Longitudinal Stability

Question 1

Longitudinal stability

Answer 1

Longitudinal stability is the quality that makes an airplane stable about its lateral axis.
A longitudinally unstable airplane has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall.

Question 2

Static longitudinal stability or instability in an airplane, is dependent upon three factors

Answer 2

●Location of the wing with respect to the center of gravity
●Location of the horizontal tail surfaces with respect to the center of gravity
●The area or size of the tail surfaces

Question 3

Center of lift

Answer 3

The center of lift in most unsymmetrical airfoils has a tendency to change its fore and aft position with a change in the angle of attack.
The center of pressure tends to move forward with an increase in angle of attack and to move aft with a decrease in angle of attack.
This means that when the angle of attack of an airfoil is increased, the center of pressure by moving forward, tends to lift the leading edge of the wing more.
This tendency gives the wing an inherent quality of instability.
Most airplanes are designed so that the wing’s center of lift (CL) is to the rear of the center of gravity.
This makes the airplane “nose heavy” and requires that there be a slight downward force on the horizontal stabilizer in order to balance the airplane and keep the nose from continually pitching downward.

Question 4

Horizontal stabilizer

Answer 4

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.
Even though the horizontal stabilizer may be level when the airplane 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 will be just enough to balance the lever.
The faster the airplane is flying, the greater this downwash and the greater the downward force on the horizontal stabilizer.
In airplanes with fixed position horizontal stabilizers, the airplane manufacturer sets the stabilizer at an angle that will provide the best stability (or balance) during flight at the design cruising speed and power setting.

Question 5

Thrustline

Answer 5

Power or thrust can also have a destabilizing effect in that an increase of power may tend to make the nose rise.
The airplane designer can offset this by establishing a “high thrustline” wherein the line of thrust passes above the center of gravity.
A very “low thrust line” would tend to add to the nose-up effect of the horizontal tail surface.
With the center of gravity forward of the center of lift, and with an aerodynamic tail-down force, the result is that the airplane always tries to return to a safe flying attitude.

Question 6

Forward CG

Answer 6

●More Stable
●Less Maneuverable

Question 7

Aft CG

Answer 7

●Less Stable
●More Maneuverable