch motion is when the nose of the airplane moves up or down.

The yaw axis is perpendicular (90 degrees) to the wings of the airplane. This axis starts at the center of gravity and points straight down. A yaw motion is when the nose of the airplane moves from side to side.

The roll axis is perpendicular (90 degrees) to the other two axes. This axis starts at the center of gravity and points towards the nose of the airplane. A roll motion is when the airplane's wings move up or down.

Control Surfaces

Every airplane has a horizontal stabilizer and vertical stabilizer. Think of these like feathers on a dart. These are what keep the airplane stable in flight. The horizontal stabilizer keeps the airplane’s nose from moving up or down. The vertical stabilizer keeps the airplane from moving side to side.

Control surfaces are movable airfoils used to control the movement of the airplane. The three control surfaces that control the airplane's rotation around each principal axis are elevator, rudder, and ailerons.

We learned that the lift produced by an airplane's wings is determined by the shape of the airfoil, the angle of attack, and the speed of the airplane. The same is true for the forces generated by each control surface on the airplane.

Elevator

How do airplanes fly up or down?

The elevator is a hinged extension of the horizontal stabilizer. Raising the elevator up or down changes the over all shape of the horizontal stabilizer’s airfoil. When the elevator is raised the air going under the horizontal stabilizer has to travel farther than the air going over it. According to Bernoulli Principal this will decrease the pressure under the stabilizer causing it to be pulled downward.

Remember Newton’s third law? When the elevator swings upwards it deflects the air that is moving past the airplane upward. This means the air is in turn deflecting the elevator downward.

OK, so we have an airplane that is balanced perfectly on its center of gravity which is always located at some point on the wings. The elevator raises up which generates a force that is pushes the tail down.

The airplane pivots on the center of gravity right? Then pushing the tail of the airplane down will make the nose of the airplane rise. In other words, the elevator controls the pitch of the airplane. When the elevator raises the nose of the airplane raises. When the elevator is lowered the nose of the airplane lowers.

If the nose of the airplane rises, the angle of attack increases. If the angle of attack increases then the lift produced by the wings increases. So by raising the elevator the airplane will gain more lift and start to climb. If the elevator is raised far enough and held in that position long enough then the airplane will do a loop. That is, if the airplane is designed to do a loop. Try that with a jet liner and you may have some issues!

Conversely if the elevator is lowered the nose will lower. That will cause the angle of attack to decrease, and the airplane will descend.

Rudder

How do airplanes fly left of right?

The rudder is a hinged extension of the vertical stabilizer. The rudder acts exactly as the elevator does except it controls the yaw of the airplane. When the rudder swings to the right it generates an aerodynamic force pushing the tail to the left. The airplane pivots on the center of gravity causing the nose to swing right. The exact opposite happens when the rudder swings to the right.

Ailerons

How do airplanes fly in a rolling motion?

Ailerons are control surfaces located on the trailing edge of each wing tip. Ailerons control the roll of the airplane. These are a bit different than the elevator and rudder. It takes two ailerons working opposite of one another to make the airplane rotate around the roll axis. Everything else is the same as elevator and rudder.

When the right aileron raises it changes the overall shape of the wings airfoil and produces an aerodynamic force pushing the right wing down. At the same time the aileron on the left wing lowers and produces a force pushing the left wing up.

The airplane always rotates about its center of gravity. So we have a force pushing the right wing down and a force pushing the left wing up. This causes the airplane to roll in the clockwise position. This is considered to be “rolling” right. When the right aileron lowers and the left aileron rises the airplane will be “rolling” to the left.

Flaps

How do airplanes fly slower when landing?

Flaps are control surfaces located on the trailing edge of the wing just inside of the ailerons. On an RC airplane they work just like ailerons except the flaps on each wing work together in the down positions. Flaps never hinge upwards like ailerons.

Flaps are used to create more lift at lower speeds when landing. Not only do flaps produce more lift, they also produce drag. This helps slow the airplane down. The flaps will also affect the pitch of the airplane. When flaps are deployed it is necessary for the elevator to compensate for any changes in pitch caused by the flaps.

Summary

To understand how do airplanes fly in the direction you want them to it is critical that you know how to control the rotation of the plane around its principal axes.

The elevator controls the rotation around the pitch axis. The rudder controls the rotation around the yaw axis. The ailerons control the rotation around the roll axis. The flaps produce more lift and drag for landing an airplane.