The Axes of an Airplane and Their Purpose

Controlling and steering an airplane is much different than driving a car. For one, stability and maneuvering is vastly different. Boats and cars both two dimensional whereas an aircraft is three dimensional. The three dimensions that determine an aircraft’s stability are its axes. The axes are what control the roll, pitch and yaw.

What are roll, pitch and yaw? Imagine, if you will, three lines. All three lines form intersecting right angles at the airplane’s center of gravity.

Roll: rotation around the front to back axis. Imagine a string running a straight, horizontal line starting from the airplane’s nose all the way to its tail. Now, if the airplane rotated about this string, it would be rolling.

Pitch: rotation around the side to side axis. Imagine another piece of string running in a straight, horizontal line across the wing span from one edge to the other. If you turned or rotated the string, the airplane would be pitching up or down.

Yaw: rotation around the vertical axis. Now, if you take the same piece of string used in the pitch example but instead of rotating, moved it back and forth this would be creating yaw.

The purposes of roll, pitch and yaw.

Roll: Rolling an airplane allows the airplane to make a turn. As covered in the previous lesson, lift is vertical. When an airplane banks or rolls into a turn, the vertical lift becomes horizontal. The horizontal component of lift is what turns the plane. The ailerons rolls the airplane and allows it to turn.

The ailerons can be found on the outer edge of the wing. One on each wing. Both ailerons move in opposite directions. So if the left aileron is up, the right one will be down and vice versa. When an aileron is up, lift is decreased bringing the wing down. When the aileron is down, lift is increased bringing the wing up. This allows the airplane to roll.

Pitch: The elevators control pitch. The elevators can be found on the horizontal tail surface of an airplane. They move up or down. Unlike the ailerons, they move in the same direction in unison. The movement of the elevators decreases or increases lift on the tail which tilts the plane’s nose up or down.

Yaw: The rudder controls yaw. The rudder can be found on the aircraft’s vertical tail fin next to the elevators. It moves from side to side. The rudder’s movement pushes the tail either to the left or right. The rudder is crucial along with the ailerons in turning an airplane smoothly.

Finding Your Stability

When you take the exam for your Private Pilot License, you’ll most likely come across the phrase longitudinal stability. When the FAA talk about longitudinal stability, they mean pitch stability. Is the airplane’s nose pitching up and down?

On any map or globe, longitude is shown as a vertical line. It’s the same with an airplane. This is why longitudinal stability refers to the pitch of the airplane. An airplane that is flying along in smooth and level flight is said to be inherently stable. An airplane that is inherently stable will require less control.

Most airplanes are designed that if you pull up on the yoke and then let go, the nose will come up and then go back down returning to equilibrium. Test it out sometime when you’re flying. Pull the yoke back. The elevators will move up and the nose will pitch up. Release the yoke and the elevators will return to neutral position. The airplane will return to equilibrium.

If the airplane oscillates or bounces up and down as if riding a wave, the oscillation or wave will eventually get smaller and smaller until the airplane returns to equilibrium. Below is a picture of an airplane oscillating.

What determines the longitudinal stability of an airplane? The location of the center of gravity in relation to the center of lift and tail down force. Most aircraft are designed so that the center of gravity is in front of the center of lift. Look at the picture below.

The center of gravity causes the airplane to want to dive down nose first. The center of lift wants to uplift the airplane as if it was attached on a string. The down force on the tail pushes the airplane downwards tail first. All three of these elements cause the airplane to stay flying in the sky. Let’s talk about these three more in detail.

The center of gravity or weight is located in front of lift. It’s the point over which the airplane is balanced. To better explain this, imagine a seesaw or teeter totter. In the center of the seesaw is a small post on which it is balanced. This post is the seesaw’s center of gravity.

Lift or center of pressure is the force that directly opposes the weight of an airplane. In other words, it is the opposite of the center of gravity. By opposing the aircraft’s weight, lift holds the aircraft in the air. Lift is produced by the motion of air rushing across the plane.

The down force on the tail is final piece that keeps the airplane stable and flying. There are two factors that attribute to the down force on the tale. (1) Design. (2) Air or wind.
The tale is purposefully designed as an upside down wing. Let me explain in more detail. The curvature of a wing is called the camber and is crucial in creating lift. Since a wing’s camber is curved, the air flowing over the top of the wing is much faster than the air rushing beneath. This causes low pressure above the wing and high pressure beneath. This large difference in pressure is one of the factors that creates lift.

Remember that the tail of a plane is just an upside down wing. Because the tail is upside down, the pressure in air is switched. The air beneath the wing is accelerating and the air above is going slower. This causes the air beneath to have high pressure and the air above to be low. This difference in pressure between both wings generates lift and helps keep the airplane balanced.

