Thrust, Stability, & Center of Gravity

Private Pilot License (PPL) Notes

This section covers the concepts of thrust, stability, and center of gravity, explaining how they affect aircraft performance and handling, especially for new student pilots learning about weight and balance theory.

Thrust and Power

1. Thrust is a force similar to lift, drag, and weight, commonly measured in pounds.
   1. Power is the amount of work performed over time.
   2. An airplane at full throttle with brakes locked has high thrust but does not move.
2. Excess thrust decreases as the airplane accelerates.
   1. Excess thrust is greatest at the start of movement.
   2. Thrust equals drag when airspeed becomes steady.

Effects of Thrust and Drag

1. In level flight:
   1. If thrust exceeds drag, speed increases.
   2. If drag exceeds thrust, speed decreases.
2. Thrust generally acts parallel to the airplane's longitudinal axis.

Weight and Center of Gravity

1. Weight opposes lift and pulls the airplane toward the center of the Earth.
   1. Includes the load of the airplane, crew, passengers, fuel, and cargo.
2. Acts at the center of gravity (CG):
   1. The point where the airplane would balance.
   2. All axes of rotation intersect at the CG.
3. Understanding CG is crucial for weight and balance theory.

Stability in Flight

1. Stability is the airplane's ability to return to its original flight attitude after a disturbance.
   1. Essential for safe and predictable flight.
2. Pilots desire a balance of maneuverability, controllability, and stability.
   1. Too much stability reduces maneuverability.
   2. Too little stability makes the airplane difficult to control.
3. Pilots can affect stability through loading.

Longitudinal Stability

1. Determined by the relationship between the center of gravity and center of lift.
   1. CG is usually ahead of the center of lift, causing nose heaviness.
   2. Nose heaviness enhances stall recovery.
2. The tail provides a downward force to balance the nose heaviness.
   1. Result of propeller slipstream, wing downwash, and stabilizer angle.
   2. Negative lift (tail downforce) must be offset by additional lift from the wings.
3. Total lift required equals weight plus tail downforce.

Lateral Stability

1. Achieved through dihedral, where wing tips are higher than wing roots.
   1. If the airplane banks, it sideslips toward the lowered wing.
   2. Lowered wing has a higher angle of attack, producing more lift.
   3. This tends to return the airplane to level flight.
2. Pilots can affect roll stability by managing fuel balance between wing tanks.

Yaw Stability

1. Function of the side area of the fuselage and vertical stabilizer.
   1. The airplane yaws around the center of gravity.
   2. Greater area aft of the CG causes the airplane to weathervane back to its original direction.
2. An aft CG reduces the area behind the CG, decreasing yaw stability.

Effects of Improper Loading

1. The CG must be within specified limits for safe operation.
   1. Improper loading affects stability on all three axes, especially longitudinally.
2. Aft CG:
   1. Reduces pitch stability and nose heaviness.
   2. Decreases tail downforce needed to balance the airplane.
   3. Can cause control issues during stalls.
3. Forward CG:
   1. Increases nose heaviness, enhancing stability.
   2. May require excessive elevator input to flare during landing.
   3. Increases total lift required, raising stall speed and reducing performance.
4. Exceeding maximum weight:
   1. Leads to higher stall speeds and longer takeoff distances.
   2. Reduces climb rate and lengthens landing roll.
   3. Can make the airplane unstable or unable to take off.

Importance of Proper Loading

1. Ensures the airplane flies with designed maneuverability, controllability, stability, and performance.
2. Pilots must carefully load the airplane within weight and balance limits.
3. Proper loading is crucial for safety and optimal aircraft performance.

Proper understanding of thrust, stability, and center of gravity is essential for safe and effective flying. By carefully managing weight and balance, pilots can ensure their aircraft performs as intended and maintain control under various flight conditions.