Today we're going to be talking about stability and what makes an airplane stable. Stability is the ability of an aircraft to correct for disturbances in its equilibrium and to return to its original flight path. For example, if you're flying your airplane and it's pushed up by turbulence, a stable airplane would have a tendency to pitch back down to its original flight path.

Most airplanes, especially training aircraft, are designed with stability in mind. Some aircraft, however, like fighters, are designed to be less stable so they can be more maneuverable. When it comes to stability, all three axes are considered. Do you remember what those are from the last lesson? Let's talk about two types of stability that affect our three axes: static and dynamic.

Static stability is the initial tendency of the aircraft after the equilibrium is disturbed. In other words, what's your airplane's immediate reaction after you make a control input or one of the three axes are changed by some outside force? Let's say, for example, you pitch your airplane up five degrees. What's the initial tendency of your airplane? If it's to pitch back down to its original attitude, this would be known as positive static stability. On the other hand, if you pitch your airplane up and it continues in that same attitude, this would be known as neutral static stability. But what if we were to pitch up to 5 degrees and the aircraft continued to pitch beyond that 5 degrees? This would be known as negative static stability. For most aircraft, this is undesirable.

Now let's talk about dynamic stability. This is an airplane's response to an upset in the equilibrium over time. Let's say you pitch the nose of your airplane up again. Initially, it may pitch below the initial pitch attitude you had before, but over time these oscillations decrease. This would be known as positive dynamic stability. If these oscillations stayed the same over time, we would call this neutral dynamic stability. These oscillations could even get worse; we would call this negative dynamic stability.

Now that you understand the different types of stability, let's talk about some of the design features that make an aircraft more stable. First, let's talk about how we get longitudinal stability on the lateral axis of our airplane. Most airplanes are designed so that the center of gravity is in front of the center of lift or the center of pressure. This intentionally makes our aircraft nose-heavy, and there's a special feature to help balance it out. The horizontal stabilizer on the tail of the airplane is designed with a slightly negative angle of attack, so it exerts a downward pressure. This downward pressure balances the airplane at the optimum cruising speed.

If our airplane slows down below that optimum cruising speed, our horizontal stabilizer, which is basically an upside-down wing, won't create enough downward pressure to balance the aircraft. This will allow the nose to pitch down. When the nose of the aircraft pitches down, it'll also gain speed. With more speed, our upside-down wing creates more downward pressure, and that downward pressure will pitch the nose of our aircraft back up. And now you can kind of see a cycle developing or an oscillation. This is where we get our dynamic stability.

Let's talk about lateral stability on the longitudinal axis for just a minute. There are several different ways we can get lateral stability, but today we're only going to talk about two: dihedral and keel effect. Some airplanes are designed so that the outer tip of the wing is higher than the wing root. We call this dihedral. Let's say your airplane rolls to the left because some turbulence hit the right wing from the bottom. When this happens, the airplane will enter a side slip. This just means that the left wing yaws slightly in front of the right wing. When relative wind hits our wings, the wing in the front has a higher angle of attack. With a higher angle of attack, that will produce more lift, which will roll us back to level flight.

Keel effect is something that affects high-wing aircraft. For these, anytime the airplane rolls, the weight of the aircraft will simply act as a pendulum and swing it back to level flight.

The last thing we'll talk about today is directional stability and how it affects the yaw axis. We achieve directional stability by the weather vane effect. This is achieved by putting a vertical fin on the tail of the aircraft. Anytime the vertical fin is not aligned with the relative wind, the relative wind will push the tail back until it aligns itself, much like a weather vane.