If you’re still confused or have more questions, I’d suggest looking up the Bernoulli effect.

 

Slow Flight
If an airplane slowed down, the wind rushing over the tail would decrease. Three elements would become unequal. The center of gravity would have more weight than the down force on the tail. This would cause the tail to come up and the nose to dive down. So, the down force on the tail is what helps make the airplane stable as far as pitch and speed are concerned.

Changing the Center of Gravity
It’s difficult to change the center of pressure in an airplane, but easy to change the center of gravity. By moving luggage around in an aircraft or adding passengers, the center of gravity is changed. But be careful when changing an airplane’s center of gravity because it can cause the airplane to become less stable.

Aft CG Limit
– Aft: towards the tail or back of airplane
– CG: Center of Gravity
There is a limit to how far aft luggage or weight can be loaded until the airplane loses stability. The same goes for how far weight can be moved towards the center of pressure or front of the airplane. When weight is moved to the aft CG limit, the airplane will become less stable because there is a decreased in down force on the tail. This will cause the airplane to have a slower stalling speed but higher cruising speed. On the other hand, when weight is moved behind the center of pressure the airplane will want to nose up and enter a stall. The aircraft will become less stable and less efficient.

Here it is broken down in bullet points.
Weight Moved Aft CG Limit
– Cruising speed decrease
– Stall speed increase
– More efficient, less stable

Weight Moved Forward CG Limit
– High stall speed
– More longitudinal stability
– More stable, less efficient

Always Know Your Instruments!

Instruments can be very confusing at first to learn and little bit intimidating. I’m still learning to master them. I’m still figuring out what exactly they all mean and how I should use them.

Today, I went under hood. Which translates to I had use foggles to instrument training. This training requires you to wear foggles so you can’t see outside the airplane. All you can see are your instruments. It forces you to learn your instruments.

Before I started my instruments training, I’d always been confused why pilots who’d flown in the clouds would talk about disorientation. They’d tell about times when they flown in clouds where they had no visibility and they quickly became confused and disoriented. I didn’t understand how anyone could feel disorientated while in a small plane where you feel every bump. I thought, wouldn’t you be able to tell if you started flying upside down or sideways? Wouldn’t gravity let you know?

Today, I realized for the first time exactly what they’d meant. When I put on the glasses, I couldn’t see my outside world. All I could see was my instrument panel. It was as if I’d flown into a cloud and couldn’t see anything around me. It was a little bit nerve racking. And I quickly realized how one could easily become disorientated.

As I watched my instruments and did a standard turn, my body could feel the plane start turning but I had no idea which way. It’s interesting that your body can feeling the motion of turning but not the direction. I never understood this until now.

Below I’ll go over the instruments and their uses.

AIRSPEED INDICATOR

An airspeed indicator shows how fast airplane is moving through the air. It calculates this by measuring the difference between total air pressure and static air pressure from the pitot tube.

ALTITUDE INDICATOR / ARTIFICIAL HORIZON

The altitude indicator or artificial horizon shows pilots where the horizon is positioned at. Pilots use the it to help them judge how an airplane is orientated. Are we turning? Are we descending or ascending? The indicator shows the airplane’s wings in relation to the horizon.

ALTIMETER

The altimeter was once called the Altitude Meter but was shortened to altimeter. It shows the airplane’s height above sea level. It can measuring the height by sensing the change in static air pressure caused by a change in altitude.

TURN COORDINATOR

When I first started flying, I’d confuse this with the Artificial Horizon. The turn coordinator is different. It doesn’t show you if the plane is level with the horizon. What it does show you is if your ailerons and rudder are coordinated. It also shows the degrees of bank your turning and your rate of turn. It helps pilots through a turn. It uses a gyroscope to show the rate and direction of a turn.

HEADING INDICATOR

This shows the direction in which the airplane is headed. The heading indicator uses a gyro to indicate the direction. It’s much better to use this than a magnet compass for direction. Magnetic compasses are prone to errors which result from the speed of an aircraft.

VERTICAL SPEED INDICATOR

The vertical speed indicator shows the airplanes rate of climb or descent by measuring how fast static pressure changes as the aircraft climbs or descends.

My advice to anyone flying, learn these instruments well. You’re life may very well depend on them. They’re important!

Video of My First Flight Lesson

This video was taken about a month or so ago of my very first flight lesson! I wasn’t very nervous since I’ve flown in small airplanes a few times before. But I was excited! Being able to see the landscape, cars and world from such a high view was thrilling. And sometimes distracting 🙂

https://youtu.be/J_aycIbcYF8