Getting the "Wright" Pitch
To gather this data, researchers will place special instruments on various parts of the airplane. Each instrument is designed to gather a specific type of information. For example, some instruments will collect information about how the air flows to the airplane. Other instruments will also measure how fast the air flows or how great the air pressure is at certain points along the aircraft. To gather the information needed by the AIAA pilots, the balance will measure the forces and where those forces are located along the center of gravity.
Remember, the center of gravity (CG) is the point where the entire weight of the airplane is considered to be concentrated. On a paper airplane you could find the general location of that point by balancing the paper airplane on your finger. The point at which the airplane balances on your finger would be considered the center of gravity (CG). All three motions (roll, pitch, yaw) pass through this point. All four forces interact on the airplane as it moves about this point.
Designing airplanes with static stability is important. (For the purposes of our discussion we are not considering military jet fighters as some form of instability is preferred which make these aircraft more quickly maneuverable when managed by computer and pilot.) Stability is the tendency of an airplane to fly with equilibrium on its flight path. To fly with equilibrium means that the sum of all forces and moments acting on the airplane will equal zero. See the graph below.
For example, let's look at an airplane that is flying straight and level. For this airplane to fly with equilibrium in its straight and level flight path, the four forces must be in balance. That means the lift will be equal to the weight and the thrust will be equal to the drag. It also means that there are no moments acting on it. These moments are trying to make the aircraft rotate about the center of gravity either by pitch, roll or yaw.
Now let's have the airplane encounter some minor turbulence. This turbulence causes the airplane to nose up or increase its angle of attack. If the airplane reacts to this disturbance by returning itself to its straight and level flight path (without the pilot having to make the adjustments), then the airplane has static stability.
Now let's have the airplane encounter some minor turbulence again. This turbulence causes the airplane to nose up or increase its angle of attack. If the airplane holds its new angle of attack after the turbulence has passed, then it is considered to have neutral static stability.
We'll return the airplane to its state of equilibrium. Let's have it encounter some more minor turbulence. This turbulence also causes the airplane to nose up. Even after the turbulence has passed, however, the airplane continues to nose up and does not automatically return to its previous flight path without the pilot having to make adjustments in the controls. This airplane is then considered to be "statically unstable".
When graphing data related to longitudinal static stability of an airplane, the graphs of airplanes with static stability have a similar slope.
Let's take a look at some actual data and graph it. The chart on the next page contains some hypothetical wind tunnel test data on a small airplane. As researchers we will be considering only the columns marked "Alpha (deg)" and "CM". The "Alpha (deg)" column tells us the angle of the nose up or down relative to the airflow. This is the angle of attack. Remember, increasing angle of attack will generally increase the amount of lift.
The "CM" column gives us information about the amount of pitching moment being generated by the airplane. A value (number) for CM that is positive means the airplane is pitching its nose up, and a negative value means the aircraft is pitching its nose down. The magnitude of the CM (the numbers themselves) are an indication of how fast the aircraft is pitching. The greater the absolute magnitude of the number, the faster the rotation (or pitching moment) is.
The pitching moment was converted into a coefficient in a manner similar to the lift coefficient and drag coefficient. This allows us to consider the pitch test done at other velocities, and for other wind tunnel models or aircraft that are otherwise identical, except for being a different scale.