Longitudinal Stability
In designing an aircraft, a great deal of effort is spent in developing the desired degree of stability around all three axes. But longitudinal stability about the lateral axis is considered to be the most affected by certain variables in various flight conditions.
Longitudinal stability is the quality that makes an aircraft stable about its lateral axis. It involves the pitching motion as the aircraft’s nose moves up and down in flight. A longitudinally unstable aircraft may have a tendency to enter an uncontrollable dive or unrecoverable stall. Thus, an aircraft with longitudinal instability becomes difficult and sometimes dangerous to fly.
CG Location
In most asymmetrical airfoils, the location of the center of pressure (CP) has a tendency to change with changes in the angle of attack. The CP tends to move forward with an increase in angle of attack and to move aft with a decrease in angle of attack.
Most aircraft are designed so that the wing’s CP is to the rear of the CG. This makes the aircraft “nose heavy” and requires that there be a slight downward force on the horizontal stabilizer in order to balance the aircraft and keep the nose from continually pitching downward. Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight negative angle of attack. The downward force thus produced holds the tail down, counterbalancing the “heavy” nose.
Pilots do not have too much control regarding the placement of the CP; they do however, have influence over the location of the CG. Generally speaking, a forward CG will result in a more stable aircraft but with reduced maneuverability and range. A rearward CG will result in a less stable aircraft but with increased maneuverability and improved range. It should be noted that the location of the CG must always remain within the allowable forward and aft limits. Training aircraft will typically be designed with added stability at the expense of maneuverability. Aerobatic or fighter aircraft will be designed with increased maneuverability and less stability.
Downwash
Even though the horizontal stabilizer may be level when the aircraft is in level flight, there is a downwash of air from the wings. This downwash strikes the top of the stabilizer and produces a downward pressure, which at a certain speed is just enough to balance the aircraft. The faster the aircraft is flying, the greater this downwash and the greater the downward force on the horizontal stabilizer (except T-tails). In aircraft with fixed-position horizontal stabilizers, the aircraft manufacturer sets the stabilizer at an angle that provides the best stability (or balance) during flight at the design cruising speed and power setting.
If the aircraft’s speed decreases, the speed of the airflow over the wing is decreased. As a result of this decreased flow of air over the wing, the downwash is reduced, causing a lesser downward force on the horizontal stabilizer. In turn, the characteristic nose heaviness is accentuated, causing the aircraft’s nose to pitch down more. This places the aircraft in a nose-low attitude, lessening the wing’s angle of attack and drag and allowing the airspeed to increase. As the aircraft continues in the nose-low attitude and its speed increases, the downward force on the horizontal stabilizer is once again increased. Consequently, the tail is again pushed downward and the nose rises into a climbing attitude.
As this climb continues, the airspeed again decreases, causing the downward force on the tail to decrease until the nose lowers once more. Because the aircraft is dynamically stable, the nose does not lower as far this time as it did before. The aircraft acquires enough speed in this more gradual dive to start it into another climb, but the climb is not as steep as the preceding one.
After several of these diminishing oscillations, in which the nose alternately rises and lowers, the aircraft finally settles down to a speed at which the downward force on the tail exactly counteracts the tendency of the aircraft to dive. When this condition is attained, the aircraft is once again in balanced flight and continues in stabilized flight as long as this attitude and airspeed are not changed.
Thrust
The location at which the thrust vector acts also impacts the longitudinal stability of an aircraft.
- If the thrust vector is above the centre of gravity any application of thrust will induce a nose down moment.
- If the thrust vector is below the centre of gravity the application of thrust will induce a nose up moment.
- If the thrust line acts through the centre of gravity, no moment is created.
