Figure 2-3. Static Stability

CHAPTER TWO

T-34C CONTACT

tendency to enter a stalled condition. Therefore, most airplanes are designed with their CG

located slightly forward of the CP to create a nose-down tendency to pitch downward away from

a stalling condition. This provides a safety feature in the characteristics of the airplane. While

the airplane is flying within its range of normal speeds, the airflow exerts a downward force on

the horizontal stabilizer; thus, at normal cruise speed it offsets the inherent nose-down tendency

of the airplane.

Generally, in straight-and-level flight, the wings on each side of the airplane have identical

angles of attack and develop equal lift. This laterally balanced condition normally keeps the

airplane level. Occasionally though, a gust of air will upset this balance by increasing the lift on

one wing, causing the airplane to roll around its longitudinal axis.

The T-34C is designed so that the outer tips of the wings are higher than the wing roots. The

upward angle thus formed by the wings is called dihedral. When a gust causes a roll, a sideslip

will result. This sideslip causes the relative wind affecting the entire airplane to be from the

direction of the slip. When the relative wind comes from the side, the wing into the wind is

subject to an increase in angle of attack and develops an increase in lift. The wing away from the

wind is subject to a decrease in angle of attack, and develops a decrease in lift. The changes in

lift effect a rolling moment tending to raise the windward wing, hence dihedral contributes to a

stable roll due to sideslip. The T-34C has positive static stability. That is, it tends to return to

the condition for which it has been trimmed.

An aircraft trimmed for steady flight is in a state of equilibrium. In other words, the sum of all

the forces and moments is zero. Displacement from this position by some outside force, such as

a gust of turbulence, creates an unbalance and causes the aircraft to demonstrate an initial

tendency. This tendency will be to (a) return, (b) be displaced further, or (c) remain in the new

attitude or position. This initial tendency is known as static stability. If the aircraft tends to

move back in the direction of its trimmed attitude, it is said to be statically stable or to have

positive static stability. If the aircraft continues to move away from trimmed equilibrium, it is

statically unstable or it has negative stability. An aircraft that stays in its displaced attitude,

neither moving positively nor negatively, is said to be neutrally stable or neutrally statically

stable (Figure 2-3).

Positive Static Stability

Tendency to return to

equilibrium

Negative Static Stability

Tendency to continue in the

displacement direction

Neutral Static Stability

Equilibrium encountered at

any point of displacement

Figure 2-3 Static Stability

2-6 INTRODUCTION TO T-34C AERODYNAMICS